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.