Efficient Oxidation of Ethanol at Ru@Pt Core-Shell Catalysts in a Proton Exchange Membrane Electrolysis Cell

Efficient electrochemical oxidation of ethanol in fuel cells and electrolysis cells is important for generating power and hydrogen, respectively, from renewable resources. PtRu alloys are most widely employed as catalysts because they provide high activities at low potentials. However, they produce acetic acid as the main product from ethanol, which results in low faradaic and overall efficiencies. In contrast, Pt provides high selectivity for the complete oxidation of ethanol to CO2, but low activities. Ru@Pt core–shell nanoparticles can improve efficiency by delivering higher activity than Pt and enhanced formation of CO2 relative to PtRu. Here, Ru@Pt catalysts have been prepared by depositing Pt onto a commercial carbon-supported Ru catalyst. The influence of the amount of Pt deposited has been investigated in H2SO4(aq) at ambient temperature and in a proton exchange membrane cell at 80 °C. Activities for ethanol oxidation were intermediate between those for commercial Pt and PtRu catalysts, providing higher currents than Pt at low potentials, and higher currents than PtRu at high potentials. Faradaic yields of CO2 (38%–48%) were greatly increased relative to the PtRu alloy catalyst (11%). This will optimize the efficiency of ethanol oxidation in PEM electrolysis and fuel cells.

There have been many studies reporting that oxidation of low molar mass alcohols in proton exchange membrane electrolysis cells (PEMEC) can produce hydrogen at much lower voltages than water electrolysis. [1][2][3][4][5] The alcohol in a PEMEC is oxidized on the surface of an anodic catalyst. A proton exchange membrane (PEM) such as Nafion™ allows the protons formed to be transferred to the cathode and reduced to form hydrogen.
Many factors influence the choice of anodic catalyst, including their ability to adsorb the alcohol molecules and complete their oxidation to CO 2 . The efficiency of the catalyst is determined by both the potential required (anode overpotential) and the amount of CO 2 generated. 4,6,7 The formation of partial oxidation products, particularly acetic acid and acetaldehyde from ethanol oxidation, decreases the amount of hydrogen produced and requires methods for waste management and emission control. A key objective in the development of catalysts for ethanol oxidation is to improve the faradiac efficiency by increasing the yield of CO 2 .
Platinum has a high ability to adsorb alcohol molecules and produce a high yield of CO 2 . 6 However, modification of Pt catalysts (e.g. alloying with Ru or surface modification) is still required to remove strongly adsorbed CO that causes high anode overpotentials. 8,9 Deposition of a few monolayers of Pt onto a more active metal such as Ru to create core-shell nanoparticles (Ru@Pt) [10][11][12] can decrease the overpotential relative to Pt, 13,14 while improving selectivity for CO 2 formation relative to PtRu alloys. 14 Ligand and lattice strain effects that result from interactions between the Pt shell and Ru core weaken the bonding of CO to the Pt surface. 10,11 The Pt shell can also increase stability and protect the Ru from dissolution during operation. 15,16 As a result, Ru@Pt catalysts have recently been used for the electrochemical oxidation of organic fuels, including methanol 14,17-27 and ethanol. 13,14,19,20,[27][28][29] Methanol electrooxidation has been studied more than ethanol because it occurs at lower overpotentials and methanol can be oxidized completely to CO 2 because it does not require cleavage of a C-C bond. However, ethanol is a much less toxic fuel than methanol and can be produced in large quantities from biomass. Additionally, a Pt@Ru catalyst capable of dissociating the C-C bond and oxidizing the adsorbed CO intermediate more easily could potentially produce higher current densities than for methanol.
Ru nanoparticle size, 13,18 crystallinity of the Ru core, 13,23,29 and Pt shell thickness 14,17,18 are the main factors that affect the catalytic activity of Ru@Pt catalysts toward the oxidation of alcohols. In nanoparticle syntheses, capping agents such as polyvinyl pyrrolidone (PVP) serve as stabilizers to prevent their overgrowth. 17 They can control the size distribution and produce uniform shell thickness. However, the active surface area of the catalyst can be significantly reduced due to the difficulty of removing capping agents. 13,19,29 Hseieh et al. 30 have reported atomically ordered Ru@Pt nanoparticles prepared by using ethanol as a reductant in an alkaline medium at a temperature lower than 100 o C. These crystalline core-shell nanoparticles prevented partial alloying at the Ru/Pt interface, which greatly increases the catalytic activity for methanol and ethanol oxidation. 13,29,30 However, large particle sizes, broadening of the size distribution, less spherical shape and non-uniform Pt shells were identified as limitations of this method. 13,29 Hu et al. 19 prepared Ru@Pt catalysts by using ethylene glycol as a reducing agent and a fast microwave technique. The resulting spherical nanoparticles had a small particle size with a narrow size distribution and uniform Pt shell thickness, but had an amorphous structure. Their studies show higher reactivity for ethanol oxidation than methanol oxidation.
The development of more efficient anode catalysts for alcohol PEMEC also benefits the growth of direct alcohol fuel cells (PEMFC such as direct methanol fuel cells and direct ethanol fuel cells), which employ the same types of cells and anodes but produce electrical power through the spontaneous reaction of oxygen at the cathode. [31][32][33][34] Chemical reaction between the fuel and the oxygen in a PEMFC due to fuel and oxygen crossover through the membrane makes it extremely challenging to determine the anode potential and the product distribution, resulting in inaccurate measurement of catalyst performance. 35,36 Consequently, anodic catalysts for fuel cells can be more accurately characterized in a PEMEC, where the anode potential is measured relative to hydrogen evolution at the cathode (i.e. a dynamic hydrogen electrode), and products from oxidation of the alcohol arise solely from the electrochemical reaction at the anode. 6,35,36 In this work, we prepared three Ru@Pt nanoparticle catalysts with different amounts (monolayers) of Pt deposited on the surface of highly crystalline, carbon-supported Ru nanoparticles (Ru/C) with a narrow particle size distribution. The aim of using crystalline Ru was to prevent partial alloying at the Ru/Pt interface, as previously reported. 29,30 Ethylene glycol was used as a reducing agent and solvent, without adding a capping agent, to form spherical nanoparticles of narrow size distribution, as shown in our previous work. 37 We report the catalytic activity of the new Ru@Pt catalysts toward ethanol electrooxidation in aqueous sulfuric acid at ambient temperature and in a PEMEC at 80°C, in order to investigate the relationship been activities under these different conditions. Measurements in the PEMEC focus on the relationships between activity and efficiency, and how these change with the composition and structure of the catalyst. A low concentration (0.1 M) of ethanol was used in order to increase efficiency for the complete oxidation of ethanol, as discussed below. Data for methanol oxidation is also reported, in order to provide further insight into the mechanistic role of the core-shell structures.
All the Ru@Pt catalysts gave significantly higher currents at low potentials than a commercial Pt/C catalyst, which indicates that deposition of Pt onto the Ru core, using ethylene glycol as a reducing agent, promotes the activity of Pt at the nanoparticle surface. As a result, these catalysts are promising anodic catalysts for use in ethanol electrolysis cells and fuel cells.
Synthesis of Ru@Pt catalysts.-500 mg of 40% Ru/C in 40 ml of EG was sonicated for ca. 40 min at 60 o C and then purged with hydrogen (5% in N 2 ) for 20 min to remove oxides and hydroxides from the Ru surface. 38 The required amount of H 2 PtCl 6 .6H 2 O dissolved in 10 ml of EG was added dropwise at ambient temperature with vigorous stirring. The temperature was gradually increased to 185 o C and held at this temperature for 2 h. The catalyst was collected using a centrifuge (5 min at 9000 rpm), washed several times with boiling water and dried at 80 o C for 8 h. The catalysts are labelled as Ru@Pt 0.6 , Ru@Pt 1.0 and Ru@Pt 1.4 according to their target atomic ratios.
Physical characterization.-A Q500 thermogravimetric analyzer (TGA) was used to determine the metal loading (mass% of Ru+Pt) on the carbon black. Ca. 6 mg of the well-dried catalyst was used under air, and the temperature was increased with a ramp of 20 o C min −1 to 800 o C. Pt:Ru atomic ratios were determined using an energy-dispersive X-ray analyzer (EDX) attached to an FEI Quanta 400 scanning electron microscope (SEM). The catalysts (ca. 2 mg) were dispersed in a mixture of water, 1-propanol and 2-propanol (0.2 ml) for application to the SEM carbon tabs.
A Rigaku Ultima IV X-ray diffractometer (XRD) with a Cu Kα radiation source (0.154 nm) was used to study the crystal structure of the catalysts. The diffracted radiation was detected by using a scintillation counter analyzer. X-ray photoelectron spectroscopy (XPS) was conducted using a VG Micro-tech Multi-lab ESCA 2000 system at Dalhousie University (Halifax, Nova Scotia, Canada) by Andrew George.
A Tecnai TM Spirit transmission electron microscope (TEM, Faculty of Medicine at Memorial University) was used to determine the average particle size, size distribution and the degree of dispersion on the carbon support. An average nanoparticle size for each catalyst was obtained by measuring the diameters of at least 80 randomly selected nanoparticles using the Image J 1.53a program. TEM samples were prepared by dispersing 1 mg of catalyst in 200 μl of a water, 1-propanol, and 2-propanol mixture by sonication for at least 1 h, then one droplet of the ink was placed on the TEM grid.
Electrochemical characterization at ambient temperature.-An SP-50 Biologic potentiostat with EC-lab software was used for cyclic voltammetry (CV) in a three-electrode glass cell, with a Pt wire counter electrode and standard calomel reference electrode (SCE). Carbon fiber paper (CFP, 0.24 cm 2 , Toray TGP-H-090) working electrodes were painted with the desired volume of catalyst ink to give a 0.3 mg cm −2 metal (Ru+Pt) loading. The catalyst ink was prepared by sonication of 2 mg of catalyst in 200 μl of a solvent mixture consisting of water, 1-propanol, and 2-propanol at volume ratios of 2:3:3. All CV measurements were made in N 2 purged 1 M aqueous sulfuric acid. Following the recording of background CV (six cycles at 100 mV s −1 ), methanol or ethanol was added to give a concentration of 0.1 M. The alcohol was homogeneously mixed by purging with N 2 for 3 min. CV for oxidation of the alcohols were obtained for three cycles from −0.25 V to 0.80 V at ambient temperature at a scan rate of 10 mV s −1 . The final anodic scan is shown in all cases.
PEM electrolysis cell for alcohol oxidation studies at 80 o C.-A commercial fuel cell (Electrochem Inc.) modified to accommodate an array of nine 0.24 cm 2 anodes was operated as an electrolysis cell in crossover mode at 80 o C, as previously described. 39 This allowed the three Ru@Pt catalysts to be tested simultaneously using three electrodes for each catalyst to assess reproducibility. In the crossover mode employed, 0.1 M of methanol or ethanol was pumped through the cathode chamber at a flow rate of 0.5 ml min −1 , in order to provide controlled diffusion to the anode through the Nafion membrane. The anode was purged with N 2 gas at 10 ml min −1 .
A single 5 cm 2 Pt black on CFP cathode was employed with a Nafion-117 membrane. The cell was operated with an MSTAT potentiostat from Arbin instruments which controlled the potential applied to each anode relative to the cathode, which acts as a dynamic hydrogen electrode (DHE). 39 Anodes were prepared by painting catalyst inks onto CFP disks (0.24 cm 2 ) to give a metal loading of 2 mg cm −2 . A 5% Nafion solution was then applied to the catalyst surface, equivalent to 30% of the total catalyst + Nafion mass. Catalyst inks were prepared by dispersion of 4 mg of catalyst in 200 μl of a mixture of water, isopropanol, and 1-propanol (2:3:3).
All anodes were set to a constant potential of 0.70 V (vs DHE) for at least 1 h before each polarization curve measurement. Polarization curves were obtained by measuring the current for 3 min at constant potentials, starting at 0.90 V. The average current for the last 60 s of each applied potential is reported. Initially, consecutive polarization curves for methanol oxidation (four) were recorded until they were consistent. Then two consecutive curves were recorded for ethanol oxidation. The final polarization curve is reported in all cases. CO 2 generated by oxidation of the alcohols at 0.50 V was monitored using an infrared CO 2 sensor (Telaire T6615) as previously described. 40 During CO 2 measurements, the N 2 flow rate was increased to 50 ml min −1 while the alcohol flow rate was decreased to 0.2 ml min −1 .

Results and Discussion
Characterization of the Ru@Pt catalysts.-The total metal (Pt +Ru) content of each catalyst was determined through TGA as the residual mass percent at 800 o C as shown in Fig. S1 (Supplementary Material) and reported in Table I. As Pt was added, the residual mass percent increased as expected, and there was reasonable agreement with the target values. However, the measured values for Ru@Pt 0.6 and Ru@Pt 1.0 are a little higher than the target values, and this may be due to the loss of some of the carbon support during collection and washing of the catalysts.
EDX spectra for the catalysts, and SEM images of the analysed areas, are shown in Fig. S2. All spectra show peaks for both Pt and Ru at 2.05 keV and 2.56 keV, respectively as expected. The Pt peak intensity increased relative to Ru as more Pt was deposited. The measured Pt:Ru atomic ratios are compared with the target ratios in Table I and show that there was quantitative deposition of the Pt.
TEM images and particles size distribution histograms of the Ru and Ru@Pt catalysts are shown in Fig. S3. The images show that the nanoparticles had an approximately spherical shape and were well dispersed on the carbon support with low agglomeration. The histograms indicate that the particle size distributions were relatively narrow, evidence of the formation of uniform Pt shells. The average diameters of the metal nanoparticles (D Ru@Pt ) are reported in Table II, together with the expected sizes of the Ru@Pt core-shell nanoparticle calculated by using Eq. 1. 13 where n Pt and n Ru are the target number of moles of Pt and Ru per particle, respectively, D Ru is the measured diameter of the Ru core (3.9 nm), M Pt and M Ru are the molar mass of Pt and Ru, respectively, and P Pt and P Ru are the densities of Pt and Ru, respectively. It can be seen that the measure sizes agree well with the expected sizes, providing strong evidence that core-shell nanoparticle structures had been formed with the target compositions. The thicknesses of the Pt shells can be calculated in Pt monolayers (ML) from D Ru , the diameter of a Pt atom (D Pt atom ) and D Ru@Pt , by using Eq. 2. The resulting values are reported in Table II.

Ru Pt Ru
Pt atom @ In all cases, more than one monolayer was deposited, which means that Pt atoms should have completely covered the surface of the Ru core. The highest coverage corresponds to more than 2 complete monolayers of Pt, which would significantly decrease the strain and ligand effects of the Ru core. 42 XRD spectra of the Ru@Pt catalysts are shown in Fig. 1, together with spectra for the commercial Pt, Ru, and PtRu catalysts for comparison. For greater clarity, the Ru and Ru@Pt spectra are offset in Figure S4, and peaks for each lattice plane are identified. Pt has an FCC structure, while the Ru core has an HCP structure. The XRD spectra of the Ru@Pt catalysts all show FCC peaks for Pt, which confirms the reduction of the H 2 PtCl 6 to Pt metal. As the thickness of the Pt shell was increased, the intensity of the Pt diffraction peaks increased relative to the Ru peaks. In addition, the positions of the Pt diffraction peaks were close to the positions for pure Pt, as shown in Table III for the Pt (111) and (220) peaks. This is strong evidence for the deposition of the Pt onto the surface of the Ru nanoparticles. 13,29 A significant strain effect is induced by the mismatch between Ru and Pt lattices, which alters the Pt-Pt interatomic distance, and a method to calculate this has been established. 42 Based on the strain values calculated from the positions of the Pt(111) peak (Table III), the Ru@Pt catalysts show compression of the Pt lattice, which decreased as the thickness of the Pt shell was increased.
The Ru diffraction peaks for the Ru@Pt catalysts appear at the same positions as for the Ru/C catalyst. This indicates that the high crystallinity of the Ru core was maintained, and that there was not significant mixing with the Pt shell. 38,43 Moreover, as shown in Fig. 1, the Pt diffraction peaks of the Ru@Pt catalysts are at significantly different positions than those of PtRu, which indicates that they were not alloys.
XPS spectra for the Ru@Pt catalysts were very similar to the spectrum for the PtRu catalyst (Fig. 2). As expected, the ratio of the intensity of the Pt peaks to the Ru peaks increased as Pt was added to the Ru core. Compositions of the metal nanoparticles (atomic Pt percentage) calculated from the relative intensities of the Pt 4 f 7/2   Table IV. Whereas the measured (apparent) Pt content was slightly lower than expected for the PtRu alloy, it was higher than expected for the Ru@Pt catalysts, indicating that Pt had been deposited onto the Ru cores. Although the measured Pt content increased as more Pt was added, the difference from the expected value decreased. This suggests that Pt did not completely cover the Ru surface, and therefore was not deposited uniformly. There were no significant differences between the Pt 4f binding energies of the three Ru@Pt catalysts and the PtRu catalyst ( Figure  S5 and Table IV), indicating that the electronic effect of the Ru core did not vary significantly. The 4f 7/2 binding energy is similar to a value of 71.06 eV reported for pure Pt nanoparticles. 44 The O 1 s regions of the XPS spectra are shown in Figure S6. Components at 529.63 eV and 530.73 eV for the PtRu catalyst were previously assigned to O 2− and OH − species, respectively, at oxidized Ru sites, while the 532.18 eV component is primarily due to C-O groups on the carbon support. 37 The intensity of the O 1 s peak decreased greatly relative to the Pt peaks as Pt was added to the core, indicating substantial coverage of the Ru surface. However, the persistence of the Ru-O/Ru-OH component at ca. 530 eV indicates that there was incomplete coverage of the core. 37 It is not clear why the O 2− and OH − components are not resolved for the Ru@Pt catalysts, but it does suggest that there are some differences in the oxide species that are present relative to PtRu. Presumably, there is also an unresolved Pt-OH component at ca. 530 eV. 45 Cyclic voltammetry in aqueous sulfuric acid.-Cyclic voltammograms of the Ru@Pt catalysts in 1 M sulfuric acid are shown in Fig. 3, together with those for the Ru, Pt and PtRu catalysts. Similar features are seen for the three Ru@Pt catalysts, with decreasing currents as the Pt shell thickness was increased. Since a constant metal (Ru+Pt) loading was employed, this can be attributed to the decreasing surface area/mass ratio of the Ru@Pt nanoparticles.
The hydrogen desorption and adsorption region, below 0.15 V in the voltammograms, provides crucial information on the nature of the surface of the catalyst nanoparticles. 37 The hydrogen desorption region for the Pt catalyst is characterized by two peaks/shoulders at ca. 0 V and −0.10 V. In contrast, the Ru and PtRu catalysts do not show these distinct features. However, all of the Ru@Pt catalysts show shapes that are characteristic of Pt with peaks at similar potentials, indicating that they had predominantly Pt surfaces. However, the persistence of higher currents than pure Pt in the 0.1 to 0.3 V region indicates that some Ru remained at the surface for even the highest Pt coverage. 46 This is consistent with the XPS results and suggests that Pt may not have been deposited evenly on the Ru cores. 13,29 Significantly, there is a cathodic peak at 0.52 V in the oxide region of the Pt voltammogram, which is due to the reduction of Pt-    OH. The Ru@Pt catalysts do not possess this feature due to the electronic and/or strain effects of the Ru on the reactivity of the Pt surface. 22,37,46 Methanol and ethanol oxidation in aqueous sulfuric acid.-Linear sweep voltammograms for methanol oxidation at the Ru@Pt, Pt and PtRu catalysts are shown in Fig. 4. As expected from other reports, the Ru@Pt catalysts were much more active for methanol oxidation than the PtRu catalyst. However, their activities were lower than for the Pt catalyst at high potentials. Changing the Pt shell thickness had relatively minor influences on the activities of the Ru@Pt catalysts, indicating dominance of the longer-range strain effects of the Ru core over electronic effects, which are much weaker for the second Pt monolayer, 18,42 and the bifunctional effect of exposed Ru.
Although the Pt catalyst produced the highest peak current, the Ru@Pt catalysts all gave lower half-wave potentials, indicating faster kinetics for methanol oxidation. This can be attributed to the interactions (ligand and strain) between the Pt shell and the Ru core. 14,18,29 The Ru@Pt 1.4 catalyst (2.5 ML) gave the highest currents at potentials above ca. 0.43 V, indicating that it was most Pt-like. However, the Ru@Pt 0.6 catalyst (1.3 ML) showed the lowest onset potential and highest currents below ca. 0.42 V, which are more similar to the PtRu catalyst. The higher currents for the Ru@Pt 0.6 catalyst relative to PtRu at all potentials can be attributed to the higher coverage of surface Pt sites, which are required for methanol adsorption and dissociation. This clearly demonstrates the value of combining the ligand and strain effects of Ru with a Pt-rich surface.
Cyclic voltammograms for ethanol oxidation at the Ru@Pt, Pt and PtRu catalysts are shown in Fig. 5. The Ru@Pt catalysts all produced much higher currents than the Pt catalyst at potentials below 0.5 V, and higher currents than PtRu above 0.40 V. Since optimizing the performances of ethanol PEMEC and PEMFC requires optimizing the balance between current at low and high potentials, 6 the Ru@Pt catalysts employed here are particularly attractive. In addition, the Pt-like peak at ca. 0.65 V for the Ru@Pt catalysts suggests that there was improved selectivity for the complete oxidation of ethanol to CO 2 . 14,37 In contrast, PtRu is much more active than Pt for partial oxidation of ethanol to acetic acid at low potentials, but produces little CO 2 . 6 As for methanol oxidation, changing the Pt shell thickness had only minor influences on the activities of the Ru@Pt catalysts. This indicates that the strain effect of the Ru core has the dominant impact on activity at low potentials. The somewhat lower performance of the Ru@Pt 0.6 catalyst suggests that the combined electronic and strain effects may be too intense for efficient ethanol adsorption. 14,47 Oxidation of methanol and ethanol in a PEM electrolysis cell at 80 o C.-Polarization curves for oxidation of 0.1 M methanol and ethanol in a PEMEC at 80 o C are shown in Fig. 6. For all of the   Ru@Pt catalysts, the onset and half-wave potentials were much lower than for the Pt catalyst, but higher than for PtRu. The improved performance relative to Pt can be attributed to the ligand, strain, and bifunctional effects of the Ru core which enhance the oxidation of the common adsorbed CO intermediate. 14 As also seen in the voltammetric data in Figs. 4 and 5, variation of the Pt shell thickness of the Ru@Pt catalysts had relatively minor effects on their performances in the PEMEC.
Since the PEMEC was operated in crossover mode, the currents are determined by the rate of steady-state mass transport at high potentials. 39 For methanol oxidation (Fig. 6A), the current became almost constant at high potentials, as expected for limitation of the current by the rate of methanol diffusion through the Nafion membrane. The variations seen for the different catalysts are due primarily to experiment uncertainly, 39 although they may include contributions from variations in the product distribution between formaldehyde, formic acid, and CO 2 .
For ethanol oxidation, it has been shown that variations in the yields of acetaldehyde, acetic acid, and CO 2 cause large variations in the mass transport limited current between catalysts, and variations with potential. 14,27,37,39  where I lim is the mass transport limited current, n av is the stoichiometry of the reaction (the average number of transferred electrons per molecule), C is the concentration of the alcohol solution, A is the electrode area, and m is the mass transport coefficient. The stoichiometry is a weighted average of the stoichiometries for each product (Eq. 4), where n i is the number of transferred electrons to form product i (for ethanol oxidation, n acetaldehyde = 2, n acetic acid = 4, and n co₂ = 12) and x i is the fraction of molecules that are converted to product i. Equation 3 provides an explanation for the differences between the methanol and ethanol polarization curves at high potentials (> 0.5 V) in Fig. 6. Whereas methanol is oxidized mainly to CO 2 (n av ∼n co₂ = 6), the main product from ethanol oxidation is acetic acid (n acetic acid = 4), and there are considerable variations in product distribution between different catalysts. 6 Mass transport limited currents for ethanol oxidation are usually lower than for methanol oxidation because of the typically lower stoichiometry and the lower diffusion coefficient (lower m) of ethanol. 39 The decreasing mass transport limited currents for ethanol at high potentials are due to decreasing yields of CO 2 (n co₂ = 12) as the potential is increased. 6 Stoichiometries for ethanol oxidation, calculated from the polarization curves in Fig. 6B by using Eq. 3 as previously described, 39 are shown as a function of potential in Fig. 7. Values below 0.5 V can not be accurately determined because of uncertainty in the influence of the electron transfer kinetics. 39 Stoichiometries for the Ru@Pt catalysts are intermediate between those of PtRu and Pt at most potentials, although values for the Ru@Pt 1.4 catalyst were significantly higher than both at potentials above 0.7 V. Whereas stoichiometry continuously decreased with potential above 0.5 V at the PtRu catalyst, it increased to a peak at the Ru@Pt and Pt catalysts. These differences show that selectivity for the complete oxidation of ethanol to CO 2 is very sensitive to the coverage of Pt at the catalyst surface, and the effect of the core on the surface electronic structure. The much higher stoichiometries at the Pt and Ru@Pt catalysts relative to the PtRu alloy clearly demonstrate the importance of a Pt-rich surface for breaking the C-C bond of ethanol.
Determination of the stoichiometry for ethanol oxidation is vital for calculating the faradaic efficiency (and fuel efficiency) of electrolysis cells to produce hydrogen and fuel cells to provide electrical power. 6 where F i is the faradaic yield of product i. Since CO 2 , acetic acid and acetaldehyde are the only products formed in significant quantities, their yields can be assumed to sum to 100%, and so the yield of only one needs to be measured if n av is known. Here, the faradaic yield of CO 2 at 0.50 V was measured using a CO 2 monitor. The results are reported in Table V, together with stoichiometries from Fig. 7 and yields of acetic acid and acetaldehyde calculated by using Eq. 5.
Yields of CO 2 from the Ru@Pt catalysts (38%-48%) were between the values obtained for PtRu (11%) and Pt (62%). Ru@Pt 1.0 provided the highest CO 2 yield for the core-shell catalysts, and also the highest n av . The primary product (on a charge basis) was acetic acid at the PtRu and Ru@Pt 0.6 catalysts, but CO 2 for Pt, Ru@Pt 1.0 , and Ru@Pt 1.4 . Acetaldehyde was a minor product (⩽12%) at all of the catalysts, and negligible for PtRu.
Anode performance and efficiency.-All of the Ru@Pt catalysts reported here provided higher performances for ethanol oxidation than a previously reported Ru@Pt 0.85 catalyst that was used under the same conditions. 14 For example, the new Ru@Pt 1.0 catalyst provided currents that were 33% and 18% higher at 0.4 V and 0.5 V, respectively. The enhanced activity can most likely be attributed to the higher crystallinity of the Ru core and/or the absence of a capping agent in the synthesis, 13,29 although the bifunctional effect of exposed Ru could also play a significant role.  The performances of the Ru@Pt catalysts for ethanol oxidation in the PEM cell are similar to those that have been reported for PtRu@Pt core-shell catalysts 37 and mixtures and bilayers of Pt and PtRu catalysts. 48 They all yield polarization curves and stoichiometries that are intermediate between those of Pt and PtRu catalysts individually. This is important for ethanol electrolysis and fuel cells because it provides an optimum balance between voltage efficiency (low anode potential) and faradaic efficiency (high stoichiometry). 48 An added advantage of the Ru@Pt core-shell catalysts is that a lower Pt:Ru ratio is required to produce a similar balance. In addition, catalysts with a Pt shell are thought to be more durable than PtRu alloys, which suffer from dissolution of surface Ru. 13,15,16 Conclusions Ru@Pt catalysts with high performances for methanol and ethanol oxidation have been conveniently prepared from a commercial carbon-supported Ru catalyst. Variation of the Pt:Ru ratio from 0.6 to 1.4 provided Pt shell thicknesses of 1.3 to 2.5 monolayers, with compressive strains ranging from 2.4% to 0.6%. Electrochemical activities for methanol and ethanol oxidation were intermediate between those for commercial Pt and PtRu alloy catalysts, providing much higher currents than Pt at low potentials, and much higher currents than PtRu at high potentials. This will optimize the efficiency of ethanol oxidation in electrolysis and fuel cells.
For ethanol oxidation, the use of a Pt shell on a Ru core greatly promotes the complete oxidation to CO 2 relative to PtRu alloys, which produce large amounts of acetic acid. Ru@Pt catalysts are particularly well suited for use in ethanol electrolysis and fuel cells to provide high activity at low potentials, control selectivity and enhance durability.