Solar driven water splitting is a promising technology to produce hydrogen fuels. Efficient oxygen evolution reaction (OER) catalysts were required for water splitting, since hydrogen evolution is severely constrained by sluggish kinetics of OER. Thus, main challenge for water splitting is to lower OER overpotential and integrating OER catalysts with solar cells. Most of previously reported OER catalysts are particles coated on conductive substrates as glassy carbon or carbon paper with polymeric binders. However, there are distinct limitations like slow charge transfer and poor stability under high current density. Thus, enormous efforts have been recently devoted to develop binder-free OER catalysts by using solution based process such as electrodeposition or hydrothermal growth on the porous metallic substrates. However, the porous substrates are not proper to deposit the semiconductor layers uniformly to fabricate solar cells. To solve such problems, Ni-Fe metal foil was electrodeposited, followed by anodization. The Ni-Fe foil has enough flatness to identify the correlation between the crystallinity and the catalytic performance of Ni-Fe OER catalysts by thermal annealing in the high vacuum. Moreover, the Ni-Fe foil is able to deposit solar cells, so it could be used as the catalytic substrates of the solar cells.
Here, we fabricated Ni-Fe oxyhydroxide film by anodizing Ni-Fe film, which exhibits low OER overpotential of 0.251 V to deliver substantial current density of 10mA/cm2 and excellent stability for 36 h in 1 M KOH solution. We also integrated the catalytic substrate with an amorphous silicon (a-Si:H) solar cell to demonstrate a monolithic photo-assisted water splitting device with a structure of Ni-Fe foil / Ag (120 nm) / Al-doped ZnO (60 nm) / p-i-n a-Si:H (250 nm) / ITO (60 nm). When the device was illuminated under 100mW/cm2 AM 1.5G solar irradiation in the alkaline electrolyte, the photocurrent generated from the amorphous silicon layer significantly lowered OER overpotential by 0.8 V. It was the first demonstration of the monolithic device which has a significant impact on large-scalable and highly efficient solar-driven water splitting devices.