(Invited) Dilute Cu Alloying in Au Nanostructures for Highly Selective and Long-Term Stability of CO2 Reduction

Abstract

Electrochemical CO2 reduction can produce value-added chemical or fuels in moderate reaction environment. For efficient CO2 reduction to a target product with high selectivity, vast research works have been carried out over a few decades. Particularly, CO2 conversion performance using Au and Ag based catalysts for CO production has reached to economically viable level according to recent techno-economic analysis [1]. In various routes to improve catalytic property, metal alloys are shown effective by tailoring intermediate binding energy of CO2 reduction reaction (CO2RR). For example, bimetallic AuCu alloys, such as AuCu, AuCu3 and Au3Cu, have shown synergetic effects which lead to higher catalytic activities (i.e. overpotential, stability) although CO selectivity is still lower than state-of-art records (over 95%) [2-4]. Herein, we demonstrate the dilute Cu alloying in Au nanostructure for highly selective CO production with long-term stability. To synthesize nanostructured Au with controlled Cu alloying, we firstly formed Cu/Au bilayers by employing the electrochemical deposition (ED) and underpotential deposition (UPD) of Cu on 200-nm-thick Au thin films on Si substrates. ED and UPD processes were conducted at applied potentials of –0.1 and 0.3 V (vs. Ag/AgCl), respectively, in 50 mM H2SO4 electrolyte with 50 mM CuSO4. We carried out electrodepositon for 10s and 60s while underpotentially deposited Cu sample were only prepared for 60s. The Cu-covered Au surfaces were then electrochemically treated for nanostructuring with the same method previously reported by our research group [5]. Specifically, we electrochemically oxidized the Cu/Au bilayers at 2.5 V (vs. RHE) for 40 minutes in 0.2 M KHCO3 solution (pH = 8.5). Then electro-reduction was sequentially followed under constant current density of –0.5 mA cm–2 for about 10 minutes in the same solution. We denote the prepared samples by Nano-ED-CuAu-10s, -60s and Nano-UPD-CuAu in accordance with the type of Cu deposition processes. Additionally, nanostructured Au without Cu was prepared for the comparison. As a result of the electrochemical treatment, nanoporous structures were formed for all samples. The heights of the nanostructures were 200, 215, 225 and 240 nm for Nano-Au, Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s. XPS investigations clearly show the appearance of Cu 2p peak and slight shift of Au 4f peak position to the higher binding energy (~0.1 eV) when Cu was incorporated in Au nanostructures. In addition, the surface concentration of Cu were found to be 2, 11 and 13% for Nano-UPD-CuAu, Nano-ED-CuAu-10s and 60s, respectively. Our nanostructured CuAu and Au exhibit low CO2RR overpotential and high selectivity for CO in CO2 saturated 0.2 M KHCO3. For instance, the selectivity of CO2RR for all electrodes is around 50 – 70% (Nano-Au : 47%, Nano-UPD-CuAu : 65% Nano-ED-CuAu-10s : 69%, Nano-ED-CuAu-60s : 57%) at low overpotential (–0.34 V (vs. RHE)). It is noted that bare Au thin film shows almost no CO production. At higher applied potential (–0.59 V), the CO selectivity increases over 90% and notably, Nano-UPD-CuAu shows almost unity CO production (~99.5%) with nearly completely suppressing H2 evolution. In addition, Cu-Au nanostructures show the remarkably improved durability as compared to nanostructured Au catalysts. Whereas the CO selectivity at –0.49 V for Nano-Au drops from 92% at 30 minutes to below 80% in about 4.5 hours, Nano-UPD-CuAu retains the same performance for about 7 hours. Surprisingly, Nano-ED-CuAu samples are stable for 12 hours showing the selective CO production over 80%. These results indicate that dilute Cu-Au alloy nanostructures can be effective to improve stability with highly selective CO production. Additionally, in the presentation, we will introduce an in-situ tailoring technique to re-active the degraded Au catalysts by utilizing Cu impurities in electrolyte during CO2 electroreduction process for robust and efficient CO2 reduction. [1] Jouny et al., Ind. Eng. Chem. Res. 57, 2165 (2018) [2] Kim et al., Nat. Commun. 5, 4948 (2014) [3] Kim et al., Appl. Catal. B, 213, 211 (2017) [4] Kim et al., ACS Energy Lett., 3, 2144 (2018) [5] Kim et al., J. Mater. Chem. A., 6, 5119 (2018)