Table of contents

The first part of this talk will discuss the application of reversible solid oxide cells (SOCs) for grid-scale energy storage, focusing on pressurized reduced-temperature cell conditions where co-electrolysis yields methane-rich fuel.  A full analysis of a storage system utilizing low-resistance thin Lanthanum Gallate electrolyte cells and large-scale underground caverns for gas storage indicates that round-trip storage efficiency > 70% can be achieved, along with storage capacity and operating cost values that are comparable or better than pumped hydroelectric storage. 

The second part of the talk will describe results on degradation mechanisms, important for SOC storage technologies to achieve sufficient long-term durability for economic viability. LSM-YSZ and LSCF oxygen electrodes have been studied in both dc electrolysis and reversing-current (alternating between electrolysis and fuel cell operation) modes; similar delamination mechanisms are observed in both cases, although stability is improved by current cycling.  A critical current density and overpotential is observed below which degradation is too slow to measure over ~ 1000 h, but above which degradation rate increases rapidly.  High efficiency storage requires SOCs characterized by low resistance at intermediate temperatures.  This makes it desirable to utilize nano-scale electrodes, but these may be susceptible to degradation by particle coarsening. Accelerated life test results for impregnated oxygen electrode materials are presented and fitted using a combined electrochemical/coarsening model, and the resulting expressions are used to predict long-term performance degradation. Life test results on Ni-based fuel electrodes are also discussed.

To address growing energy demands while reducing CO₂ emissions and the environmental footprint of dirty solid fuels such as coal, it is essential to develop advanced technologies with high efficiencies that also facilitate easy carbon capture.  To this end, converting a solid fuel to clean hydrogen in a fuel cell with spontaneous and simultaneous generation of electricity while producing capture-ready CO₂ offers attractive opportunities and advantages. The steam-carbon fuel cell (SCFC) depicted in Fig. 1 addresses this need through higher efficiencies, and because the solid fuel is kept separate from the steam in the cathode compartment by an ionically conducting electrolyte membrane, the effluent anode product gas is a concentrated CO₂ stream that is capture ready.  Moreover, due to the downhill chemical potential gradient of oxygen across the electrolyte, steam electrolysis and electricity generation takes place spontaneously under a driving force of > 0.5 V. 

Recently our group has published work demonstrating the feasibility of hydrogen production using activated carbon as the solid fuel in a steam-carbon fuel cell.¹ While this work has demonstrated the potential of this technology, the ultimate goal is to convert real fuels such as coal or biomass, into clean fuel and electrical energy.  Biomass and especially coal contain a multitude of contaminants, many of which are potent poisons for the Ni-based cermet anodes typically used in these cells.  In particular, the high sulfur content of these fuel sources present a problem, as concentrations as high as 3000 ppm H₂S can be generated during fuel gasification, which would be fatal to cell operation. Furthermore, supported Ni-cermet anodes may show susceptibility to H₂S poisoning at concentrations as low as 1 ppm,² making their direct use with these fuels unfeasible. 

However, through a two-prong approach adopted in the present work, progress towards the ultimate goal of converting coal to clean energy can be realized. The first prong of our approach involves the use of alkaline-metal based sulfur sorbents³ that can chemically remove sulfur and reduce the H₂S concentration to less than 10 ppm. The second prong involves the development of sulfur tolerant anode materials to complement the first and improve anode catalyst lifetime and performance under low sulfur conditions.

Herein we report our progress on advancing this technology.  Through the use of sulfur sorbents that are highly dispersed on the solid fuel, it is possible to dramatically reduce the H₂S and COS contents of the syngas in situ and hence, mitigate the sulfur burden on the catalytic anode material.  Moreover, high dispersion makes it possible for high sorbent utilization that greatly reduces the need for the total amount of sorbent used.  Effects of dispersion and carbonaceous support preparation and impregnation on sulfur take up capacity are examined by TGA and gas analysis. Additionally, a variety of perovskite-based candidate materials are studied and evaluated for sulfur tolerance.  Implementation of promising compositions into the membrane electrode assemblies (MEA) is underway; and their electrochemical behavior and impact on fuel cell performance will be presented. 

If successful, this work hopes to achieve a large step towards realizing the technology for converting dirty fuels such as coal to green energy, and provides an impetus for better understanding of sulfur poisoning mechanisms, development of sulfur tolerant anode materials, and the most effective means of utilizing sulfur sorbents. Through this technology, it may be possible to take dirty fuels containing high concentrations of sulfur, and convert them into green energy in the form of hydrogen and electricity.

Figure 1

Biomass is a potential renewable source for liquid fuels and most commodity chemicals. Non-edible lignocellulosic biomass residue such as agricultural and forest wastes can be converted to liquid fuels via bio-oil production by fast pyrolysis. A variety of challenges related to unwanted characteristics of bio-oil need to be overcome for practical use of this approach. One such challenge is the high oxygen content (35 – 40 wt%) of bio-oil. The conventional approach to remove oxygen is the hydro-deoxygenation process that uses high pressure hydrogen. Typical bio-oil is biphasic and only the organic phase is processed in subsequent upgrading steps, leaving behind valuable carbon-containing material in the aqueous phase. Ceramatec and its partners, Pacific Northwest National Laboratory (PNNL), Technology Holding LLC, and Drexel University, are investigating an electrochemical process to remove oxygen from bio-oil components without the use of hydrogen. Model compounds have been tested using an oxygen ion conducting ceramic membrane based electrochemical cells operated in the temperature range of 500 – 600 °C. Under an applied electric potential, only oxygen ions are transported through the membrane. In addition to direct removal of oxygen from the oxygenated hydrocarbons, indirect removal of oxygen via reaction with hydrogen that is generated by the electrolysis of steam present in the feed is also expected to aid in the electrodeoxygenation (EDox) process.

Feasibility tests were performed using water soluble model compounds, guaiacol that has two oxygen containing functional groups, a phenolic (−OH) and methoxy (−OCH₃) groups and syringol that has a phenolic and two methoxy groups. These compounds are produced by pyrolysis of lignin. The selected model compound is co-fed with steam without using external hydrogen in the feed. A button cell configuration was used as the test vehicle with a nickel-cermet cathode and a ferrite-cobaltite perovskite anode. The results show that the EDox process produces a variety of compounds that have lower or no oxygenated functional groups. The liquid products were analyzed using GC-MS. Analysis shows that, on weight basis, a reduction of 25% and 47% of oxygen content occurred for guaiacol and syringol feed. Gas phase hydrocarbon products such as methane, ethane, propane, and propene were also identified but not included in estimating oxygen removal.

A deoxygenation trial of aqueous phase from the pyrolysis of yellow pine oil received from PNNL was also performed. Based on the analysis of liquid products collected, over 24% of oxygen removal was accomplished.

Test system modifications are planned to allow complete mass balance of the process. Electrode materials and test conditions will be modified to evaluate the effect on oxygen removal and product selectivity.

Acknowledgment:  This material is based upon work supported by the Department of Energy under Award Number DE-EE0006288.

Disclaimer:  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.  Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

Recently the development of alternative feed stocks for fuels and chemicals has received growing interest to reduce greenhouse gas (GHG) emissions from a variety of sources. One large source of GHG emissions is long haul transportation of people and goods by ground, sea, and air. For the foreseeable future, long haul transportation will continue to depend on infrastructure compatible hydrocarbon fuels. The use of fuels synthesized from alternative renewable feed stocks such as biomass is one way to reduce the GHG emissions from these modes of transportation. Processes currently being commercialized to convert biomass into fuels and chemicals rely on high energy reactions that require catalysts and/or hydrogen. These restraints require centralized processes and result in high conversion cost.

In order for biomass based fuel and chemicals to have a significant market penetration they need to compete economically with petroleum based streams, thus finding ways to reduce the cost of the biomass conversion is essential. As part of a project to produce infrastructure compatible fuel and chemicals Ceramatec is developing an electrochemical process, adapted from the Kolbe electrolysis. This process converts fatty acids from a biomass origin into long chain hydrocarbons, which can be further converted into lubricants and fuel using existing infrastructure.

The conventional Kolbe electrolysis has been investigated since 1849 when Kolbe applied it to the synthesis of different hydrocarbons. The reaction pathway is generally considered to follow a mechanism in which a one electron oxidation at an electrode surface generates a radical and CO₂. The radical can then undergo homo- or hetero- coupling with other radicals generated at the surface of the electrode.[1] While it has been extensively studied, the Kolbe electrolysis has never been widely commercialized because of low efficiency and yield caused by the high over potentials and reactivity of the electrolyte that are conventionally used.

To circumvent these shortcomings, Ceramatec's decarboxylation process uses a two compartment electrochemical reactor, where the compartments are separated by a ceramic membrane.[2] The separation afforded by the membrane (NaSelect®) is such that Na-ions are exclusively (>99% efficiency) transferred between the compartments, allowing different electrolytes to be used for the anolyte and catholyte. This affords: 1) the anolyte to be customized for the electrolysis of interest, and 2) the catholyte to be designed to have high conductivity and a low reduction potential. The latter helps reduce the overall cell potential, improving the energy efficiency of the process. The use of this membrane requires the anolyte to contain sodium salts of carboxylic acids, thus the anodic reaction can be represented generically by the reaction below.

 2RCO₂Na → R-R + 2CO₂ + 2e ^-+ 2Na⁺

 The Na-ions that are produced in the anolyte are then transferred across the membrane to the cathode compartment, where the corresponding reduction reaction occurs, as shown below using water as an example.

2H₂O + 2e^- +2Na⁺ → 2NaOH + H₂

The sodium hydroxide produced in the cathode compartment can be used to saponify the fatty acid feed stock, making it a closed loop system for sodium. Also, the use of the two compartment reactor causes the distance between the anode and cathode to be separated into three regions: 1) the distance between the anode and the membrane, 2) the membrane thickness, and 3) the distance between the membrane and the cathode. This separation decreases the length of diffusion through the anolyte, permitting the anolyte conductivity to be sacrificed for improved oxidative stability.

We have used these benefits to optimize the decarboxylation process, obtaining saturated hydrocarbons with yields and electrical efficiency over 80% while maintaining a low power consumption of 1.8 kWh/L ($0.18/L). These oxygen free hydrocarbons are produced using a modular process without the use of hydrogen. This process has also been scaled up in a self-contained portable pilot unit designed to produce over 1 L/day of saturated hydrocarbons from a tubular electrochemical reactor. The pilot unit is being used to address scale-up issues at a modular level and obtain data that is needed to help determine the economic feasibility of the process on an industrial scale.

The results of this reactor optimization and scale-up will be discussed. Also, results using the membrane reactor to produce different high value chemicals will be shared.

Acknowledgement

This material is based upon research supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, under Agreement No. 2012-10008-20263.

References

[1] H. J. Schafer, Topics in Current Chemistry, Electrochemistry IV, 1990, 152, 91.

[2] M. Karanjikar, S. Bhavaraju, A. V. Joshi, P. Chitta, D. J. Hunt, U.S. Patent 8,647,492, Feb. 2014.

Electrodes used in natural gas-to-chemicals processing must demonstrate unique performance attributes in challenging high-temperature, reducing environments.  Materials must be stable, have high electronic conductivity, and be chemically inert to methane and other hydrocarbons. Ceramic-metal composites (cermets) made of nickel and the electrolyte material, used as the fuel-side electrodes in protonic ceramic fuel cell, are not suitable as their performance decreases rapidly due to chemical reaction with the natural gas fuel. This leads to carbon-deposit formation, poisoning at the Ni phase and eventual electrode fracture [1]. These issues motivated the development of coke-resistant electrodes and catalysts. The coking rate of copper at 800 °C is reported to be two orders of magnitude lower than that of Ni [2].  This study presents a detailed electrochemical characterization of Cu electrodes on BaCe_0.2Zr_0.7Y_0.1O_3-d proton-conducting ceramic electrolyte (BCZY27).

High-quality dense BCZY27 electrolytes with large grains were successfully prepared using solid-state reactive sintering [3-4]. Cu electrodes were deposited by various techniques such as sputtering and screen-printing/painting, resulting in electrodes with different thicknesses and microstructures, making it possible to understand the chemical and electrochemical mechanisms taking place at the electrodes. Impedance spectra were recorded on symmetric cells (electrode/BCZY27/electrode) over a wide temperature range (400 - 700 °C) and in various atmospheres (dry and moist H₂, dry methane). Figure 1 displays the ASR of 500 nm sputtered porous Cu electrodes in dry and moist H₂. With this electrode architecture, lower ASR are obtained in dry H₂, while dense Cu electrodes lead to the opposite trend. Stability tests were also performed to quantify the degradation of the electrodes in hydrogen and methane at 700 °C by using both the ASR and the post-testing micrographs. In this work, the ASR of the various Cu electrodes are compared.

References:

1. C.M. Finnerty, N.J. Coe, R.H. Cunningham, R.M. Ormerod (1998) Catalysis Today 46 137.

2. P.R.S. Jackson, D.J. Young, D.L. Trimm (1986) J. Mater. Science21 4376.

3  W.G. Coors (2011) "Co-ionic conduction in protonic ceramics of the solid solution, BaCe_xZr_(1-x)Y_(y-x)O_3-d; part I: fabrication and microstructure, ceramic materials", Book 3. Intech, Croatia.

4    S. Ricote, N. Bonanos, A. Manerbino, W.G. Coors (2012) Int. J. Hydrogen Energy37 7954.

Figure 1

Non-fossil fuel based, low carbon footprint fuels are needed to ameliorate the effects of anthropogenic climate change. We have previously demonstrated molten hydroxide electrolytes for solar water splitting to hydrogen fuel, and molten carbonate electrolytes for solar carbon dioxide splitting to carbon or carbon monoxide fuels. Solid carbon (as coal) is used as the starting point to generate CO and hydrogen for the Fischer-Tropsch generation of a variety of fuels, such as synthetic diesel. However, that process is carbon dioxide emitting intensive. In this study we present the first molten electrolyte sustaining electrolytic co-production of both hydrogen and carbon products in a single cell (1). Here, hydrogen and carbon products are produced without carbon dioxide emissions and instead produced from water and carbon dioxide. The demonstrated functionality of hydroxide and carbonate electrolytes to co-generate hydrogen and carbon fuels at low electrolysis potentials, and from water and CO₂starting points, provides a significant step towards the development of renewable fuels.

• Fang-Fang Li, Shuzhi Liu, Baochen Cui, Jason Lau, Jessica Stuart, Baohui Wang, and Stuart Licht, "A one-pot synthesis of hydrogen and carbon fuels from water and carbon dioxide," recommended for publication Nov. 20, 2014, Advanced Energy Materials

Figure 1

The high ionic conductivity of the high-temperature, cubic phase of bismuth oxide has been recognized for several decades. Significant effort has been directed over the years to stabilize this phase at temperatures of interest for operation of solid oxide electrochemical cells, with Y and Er having emerged as promising substituents to achieve this goal. While stabilized bismuth oxide is not suitable as the membrane material for an electrochemical cell due to its instability under reducing atmospheres, its high conductivity, one to two orders of magnitude greater than that of yttria-stabilized zirconia, renders it attractive as a component in a composite air electrode. Here we describe oxygen electrocatalysis in both random composite structures of yttria-stabilized bismuth oxide (YSB) and lanthanum strontium manganite (LSM) and defined structures with patterned metal electrodes on YSB. The results show that, beyond the high ionic conductivity of YSB, the material provides inherent electrochemical activity for oxygen reduction/evolution reaction.

Versa Power Systems (VPS) is a developer of Solid Oxide Fuel Cell (SOFC) technology focused on SOFC stack development for commercial applications.  In recent years VPS has been developing reversible SOFC (RSOFC) materials systems with a view to future commercial development of RSOFC stacks.  VPS has demonstrated significant technical improvements in RSOFC materials technology in a relatively short timeframe, and has begun to test these at the stack level.  This paper will provide an update of RSOFC activities including cell and stack testing in both steady-state electrolysis operation and operating with RSOFC cycles (fuel cell and electrolysis cycling).

Predominantly ionic conductors and mixed ion and electron conductors (MIEC) have numerous applications in gas permeable membranes, solid oxide fuel cell (SOFC), solid oxide electrolysis cell (SOEC) to name a few. Both ionic and electronic conductivities of MIEC are of interest. A traditional way to measure partial conductivity is the Hebb-Wagner polarization method [1]. In the original work by Hebb, electronic conductivity of Ag₂S was measured as a function of position by measuring position-dependent electric potential. Application of the Hebb-Wagner method to materials such as YSZ involves fitting to the polarization equation. Since in most studies position-dependent electric potential cannot be readily measured. Most measurements are made under applied voltage large enough so that parts of the sample exhibit n-type and p-type conduction as well as the intrinsic region in which the electronic conductivity is exceptionally low. This may lead to an underestimate of electronic conductivity under certain conditions.

We recently investigated a steady state permeation technique to measure the electronic conductivity in YSZ [3]. A YSZ disc sample with embedded Pt probe is made using die-pressing and sintering at 1500°C. Pt paste is applied on both surfaces of the sample, and the rest of the bare YSZ surface is covered by a sealant glass. Measurements are made under very low applied voltages, as low as 0.01 V. Under such a low applied voltage, the entire sample is under a nearly uniform oxygen chemical potential and thus exhibits a nearly constant electronic conductivity. In that study, electronic conductivity of YSZ was measured in air, which corresponds to p-type conduction. Here we report a transient technique for the measurement of electronic conductivity of YSZ using a sample with an embedded electrode and a cavity.

The electrical circuit used in the transient technique is shown in Figure 1. During charging, oxygen ions migrate through YSZ electrolyte from electrode II to electrode I, and electrons migrate through external circuit from electrode I to electrode II. Effectively, neutral oxygen gas is pumped into YSZ (changing its stoichiometry slightly) and into cavity between YSZ and Pt probe. The kinetics of this process is dictated by oxygen ion conductivity, and the corresponding ionic resistance can be obtained. During discharge, both oxygen ions and electron migrates through the YSZ electrolyte, and neutral oxygen dissipates until its partial pressure inside the sample reaches the same value outside the sample. The kinetics of this process is limited by electronic conductivity, which can be obtained from the time dependence of the potential across the embedded electrode and the surface electrodes. Figure 2 shows the time dependence of the applied voltage, the measured current,the measured Nernst voltage, and the calculated electronic resistance of the YSZ sample at 650°C. Figure 3 shows an Arrhenius plot of the electronic resistance of YSZ. The corresponding activation energy is measured to be 1.9 eV. The results of the transient technique described here agree well with our previous steady state permeation technique.

Funded by in part DOE EFRC Grant Number DE-SC0001061 as a flow-through from the University of South Carolina and in part by DOE under Grant DE-FG02-06ER46086.

References:

1. M. Hebb, J. Chem. Phys., 20, 185 (1952)

2. I. Riess, Solid State Ionics, 91, 221 (1996)

3. L. Zhu, L. Zhang, A. Virkar, submitted to J. Electrochem. Soc.

Fig. 1a: Schematic plot of test circuit during charging and discharging stage; 1b and 1c: Schematic plot of oxygen ion and electron migration path, as well as electrode reactions.

Fig. 2a: Applied voltage; 2b: The measured current; 2c: Measured Nernst potential; 2d: calculated electronic resistance during discharging stage.

Fig. 3: Arrhenius plot of electronic resistance.

Figure 1

This work investigates the Ni/YSZ and chromium oxide (Cr₂O₃) coated Ni/YSZ cathodes as electrodes of solid oxide electrolysers for biogas reforming. XRD, TEM, SEM and XPS confirm the formation of NiCr₂O₄ on Ni surface for the coated samples while it is in-situ reduced to a core-shell structure with Ni metal coated with Cr₂O₃. The electrical properties of Ni/YSZ and Cr₂O₃-coated Ni/YSZ are investigated and correlated to their electrochemical performances. The R_p of the symmetric cell with Cr₂O₃-coated Ni/YSZ is smaller than the R_p with Ni/YSZ in methane atmosphere. Carbon fibers is observed in the Ni/YSZ electrode, rather than in the Cr₂O₃ loaded Ni/YSZ electrode after exposing in CH₄-CO₂ (1:1) atmosphere at 800 °C for 1 h. Electrochemical reforming of dry CH₄-CO₂ (1:1) mixture is successfully achieved in oxide-ion-conducting electrolysers with Ni/YSZ and Cr₂O₃-coated Ni/YSZ cathodes, respectively. Moreover, it is found that the coating of Cr₂O₃ on Ni further improves CH₄-CO₂ conversion. The higher methane and carbon dioxide conversions has demonstrated the superiority of direct electrochemical reforming compared with the low conversions under open circuit condition in oxide-ion-conducting solid oxide electrolysers.

B-site doped, A-site deficient perovskite titanate oxides with formula  (M = Ni²⁺ or Fe³⁺; x = 0.06; g = (4-n)x/2) were employed as solid oxide electrolysis cell (SOEC) cathodes for hydrogen production via high temperature steam electrolysis (HTSE) at 900 °C. A proportion of B-site dopants were exsolved at the surface in the form of metallic nanoparticles under SOEC operating (reducing) conditions, due in part to the inability of the host lattice to accommodate vacancies (introduced (d) oxygen vacancies () and fixed A-site () and inherent (g) oxygen vacancies) beyond a certain limit. The presence of electrocatalytically active Ni⁰ or Fe⁰ nanoparticles and higher  concentrations dramatically lowered the activation barrier to steam electrolysis and led to sharp rises in oxide ion mobility compared to the parent material (x = 0). La_0.4Sr_0.4Ni_0.06Ti_0.94O_2.94 demonstrated discontinuous temperature dependence possibly due to oxygen vacancy trapping at lower temperatures.

Planar, fuel electrode-supported cells were fabricated utilizing a thin (~25 μm) BaCe_xZr_0.9-xY_0.1O_3-δ electrolyte on a 1-mm-thick BCZY27/Ni fuel electrode [1]. Three electrolyte compositions were studied: x = 0, 0.1 and 0.2 referred to as BZY10, BCZY18 and BCZY27 respectively. Two different air electrodes were prepared: painted Au or a 40-μm-thick porous BCZY27 backbone infiltrated with La₂NiO_4+δ nano-particles as an electro-catalyst. As shown in Figure 1, which is an example of the cell microstructure for the infiltrated backbone, the adhesion between the electrolyte and the electrodes as well as the density of the electrolyte are satisfactory.

These devices were tested over a wide range of conditions. Both fuel-cell and electrolyser modes of operation were explored over a range of temperature spanning 550-750 °C. Flux of hydrogen was continuously monitored with a gas chromatograph and a mass spectrometer in parallel. Figure 2 displays the hydrogen fluxes in electrolysis operation measured with a BCZY27 electrolyte at 700 °C with 5% H₂, 3% H₂O (balance Ar) at the fuel electrode and 21%, 7% H₂O (balance Ar) at the air electrode. The measured hydrogen fluxes are far below the Faradaic fluxes. The difference between the measured fluxes and the Faradaic fluxes increases with increasing current. Similar results were reported by Sakai et al. [2] and Matsumoto et al.[3]. 

Oxide ion conduction is negligible as no increase of steam content was measured at the fuel electrode. Therefore, the low Faradaic efficiency can only be compensated by an electronic leakage. This work draws a comparison of the hydrogen fluxes with the percentage of cerium in the BaCe_xZr_0.9-xY_0.1O_3-δ membrane.

References:

1. S.M. Babiniec, S. Ricote, N.P. Sullivan, submitted to Journal of Power Sources

2. T. Sakai, S. Matsushita, H. Matsumoto, S. Okada, S. Hashimoto, T. Ishihara, Int. J. Hydrogen Energy 34 (2009) 59

3. H. Matsumoto, T. Sakai, Y. Okuyama, Pure Appl. Chem. 85 (2013) 427

Figure 1

Solid oxide electrolysis cells (SOECs) are considered as an effective way of converting renewable energies to chemical energy in the form of hydrogen. Using this conversion as energy storage, we can solve the site-specific and intermittent problems for renewable energies, such as solar and wind energy. During the SOEC working condition, H₂O is split into H₂ and O₂ by applying voltage. Compared with low temperature electrolysis cells, SOECs that work at high temperatures can save electricity with the compensation from heat sources [1]. However, conventional SOECs using oxygen-ion conducting electrolytes have several problems. First, the working temperature is quite high due to the use of yttria-stabilized zirconia (YSZ) as electrolyte, which possesses adequate conductivity only at high temperatures. Second, the produced H₂ is mixed with H₂O, needing further separation. Third, the Ni-based fuel electrode materials trend to be oxidized by H₂O during operation. To solve these problems, proton-conducting oxides are proposed as alternative electrolytes that show several advantages and can avoid the mentioned problems occurring for conventional oxygen-ion SOECs [2]. However, current proton-conducting SOECs focus on the use of BaCeO₃-based electrolytes, which have been demonstrated to be unstable in the presence of water. In this talk, chemically stable BaZrO₃-based electrolyte material used for proton-conducting SOECs is presented. Proton-conducting SOECs with BaZrO₃-based electrolyte show a good chemical stability, together with reasonable cell performance and a superior long-term stability. The possibility of applying proton-conducting SOECs for synthesizing CH₄ by co-electrolyzing CO₂ and H₂O will be also discussed.

Reference

1. A. Hauch, S. D. Ebbesen, S. H. Jensen and M. Mogensen, J. Mater. Chem., 2008, 18, 2331-2340.

2. Lei Bi, Samir Boulfrad and Enrico Traversa, Chem. Soc. Rev., 2014, 43, 8255-8270.

This report presents hydrogen permeation measurements through dense composite-ceramic membranes. The high-temperature operation of ceramic hydrogen-separation membranes makes them suitable in many hydrogen-production applications, such as catalytic partial oxidation of methane (CPOX), steam reforming of methane, the gasification of carbonaceous materials, and methane dehydroaromatization (MDA). Hydrogen permeation is measured through BaCe_0.8Y_0.2O_3-δ–Ce0_.8Y_0.2O_2-δ (BCY-YDC) membranes fabricated by solid-state reactive sintering with 1 wt.-% NiO and standard ceramic-processing methods. Hydrogen gas is incorporated into the membrane as protonic defects; proton transport must be compensated by electron transport in order for protons to re-associate into hydrogen gas on the opposite side of the membrane. BCY serves as the proton conductor, and YDC serves as the electronic conductor.

For hydrogen-permeation testing, the composite-ceramic membranes are hermetically sealed inside of a ceramic manifolding assembly utilizing a spring-compression system with vermiculite seals. Hydrogen and helium (for leak detection) are fed to the permeant (feed) side of the membrane using high-precision mass flow controllers, while argon is fed to the permeate (sweep) side of the membrane. All gases are humidified using a room-temperature bubbler, which results in ~3 mol.-% steam. The permeate gas composition is continuously measured using a gas chromatograph calibrated for low levels of hydrogen, helium, nitrogen, and oxygen (balance argon).  The hydrogen-permeation rate is quantified through the hydrogen mole fraction measured in the permeate exhaust. 

Hydrogen permeation rates are measured as a function of temperature and the hydrogen partial-pressure gradient. The permeation rate is found to increase exponentially with increasing temperature, and linearly with an increasing hydrogen partial-pressure gradient. The hydrogen permeation rate through a BCY-YDC membrane over several days is shown in Figure 1. No degradation in performance is observed at 900 °C and a 0.1 atm hydrogen partial-pressure gradient. However, when the gradient is increased to 0.5 atm, the permeation rate is found to decrease over time. Following testing, grain-boundary fractures and metallic nano-particles are observed in scanning electron micrographs of the membrane. This, in addition to performance degradation, may indicate that the materials are not stable in highly reducing environments. However, X-ray diffraction patterns of the membrane after testing do not reveal the formation of tertiary phases.  

Figure 1: Hydrogen permeation through a BCY-YDC membrane at 900 °C with a 0.1 and a 0.5 atm hydrogen partial-pressure gradient.

Figure 1

This presentation details the first report of a dense BaCe_0.8Y_0.2O_3-δ–Cu (BCY-Cu) ceramic-metallic (cermet) membrane for high-temperature hydrogen separation. Such a composite may have applications in catalytic partial oxidation of methane (CPOX), steam reforming of methane, the gasification of carbonaceous materials, and methane dehydroaromatization (MDA). BCY serves as the proton conductor, while Cu provides electronic conductivity for electronic-charge compensation. A novel molten-copper infiltration technique is used to form a dense cermet membrane by infiltrating a porous BCY skeleton.

The BCY skeleton fabrication begins with mixing appropriate amounts of BaCO_3, CeO_2, and Y₂O₃ powders with a mortar and pestle, followed by calcination in air. The phase-purity of the resulting powder is verified by X-ray diffraction spectroscopy. Binder is added to the powder via wet-milling with water, followed by pan drying. The powder is crushed with a mortar and pestle, and sieved to achieve a uniform particle size distribution. Pellets are formed by uniaxial dry-pressing, and subsequently sintered at 1600 °C in air, creating a skeleton with approximately 50 % open porosity (Figure 1a). 

This porous BCY skeleton is then infiltrated with copper. Cu and CuO powders are mixed with a mortar and pestle to form a powder containing 8 at.% O. This powder is uniaxially pressed into pellets, which are placed on top of the BCY skeleton. The skeleton and Cu-CuO pellet are placed into a controlled-atmosphere furnace, and heated to 1200 °C in a 330-ppm-oxygen environment (balance argon). Under these conditions, the liquid Cu spontaneously infiltrates the BCY skeleton. While dwelling at 1200 °C, the gas environment is changed to a 10 mol.% hydrogen environment (balance argon), and the sample is cooled.

Cu will not wet, nor infiltrate, a BCY skeleton in a reducing environment, which necessitates the initial heating in the oxidizing environment. Once the Cu-CuO alloy infiltrates the BCY skeleton at high temperature, the environment can be switched to a reducing environment without the Cu-CuO alloy defiltrating the BCY skeleton, due to capillary pressure. The reducing environment removes the O from the alloy, leaving pure Cu metal. An electron micrograph of a polished-cross section of the resulting membrane (Figure 1b) reveals that the cermet is nearly fully dense. Our current efforts focus on hydrogen permeation measurements through these novel cermets.

Figure 1: (a.) Scanning electron micrograph of BCY skeleton after sintering, and (b.) scanning electron micrograph of BCY-Cu membrane, in which the dark phase is Cu and the light phase is BCY.

Figure 1

Hydrogen infrastructure is becoming a major challenge for widespread introduction of fuel cell vehicles.  Electrolysis is therefore rapidly gaining international interest, particularly in the context of the penetration level of renewable energy sources on the electrical grid in places such as Europe.  Electrolysis technology can cross-link different infrastructures through capture of excess renewable power (solar or wind): the electrical grid, transportation, and chemical processing.  Electrolysis based on ion exchange membranes offers several advantages vs. traditional liquid electrolyte systems that use concentrated potassium hydroxide.  Proton exchange membrane electrolysis technology is commercially mature, and can provide higher turndown capability, lack of corrosive electrolyte, system simplicity and ease of maintenance.  PEM electrolysis is already cost competitive on an equal output capacity basis vs. other sources of hydrogen for industrial applications, but overall lifecycle cost needs to be reduced for energy markets.  This talk will focus on the material challenges in PEM systems, from the catalyst and membrane materials and supports to bipolar plates and coatings. 

In water electrolyzers, the overvoltage of the oxygen evolution catalyst is a key efficiency loss, typically contributing over 300 mV of overpotential in proton exchange membrane systems.  In addition, the catalyst loading is very high, in order to maintain activity throughout operating lifetimes surpassing 50,000 hrs, due to the lack of stability of most catalyst supports in acidic environments at electrolysis potentials.  While catalyst cost is not currently a key driver in the overall system cost, as other costs are decreased through system scale up and improvement in other processes, catalyst utilization must be improved in order to meet overall cost targets.  Therefore, research must focus not only on composition of the catalyst but also electrode structure and application method.  Proton has shown that catalyst composition, process conditions, and electrode formulation can all improve performance vs. current commercial baselines.  Catalyst loadings also have the potential to be significantly decreased without loss in performance.

The membrane is also a large contributor to the efficiency losses, especially for the much thicker membranes typically used currently vs. state of the art fuel cells, as electrolysis membranes must withstand substantial differential pressure, while fully hydrated.  However, they do not typically undergo freeze-thaw cycles, or changes in hydration.  Conductivity under hot, dry conditions is also not a primary concern.  Therefore, the material challenges and solutions are specific to electrolysis, although much of the understanding from fundamental PEM fuel cell research can be applied.  With the right combination of material properties tailored for electrolysis, Proton has demonstrated that membrane thicknesses of 50-75 microns are very feasible even at 400 psi differential pressure operation.

Finally, the bipolar assembly is the biggest cost driver, exceeding the cost of the membrane electrode assembly, primarily due to the aggressive conditions it is designed to withstand.  On the hydrogen side of the cell, the bipolar plate has to be resistant to hydrogen embrittlement, while on the oxygen side of the cell, the plate has to be corrosion resistant at potentials of 2V, in the locally acidic environment of the PEM electrode.  These constraints therefore severely limit the selection of suitable materials for this component.  Titanium is a common choice, but adds expense not only in the base material but also in the manufacturing, since titanium is typically very difficult to work.  Alternate methods of fabrication and coatings have enabled reduction of over half of the metal in the stack over the last several years, in addition to reduction in scrap.

This talk will discuss progress in each of these areas and implications in development of megawatt scale electrolysis.  Progress in system development will also be discussed.

The hybrid sulfur thermochemical cycle has seen much attention lately because of its potential to provide clean hydrogen on a large scale at a higher efficiency than water electrolysis. The two step HyS process relies on high temperature decomposition of H₂SO4 to SO₂, O₂, and H₂O, and the low temperature electrochemical oxidation of SO₂ in the presence of water to produce H₂SO₄ and H₂. Because of internal recycling of the sulfur compounds, the overall process is the decomposition of water to form H₂ and O₂. This is an interesting process because the high temperature decomposition step could be coupled to next generation power plants or high-temperature solar arrays to enable the production of H₂for other applications.

                For a gas-fed anode using a proton exchange membrane such as Nafion in the electrolyzer, we have predicted water transport and used that to calculate cell voltages and sulfuric acid concentrations as a function of operating and design variables. Acid-doped polybenximidazole (PBI) membranes are an alternative to Nafion because they do not rely on water for their proton conductivity, and therefore they offer the possibility of operating at high acid concentrations and higher temperatures to minimize voltage losses. Early studies relied on doping the PBI membranes with concentrated solutions of phosphoric acid to increase membrane conductivity. However, leaching of the phosphoric acid resulted in a gradual loss of conductivity.

                More recently, an alternative casting and doping procedure was developed for PBI membrane fabrication. Sulfonated PBI (s-PBI) membranes can be prepared using the same process starting with sulfonated monomers to impart an additional acid moiety in the polymer structure to enhance conductivity. In our research, we were able to use s-PBI membranes in the HyS electrolyzer and compared it to data collected from a Nafion-based cell.

                We have successfully operated the HyS electrolyzer using sulfuric acid-doped s-PBI membranes. We have shown that despite the relative thickness of s-PBI, the area-specific resistance of s-PBI compares favorably with Nafion and is not adversely affected by the sulfuric acid concentration at the anode. Also, s-PBI membranes provide the option of operating the cell at significantly elevated temperatures to reduce kinetic resistance. Data has also been compared to a model that has been developed to predict HyS electrolyzer performance under a variety of different operating conditions.

• Garrick, T.R.; Gulledge, A.L.; Staser, J.A.; Benicewicz, B.C.; Weidner, J.W. ECS Transactions2014, 61(28), 11-17

• Jayakumar, J.V.; Gulledge, A.L.; Staser, J.A.; Benicewicz, B.C.; Weidner, J.W. ECS Electrochemistry Letters2012, 1(6), F44-F48

• J. Staser, R. P. Ramasamy, P. Sivasubramanian, and J. W. Weidner, Electrochemical and Solid-State Letters, 10, E17 (2007)

• J. A. Staser and J. W. Weidner, J. Electrochem. Soc., 156, B16 (2009)

• J. A. Staser, K. Norman, C. H. Fujimoto, M. A. Hickner, and J. W. Weidner, J. Electrochem. Soc., 156, B842 (2009)

• J. A. Staser, M. B. Gorensek, and J. W. Weidner, J. Electrochem. Soc., 157, B836

• M. B. Gorensek, J. A. Staser, T. G. Stanford, and J. W. Weidner, Int. J. Hydrogen Energy, 34, 4701 (2009)

• J. A. Staser, M. B. Gorensek, and J. W. Weidner, J. Electrochem. Soc., 156, B16 (2009)

The electrochemical production of syngas would enable production of transportation fuels from carbon dioxide (CO₂), water and renewable energy, but a suitable process does not exist.  CO₂ reduction to carbon monoxide (CO) has been a particular problem, in that high overpotentials are often needed to drive the CO₂ reduction process, so the cost of the process was too high to be economic. Recently, we discovered a helper catalyst, an imidazolium-based ionic liquid (IL), by which CO₂ can be reduced to CO at an overpotential as low as 0.17 V on silver (Ag) particle catalysts [1]. The IL suppresses hydrogen evolution on the Ag electrode to a degree tunable by modifying, for instance, the pH and water content of the IL electrolyte.

In previous work, the currents were too low to be practical.  In the present paper, we present results using a newly developed DX4-13 membrane.   We find that the new membranes allow us to obtain 300 mA/cm² of CO current at room temperature and atmospheric pressure. 

We also find that by changing the membrane composition, or varying the pH, we can produce syngas from a single electrolyzer. We have observed syngas production from a single cell running at room temperature at 100 mA/cm² total current, however H₂production is reduced at higher currents due to CO poisoning of the hydrogen production process.   These and other results will be discussed.     

[1] Rosen, B. A.; Salehi-Khojin, A.; Thorson, M. A.; Zhu, W.; Whipple, D. T.; Kenis, P. J. A.; Masel, R. I.  Science, 2011, 334, 643.

We report the activity and selectivity of electrochemical CO₂ reduction to carbon monoxide and formate on rutile (110) and anatase (101) TiO₂. Our studies show that the rutile TiO₂ phase was found to be more selective, with Faradaic efficiencies of ~ 15% and ~ 18% for CO and formate, while the anatase efficiencies were less than ~2%.  The phase dependent efficiency exhibited by the electrochemical reaction is opposite to previously reported photochemical CO₂ reduction efficiency, which showed a greater activity from anatase TiO₂. In order to evaluate the role of oxygen vacancy and Ti³⁺ sites in the catalytic activity XPS was performed on the catalysts after CO₂ reduction. The XPS showed oxygen vacancies and Ti³⁺ defects in both the anatase and rutile phases. This implies that while the presence of surface defects may be necessary for the electro reduction of CO₂ on TiO₂ the efficiency determining property is the catalyst's underlying atomic structure. Our recent results using DFT to elucidate a potential mechanism for this phenomenon, comparing the binding energies of CO₂ reduction adsorbates on rutile and anatase TiO₂, will also be discussed.

The electrocatalytic reduction of CO₂ to fuels or chemicals is an attractive alternative to fossil- or biomass-based feedstocks. However, since the reduction typically requires protons in the reaction pathway, the evolution of H₂ is a common side reaction; in practice H₂ often dominates the observed products. In this talk, I will discuss ways in which our group is understanding and optimizing electrocatalysts for selectivity between these reactions. We will discuss ways in which we identified and optimized the expression of active edge sites in nanostructured Au catalysts for CO evolution, providing extremely high selectivities at low overpotentials. Also discussed will be the fundamental adsorbate-adsorbate interactions that drive reactivity and selectivity across various catalyst surfaces, including on Cu, on Mo, and on carbides. Strategies to overcome these limitations and provide low-overpotential CO₂ reduction will be discussed.

Introduction

Carbon dioxide (CO₂) is a readily available industrial byproduct that can be used to produce useful chemical compounds.  Electrochemical reduction is a highly effective method for converting CO₂ to CO, organic acids, low molecular weight hydrocarbons and alcohols.  Product distribution from the reduction of CO₂ is highly dependent on the composition and preparation of the catalysts and reduction potential, as such, multiple reduction pathways are observed depending on the catalysts material and the preparation methods.

Copper-based catalysts have been found to produce higher molecular weight hydrocarbons, such as ethylene, methanol and ethanol.  Here we demonstrate a high surface area Cu catalysts that has a preferential selectivity towards the formation of C2 and C3 species.  This paper will also discuss our attempts to understand the mechanistic pathway using in-situ vibrational spectroscopy under reaction conditions.

Experimental

A copper/aluminum alloy was used as the starting material for producing a high surface area copper catalyst.  The aluminum was removed through an etching procedure in strong base. The resulting nanoporous copper was crushed and mixed with Nafion, a binding agent, then casted over a copper foil substrate.  Carbon dioxide electroreduction was performed by placing the catalyst in a specially designed flow cell. A bubbler in the cell was used to deliver CO₂ to saturate the electrolyte, 0.1 M KHCO₃, at a rate of 10 mL/min. The catalyst acts as the working electrode in this electrochemical cell. The product distribution from the electroreduction process was collected at a series of potentials. Gaseous products were identified and quantified using gas chromatography coupled with both mass spectrometry and a thermal conductivity detector.  Liquid products were analyzed using nuclear magnetic resonance.

                The surface morphology and composition of the catalyst was characterized using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS).  The SEM images show a rough surface that retains a porous structure before and after experiments. XPS studies incorporated depth profiling, and show that ruthenium from the surface of the material migrates to the bulk after electrolysis.

The discovery of new structural motifs that promote high electro-reduction activity is essential for the development of electrochemical fuel synthesis. Polycrystalline Au is the most active bulk material for electrochemical CO₂ reduction to CO and serves as a useful model system to evaluate new structures [1]. We recently developed a catalyst called oxide-derived Au that has even higher selectivity than bulk Au for CO₂ reduction to CO at very low overpotentials [2]. Oxide-derived Au is a nanocrystalline material with a dense grain boundary network. Quantifying the effect of the grain boundaries in oxide-derived Au is challenging because extracting TEM samples from the oxide-derived Au films is inefficient. Here we describe the synthesis of grain-boundary rich Au catalysts by vapor deposition. The catalysts can be directly studied by TEM with minimal sample preparation. We compared the CO₂ reduction activity of as-deposited catalysts to catalysts that were annealed at various temperatures. The annealing process has very little impact on the electrochemical surface area or the distribution of Au surface facets, as determined by Cu and Pb underpotential deposition studies. Annealing does, however, reduce the grain boundary density, which is quantified by counting boundaries in a large number of individual particles using TEM. In the low overpotential regime (–0.3 V to –0.4 V vs RHE), there is a linear correlation between grain boundary density and specific current density (surface area normalized) for CO₂ reduction to CO. This quantitative correlation between defect density and catalytic activity suggests that grain boundary engineering is a fruitful avenue to explore for the development of practical CO₂ reduction catalysts.

References:

(1) Hori, Y. In Modern Aspects of Electrochemistry; Vayenas, C. G., White, R. E., Gamboa-Aldeco, M. E., Eds.; Springer: New York, 2008; Vol. 42, p 89.

(2) Chen, Y.; Li, C. W.; Kanan, M. W. J. Am. Chem. Soc.2012, 134, 19969−19972.

The sustainable civilization and long-term economic development rely on the security of energy. The conversion of carbon dioxide into fuels or commodity chemicals, incorporated with intermittent renewable energy sources like solar and wind, is an attractive venture that could offer an alternative solution to both the contemporary energy crisis and environmental issues. The recycling of carbon dioxide together with water by utilizing renewable electricity in the low temperature electrolysis process provides a diversity of carbon-neutral fuel products depending on the catalyst system.The development of active and stable catalyst system with affordable cost for the low temperature electrolysis remains a major challenge. We developed carbon-based two dimensional materials as metal-free catalysts for efficient, selective and sustainable electroreduction of carbon dioxide into carbon monoxide.

Hydrogen is not only an ideal energy carrier, but also an important agent for many industrial chemical processes. The economical production of hydrogen from electrochemical water splitting is strongly dependent on the affordable catalysts with promising activity to replace platinum-group metals. Recently, layered molybdenum and tungsten have attracted substantial interest to catalyze hydrogen evolution reaction (HER) with their suitable differential free energies for intermediate adsorbed *H at the edge sites. We aim to develop a novel series of transition-metal dichalcogenides with surface activity for HER with a combination of theoretical prediction and experimental verification.

The electrochemical reduction of CO₂ is a reaction of much current interest as a possible reaction for energy storage.¹ To date, formic acid, carbon monoxide, methanol, and oxalic acid have been prepared by this way. However, the key technological challenge for electrochemical reduction of CO₂ is the preparation of the electrode with high catalytic activity, high selectivity and long term stability. Considering these difficulties, developing new material synthesis technology to give innovative new catalysts with optimal performance is the priority.

 Tin (Sn) and copper (Cu) are considered as promising electrocatalysts to convert carbon dioxide (CO₂) to fuels (e.g. formate, methanol or hydrocarbons) because of their low cost, easy availability and reasonable overall Faradaic efficiency towards fuel production.² However, the deactivation of Sn metal electrodes during CO₂ reduction is very fast, and requires at least 0.86 V of overpotential to attain a CO₂ reduction partial current density of 4−5 mA cm^-2 in an aqueous solution saturated with 1 atm CO₂.³ Combined with the advantages of metal tin, we here report a simple one step synthesis of crystalline SnO₂ nanosphere with good electrochemical performance of high catalytic activity and high selectivity.

Unmodified Cu foil electrodes are prepared by etching briefly in 1.0M HCl, followed by ultrasonic cleaning in sequential baths of 2-propanol, acetone and de-ionized H₂O. The single-layer Cu₂O electrode is prepared by electrodepositing a Cu₂O layer onto Cu foil using a mixed CuSO₄/ lactic acid bath, adjusted to pH = 9 using NaOH and held at 65̊C for a deposition time of 30 minutes. The bi-layer electrode is prepared by first thermally oxidizing a Cu foil in air at 400̊C for 4 hours, followed by a subsequent 10 minute electrodeposition step.

The CO₂ reduction measurements are performed using a two compartment cell with a Nafion membrane separator, a Pt wire counterelectrode, and Ag/AgCl reference electrode. The working electrolyte is aqueous 0.5M KHCO₃ that is bubbled continuously with CO₂. Product concentrations are determined using gas chromatography. Gas phase products (H₂, CO, CH₄, and C2H₄) are sampled directly from the CO₂ purge gas leaving the reactor. Liquid products (mainly CH₂H₅OH) are measured by taking syringe samples of the electrolyte at 15 minute intervals.

X-ray diffraction measurements confirm that the layers grown either thermally or by electrodeposition consist mainly of Cu₂O. After being exposed to CO₂ electroreduction conditions, the XRD peaks for Cu₂O on the single-layer electrode appear to be completely removed, suggesting the electrode may have become fully reduced to Cu metal. In contrast, the bi-layer electrode still exhibits XRD peaks for Cu₂O and CuO following CO₂electroreduction, suggesting the inclusion of the thermal oxide layer has increased the electrochemical stability of the bi-layer electrode.

SEM images of the initially deposited single-layer electrode show cubic morphology characteristic of Cu₂O crystallites with a relatively large particle dimension (ca. 1μm). Following CO₂ electroreduction, the primary morphology remains intact, although many of original crystallites have broken apart, and the particle surfaces show a modest degree of roughening. For the bi-layer electrode, the initial thermal oxide layer shows a more random crystallite morphology with a some what smaller length scale (500 nm). The subsequent electrodeposited Cu₂O overlayer reflects the same smaller length scale but with a more sharply define cubic habit. Following CO₂electroreduction, the surface of the bilayer sample undergoes restructuring in a manner similar to the single-layer electrode, but with a more sharply defined morphology (i.e., well-resolved cubes with length scale about 100 nm).

The CO2 electroreduction measurements are performed at −1.745 V_SCE. As expected, the control experiment using an unmodified Cu foil produces CO and CH₄ as the greatest CO₂ reduction products (formation rates of 8.3 μmole CO /cm²/hr and 2.8 μmole CH₄/cm²/hr, respectively). In addition, the C₂H₄ formation rate was much lower (0.3 μmole C₂H₄/cm²/hr), such that the C₂H₄/CH4 ratio was only 0.1 . By comparison, the single-layer electrode produces CO and C₂H₄ as the largest CO₂ reduction products (formation rates of 4.3 μmole CO/cm²/hr and 2.5 μmole C₂H₄/cm²/hr, respectively). The CH₄ formation rate is reduced to 0.06 μmole CH₄/cm²/hr, so that the C₂H₄/CH₄ ratio is reversed to a value of 42. This effect becomes even more pronounced for the bilayer electrode. The CO and C₂H₄ formation rates increase to 15.6 μmole CO/cm²/hr and 9.1 μmole C₂H₄/cm²/hr, while the CH₄ formation rate decreases slightly, to 0.04 μmole CH₄/cm²/hr. The increased CO and C₂H₄ formation rates may be partially the result of higher electrode surface area, as suggested by a higher current density observed with the bilayer electrode. However, the C₂H₄/CH₄ratio has clearly increased to over 200, indicating that the bilayer electrode provides a higher selectivity for carbon-carbon coupling than for single carbon species hydrogenation.

Our present results support a model in which the Cu₂O particles are converted to metallic Cu during CO₂ reduction. The morphological evolution of the starting Cu₂O nanoparticles during this reduction leads to the formation of more highly dispersed Cu clusters on the surface of the converted particles. The dispersed Cu clusters are expected to contain a higher concentration of more open crystal faces and lower co-ordination surface atoms, which leads to the observation of a higher C₂H₄/CH₄ratio, relative to low-index planar Cu surfaces. Additional work is underway to further characterize these dispersed Cu clusters, and to explore methods to improve their stability during prolonged electrochemical operation.

This material is based upon work supported as part of the Center for Atomic Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001058.

Although the vast majority of fuels and hydrocarbon products are presently derived from petroleum, there is immense interest in the development of alternate routes for synthesizing these products by hydrogenating carbon oxygenates. Electrochemical methods of reducing carbon dioxide could serve as a method of storing electrical energy derived from intermittent sources like solar and wind if efficient catalysts with high hydrocarbon selectivity are developed. Although metals in the form of foils are increasingly well-characterized as electrocatalysts for carbon dioxide reduction, the activity and stability of their nanoscale counterparts remain poorly understood. We present an understanding of the electrochemical conditions and catalyst architectures that afford control over the selectivity of copper nanoparticles for electrochemical methanation. Highly dispersed copper nanoparticles supported on glassy carbon exhibit enhanced Faradaic efficiencies for methanation compared to copper foils. The improved hydrocarbon selectivity for the copper nanoparticles is due to an underlying difference in the mechanism by which electrochemical carbon dioxide reduction proceeds on the nanoparticle surface. Our understanding of highly dispersed copper nanoparticles for electrochemical methanation is a first step towards their incorporation into membrane-electrode assemblies in practical electrolyzers.

Electrochemical CO₂ reduction holds the potential to produce energy-dense liquid fuels using renewable energy sources. Previous works have shown that Cu provides some of the highest yields for valuable products such as methane or ethylene, yet the mechanisms for selectivity are not well understood.  Of course, nature performs similar reactions using enzymes including nanocluster metal alloys and ligands with extraordinary selectivity.   Here we discuss the effects of various ligands at Cu, Au and Au-Cu alloy electrocatalysts.

From the literature, results of CO₂ electroreduction on bulk Cu, Au and Au-Cu alloys are known (1-3) as are the size effects of Cu and Au nanoparticles (4, 5), but the effect of chemical surface modification is still unknown, as is the size effect of gold/copper alloy nanoparticles. 

In this work we demonstrate the effect of chemical surface modification and the size effect of Au-Cu alloy nanoparticles with the aim of combining the two effects to improve CO conversion and hydrogenation.  The effect of surface chemistry is studied using planar electrodes modified by organic compounds.  Copper and gold foils are exposed to strongly adsorbed ligands such as alkylthiols and glutathione (GSH) and subsequently used as electrocatalysts.  As shown in Figure 1, where copper foil was modified with GSH, the adsorbed ligands strongly affect the product yields and selectivity.  At the optimal CO yield potential, the yield of CO on the GSH-modified copper was 2.4 times that of blank bulk copper. Although the yield of CH₄ has been decreased by a factor of 0.5 in exchange, the overall yield of gas product has been increased by a factor of 2 on the surface engineered electrode. 

The size effect of Au-Cu nanoparticles is investigated using nanoparticles synthesized via the Brust-Schiffrin method immobilized in various binder inks as electrocatalysts for CO₂reduction.  Ultimately, we consider the potential to break scaling relationships associated with conventional metal electrodes through the use of size control, alloys and ligands. 

References

1.            Y. Hori, A. Murata, K. Kikuchi and S. Suzuki, J. Chem. Soc., Chem. Commun., 728 (1987).

2.            Y. Hori, K. Kikuchi and S. Suzuki, Chemistry Letters, 1695 (1985).

3.            J. Christophe, T. Doneux and C. Buess-Herman, Electrocatalysis, 3, 139 (2012).

4.            D. R. Kauffman, D. Alfonso, C. Matranga, H. Qian and R. Jin, Journal of the American Chemical Society, 134, 10237 (2012).

5.           R. Reske, H. Mistry, F. Behafarid, B. Roldan Cuenya and P. Strasser, Journal of the American Chemical Society (2014).

Figure 1

Conversion of CO₂ to value added chemicals can provide an attractive alternate pathway for making petrochemicals without petroleum based raw materials, storing renewable energy in chemical form, as well as providing greater acceptance to CO₂ as a valuable resource of chemical manufacture. This work deals with the electrochemical reduction of CO₂ to formate / formic acid, with specific focus on economic feasibility of this process. The current density or rate of production of product directly affects the overall capital expenditure of a large scale process by affecting the reactor requirements. In this paper, the various approaches studied to enhance the electrochemical surface area of the electrodes will be presented. Electroplating of tin of carbon fiber substrates provided a 5-10 times area enhancement. Nano particles of tin (~ 5 nm size) were formed on high surface area activated carbon black (Vulcan XC-72).  The porous electrode structure is optimized based on the binder content and loading on the base substrate. Studies were also performed to ascertain the effects of tin oxide on the reduction of CO₂ to the desired product. The time evolution of the performance of such electrodes with respect to both, the overall current density as well as the selectivity towards formate product will be presented. Characterization of the electrode and particle morphology based on 'before' and 'after' experiment SEM and TEM analysis will also be presented.

Since the strong O=O bond (498kJ mol^-1) with very slow oxygen reduction reaction (ORR) rate at the cathode has a much larger overpotential (or polarization) than that of hydrogen oxidation reaction (HOR) at the anode, the cathodic ORR has become the most challenging step in a fuel cell¹. Up to now, Pt is still the best catalyst candidate for the ORR due to its high catalytic activity. As yet, there are still two major challenges for the Pt-based catalysts, one is high cost, and the other is insufficient electrochemical durability^2-3. Therefore, the major effort in fuel cell research and development have been put on reducing Pt loading by exploring more effective synthesis technologies, and/or replacing Pt metal using other non-precious metals such as Fe, Co and Cu.  

In this regard, carbon-based transition metal macrocycle complexes catalysts have become one of the most promising candidates to replace Pt/C catalyst, especially these catalysts synthesized after thermal treatment show high catalytic activity^3,4. For revealing the mechanism of ORR and improving the ORR activity, one should know the active site in these catalysts. So far, there is a consensus that carbon, nitrogen and transition metals are all considered to be indispensable elements for the ORR activity⁵. However, for the transition metals, there is few report clearly point out whether it is a part of the active sites in these macrocycle complexes. In this work, with carbon-supported copper phthalocyanine tetrasulfonic acid tetrasodium salt, CuTSPc/C, as a target, the role of transition metal in macrocycle complexe was investigated by acid-leaching procedures.

Non precious metal (NPM) catalysts are considered as promising candidates ofreplacing Pt for the oxygen reduction reaction (ORR) inpolymer electrolyte membrane fuel cells for its low cost and rich resource. It has been reported that the substitution of some carbon atoms withheteroatoms, such as N, S, P, I and B, is an effective way totailor NPM catalysts' electron-donor properties thus effectively weakening the O−O bonding¹ and leading to a high ORR catalytic activity. Up to now, extensive research efforts have been made to explore the N doped non-noble metal oxygen reduction catalysts² since nitrogen has electronegativity higher than other elements³. In our recent work, we found that sulfur-doping could lead to increased catalyst porosity and therefore promote mass transport^4.5. As yet, the S doped ones, in particular the S and N co-doped carbon materials are less reported^6-8. On the other hand, the specific surface area and porous structure, which determine the accessible part of active sites and the transport properties of ORR-relevant species (H⁺, e⁻, O₂, H₂O), are believed to play the important role in the performance of NPM catalysts.In this regard, template method has drawn great attention to obtain the specified morphologyand predetermined microstructure^9,10. Based on the above conceptions, in this work, we report a novel kind of heteroatom-doped carbon catalyst from N-containing polymer and sulfate by template method and acid leaching.Poly(diallyldimethylammonium chloride)(abbreviate as PDDA) was employed as sources of nitrogen and carbon, ferrous sulfate as precursor of sulfur and metal, while the nanoscale silica as sacrificial supports to create pores.

The doped mesoporous carbon as the cathotdic electrode materials for oxygen reduction reaction (ORR) have drawn great attention due to the high specific surface area and porous structure, which can facilitate the accessible part of active sites and the transport properties of ORR-relevant species.^1,2 For obtaining the specified morphology and predetermined microstructure, the mesoporous carbon were usually synthesized using a sacrificial support method (SSM), during which the excess amount of hydrofluoric acid (HF) have to be added to vacate the silica and other metal-containing precursors.^2,3 The waste liquid generated in the process of centrifugal brings a lot of damage for the environment. Therefore, try to develop and/or find a green and effective method for the preparation of mesoporous/microporous carbon materical is no doubt an urgent issue.

Here, by combining a simple two-step graphitization of the impregnated carbon with sodium hydroxide (NaOH), a green and efficient  procedure has been demonstrated for the synthesis of nitrogen-doped hierarchical mesoporous/microporous carbon material using poly(ethyleneimine) (PEI) and ferrous sulfate (FeSO₄ 7H₂O) as precursors. The obtained doped mesoporous carbon shows very high catalytic activity toward oxygen reduction reaction (ORR). Additionally, the suction filtration can be adopted  rather than centrifuge and, 50% more yield of cathotdic electrode material can be obtained. Therefore we can completely use sodium hydroxide instead of hydrofluoric acid to vacate the silica, which is very green and environmentally friendly.

Electric double layer capacitors (EDLCs) have attracted widespread attention in recent years owing to their ability to supply high power in short-term pulse and long cycling durability, which make them very good energy storage devices for applications such as hybrid power sources for electrical vehicles, portable electronic devices, uninterruptible power supply (UPS), and pulse laser techniques^1-3. The electrical properties of a supercapacitor are mainly determined by the electrode material. Various materials are used in EDLCs such as carbon-based material, transition metal-oxides, and conductive polymers, in which mesoporous carbon are the most competitive electrode material for its high specific surface and low cost. In this paper, using simple template method with poly(ethyleneimine) (PEI) as sources of C, ferrous sulfate as precursor of sulfur and metal and, the nanoscale silica as sacrificial supports to create pores, N- and S- dual-doped mesoporous carbon materials are synthesized and successfully applied to EDLC with high specific capacity and cyclic stability

Introduction

The electrochemical conversion of CO₂ has been studied for many years, and copper is the only metal catalyst found to produce hydrocarbons¹. This work examines the behavior of a nanoporous copper/M electrode when used as a catalyst for the electroreduction of CO₂.

Materials and Methods

A copper/aluminum alloy was used as the starting material for producing a high surface area copper catalyst. The aluminum was removed through an etching procedure in strong base. The resulting nanoporous copper was crushed and mixed with Nafion, a binding agent, then casted over a copper foil substrate. After drying, transition metal (M) was galvanically displaced onto the copper to form nanoporous copper/M. Carbon dioxide electroreduction was performed by placing the catalyst in a specially designed flow cell. A bubbler in the cell was used to deliver CO₂ to saturate the electrolyte, 0.1 M KHCO₃, at a rate of 10 mL/min. The catalyst acts as the working electrode in this electrochemical cell. The product distribution from the electroreduction process was collected at a series of potentials. Gaseous products were identified and quantified using gas chromatography coupled with both mass spectrometry and a thermal conductivity detector. Liquid products were analyzed using nuclear magnetic resonance.

The surface morphology and composition of the catalyst was characterized using scanning electron microscopy (SEM) and x-ray photoelectron spectroscopy (XPS). The SEM images show a rough surface that retains a porous structure before and after experiments. Depth profiling studies with XPS show that the transition metal (M) from the surface of the material migrates to the bulk after electrolysis.

Results and Discussion

Identification of reduction products reveal that the catalyst is selective toward C₂ species such as ethane and ethanol. This is a phenomenon that is also observed on nanoporous copper without the transition metal, while the addition of a transition metal changes the product selectivity. The only C₁ species produced were carbon monoxide and formic acid, while no methane and methanol was detected. The most interesting product observed was n-propanol, a C₃ species.

This work provides further insight into how selectivity of the CO₂ reaction can be altered by tuning the catalyst properties. The use of a nanoporous copper catalyst provides an increased surface area and introduces many step edges, which are sites that are considered to be especially electrochemically active². The role of the transition metal has yet to be determined, however future work will focus on optimizing the selectivity as a function of controlling the Cu/M (where M = transition metals) composition in the nanoporous copper framework.

References

1] Hori, Y.; Kikuchi, K.; Suzuki, S. Chem. Lett. 1985, 1695.

2] Billy, Coleman, Walz, Co, US 62/058,121.

Figure 1

There is a growing interest in electrolysis cell technology for producing hydrogen that is today considered as an important fuel in the future systems. Electrolysis in molten carbonate salts at high temperature is a promising method for hydrogen- and/or syngas (H₂+CO₂) production. Due to the favorable thermodynamic and kinetic conditions, high-temperature electrolysis will gain higher overall efficiency and require lower applied voltage when compared to low-temperature electrolysis. Electrolysis in molten carbonates has been investigated by some authors mainly by converting CO₂into CO [1-3]. In our previous study, the feasibility of running the molten carbonate fuel cell reversibly with conventional state-of-the-art cell components is evidenced [4]. However, the Ni hydrogen electrode shows slightly higher polarization losses in water electrolysis than in hydrogen oxidation. For the development of molten carbonate electrolysis cells it is important to figure out the activity of the Ni electrode for water electrolysis in molten carbonates.    

In the present study the kinetics and reaction mechanism of hydrogen production in the Ni porous electrode in a molten carbonate electrolysis cell is investigated. For this purpose the electrochemical reaction orders of hydrogen, carbon dioxide and water were determined on basis of the steady-state polarization data. Within the temperature range of 600-650 °C the electrochemical reaction order of hydrogen is not constant; the value was found to be 0.49-0.44 at lower H₂ concentration, while increasing to 0.79-0.94 when containing 25-50% H₂, i.e. not strongly depending on the temperature. On the other hand the partial pressure dependence of CO₂ exhibits a larger influence by temperature increasing from 0.62 to 0.86 when the temperature rose from 600 to 650 °C. The reversed water-gas shift reaction has little or almost no impact on the electrochemical reaction order of hydrogen and carbon dioxide. The electrochemical reaction order of water shows two cases as does hydrogen. At lower water concentration, 10-30%, the reaction order is in the range of 0.47-0.67 while it increases to 0.83-1.07 with 30-50% water at 600-650 °C. When taking the shift equilibrium into account, the reaction order of water is constant and the value is approaching to that of higher water content. The activation energy of the Ni porous electrode for hydrogen production is in the range of 55-100 kJ·mol^-1, which indicates that the Ni electrode is under mixed-control.

Reference

[1] W.H.A. Peelen, K. Hemmes, and J. H. W. De Wit, Electrochim. Acta, 43, 763 (1997).

[2] V. Kaplan, E. Wachtel, K. Gartsman, Y. Feldman, and I. Lubomirsky, J. Electrochem. Soc., 157, B552 (2010).

[3] D. Chery, V. Albin, V. Lair, and M. Cassir, Int. J. Hydrogen Energy, 39, 12330 (2014).

[4] L. Hu, I. Rexed, G. Lindbergh, and C. Lagergen, Int. J. Hydrogen Energy, 39, 12323 (2014).

The alkaline membrane fuel cells (AMFCs), which use the anion-exchange membranes(AEMs) as electrolytes have attracted considerable attention owing to their higher reaction kinetics, lower fuel crossover, reduced CO poisoning, and use of non-precious metal catalysts. ¹ In the development of AMFCs, the AEMs are obviously the key issues to make a breakthrough in AMFC performances. However, the conductivity and stability of AAEMs are still far less than commercial membrane Nafion which has commonly been used to proton-exchange membrane fuel cells (PEMFCs).²

More recently, imidazolium-based AEMs are of interest due to the five-membered heterocyclic ring and π conjugated structure of the imidazolium cation, which is expected to have good stability in alkaline condition.^3-4 We here report novel series of alkaline anion-exchange membranes: Chitosan / poly (3-methyl-1-vinylimidazolium chloride)-Co-(1-vinylpyrrolidone) membranes (Cs/PMViC-Co-VP)and PVA/ poly (3-methyl-1-vinylimidazolium chloride)-Co-(1-vinylpyrrolidone) (PVA/PMViC-Co-VP)membranes.

The membranes were prepared by a solution-casting method, where 1g chitosan (degree of deacetylation = 80.0-95.0,supplied by Sinopharm Chemical Reagent Co. Ltd. China) was dissolved in 50 mL of 2% aqueous solution of acetic acid. Meanwhile, PVA (99% hydrolyzed, average molecular weight M_w = 86,000-89,000; Aldrich) was fully dissolved in water to make a 10% solution at 70^oC. Then PMViC-Co-VP (supplied by Aldrich) was mixed with the above chitosan solution and PVA solution, respectively. At last, 0.5 mg GA was added in two types of composite solutions for cross-linking reaction. Membranes were obtained with a thickness about 50-120 µm. The The hydroxyl ion conductivity of the obtained membrane is studied using AC impedance technique.

To overcome the barrier of  high cost caused by the exclusive use of Pt-based catalysts, the development of non-precious  metal catalysts (NPMCs) to replace Pt in polymer electrolyte membrane fuel cells has become the goal of intensive research in recent years.^1,2 Some of these NPMCs have shown remarkable catalytic activity towards ORR. Among these NPMCs, carbon-supported transition metal/nitrogen ( M/N/C, M = Fe, Co, Mn, etc.) materials have gained increased attention due to their promising catalytic activity and high durability.^3-5 However, the role of transition metal playing in the catalysts' active sites is still a subject of controversy. In order to further clarify the nature of the active sites of NPMCs, in this, with Co(SO₄)₇H₂O as the metal precursor and N,N-bis (salicylidene) ethylenediamine (Salen) as the nitrogen precursor, carbon-supported non-precious metal catalysts, Co-Salen/C, were synthesized using a facile thermal annealing approach. The catalysts were heat-treated at different temperatures (from 600- 1000^oC) to optimize oxygen reduction reaction (ORR) activity. To clarify the significance of metal (Co) content for the ORR enhancement, the catalyst was further processed by acid leaching.  

The electrocatalytic activity and electron transfer mechanism were demonstrated in oxygen-saturated alkaline electrolyte by cyclic voltammetry (CV), linear sweep voltammetry (LSV) as well as rotating disk electrode (RDE) techniques. Scanning electron microscope-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) measurements were used to identify the structure and composition of the catalysts.

Supercapacitors are one of the most promising electrochemical energy-storage device and have been widely applied due to their advantages of large power density, long cycle stability and environmentally friendly^[1]. In the past few decades, many metal oxides were investigated as active electrode material for supercapacitor, among which ruthenium oxide (RuO₂) is the best candidate^[2-3]_. However, the high cost and toxic nature of RuO₂  has  limited its practical production and commercial applications. 

Aiming at new cost-effective, high performance electrode material for supercapacitors, partial or complete replacement of precious metal has been under great effort by many research groups^[4]. Manganes oxide (MnO₂) is generally considered to be the most promising transition metal oxide for the next-generation supercapacitors by virtue of its high energy density, low cost, environmental friendliness, and natural abundance. For obtaining the specified morphology with increased surface area and predetermined microstructure, the MnO₂ has been proposed to be synthesized by various methods such as hydrothermal method^[5] and  microwave method^[6] to  realize the improved reaction kinetics. However, these methods are relatively complex, which inevitably adds the cost. In this work, use glucose reducing potassium ermanganate, the mesoporous MnO₂ can be easily prepared at ambient conditions with high production rate. Thus a very simple, economic and green synthesis procedure is realized compared to other procedures reported in the literature. In addition, the as-prepared mesoporous MnO₂ exhibits high-performance in electrochemical capacitors.

Anion-exchange membrane fuel cells (AEMFCs) researches have recently made a noticeable comeback from proton-exchange membrane fuel cells (PEMFCs) dominant era, since the AEMFCs can get faster electrokinetics, lower fuel crossover, reduced CO poisoning, and use of non-precious metal catalysts^1,2. In the development of AEMFCs, the conductivity of  alkaline anion-exchange membranes (AEMs) is considered to be the most chanellagen, since the mobility of OH^- is only 1/4 of that of the H⁺ transportation (membranes that conduct H⁺ cations), the membranes that can have high OH^- conductivity are highly desired for making a breakthrough in AEMFCs performances³. In cases where much effort has been undertaken for the above purpose and some promising candidates have been proposed such as quaternized polysulphane (PS)⁴, poly(ether imide) (PEI)⁵ and radiationgrafted PVDF and FEP⁶. However, these polymers were generally of high price and their quaternization is complex process. 

Chitosan (CS) is a low-cost biopolymer, which has very good mechanical and chemical stability along with good film formation property⁷. However, since CS is just a weak alkaline conductivity, it cannot be directly used to fuel cell membrane. In this work, we report a novel series of alkaline anion-exchange membranes: Chitosan /Bis(2-chloroethyl)ether-1,3-bis[3-(dimethylamino)propyl]urea copolymer (PUB) membranes. By incorporation of PUB, the OH^- conductivity of the CS/PUB membrane was found to be greatly improved and reached high up to 8.610×10^-3S cm^-1 at 70^oC.

One of the greatest challenges for any manned space mission is the amount of oxygen that is required, both for life support and as a chemical oxidant.  It is not feasible to carry all the oxygen needed for an extended mission to any planet. In the case of Mars, an oxygen generator may be used to separate and collect oxygen from a planet's atmosphere.  The Martian atmosphere is composed of ~95% CO₂, which may be electrolyzed to produce oxygen (O₂) and carbon monoxide (CO) (and possibly water via reaction with H₂) for human consumption or use as a chemical oxidant for power generation systems.  There are a few established methods for generating oxygen from oxygen-containing gases, but these methods have significant power requirements, and the systems produce lower purity oxygen (<97%).  A few authors have reported work on the fabrication and testing of high purity oxygen generators using various selective solid-oxide electrolysis (SOE) membranes.  Solid-oxide electrolysis membranes function by the movement of oxygen ions through a solid electrolyte membrane due to an applied field or pressure driving force.  Many recent works have centered on the co-electrolysis of CO₂/H₂O, but the study of direct electrolysis of pure CO₂ to O₂ and CO is still limited.

The objective of this work was to investigate cermet electrodes for use in solid-oxide electrolysis cells (SOECs) for the efficient reduction of CO₂.  This work includes the investigation of the optimal electrode microstructure that is required for performance stability.  In this work, palladium (Pd)- and platinum (Pt)-based compositions were mixed with yttrium-stabilized zirconia (YSZ) and gadolinium-doped ceria (GDC) to form cermet composite compositions.  The compositions were synthesized by a mixed-oxide synthesis process in which the precious metal component is mixed at various ratios with the electrolyte powders.  The particle size distribution of the electrolyte powders were varied (by combining different powder lots with different particle size distributions) in order to alter the final pore size and electrode continuity throughout the final sintered film. In addition to the particle size distribution effects on the microstructure, various levels of Pd levels within the electrolytes were investigated.  Screen-printing inks were prepared for the powders produced.  The electrode compositions were screen-printed and sintered on YSZ electrolyte substrates (~150 mm thick) to produce symmetrical cells for electrolysis testing.  Platinum electrode mesh were attached to each side of the sample with small amount of the similar ink.  Electrode interfacial polarization measurements were completed on the compositions by impedance spectroscopy (at temperatures 650-850°C) using a Solartron 1260 in air and CO₂.   Four probe current-voltage (I-V) measurements were completed in dry CO₂ on the symmetrically printed cells.  The current density versus the applied voltage were related to the calculated oxygen separation efficiency. The optimal Pd-based electrodes demonstrated O₂ production from dry CO₂ at higher level than previous reported SOE cells, with production rates as high as 9.8´10^-5 mol×cm^-2×min^-1 (for 1.5 DCV) at 800°C.

 Acknowledgements:

A portion of this work was sponsored by the NASA West Virginia Space Grant Consortium (WVSGC) and by the WVU Statler College of Engineering and Mineral Resources. The authors would also like to acknowledge West Virginia University Shared Research Facilities for support through materials characterization.

La_0.75Sr_0.25Cr_1-xMn_xO₃ perovskite family, a mixed ionic electronic conductor has promising attributes to function as fuel electrode and electrolyte membrane in high temperature electrochemical devices including solid oxide fuel cell as well as oxygen transport membrane systems. In this study, role of oxygen partial pressure on the sintering behavior of (La_0.75Sr_0.25)_0.95Cr_0.7Mn_0.3O₃ is investigated at 1400°C (simulating device fabrication condition). Unlike conventional chromite, results show a decrease in the density during exposure to reducing atmospheres. Chronology of microstructural development is presented along with mechanisms for the role of exposure conditions on the compound formation. Electrochemical impedance spectroscopy is performed on LSCM73+8YSZ//8YSZ//LSCM73+8YSZ symmetrical cells under constant bias for 80h at ~950°C (simulating device operating condition) in both oxidizing (air) and reducing atmosphere (Ar-3%H₂-3%H₂O). Stable and higher performance is obtained in oxidizing atmosphere when compared to reducing atmosphere. Sr-segregation on the LSCM73 surface is attributed to the lower performance in reducing gas atmosphere. Bulk, surface and the electrode/electrolyte interfaces are examined during post-test characterization of the tested cells. No electrode delamination as well as no interface layer or compound formation is identified in the bulk and/or interface in both oxidizing and reducing atmosphere.

The worldwide demand for energy is continuously growing and is expected to be met largely by fossil fuels. However, several aspects related to i) the depletion of fossil fuels, ii) the environmental and public health concerns arise from the associated emissions, and iii) the lower energy conversion efficiency in conventional power plants, render the development of alternative, energy-efficient technologies increasingly demanding.    

Fuel cells have emerged as a promising energy conversion technology, able to satisfy the needs of high efficiency and low environmental footprint [1]. Although H₂ can be considered as the ideal fuel cell feedstock, several barriers dealing with its limited availability in nature as a free substance, the absence of relevant infrastructure and the associated difficulties with its storage and distribution prevent the development and commercialization of H₂-fed fuel cells [2].

Commercial-grade liquid hydrocarbons, such as gasoline, comprise the most proper feedstock for on-board reformation into hydrogen, due to their well-established infrastructure for storage and transportation. SOFCs due to their high operating temperature, can be potentially operated directly (internal reforming) on conventional hydrocarbons fuels [2]. However, the development of high efficiency electro-catalysts, with adequate activity, electron conductivity and carbon tolerance should be developed.

Ni-based anode materials are usually employed in SOFC applications, since they exhibit adequate catalytic activity toward reforming reactions and thermal stability [3]. However, Ni-based anodes are prone to carbon poisoning when directly exposed to hydrocarbon fuels [3]. Recently, the substitution of Ni cermets by Cu-based catalysts in direct hydrocarbons fuel cells has been proposed, owing to their electrochemical activity and resistance to carbon [2]. It has been demonstrated that the employment of Cu-CeO₂/YSZ composites as anode materials results in a stable power generation in direct hydrocarbons fuel cells [2]. 

In the present communication, iso-octane, a common surrogate for gasoline, is employed as a fuel in a Cu-CeO₂/YSZ/Pt type SOFC reactor, at atmospheric pressure. The effect of cell temperature, reactants' partial pressure and imposed overpotentials on products' distribution and overall catalytic and electro-catalytic activity was examined. Furthermore the as prepared cell was electrochemically characterized employing typical fuel cell measurements and AC impedance spectroscopy studies. 

Iso-octane was efficiently reformed by H₂O to syngas over Cu/CeO₂ electrodes. In all cases examined, no carbon was detected, which was attributed to the favoured gasification of carbonaceous deposits by H₂O. Cu-CeO₂ exhibited high catalytic activity toward the electro-oxidation of all combustible species that are present at the anode. Under "fuel cell" mode of operation, the achieved power densities were substantially increased with increase in temperature and P_i-C8H18/P_H2O feed ratios (Fig. 1). AC impedance spectra showed contributions both from charge and mass transfer processes depicted at high and low frequencies, respectively. Increase in temperature reduced both the Area Specific Resistance (E_act= 1.15 eV) and interfacial resistance, with the latter being attributed to charge transfer processes (E_act= 1.59 eV) and diffusion limitations (E_act= 1.07 eV). The effect of H₂O and i-C₈H₁₈ concentrations was more intense in the case of the resistance associated to mass transfer limitations, while both the ASR and the resistance attributed to the charge transfer processes at electrode and electrode/electrolyte interface were almost independent of reactants feed concentration. 

References

[1]  Singhal SC, Kendall K, High Temperature Solid Oxide Fuel Cells: Fundamentals, Design and Applications, Elsevier Advanced Technology, 2003.

[2]  Gorte RJ, Vohs JM, McIntosh S. Solid State Ionics 2004;175(1-4):1-6.

[3]  He H, Vohs JM, Gorte RJ. J Power Sources 2005;144(1):135-140.

Figure 1

There has been growing interest in the electrochemical reduction of carbon dioxide (CO₂), a potent greenhouse gas and a contributor to global climate change, and its conversion into useful carbon-based fuels or chemicals. Numerous homogeneous and heterogeneous catalytic systems have been proposed to induce the CO₂ reduction and, depending on the reaction conditions (applied potential, choice of buffer, its strength and pH, local CO₂ concentration or the catalyst used), various products that include carbon monoxide, oxalate, formate, carboxylic acids, formaldehyde, acetone or methanol, as well as such hydrocarbons as methane, ethane, and ethylene, are typically observed at different ratios. These reaction products are of potential importance to energy technology, food research, medical applications and fabrication of plastic materials.

Given the fact that the CO₂ molecule is very stable, its electroreduction processes are characterized by large overpotentials, and they are not energy efficient. To produce highly efficient and selective electrocatalysts, the transition-metal-based molecular materials are often considered. Because reduction of CO₂ can effectively occur by hydrogenation, in the present work, we concentrate first on such a model catalytic system as nanostructured metallic palladium capable of absorbing reactive hydrogen in addition to the ability to adsorb monoatomic hydrogen at the interface. We are going to demonstrate that palladium nanocenters can be generated within the coordination architecture of tridentate Schiff-base-ligands by electrodeposition from the supramolecular complex of palladium(II), [Pd(C₁₄H₁₂N₂O₃)Cl₂]₂MeOH. The resulting Pd nanoparticles (diameters, 5-10 nm) are stabilized and activated by nitrogen coordination sites, and the electrocatalytic system exhibits appreciable activity toward reduction carbon oxide (IV) in 0.1 mol dm^-3 KHCO₃.  The respective voltammetric peak currents are ca. three times larger than those observed at conventional palladium nanoparticles (diameter, ca. 10-20 nm) under analogous experimental conditions and at the same loading of palladium (100 μg cm^-2).  Despite that fact that the degree of agglomeration of nanostructured palladium is much lower when it has been generated within the macromolecular network, it is reasonable to expect that some specific interactions between nitrogen coordination centers and metallic Pd exist. The process of electrosorption of hydrogen at the Schiff-base-ligand supported palladium nanostructures seems to be more reversible (when investigated in KHCO₃) and dominated by the hydrogen absorption rather than the surface adsorption phenomena (characteristic of conventional Pd nanoparticles).

We are also going to address the possibility of utilization of the tridentate Schiff-base-ligand coordination architectures as matrices for cobalt and cooper catalytic sites during electrooxidation of carbon dioxide.

We acknowledge collaboration with Adam Gorczynski, Maciej Kubicki, and Violetta Patroniak from Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland that has led to fabrication of supramolecular coordination compounds.