High-performance bimetallic Ni-Mn phosphate hybridized with 3-D graphene foam for novel hybrid supercapacitors

Nickel-manganese phosphate/graphene foam (GF) composite was successfully synthesized through a simple and direct hydrothermal approach, serving as highly efficient electrodes for hybrid device. Ni and Mn are both redox metals owing to their multiple oxidation states however, Ni-based materials possess higher capacity due to its strong faradic mechanism while Mn-based materials offer pseducapacitive nature and GF for its ability to improve the surface area as well as the electrical conductivity of the composite. Consequently, the prepared NiMn(PO₄)₂/GF composite achieved a maximum specific capacity of 97 mAh g ^{− 1} at 0.5 A g ^{− 1} in a 3-electrode configuration setup compared to 63 mAh g ^{− 1} for the pristine material. Thus, NiMn(PO₄)₂/GF considered as a positive electrode in a hybrid device with carbon derived from Capsicum (bell pepper) seeds “peppered”-activated carbon (ppAC) as a negative electrode. The hybrid device reported a maximum energy density of 35.42 Wh kg⁻¹ corresponding to a power density of 538 W kg⁻¹ at 0.5 A g ^{− 1}. The NiMn(PO₄)₂/GF//ppAC hybrid device also showed high stability of 97.8% capacity retention over 10,000 cycles at 6 A g ^{− 1}, ∼93% float time efficiency after 120 h and could still preserve ∼1.17 V against its initial cell potential of 1.5 V after 60 h of self-discharge. The outstanding performance of NiMn(PO₄)₂/GF material in a half and full cell makes it a good candidate for supercapacitor application.