Grain Boundary Engineering of Cu–Ag Thin-Film Catalysts for Selective (Photo)Electrochemical CO₂ Reduction to CO and CH₄

Abstract

We investigated the relationship between grain boundary (GB) oxidation of Cu−Ag thin-film catalysts and selectivity of the (photo)electrochemical CO 2 reduction reaction (CO 2 RR). The change in the thickness of the Cu thin film accompanies the variation of GB density, and the Ag layer (3 nm) has an island-like morphology on the Cu thin film. Therefore, oxygen from ambient air penetrates into the Cu thin film through the GB of Cu and binds with it because the uncoordinated Cu atoms at the GBs are unstable. It was found that the Cu thin film with a small grain size was susceptible to spontaneous oxidation and degraded the faradaic efficiency (FE) of CO and CH 4 . However, a relatively thick (≥80 nm) Cu layer was effective in preventing the GB oxidation and realized catalytic properties similar to those of bulk Cu−Ag catalysts. The optimized Cu (100 nm)−Ag (3 nm) thin film exhibited a unique bifunctional characteristic, which enables selective production of both CO (FE CO = 79.8%) and CH 4 (FE CH4 = 59.3%) at a reductive potential of −1.0 and −1.4 V RHE , respectively. Moreover, the Cu−Ag thin film was used as a cocatalyst for photo-electrochemical CO 2 reduction by patterning the Cu−Ag thin film and a SiO 2 passivation layer on a p-type Si photocathode. This novel architecture improved the selectivity of CO and CH 4 under light illumination (100 mW/cm 2 ).

Silver Halide Catalysts on GaN Nanowires/Si Heterojunction Photocathodes for CO2 Reduction to Syngas at High Current Density

“…To date, the integration of semiconductors and cocatalysts has achieved remarkable advances for syngas production by a simultaneous CO 2 RR and the H 2 evolution reaction (HER) in an aqueous electrolyte under solar light and applied bias conditions. For instance, cocatalysts of Au nanoparticles, , grain-boundary-rich Au, nanoporous Au, Au 3 Cu, Ag, , and CuAg achieved selective CO production on ZnTe, TiO 2 /InP nanopillars, p–i–n amorphous Si solar cells, n + –p Si nanowires (NWs), and p-type planar Si photocathodes. Many other photocathodes also evidenced the crucial role of cocatalysts in controlling the selectivity of products.…”

“…Hence, finding an appropriate cocatalyst on a semiconductor photocathode can further improve the catalytic activity toward syngas production. Recently, Ag-based electrocatalysts fabricated by electrochemical anodization of Ag by O, , Cl, , Br, and CO 3 − dramatically enhanced catalytic selectivity of CO 2 RR to CO. However, converting the metal catalysts into other compound materials is not simple on the photocathodes due to unwanted chemical reactions of underlying semiconductors in the solutions or biasing conditions. , In particular, the electrochemical anodization method converts not only the catalysts but also oxidizes the Si photocathode to SiO 2 and degrades activity and stability. , III–V compound semiconductors such as GaAs and InP are readily corroded by the accumulation of surface holes under light illumination. − Therefore, it is necessary to develop a photocathode capable of withstanding the harsh reaction conditions in the process of forming/incorporating a suitable cocatalyst.…”

Insight into the Activity and Selectivity of Nanostructured Copper Titanates during Electrochemical Conversion of CO2 at Neutral pH via In Situ X-ray Absorption Spectroscopy

“…Although the Cu−O bond is thermodynamically unstable under reducing conditions, the stability of the Cu−O bond under CO 2 RR conditions has been reported previously for both bulk and nanostructured Cu catalysts. 11,53−55 Such stability has been linked to changes of the electronic structure of Cu because of dopant materials, 56 alloying, 57,58 strain effects, strong metal support interactions, 59 or even spill-over of oxygen. Indeed, the synthetic method employed in production of the catalyst will also impact the stability of Cu−O.…”

Development of catalysts and electrolyzers toward industrial-scale CO2electroreduction

“…Recently, an oxidation-resistive CuAg thin film was reported, which electroreduced preferentially to CH 4 (FE of 59.3%) at −1.4 V vs. RHE. 90 Despite the majority of the alloys for CH 4 production being Cu-based, some alloys without Cu were also reported. For example, Verma et al 91 prepared a bimetallic AgCo electrocatalyst by simultaneously reducing Co(NO 3 ) 2 and AgNO 3 with sodium borohydride.…”