Electrochemical Surface Area Quantification, CO2 Reduction Performance and Stability Studies of Au and Au-Cu Aerogels
Authors: Chauhan Piyush , Hiekel Karl , Georgi Maximilian , Diercks Justus Sebastian , Herranz Juan , Eychmüller Alexander , Schmidt Thomas J. 
Published: July 2022

The efficient scale-up of CO2-reduction technologies is a pivotal step to facilitate intermittent energy storage and for closing the carbon cycle, and thus there is a strong need for catalysts with minimal side reactions and high specific surface areas. While the latter requirement is usually fulfilled by nanostructured catalysts, it is often combined with the use of carbon supports that improve the nanoparticles’ dispersion and concomitantly increases the catalysts’ electrochemical surface area (ECSA), but can also shift the product selectivity towards undesirable H2 due to the intrinsic H2-evolution activity of C-surfaces.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 hypothesis, 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 present our work employing Au aerogels with ≈ 5 nm web thickness for CO2 electroreduction. Additionally, polycrystalline Au and a commercial catalyst consisting of Au nanoparticles supported on a carbon black (20 % Au/C) were used as benchmarks for comparison of our results. The ECSAs of these materials were first quantified using the surface oxide reduction method, followed by more accurate copper underpotential deposition (Cu-UPD) measurements 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.5 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 these electrochemical processes, which were conducted directly on finder TEM grids.6 We show that Au aerogels exhibit a CO faradaic efficiency (FECO) of ≈ 97 % at ≈ − 0.48 VRHE in 0.5 M KHCO3, with suppressed H2 production as compared to Au/C across the entire potential range (refer Fig. 1a). An in-depth analysis of the partial H2 current densities of Au aerogel, Au/C and the carbon material alone proved the aforementioned H2 suppression to be an effect of Au nanoparticle size and shape, and not of the absence of carbon support in the aerogels.

Complementarily, and inspired by the prospects to alter the selectivity trends as a result of surface d-band tuning upon alloying,7 and by reports of enhanced CO2-to-CO conversions observed for Au-Cu materials,8 the second part of the talk features our CO2 reduction results using Au3Cu and AuCu alloy aerogels. Firstly, the quantification of the electrochemical 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, polycrystalline Cu and Cu aerogel 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- and Cu-aerogels. Following this, IL-TEM and STEM/EDX were used to determine if the alloy aerogels’ nanostructure and local composition undergo drastic changes during the process of CO2 electroreduction. To investigate the effect of dissolution of surface Cu during conditioning on the CO2RR selectivity, additional CO2RR measurements were conducted without the conditioning step and with conditioning and CO2-reduction tests being performed in separate cells. Au3Cu and AuCu aerogels show a similar qualitative trend towards CO selectivity, with a peak FECO of ≈ 96% and ≈ 92% at ≈ − 0.48 VRHE (refer Fig. 1a) in 0.5 M KHCO3, respectively. However, AuCu aerogels exhibit higher CO partial current densities than Au3Cu and Au aerogels when normalized with respect to the Au loading of the respective catalysts.

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.

Figure 1