Membrane-less photoelectrochemical devices for H2O2 production: efficiency limit and operational constraint

Abstract

Three types of membrane-less photoelectrochemical (PEC) devices for solar-driven hydrogen peroxide (H₂O₂) production have been proposed, including (a) a H₂O₂ electrolyzer; (b) a photoelectrode-based PEC device; and (c) particulate photocatalysts in a suspension reactor. These devices can produce H₂O₂ from sunlight, water, and oxygen in air as the only inputs. Their respective efficiency limits were calculated by using the measured performance of H₂O₂ evolving electrocatalysts and by assuming idealized 100% selectivity for H₂O₂ accumulation, and by using the energy-conversion performance of light absorbers at their Shockley–Queisser limit. Multi-physics models for the three device configurations were employed for evaluating their electrochemical polarization behaviour as a function of the operating current density and for quantifying the respective contribution from the catalyst overpotential loss, the resistive loss between the cathode and anode, and the Nernstian potential loss due to pH gradients across the electrolyte. For particulate photocatalysts in a suspension, the concept of a non-zero light intensity threshold was for the first time applied to account for net positive H₂O₂ accumulation, which implies that the most efficient photocatalyst suspension should not fully absorb sunlight. The maximum solar-to-H₂O₂ conversion efficiency for a photoelectrode-based device was found to be 20.0%, fundamentally limited by the photocurrent density that is matched by dual absorbers in tandem series under 1-sun illumination. The maximum solar-to-H₂O₂ conversion efficiency for a particulate photocatalyst suspension can achieve 24.5–27.5% with a single-absorber band gap of 1.5–1.7 eV, depending on particle sizes. The modelling outcomes emphasize the importance of water-oxidation selectivity for achieving H₂O₂ accumulation towards molar concentration levels, and should guide the PEC H₂O₂ device implementation as the activity and selectivity of H₂O₂-evolving electro-catalysts continue to improve.