3.2 Non-noble metal
Despite the prominent PEC CO2RR of noble metal-based co-catalysts, their high cost and scarcity limit large-scale CO2 conversion. Hence, the development of inexpensive catalysts with sufficiently high activity and stability is essential. Bi- and Sn-based co-catalysts are well known for their particularly high selectivity for the reduction of CO2 to formate (HCOOH). Choi et al. prepared heterojunction Sn-coupled p-Si NWAs using an Ag-catalyzed electroless chemical etching method.[94] HR-TEM and HAADF-STEM measurements indicated the uniform distribution of Sn nanoparticles on the wire array. These heterojunction wire/Sn arrays show extraordinary PEC performance toward HCOOH, compared with planar p-Si and wire arrays with FE of 40 % and 88 % in single-cell and H-type cells, respectively. Recently, Ding et al. prepared Si/Bi photocathodes with an enhanced interface through the Bi3+-assisted chemical etching of Si wafers and assessed their PEC CO2 reduction performance.[95] The optimized Si/Bi photocathodes exhibit outstanding catalytic activity, with a positive onset potential, large photocurrent density of 10 mA cm−2 under 0.5 sun, and high formate FE of up to 90 % (Figure 4(G)). Moreover, the photocurrent density was improved up to 12 mA cm−2when the Si surface was exposed using the photolithography method (Figure 4(H)). Ma et al. demonstrated core-shell-structured Ni@In co-catalyst loaded p-type Si nanowire arrays (SiNWs) for the reduction of CO2 to formate.[96] P-type SiNWs were synthesized via a metal-assisted chemical etching method, and Ni@In/SiNWs photocathodes were subsequently fabricated by a photodeposition approach. As a result, compared with pristine SiNWs, Ni@In/SiNWs catalyst showed better performance for CO2to HCOOH conversion with a formation rate of 58 μmol h−1 cm−2 as well as high FE, 87 % at −1.2 V vs. RHE.