Operating photoelectrochemical water splitting cells at elevated pressure

Direct production of pressurized green hydrogen via photoelectrochemical (PEC) water splitting cells presents a promising technological approach, as it minimizes the need for mechanical compression in downstream H2 supply chains and mitigates the challenges associated with gas bubbles. However, PEC water splitting demonstrations to date have been limited to atmospheric pressure. To bridge this gap, we designed, constructed, and tested the first high-pressure flow cell for PEC water splitting. Two different configurations are examined. First, in a back-illuminated BiVO4-based PEC cell, operating at increased pressure largely prevents the photocurrent from saturating when increasing the illumination intensity. Specifically, at a solar concentration of 10, increasing the operating pressure from 1 to 5 bar causes the photocurrent to increase from 3 to almost 7 the photocurrent under 1 sun. Direct operando imaging of the electrode surfaces confirms that this improvement is primarily caused by suppressed gas bubble evolution. Conversely, in a front-illuminated platinized triple-junction (3J) III-V-based PEC cell the photocurrent scales proportionally to the illumination intensity and does not saturate. We attribute this to the combination of the dispersed nature of the nanoparticulate Pt catalyst and the long charge carrier diffusion length in the 3J III-V material. As a result, operation at 8 bar exhibits negligible impact on photocurrent, enabling hydrogen production at elevated pressure without loss of performance. Overall, our findings demonstrate a new pathway for scaling up PEC water splitting cells.