Electrochemical Surface Area Quantification, CO₂ Reduction Performance and Stability Studies of Au and Au-Cu Aerogels

To boost the valorization of electrochemical CO₂ reduction technologies, there is a strong need for active catalysts with minimal side reactions and high specific surface areas. While the latter requirement is usually fulfilled by nanostructured catalysts, it is often realized by using porous carbon supports which improve the dispersion of nanoparticles at the cost of shifting the product selectivity towards undesirable H₂.¹ This problem could be circumvented by employing unsupported aerogel catalysts, which are tridimensionally interconnected nanodomain networks (e.g., nano-wires or -particles) of highly porous materials with large surface areas.²

With this consideration, and motivated by the high activity and selectivity of Au for converting CO₂ to CO³ and the high economic value of CO as a reduction product,⁴ in the first part of the talk we will present our efforts on employing Au aerogels for CO₂ electroreduction. Specifically, Au aerogels with a web thickness of ≈ 5 nm were electrochemically investigated, along with polycrystalline Au and a commercial catalyst consisting of Au nanoparticles supported on a carbon black (20 % Au/C) that provided a benchmark for comparison of our results. The electrochemical surface areas (ECSAs) of these materials were first quantified using the surface oxide reduction method, followed by more accurate copper underpotential deposition (Cu-UPD) measurements in aqueous H₂SO₄ electrolyte in a rotating disc electrode (RDE) configuration. Further, their CO₂ reduction performance was assessed using an in-house developed, two compartment electrochemical cell, coupled to an online gas chromatograph for detection of gaseous products and subsequently subjected to ion chromatography (IC) analysis for liquid product quantification. Finally, to test the structural stability of all Au nanocatalysts during electrochemical conditioning and CO₂ electroreduction, identical location transmission electron microscopy (IL-TEM) was employed before and after the electrochemical processes, which were conducted directly on finder TEM grids.⁵ We show that Au aerogels exhibit a CO faradaic efficiency (FE_CO) of ≈ 97% at ≈ -0.55 V_RHE in 0.1 M NaHCO₃, with suppressed H₂ production as compared to 20% Au/C across entire potential range.

Complementarily, and inspired by the prospects to alter the selectivity trends as a result of surface d-band tuning upon alloying,⁶ and by reports of enhanced CO₂-to-CO conversion observed for Au-Cu materials,⁷ the second part of the talk focusses on our on-going research on CO₂ reduction using Au₃Cu and AuCu alloy aerogels. Firstly, the quantification of the electrochemically available surface area was carried out by lead underpotential deposition (Pb-UPD) on Au-Cu aerogels with 4-6 nm web thickness. To establish a benchmark for the ECSA results, polycrystalline Au, Au aerogel and polycrystalline Cu were also subjected to Pb-UPD measurements. Similar to the Au aerogel, the alloys were next assessed for their CO₂ reduction performance at various potentials and were compared with the baseline established by Au aerogels. Following this, IL-TEM was used to determine if the alloy aerogel nanostructure undergoes drastic changes during the process of CO₂ electroreduction. Au₃Cu aerogels show a shift in CO selectivity to lower CO₂ reduction overpotentials, with a peak FE_CO of ≈ 97% at ≈ -0.4 V_RHE in 0.5 M KHCO₃.

In summary, this contribution will present a detailed electrochemical study of the applicability of aerogel catalysts for CO₂ reduction, including a comparison with well-documented polycrystalline and commercial catalysts and outlining the effects of alloying on the activity and stability of these materials.

References

1. O.A. Baturina et al., ACS Catalysis 2014, 4, 3682.

2. B. Cai et al., Adv. Energy Mater. 2013, 3, 839.

3. Y. Hori et al., Electrochim. Acta 1994, 39, 1833.

4. J. Durst et al., CHIMIA 2015, 69, 769.

5. K. Schlögl et al., J. Electroanal. Chem. 2011, 662, 355.

6. K. Liu et al., ACS applied materials & interfaces, 2019, 11, 16546-16555.

7. D. Kim et al., Nat. comm. 2014, 5, 1-8.