*Article* **Preparation of a Honeycomb-like FeNi(OH/P) Nanosheet Array as a High-Performance Cathode for Hybrid Supercapacitors**

**Chenliang Li, Ruizhi Li \* and Yingke Zhou \***

The State Key Laboratory of Refractories and Metallurgy, Institute of Advanced Materials and Nanotechnology, College of Materials and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China; lcl11905991@hotmail.com

**\*** Correspondence: rzli@wust.edu.cn (R.L.); zhouyk@wust.edu.cn (Y.Z.)

**Abstract:** Polymetallic transition metal phosphides (TMPs) exhibit quasi-metallic properties and a high electrical conductivity, making them attractive for high-performance hybrid supercapacitors (HSCs). Herein, a nanohoneycomb (NHC)-like FeNi layered double hydroxide (LDH) array was grown in situ on 3D current collector nickel foam (NF), which is also the nickel source during the hydrothermal process. By adjusting the amount of NaH2PO2, an incomplete phosphated FeNi(OH/P) nanosheet array was obtained. The optimized FeNi(OH/P) nanosheet array exhibited a high capacity up to 3.6 C cm−<sup>2</sup> (408.3 mAh g−1) and an excellent long-term cycle performance (72.0% after 10,000 cycles), which was much better than FeNi LDH's precursor. In addition, the hybrid supercapacitor (HSC) assembled with FeNi(OH/P) (cathode) and polypyrrole (PPy/C, anode) achieved an ultra-high energy density of 45 W h kg−<sup>1</sup> at a power density of 581 W kg−<sup>1</sup> and an excellent cycle stability (118.5%, 2000 cycles), indicating its great potential as an HSC with a high electrochemical performance.

**Keywords:** polymetallic transition metal phosphides; hybrid supercapacitor; FeNi(OH/P) nanosheet array
