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Despite the significant impact of sodium (Na) storage systems in terms of natural abundance and environmental friendliness, high-performance pseudocapacitive mterials in organic electrolytes remain challenging. Here, we demonstrate the pseudocapacitive Na-ion storage of hierarchically structured, phosphorus-incorporating steam-activated nanoporous carbons (P-aCNs) with improved rate and cyclic capabilities in organic electrolytes. The P-aCNs with a hierarchical honeycomb structure are derived from lignocellulosic biomass via a proposed synthetic process. The prominent pseudocapacitive behaviors of the P-containing groups in organic Na-ion electrolytes are confirmed by the surface area-independent and surface-confined capacitances, distinctive redox waves, and strong binding with Na-ions. In particular, the P-aCN demonstrates the cyclic stability of 96.0% over 100 000 cycles in the full cell, achieving a high capacitance of 265.43 F g–1 and rate capability of 75%. These Na-ion pseudocapacitive features of P-aCNs arising from the hierarchical interconnected porosity and the redox-active P=O bonds are comprehensively investigated by experimental and computational analyses.

Non-stoichiometry and defect engineering of nanostructured materials plays a vital role in controlling the electronic structure, which results in enhancing the electrochemical performances. Herein, we report three-dimensional (3D) oxygen-deficient flower-like non-stoichiometry zinc cobaltite (Zn 1.5 Co 1.5 O 4−δ ) for hybrid supercapacitor applications. In particular, XRD, XPS and Raman analyses confirm the oxygen-deficiency of the nonstoichiometry Zn 1.5 Co 1.5 O 4−δ . The oxygen-deficient non-stoichiometry and 3D hierarchical porous structure of Zn 1.5 Co 1.5 O 4−δ offer the efficient utilization of abundant electrochemical active sites and the rapid transportation of ion/electron. Accordingly, the Zn 1.5 Co 1.5 O 4−δ electrode achieves the high specific capacitance of 763.32 F g −1 at 1 A g −1 , which is superior to those of ZnCo 2 O 4 (613.14 F g − 1), Co 3 O 4 (353.88 F g −1 ) and ZnO (248.59 F g −1 ). The hybrid supercapacitor cells, configuring Zn 1.5 Co 1.5 O 4−δ as the positive electrode and activated carbon as negative electrodes, respectively, deliver the maximum energy/power densities (40.49 W h kg −1 at 397.37 W kg −1 and 20.87 W h kg −1 at 50.08 kW kg −1 ) and outstanding cycle stability with capacitance retention of 89.42% over 20 000 cycles.