A precious metal-free solar water splitting cell with a bifunctional cobalt phosphide electrocatalyst and doubly promoted bismuth vanadate photoanode

A bifunctional cobalt phosphide (CoP) electrocatalyst is applied to a doubly promoted BiVO4 photoanode as an oxygen evolution as well as to a cathode as a hydrogen evolution reaction (HER) catalyst to establish a photoelectrochemical (PEC) water splitting cell made of only earth abundant elements without any precious metals. Although the intrinsic HER activity of CoP is lower than Pt/C, CoP can replace Pt as the cathode of PEC water splitting cells without any significant loss in performance. During the water splitting reaction, the cathode remains as CoP, but CoP loaded on the BiVO4 photoanode turns into amorphous CoOx-HPOy with its chemical state and performance very similar to those of well-known cobalt phosphate electrocatalysts. A tandem cell composed of CoP/hydrogen-treated, 1% Mo-doped BiVO4 as the photoanode, a CoP/Ni foam as the cathode and a 2-piece, series connected crystalline Si solar cell as the bias power generator demonstrates stable unassisted water splitting with a solar-to-hydrogen conversion efficiency of 5.3% under simulated solar light, which represents the highest among reported precious metal-free unassisted solar water splitting devices

Demonstration of a 50 cm(2) BiVO4 tandem photoelectrochemical-photovoltaic water splitting device

In this paper, we demonstrate a new benchmark for a large area photoelectrochemical-photovoltaic (PEC-PV) solar water splitting device with a metal oxide-based top absorber. The stand-alone 50 cm(2) device consists of cobalt phosphate-coated tungsten-doped BiVO4 (CoPi/W:BiVO4) photoanodes combined with series-connected silicon heterojunction (SHJ) solar cells. We highlight the performance limitations for large area BiVO4 photoanodes and present initial attempts in overcoming these challenges. Specific challenges encountered are (i) the high resistivity of the FTO substrate, (ii) non-uniform CoPi deposition, and (iii) limited ionic conductivity of the 0.1 M phosphate buffer electrolyte typically used for small area BiVO4 devices. The former two problems were overcome by applying Ni lines to the FTO substrate, and the latter to some extent by increasing the electrolyte concentration to 2.0 M. Despite the high buffer concentration, the overall performance of the large area photoelectrodes was found to be limited by H+/OH- transport in this near-neutral pH electrolyte. This limitation results in H+/OH- depletion towards the center of the large area electrode and significant potential drop, which can be overcome by implementing a cell design with a small electrode-area-to-electrolyte-volume ratio. Our optimized photoanodes were then integrated into tandem PEC-PV devices in either a single or dual photoanode configuration. These 50 cm(2) PEC-PV devices demonstrate solar to hydrogen (STH) efficiencies of 1.9% (single CoPi/W:BiVO4 and 2-series connected SHJ cells) and 2.1% (dual CoPi/W:BiVO4 and 2-series connected SHJ cells). Optimized small area (0.24 cm(2)) PEC-PV devices based on a similar configuration show a STH efficiency of up to 5.5%. Our results illustrate the challenges involved in the scale-up of solar water splitting devices and underline the importance of increased electrochemical engineering efforts in this developing field