Niobium oxide anode materials with suppressed activity toward hydrogen evolution reaction for aqueous batteries

The hydrogen evolution reaction is the most prominent parasitic reaction for aqueous battery chemistries. Although water-in-salt electrolytes show greatly enhanced electrochemical stability, increasing the voltage of aqueous batteries further by lowering the potential of the negative electrode remains a major challenges due to reductive water splitting. Here, we systematically investigate twelve niobium-based anode materials that show much lower activity towards hydrogen evolution reaction than classic titanium-based anode materials such as lithium titanate (Li4Ti5O12) or titanium dioxide and are therefore a much better choice for aqueous batteries. We confirm Zn2Nb34O87 to be the most suitable anode material for aqueous batteries among these niobates and present full-cell cycling data with LiMn2O4 and LiNi0.8Mn0.1Co0.1O2 cathodes in a water-in-salt/ionic liquid hybrid electrolyte. Furthermore, we compare the catalytic activities of Zn2Nb34O87 and Cu2Nb34O87, with the latter being incompatible with aqueous batteries, and discuss the origin of the large difference in activity toward hydrogen evolution reaction.