Hierarchical Porous Nickel Anode with Low Polarization at High Current Density

Abstract

Metal anodes have attracted much interest in metal batteries because of their high theoretical capacity and outstanding electrochemical performance. However, practical applications of metal anodes are often limited by polarization effects, leading to limited reaction capacity, lower energy efficiency, and shorter cycle life. Herein, a hierarchical porous nickel (Ni) is proposed as a promising anode with extremely low polarization at a high current density and reaction capacity. This Ni anode has a large specific surface area, and fast reaction kinetics can be realized. As a result, this porous Ni anode is capable of achieving an extremely low polarization voltage of ~10 mV at 40 mA cm⁻²@40 mAh cm⁻². In contrast, the polarization voltage of the Ni plate anode is ~100 mV at 40 mA cm⁻²@40 mAh cm⁻². Additionally, symmetrical coin cells made in a unique way can be maintained for 800 h at 40 mA cm⁻²@40 mAh cm⁻² and 400 h at 80 mA cm⁻²@80 mAh cm⁻². This porous Ni anode that can achieve low polarization at a high current density/high capacity provides a new direction for developing high-performance nickel metal batteries.

1. Introduction

With the increasing energy demand and rapid depletion of fossil fuels, the development of sustainable green energy and the research on corresponding energy storage technologies have become a worldwide focus, and the development of inexpensive, high energy density, and highly efficient rechargeable aqueous batteries is imperative for portable electronic devices and grid-scale energy storage applications [1,2,3,4]. Particularly, it is highly promising for a high-performance battery system with high capacity or high current density. However, compared with cathodes such as Mn-based [5], V-based [6], or small molecule organics [7], the choice for anodes is very limited because of the challenge associated with achieving reaction at a high capacity and high current density. Metal anodes are a good choice because of deposition/dissolution reactions as well as the low cost, and abundant sources [8,9,10]. However, the currently developed lithium metal anodes or zinc metal anodes can achieve high capacities of up to 10 mAh cm⁻² or high current densities with a low capacity for fast charging performance. This limited performance mainly arises from relatively sluggish reaction kinetics that cannot achieve a high capacity at a high rate. Wang et al. [11] revealed that electrolyte engineering (e.g., Cl⁻-rich electrolytes for suppressing Ni(OH)₂ passivation) could be helpful in achieving reversible Ni anode reaction, in which the plating/stripping current could be extended to 50 mA cm⁻² and the areal capacity reached 100 mAh cm⁻². There is still a challenge for an ideal anode with compatible operation at a high current density and a high capacity.

Recently, constructing three-dimensional (3D) structures and nanostructures has emerged as a promising strategy to enhance ion accessibility and improve electrochemical stability [12,13,14,15,16,17,18,19,20]. For example, Guo et al. [12] prepared metal/SiO_x/nitrogen-doped carbon (NC) superstructures via electrospray carbonization, alleviating the strain caused by volume change and reducing the resistance between nanoparticles. Also, they have higher electrical conductivity [12]. Wu et al. [13] designed and prepared a NiO nanosheet array laminated composite grown on a loose dendritic β-NiS@Ni₃S₂ framework, which was employed as a standalone integrated anode for high-performance lithium-ion batteries. They converted Ni foams in situ into loose dendritic β-NiS@Ni₃S₂ electroactive materials via a one-step solvothermal method. The direct use of 3D nanostructures based on active materials as conductive substrates provides a large contact area of electrode/electrolyte with 3D interconnection networking ionic/electronic pathways, contributing to efficient reaction kinetics during charging/discharging [13]. This polarization enhancement from porous structure also holds great potential in Ni anodes because of the abundant active sites, improved wettability of the electrolyte, and multiple paths for ion transport. In addition, the inhomogeneous effect from volume changes during charging/discharging can also be retarded, thus improving the cycling stability and multiplicity performance of the electrode [21,22].

Herein, we proposed a facile preparation of hierarchical porous Ni anodes. The hierarchical porous Ni anodes are featured with extremely low polarization at a high current density, and can be prepared in only 100 s. The hierarchical porous structure endows the electrode with a large specific surface area and fast reaction kinetics. As a result, porous Ni anode exhibited extremely low polarization voltages of ~10 mV at 40 mA cm⁻²@40 mAh cm⁻², respectively. In contrast, the Ni plate (Dongguan Kelude, Dongguan, China) anode demonstrated a higher polarization voltage of ~100 mV at the same condition. In addition, the symmetrical coin cell can be maintained for 800 h at 40 mA cm⁻²@40 mAh cm⁻² and 400 h at 80 mA cm⁻²@80 mAh cm⁻².

2. Materials and Methods

2.1. Preparation of Hierarchical Porous Ni Anode

The electrochemical Ni plating process was conducted on a CHI760E electrochemical workstation (CH Instruments, Shanghai, China). In brief, nickel was deposited on copper mesh using the three-electrode system. The copper mesh (Anping county chulin metal wire mesh product, Hengshui, China) was immersed in 1 M H₂SO₄ (all chemicals from Chengdu Kelong, Chengdu, China) for 10 min, followed by washing with deionized (DI) water. Then, the clean copper mesh served as a working electrode, together with a saturated calomel electrode and a Pt plate electrode as the reference electrode and counter electrode, respectively. The three-electrode system with the electrolyte of 2 M NaCl, 2 M NH₄Cl, and 0.1 M NiCl₂ was then connected to the electrochemical station, and a cathodic voltage of 6 V for 100 s was set as the deposition program. After deposition, the porous Ni electrode washed with DI water was immediately transferred to a vacuum for drying. The dried porous Ni electrode was finally sliced using a slicer and loaded into a symmetrical coin cell (with identical electrodes assembled in a symmetric configuration for electrochemical testing).

2.2. Preparation of Ni-MnO₂ Full Cell

In the assembly of the Ni-MnO₂ full cell, carbon cloth is used as the current collector for the cathode, the anode is a hierarchically porous nickel electrode, and the electrode reaction area is 4 cm². The electrolyte for the cathode is a solution containing 1 M MnSO₄ and 0.5 M H₂SO₄, while the electrolyte for the anode is a solution composed of 2 M NiCl₂ and 0.1 M H₃BO₃. The electrode test is carried out in an H-type electrolytic cell.

2.3. Structure and Composition Characterizations

The morphologies and microstructures of the samples were characterized by field-emission scanning electron microscope (FESEM, ZEISS Gemini 300, Carl Zeiss AG, Oberkochen, Germany). The crystal structures were measured using X-ray diffraction (XRD, XRD-6100, Shimadzu Japan, Kyoto, Japan).

2.4. Electrochemical Measurements

A cyclic voltammetry (CV) test was conducted on a CHI760E electrochemical workstation. Symmetrical coin cells were cycled at a Land cycler (Wuhan Kingnuo Electronic Co., Wuhan, China), and the working conditions are cycling at a current density of 40 mA cm⁻² with an areal capacity of 40 mAh cm⁻² (40 mA cm⁻²@40 mAh cm⁻²) and a current density of 80 mA cm⁻² with an areal capacity of 80 mAh cm⁻² (80 mA cm⁻²@80 mAh cm⁻²).

3. Results and Discussion

We used the electrochemically deposited porous Ni on copper mesh as the experimental group and the normal Ni plate and Ni foam (Dongguan Kelude, Dongguan, China) as the control group. This experimental design is based on two key considerations. First, the copper mesh is selected as the substrate because of its excellent electrical conductivity, which can ensure efficient charge transfer during electrochemical reactions. Second, the 3D mesh structure not only provides strong mechanical support for the nickel anode but also promotes the uniform penetration of the electrolyte. Both of these factors are crucial for achieving stable cycling performance. We previously reported the electrochemical deposition of porous Ni, which resulted in a microporous structure of about 5 μm in size and improved the capacity and cycling stability of nickel–zinc batteries [23,24]. As shown in Figure 1, the i-t curve of nickel electrodeposition is presented.

The morphology of the porous nickel electrode obtained by electrodeposition is shown in Figure 2a,b. The surface of the porous Ni electrode on copper mesh is evenly covered with large micropores (mostly about 10 μm) and small nanopores (the inset in Figure 2b), which were reported previously and are composed of a hierarchical pore structure. It is hypothesized that the small nanopores on the porous Ni surface are produced by the gases generated during the electrochemical reduction process. Moreover, due to the large current density given for both Ni deposition and the hydrogen reduction reaction, relatively uniform and sufficient electrons are provided; hence, a certain number of bubbles with similar size are roughly generated at the same time, forming a regular porous structure. As shown in Figure 2b,c, the diameter variation of the copper mesh fibers before and after electrodeposition is demonstrated. The calculated thickness of the electrodeposited layer is approximately 13.20 μm. The hierarchical porous structure resulting from electrodeposition increases the specific surface area and further promotes the penetration of the electrolyte.

In contrast, a normal Ni plate exhibits only flat surfaces with limited surface area (Figure 3a,b). Traditional nickel foam merely displays micron-scale pores formed by thick metallic struts (Figure 3c,d), lacking hierarchical porosity, which results in significantly lower specific surface area compared with that of the hierarchical porous structure. The limited surface area of these conventional structures (flat Ni plate and non-hierarchical Ni foam) inevitably restricts their electrochemical performance. In comparison, the developed hierarchical porous architecture not only overcomes these limitations but also creates a 3D interconnected network. The high specific surface area provides sufficient active sites for electrochemical reactions, which is promising to promote electrolyte penetration and ion transport and increase the reaction rate of the electrode.

Regarding the analysis of the enhanced electrochemical performance, we compared the CV curves of porous Ni and the Ni plate at a sweep rate of 10 mV s⁻¹ and then calculated their Tafel slopes to measure the reaction kinetics. As shown in Figure 4a, the porous Ni electrode exhibited a much larger surface area of the active electrode than that of the nickel plate. Particularly, the reduction peak potential is more positive than that of the nickel plate, which indicates a better nickel deposition environment. Additionally, comparing the deposition part from the CV curve with the Tafel slope is an ingenious approach to measuring the reaction kinetics directly [25]. Herein, the Tafel slope is obtained from the equation η = a + b $\times$ log|J|, where η is the overpotential, a and b are constants, J is the current density, and b is known as the Tafel slope, its unit is mV dec⁻¹. The unit of the Tafel slope represents the amount of change in overpotential required for each tenfold change in current density, so a smaller Tafel slope represents faster reaction kinetics [26]. The calculated Tafel slope for the porous Ni electrode is 14.49 mV dec⁻¹, while the Tafel slope for the Ni plate is 28.71 mV dec⁻¹, referring to faster reaction kinetics (Figure 4b). Furthermore, the porous nickel on the copper mesh was confirmed by the XRD pattern, as demonstrated in Figure 4c. The peaks of the Ni phase at 44.48°, 51.84°, and 76.36° and Cu peaks at 43.30°, 50.44°, and 74.14° clearly matched well with the referenced crystal information of Ni (PDF#03-065-2865) and Cu (PDF#00-004-0836). Accordingly, the prepared hierarchical porous Ni on Cu mesh can achieve superior reaction kinetics and large reaction capacity for the anode reaction of Ni: Ni − 2e⁻⟶Ni²⁺, which holds great promise for high-current and high-capacity anode reaction.

To investigate the electrochemical performance of the porous Ni electrode, we prepared symmetric coin cells with the related Ni structure, and the electrolyte was 2 M NiCl₂ and 0.5 M boric acid. Generally, the polarization voltage of a symmetric cell can reflect the reaction polarization condition. As shown in Figure 5a, cells with porous nickel were able to maintain polarization voltages of about 10 mV at a high current density and a capacity of 40 mA cm⁻²@40 mAh cm⁻². In contrast, cells with nickel plates exhibited a much higher polarization voltage of around 100 mV for the same condition, while the Ni foam-based symmetric coin cells demonstrated a polarization voltage of ~80 mV. Higher polarization voltages will result in lower energy efficiency for the full cell and potentially limit the power density. The comparison of the polarization voltages (~10 mV, ~80 mV, and ~100 mV) between symmetric cells with porous Ni electrodes, Ni foam, and Ni plate electrodes also suggested a faster charge transfer rate and lower reaction energy barrier brought by the hierarchical porous structure. Furthermore, the porous Ni can be maintained for 800 h at a current density of 40 mA cm⁻² and 400 h at current densities up to 80 mA cm⁻² in Figure 5b,c, whereas Ni plate anodes exhibit significantly shorter lifespans of only ~100 h (40 mA cm⁻²) and ~130 h (80 mA cm⁻²).

By comparing the SEM images of the initial porous nickel anode (Figure 2b) with those after cycling (Figure 6), it is found that after cycling at a current density of 40 mA cm⁻², the morphology of the anode is well preserved, and no obvious structural changes or dendritic growth are observed. For the anode cycled at a current density of 80 mA cm⁻², although the basic porous skeleton structure is maintained, obvious structural changes have occurred. The decline in cycling performance (manifested as scattering) under the condition of 80 mA cm⁻² may be related to the deterioration of initial structural [27], while the excellent cycling stability under the condition of 40 mA cm⁻² benefits from the abundant active sites and high specific surface area provided by the hierarchical porous structure.

In addition, the assembled Ni-MnO₂ full cell exhibited an extremely high coulombic efficiency (CE) of 99.27% when the charging current density is 5 mA cm⁻² and the discharging current density is 1 mA cm⁻² (Figure 7a). In contrast, the CE values of the nickel plate and the nickel foam are only 90.81% and 95.48%, respectively, under the same conditions (Figure 7b). All these performances indicate that the Ni anode with a hierarchical porous structure can achieve stable low-polarized electrochemical performance at a high current density.

4. Conclusions

In summary, a Ni anode possessing an extremely low polarization at high current densities was achieved by a hierarchical porous structure. By using the bubble-template strategy, a densely distributed porous structure could be obtained in just 100 s. This structure endowed the electrode with a large specific surface area, providing sufficient active sites for electrochemical reactions and significantly improving the anode’s reaction kinetics. As a result, the porous Ni anode exhibited low polarization voltages of ~10 mV at a high current of 40 mA cm⁻² and a high capacity of 40 mAh cm⁻². In contrast, the Ni plate anode demonstrated a high polarization voltage of ~100 mV under the same condition. Meanwhile, the symmetrical cells with porous Ni could be stably operated at a high current density/capacity of 40 mA cm⁻²@40 mAh cm⁻² and 80 mA cm⁻²@80 mAh cm⁻² for 800 h and 400 h, respectively. This optimized design of a hierarchical porous structure provides a time-saving and facile pathway to prepare nickel anodes with a stable low-polarized electrochemical performance at a high current density.