Low-crystalline FeO_x@PPy hybridized with (Ni_0.25Mn_0.75)₃O₄@PPy to constructed high-voltage aqueous hybrid capacitor with 2.4 V

Abstract

The operating voltage of aqueous hybrid capacitors are generally limited to 2 V due to the decomposition of water, which significantly impede the progress of energy density. Herein, the porous low-crystalline FeO_x nanorod array on carbon cloth is prepared by the novel electrochemical Li⁺ pre-insertion method, and a 2.4 V high-voltage aqueous hybrid capacitor device is successfully obtained after matching with the nickel doped (Ni_0.25Mn_0.75)₃O₄@PPy nanoprisms array. The low-crystalline structure of FeO_x preserved during the first Li⁺ insertion and space created via the elimination of low-crystalline Li₂O dramatically provides sufficient electronic and ionic transfer channels. In addition, surface polypyrrole (PPy) stabilization is employed to further enhance electron conductivity and electrode stabilization. Benefitting from increasing active sites, fast ion diffusion and electron transfer the obtained low-crystalline FeO_x@PPy electrode exhibits improved electrochemical performance, especially for capacitance and stability. Moreover, the aqueous hybrid capacitors (Ni_0.25Mn_0.75)₃O₄@PPy//FeO_x@PPy device delivers a high energy density of 72.4 Wh kg⁻¹ with the ultra-high voltage, and admirable cycling stability (94.7% retention after 4000 cycles). Our work highlights the novel electrochemical Li⁺ pre-insertion method to achieve superior low-crystalline electrodes materials and designs the high-voltage aqueous hybrid energy storage devices.

Introduction

The awful aggravation of environment pollution and energy crisis have urgently triggered the development of energy storage system forward for its standing at the forefront of seeking renewable energy alternatives [1,2]. As a promising alternative for advanced energy storage device, supercapacitors (SCs) also called electrochemical capacitors have witnessed extensive expansion owing to their attractive superiorities including ultrahigh power density (P), long lifespan, superior safety and low maintenance cost [[3], [4], [5]]. SCs can be generally classified into two categories according to their intrinsic charge storage mechanism, one is electric double-layer capacitors (EDLCs) which depends on the electrochemical reversible adsorption/desorption of cations and anions at the electrode/electrolyte interfaces; another is pseudocapacitor which relates to the reversible surface Faradic redox reactions [[6], [7], [8], [9]]. In reality, pseudocapacitors generally possess much higher capacity than EDLCs originated from fast and reversible redox reactions.

Nevertheless, prior to widespread application, its relatively low energy density (E) compared with commercial lithium ion batteries (LIBs) remains to be enhanced without sacrificing power density [10,11]. Generally, two predominant strategies have been proposed to address the above issue: 1. boosting the capacitance of electrodes. Considerable efforts have been focused on the rational design of electrodes to provide more redox active sites, facilitate electron transfer and ion diffusion rates, such as constructing hierarchical nanostructure [8,9,12], doping exotic element [13,14], engineering intrinsic defects [15,16], functionalizing surface [17]. 2. broadening the working voltage of devices. According to the formula E = 1/2CV², in which C, V represents specific capacitance and operating potential respectively, thus, it is more effective by enlarging cell operating potential instead of maximizing specific capacitance to promote energy density [18,19]. On one hand, the cell operating voltage is greatly dominated by the decomposition of electrolyte and interaction between electrolyte and electrodes. Generally, organic electrolyte possesses larger operating voltage than aqueous electrolyte owing to the theoretical decomposition of water at 1.23 V in aqueous electrolyte, normally, the lower concentration of H⁺ or OH^– in neutral electrolyte endowing higher overpotential than acid or alkaline ones, which leads to the larger operating voltage. However, the flammability, high-cost, slumped ion diffusion rates deeply impede its practical application. On the other hand, constructing hybrid capacitor configuration can also achieve larger operating voltage ascribed to the electrodes possess different nature for positive and negative polarization [20].

Iron oxide was regarded as a promising anode candidate used for aqueous electrochemical energy storage device owing to its features of remarkable high theoretical capacity, negative operating potential, co-friendliness and natural abundance [21,22]. Despite the notable achievements on the electrochemical performance, it still suffers from low specific capacitance, poor cycling durability and structural instability ascribed to its lower electron conductivity and structural instability [23]. On the other hands, Fe₂O₃ also attracted widespread concern as promising anode materials in secondary lithium ion batteries [24], the reaction mechanism has been demonstrated to follow “conversion-type”: when Li⁺ inserted, Fe₂O₃ is reduced into metallic Fe nanoparticles (<2 nm) which dispersed in the amorphous Li₂O matrix, in the followed desertion process, the Li₂O composed and metallic Fe nanoparticles were oxidized to FeO, fortunately, the low-crystalline structure formed during Li⁺ insertion preserved in the following charge-discharged process. Recently, amorphous structure or low-crystalline structure have been demonstrated beneficial the electrochemical performance of supercapacitors [[25], [26], [27]], the loose arrangement of atom can accommodate the volume expansion during the redox reactions, which enhance the cycling durability; on the other hand, long-range disorder can dramatically increase redox sites due to facilitating the electrolyte diffusion, achieving higher specific capacity.

Based on the above considerations, herein, an electrochemical Li⁺ pre-insertion method was proposed to transform well-crystalline Fe₂O₃ nanorod array directly grown on carbon cloth (CC) into porous metallic low-crystalline Fe nanorods array, according to the conversion-type lithiation mechanism, the iron oxide was transformed into pseduamorphous Fe⁰/Li₂O compound. Afterward, ethanol and deionized water were used to get rid of residential Li₂O and naturally oxidized to low-crystalline FeO_x. To enhance the electrode structural stability and conductivity, polypyrrole (PPy) nanocoating was introduced on FeO_x to obtain porous low-crystalline FeO_x@PPy electrodes [28,29]. Electrochemical measurement reveals this elaborate material engineering approach can tremendously optimize the electrochemical performance of electrodes, enhanced specific capacitance of 592.8 F g⁻¹ at 1 A g⁻¹, as well as improved cycling durability, is achieved. To assemble aqueous hybrid capacitor device with high energy density, Ni doped manganese oxide nanoprism array stabilized by PPy with high operating potential from 0 to 1.3 V (vs SCE) was used as a cathode. The introduction of Ni atom and PPy stabilization can dramatically suppress water decomposition and increase electron conductivity [30,31]. As the fabricated aqueous device can operate stably at high-voltage range of 0–2.4 V and exhibits ultrahigh energy density of 72.4 Wh kg⁻¹ at power density of 0.676 KW kg⁻¹ as well as outstanding cycling durability life of 4000 cycles.