Neutral water electrolysis unifies green hydrogen and renewable natural gas production from biogas

Power-to-gas approaches that convert surplus renewable electricity into green hydrogen (H₂) and renewable natural gas (RNG, i.e., CH₄) represent promising energy storage technologies, featuring long-term gas storability, versatility, and low carbon footprint. A new power-to-gas approach is presented that couples H₂ and RNG production from biogas via neutral water electrolysis (NWE). The process leverages the inherent capability of the water electrolysis reaction to produce acids and bases from pH-neutral electrolytes, facilitating pH-mediated carbon capture and release. NWE achieved the same H₂ production rates and purities as a reference alkaline water electrolysis (AWE) cell, while concurrently upgrading synthetic biogas (65 % CH₄, 35 % CO₂) to pipeline-ready RNG with maximum CH₄ concentrations of >99 %. NWE's added energy consumption for CO₂ removal (relative to AWE) achieved economically competitive levels as low as 1.03 kWh/kg_CO2 (excluding pump energy demand), while simultaneously benefitting from chemical-free operation with no waste brine production, a compact design (same as AWE), and exclusively electrolytic pH control without the need for costly and current-limiting bipolar membranes.