Bifunctional NiFe inverse opal electrocatalysts with heterojunction Si solar cells for 9.54%-efficient unassisted solar water splitting

Photovoltaic cell-electrolyzer combined systems have recently been considered a most promising route to commercialization of photoelectrochemical (PEC) water splitting although efficiency, stability, and cost remain major challenges. Herein, we develop a highly efficient and low-cost, bifunctional NiFe electrocatalyst that uses NiFe inverse opal nanostructures for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). From their increased surface area, NiFe inverse opal structures can reduce the overpotential by similar to 70 mV and similar to 90 mV for OER and HER, respectively. In addition, correlations between increased surface area of inverse opal 3-dimensional nanostructures and electrocatalytic activity of OER and HER are explored. Furthermore, a NiFe inverse opal structure with an optimized number of thickness layers reduces the overpotential of water splitting by similar to 160 mV. When integrated to four series-connected Si heterojunction (SHJ) solar cells, the NiFe inverse opal electrolyzer achieves a 9.54% solar-to-hydrogen conversion efficiency over 24 h under a zero-bias condition. The combined PEC water splitting investigated in this work is expected to provide a basis for the design of highly efficient renewable and sustainable water electrolysis systems that incorporate earth-abundant, low-cost materials.