Solid-solution Ru Cu1-O2 nanocrystals: A promising negative electrode for high-energy-density aqueous hybrid supercapacitors

• Solid-solution Ru x Cu 1- x O 2 nanocrystals deliver Cu-dependent pseudo-capacitance within a potential range of −0.9 V to 0.0 V (vs. SCE). • The Cu species enhances the electrochemical utilization of RuO 2 , reaction kinetics, conductivity, and hydrogen evolution overpotentials. • The optimized RuCu82 electrode offers a large specific capacitance of 689 F g −1 at 0.5 A g –1 . • The RuCu82 electrode exhibits a high specific capacitance of 313 F g −1 at 2.0 A g −1 at −20 °C. • The constructed NiCoO 2 //RuCu82 aqueous hybrid capacitors display superb electrochemical stability and high energy density . The low specific capacitances (SCs) of traditional carbonaceous negative electrodes significantly limit the enhancement in energy density of aqueous hybrid supercapacitors (AHCs). It is still hugely challengeable to explore a candidate with large SCs, which can stably operate in the negative potential region meanwhile. For this propose, we design and fabricate solid-solution Ru x Cu 1- x O 2 nanocrystals (NCs), which exhibit competitive SCs and electrochemical stability within the potential range from -0.9 V to 0.0 V in the aqueous KOH electrolyte. The incorporation of Cu enhances the electrochemical utilization of RuO 2 , reaction kinetics, electronic conductivity, and hydrogen evolution overpotentials, which are all highly dependent upon the added contents of Cu species. The optimized Ru 0.8 Cu 0.2 O 2 (RuCu82) electrode of a high mass loading of 5 mg cm –2 reveals the best electrochemical capacitances in terms of reversible SCs and capacitance degradation at room temperature and -20 °C. Furthermore, the reversible K + -(de)intercalation induced pseudocapacitance is proposed for electrochemical charge storage process of RuCu82. In particular, remarkable specific energy of 59.1 Wh kg –1 at 400 W kg −1 and excellent cycling stability are achieved in the assembled NiCoO 2 //RuCu82 AHCs. Our contribution here presents a new promising negative electrode platform with high SCs and electrochemical stability for next-generation AHCs.