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An electrode material is the electrochemically active solid that stores or transfers charge at one pole of a cell, a capacitor or an electrolyser, and it is the component that sets the capacity, the rate capability and the cycle life. Two limiting behaviours are distinguished: charge held in the electrical double layer of a high-area conductor, and charge stored by a fast, reversible redox reaction within the solid. Porous carbons are the standard double-layer material, and activated carbon made from a biopolymer precursor is typical of the biomass-derived carbons now examined to lower cost ^[1]. Adding a conductive metal phase improves the power response of such a carbon electrode, particularly in thick printed layers where the electronic resistance of the film itself becomes appreciable ^[2]. Transition-metal chalcogenides store charge faradaically and reach higher capacitance than carbons of the same area, at the cost of a narrower voltage window and slower kinetics ^[3]. Hybrid and composite solids combine a conductive framework with a redox-active component, among them two-dimensional organic-inorganic hybrids ^[4]. Ferrite particles dispersed in a functionalised polymer follow the same logic ^[5]. Which figure of merit matters depends on the device, so reported capacitances are comparable only at comparable rates.