**1. Introduction**

The energy crisis and global climate change have stimulated the search for sustainable energy. Numerous efforts have been made to develop energy storage devices that are efficient, economical, safe, and environmentally friendly in order to meet the future development of a low-carbon, sustainable economy. Batteries have a high energy density, but a low power density and poor cycling stability. Supercapacitors have a higher power density, faster charging time, and better long-term stability than batteries. However, supercapacitors have limited industrial applications because of their low energy density. By combining the advantages of batteries and supercapacitors, researchers proposed a hybrid supercapacitor (HSC) with a battery-type electrode and a capacitive electrode. Compared with carbon-based materials storing energy via a double electric layer, battery-type electrode materials can generate extreme energy density based on an active redox reaction [1,2]. Therefore, the performance of HSC is highly dependent on the efficiency and stability of the battery-type electrodes.

Recently, it has been demonstrated that transition-metal phosphides (TMPs) have garnered significantly more research because of their higher conductivity, electrochemical activity, and structural stability [3,4], all of which make them potentially useful in energy storage devices. TMP is a triangular prism made of metal bonds or covalent chemical bonds [5]. By increasing the number of metal atoms situated in the center of the prism's vertical plane, a nine-fold tetrakaidecahedron structure centered on phosphorus atoms is formed. The large channels and open framework of TMPs provide effective electron/ion transport. Because of the rapid reduction in ionic properties and the "P" dragging electron density in the sublattice presence of delocalization, TMPs have more free electrons, which is the fundamental reason for their higher conductivity. After the introduction of metals

**Citation:** Li, C.; Li, R.; Zhou, Y. Preparation of a Honeycomb-like FeNi(OH/P) Nanosheet Array as a High-Performance Cathode for Hybrid Supercapacitors. *Energies* **2022**, *15*, 3877. https://doi.org/ 10.3390/en15113877

Academic Editors: Alon Kuperman and Alessandro Lampasi

Received: 29 April 2022 Accepted: 22 May 2022 Published: 24 May 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

into the monometallic phosphide, the resulting polymetallic (or metal-rich) transition metal phosphide (e.g., MP or M2P, where M is the transition metal) exhibits metallic properties (chemical stability), multiple redox centers, and the ability to carry more free electrons, resulting in a significantly increased electrical conductivity (compared with phosphorusrich or monometallic phosphides) [6]. However, phosphorus-rich TMPs with abundant P−P bonds are unsuitable as energy storage materials because of the electron accumulation around the P atoms and decreased electrochemical performance [7]. Thus, understanding how to select appropriate metals and combine them to form multi-component metal phosphides with synergistic effects is critical for optimizing the electrochemical performance of electrochemical energy storage devices using TMPs as electrodes.

Recently, it was discovered that one of most effective tactics to improve the electrochemical performance is to combine Ni and Fe to form bimetallic phosphide, which improves the conductivity of Fe/Ni-based compounds [8,9]. The electrochemical properties of Ni and Fe are quite similar; the ionic radius of Fe3+(0.65 Å) is close to that of Ni2+(0.69 Å) [10,11]. As a result of the formation of hydroxyl oxides during charge storage, Ni and its compounds in various morphologies (e.g., nanoparticles, nanowires, and thin films) are typical supercapacitor electrode materials with an excellent redox behavior. Because of its high theoretical capacity (1951.2 C g−1) and good electronic conductivity (1.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> S cm−1) [12], Ni2P is considered as a promising material for energy storage. However, because of structural degradation, the stability of FeNi-based materials is still unsatisfactory [9]. In recent years, many binder-free nanoarray electrodes with self-contained nano-units and porous structures have been investigated to minimize the volume expansion and improve the electrochemical performance. As a result of phosphorus's lower electronegativity and reduced bond ionicity, Fe/Ni bimetal phosphides give a series of various redox couples (Fe0/Fe2+/Fe3+, Ni0/Ni1+/Ni2+) with better metalloid properties [13]. Although there have been many reports about Ni/Fe bimetal phosphides as high-performance electrocatalysts for water splitting [14–16], their use as electrodes in SCs is unclear [17].

In this study, we designed, synthesized, and developed self-supported nanohoneycomb (NHC)-like FeNi(OH/P) electrodes using a simple two-step procedure, in which incomplete phosphated FeNi(OH/P) nanosheet arrays were oriented and anchored on 3D nickel foam without any bridging agent. The large contact areas and electron pathways of the unique interconnected NHC-like FeNi(OH/P) nanosheet array could provide high-efficiency electronic transmission for the charge storage. Furthermore, Ni/Fe bimetal phosphides have high density redox centers and a high conductivity. This study also describes a hybrid supercapacitor (HSC) with a cathode electrode made of bimetallic transition phosphides and an anode electrode made of polypyrrole/C (PPy/C). By optimizing gel electrolytes, the HSC exhibited excellent capacity retention. This work provides a new strategy for designing a cost-effective, reliable, and high-performance asymmetric supercapacitor for energy storage applications.

#### **2. Materials and Methods**

#### *2.1. Materials*

Nickel chloride hexahydrate (NiCl2·6H2O), ferric chloride hexahydrate (FeCl3·6H2O), sodium chloride (NaCl), and sodium hypophosphite (NaH2PO2·H2O) were purchased from Sinopharm Chemical Reagents Co., Ltd. All of the reagents were of analytical grade and were used without further purification.

#### *2.2. Synthesis of Materials*

#### 2.2.1. Synthesis of FeNi LDH's Precursor

First, the nickel foam (NF, 5.2 <sup>×</sup> 3.7 cm2) was cleaned with 2 M HCl, absolute ethanol, and deionized water by sonication for 15 min in sequence to remove the oxide layer on the surface. Then, 0.27 g FeCl3·6H2O [18,19] and 0.097 g NaCl were dissolved in 70 mL of deionized water by magnetic stirring for 30 min. The mixed solution and a piece of the

pretreated NF were transferred to a 100 mL Teflon-lined stainless autoclave and kept at 120 ◦C for 10 h. The Ni foam coated with the FeNi LDH sample was washed alternately with ethanol and deionized water several times, and dried for 5 h at 60 ◦C. The mass density of the FeNi LDH nanosheet array (0.57 mg cm<sup>−</sup>2) on the NF was determined by subtracting the weight of the NF (after acid etching) from the weight of the FeNi LDHs with the NF.

#### 2.2.2. Synthesis of the FeNi(OH/P) Sheet Array Electrode

The NaH2PO2 and Ni foam-supported FeNi LDH were placed upstream and downstream of the tube furnace, respectively. The samples were heated to 300 ◦C at a heating rate of 2 ◦C min−<sup>1</sup> for 2 h under an Ar flow. The FeNi(OH/P) was collected after the furnace was cooled to an ambient temperature. To explore the effect of the phosphorization degree, we considered NiFe(OH/P) with different amounts of NaH2PO2 (0 g, 0.6 g, 1.2 g, and 1.8 g, defined as FeNi(OH/P)-0, FeNi(OH/P)-0.6, FeNi(OH/P)-1.2, and FeNi(OH/P)-1.8, respectively). After phosphating, the mass increased by 1.87 mg cm−<sup>2</sup> on average. The mass density of the FeNi(OH/P)-1.2 nanosheet array on the NF was 2.44 mg cm−2.

#### 2.2.3. Assembly of Hybrid Supercapacitor Device

All-solid-state hybrid supercapacitors (HSCs) were assembled by employing the asfabricated FeNi(OH/P)-1.2 and PPy/C as the cathode and anode electrode, respectively, and PVA/KOH/PAAS as the electrolyte.

For the preparation of PPy/C, PPy aerogel was heated to 500 °C at a rate of 5 °C min−<sup>1</sup> under an argon atmosphere. The preparation of PPy aerogel was mentioned in previous articles from our group [20]. PPy aerogel was synthesized through PPy hydrogels, which were obtained by oxidizing the pyrrole monomer with ammonium peroxysulfate (APS) in a Methyl orange (MO) solution. The mixture of PPy/C (80 wt%), acetylene black (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) was coated onto an NF (1 <sup>×</sup> 1 cm2) and then dried at 80 ◦C for 8 h to obtain the PPy/C electrodes. The mass loading of PPy/C powder attached to the NF was 8.4 mg cm−2. The gel electrolyte was prepared as follows: 6 g PVA (1799 type) and 5 g KOH were dissolved in 60 mL deionized water at 90 ◦C, then 2 mL PAAS was added dropwise into the above solution with stirring and maintained at 65 ◦C for 15 min. The two electrodes were coated with a PVA/KOH gel electrolyte to obtain HSCs with a thickness of about 2.1 mm.

#### *2.3. Morphology and Structure Characterization*

The micromorphology and structure of the as-synthesized materials were analyzed using a scanning electron microscope (SEM, PHILIPS XL30 TMP) and transmission electron microscope (TEM, JEM-2000 UHR SETM/EDS). Energy dispersive X-ray spectroscopy (EDX), elemental mapping, and selected area electron diffraction (SAED) were performed on the same FE-TEM microscope. The phase characteristics of the as-synthesized materials were identified using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS, AXIS SUPRA+). Mass weighting was recorded with a semi-micro balance (ESJ200-4B) with an accuracy of 0.01 mg.
