Water electrolysis is the dominant approach for producing green hydrogen from renewable energy, providing an alternative to fossil fuels. Conventional water electrolysis faces many problems due to the intermittent nature of renewable power and the need for a membrane to avoid the formation of an explosive gas mixture. To avoid the use of a membrane, the decoupled water electrolysis approach has been introduced, where one catalyst electrode is replaced by a solid redox mediator. So far, redox mediators for decoupled electrolysis have been prepared using fluoropolymer binders, which pose a serious environmental burden and are facing a ban. This is a limiting factor for further implementation of emerging decoupled water electrolysis technology. In this work, we report a binder-free charcoal electrode for use as a redox mediator electrode in acid and alkaline decoupled water electrolysis. The binder-free redox mediator electrodes were prepared by a solvent-free spark plasma sintering. The sintering temperature was optimised to obtain a volumetric redox mediator with a high specific capacitance of 175 F g-1 and a specific charge capacity of 41 mAh g-1 at a current density of 50 mA g-1. The observed values are higher than reported in the literature for volumetric carbon-based supercapacitor materials.