Decoupled Water Electrolysis via VO₂⁺/VO²⁺ Redox Mediator for 35 MPa High-Pressure Hydrogen Production

Proton exchange membrane electrolysis cell (PEMEC) is highly anticipated for high-pressure hydrogen production, which can eliminate the mechanical gas compression process and thereby cut the capital expenditure cost. However, under high-pressure conditions, the H₂ crossover could induce an increased H₂ content in O₂ to the explosion limits, which has hindered the development of high-pressure PEMEC. Herein, a decoupled water electrolysis (DWE) system consisting of a PEMEC for high-pressure H₂ generation and an H-cell for low-pressure O₂ generation was designed. The flowed electrolyte containing the VO₂⁺/VO²⁺ redox couple was employed as the mediator. The DWE system could output H₂ at a pressure of 35 MPa without H₂-O₂ mixing. At various current densities and cathodic pressures, the H₂ crossover rate (r_c) was evaluated. The r_c was increased with the elevated pressure and decreased current densities and could reach nearly 1 mL min^-1 cm^-2 at 35 MPa when the circuit was disconnected. This work demonstrated the feasibility of the DWE system, which could offer a new horizon for high-pressure H₂ production by PEM water electrolysis.