Metal phosphide- and phosphate-based electrodes and their graphene composites for hybrid supercapacitors: A critical review and perspectives

Hybrid supercapacitors combine faradaic and electric double-layer charge storage to balance energy density, rate capability, and cycling retention. Reported performance, however, varies with active-material mass loading, electrolyte, potential window, cell configuration, and normalization. This review critically compares transition-metal phosphide- and phosphate-based electrodes and their graphene composites, focusing on the contribution of graphene to composite performance and the translation of individual-electrode results to two-electrode devices. Phosphides extend to higher reported three-electrode specific capacities, and phosphide devices have a higher median reported energy density. Nevertheless, the families show overlapping three-electrode specific-capacity and device energy-density ranges, while their median cycling-retention values are similar despite different distribution widths. Phosphide-graphene and phosphate-graphene composites likewise show overlapping three-electrode specific-capacity and device energy-density ranges. High specific capacities measured for individual electrodes in three-electrode configurations therefore do not consistently translate into high device energy density or cycling retention. Because studies differ in active-material mass loading, testing conditions, cell configuration, and normalization, the distributions do not establish an intrinsic advantage for either family. Matched comparisons show that graphene can increase electrochemical response, but do not isolate its contribution from accompanying changes in active-material morphology and electrode architecture. Where reported, active-material mass loading did not exceed 3 mg cm -2 , and dry active-layer thickness was generally not reported. Evidence that performance persists at increased active-material mass loading or in charge-balanced packaged cells normalized to total mass or volume remains limited. Metal phosphides, phosphates, and their graphene composites therefore remain laboratory-stage electrode candidates whose commercial readiness has yet to be demonstrated.