A tri-material trifecta: designing a high-performance electrode for asymmetric supercapacitors and an electrocatalyst for HER applications with Ni3(PO4)2–NbPO4/CNT nanocomposites

Abstract

Developing novel energy storage technologies is critical for increasing the efficiency of electrochemical energy storage systems. To fulfill this demand, supercapatteries are being introduced as hybrid electrochemical energy storage devices that merge the benefits of supercapacitors and rechargeable batteries into one device. In this study, we synthesized a novel Ni₃(PO₄)₂–NbPO₄ electrode material, and carbon nanotubes (CNTs) were incorporated into it. The crystal structure and surface morphology of the Ni₃(PO₄)₂–NbPO₄/CNT electrode were investigated by XRD and SEM, respectively. The Ni₃(PO₄)₂–NbPO₄/CNT electrode provided 208 mA h g⁻¹ capacity in a three-cell set-up and 38 mA h g⁻¹ in a two-cell set-up. The supercapattery (Ni₃(PO₄)₂–NbPO₄/CNT//AC) delivered a maximum of 47 W h kg⁻¹ energy density. The Ni₃(PO₄)₂–NbPO₄/CNT device delivered a maximum of 7100 W kg⁻¹ power density while retaining an energy density of 8 W h kg⁻¹. The Ni₃(PO₄)₂–NbPO₄/CNT device can retain 85% capacity (after 10 000 cycles). The remarkable electrochemical performance of the Ni₃(PO₄)₂–NbPO₄/CNT composite can be predominantly attributed to the powerful synergistic effect arising from the heterojunctions formed between Ni₃(PO₄)₂ and NbPO₄. Moreover, when applied in the HER, the Ni₃(PO₄)₂–NbPO₄ composite exhibited 133 mV overpotential and 57.7 mV dec⁻¹ Tafel slope.