Photoelectrochemical Water Splitting at Elevated Pressure: A Model-Based Evaluation

Abstract

Photoelectrochemical (PEC) water splitting is one of the few truly renewable pathways towards “green” H2. Although PEC water splitting demonstration devices have been mainly operated at atmospheric pressure, most applications and processes that use H2 require it to be supplied at elevated pressure. Although PEC-generated H2 can be pressurized afterwards with e.g. mechanical compression, operating the PEC water-splitting device itself at elevated pressure offers an intriguing alternative and several possible advantages. For example, bubble formation is strongly reduced at elevated pressure, which lowers bubble-induced electrode deactivation and bubble-induced product crossover rate.[1,2] In addition, the optical reflection and diffraction losses induced by bubbles can be minimized.[3] Finally, the electrochemical production of hydrogen at higher pressure requires only a small (though not insignificant) increase of the thermodynamic cell voltage (29 mV for 10-fold increase in pressure).[4] The main disadvantage of a higher pressure is the increasing mechanical demands on the cell design, and the challenge is to find a pressure range at which optimum conditions will be achieved.To evaluate these pressure-induced effects, we have developed a two-dimensional multiphysics model that considers semi-empirical pressure-dependent bubble characteristics (e.g., bubble formation efficiency, bubble diameter) and their impacts on the multi-phase flow, electrochemistry, and mass transport in a membrane-free PEC water-splitting device. Based on the simulation results, a quantitative comparison between two technical approaches to generate pressurized H2 from a PEC device, namely operating PEC water-splitting at atmospheric vs. elevated pressure, has been conducted, as shown in the schematic diagram in Fig. 1a. Various performance loss factors in a PEC water-splitting device were considered, including bubble-induced product crossover, optical loss, and pH gradient-induced concentration overpotential. These losses (translated to kWh unit) were then compared with the specific work required to mechanically compress H2 from the collection pressure (p 1 in Fig. 1a) to 700 bar (p 2 in Fig. 1a). Our results show that bubble-induced optical loss is the predominant factor among all the losses considered. The overall loss diminishes as pressure increases, as shown in Fig. 1b. By comparing the benefit of conducting PEC water-splitting at elevated pressure (defined as the amount of losses avoided vs. operation at 1 bar) with the thermodynamic cell voltage penalty (see Fig. 1c), we found that no significant benefit can be gained by operating beyond 6 – 8 bar. Therefore, this pressure range represents a desirable range for the operation of a membrane-free PEC water-splitting device. Keywords : (photo)electrochemistry, water splitting, elevated pressure, multiphysics modeling, quantitative evaluation.