Screening Selection of Hydrogen Evolution‐Inhibiting and Zincphilic Alloy Anode for Aqueous Zn Battery

Abstract The hydrogen evolution reaction (HER) and Zn dendrites growth are two entangled detrimental effects hindering the application of aqueous Zn batteries. The alloying strategy is studied to be a convenient avenue to stabilize Zn anodes, but there still lacks global understanding when selecting reliable alloy elements. Herein, it is proposed to evaluate the Zn alloying elements in a holistic way by considering their effects on HER, zincphilicity, price, and environmental‐friendliness. Screening selection sequence is established through the theoretical evaluation of 17 common alloying elements according to their effects on hydrogen evolution and Zn nucleation thermodynamics. Two alloy electrodes with opposite predicted effects are prepared for experimental demonstration, i.e., HER‐inhibiting Bi and HER‐exacerbating Ni. Impressively, the optimum ZnBi alloy anode exhibits one order of magnitude lower hydrogen evolution rate than that of the pure Zn, leading to an ultra‐long plating/stripping cycling life for more than 11 000 cycles at a high current density of 20 mA cm−2 and 81% capacity retention for 170 cycles in a Zn‐V2O5 pouch cell. The study not only proposes a holistic alloy selection principle for Zn anode but also identifies a practically effective alloy element.

solution.Electrodeposition of anodes was carried out at constant current density j=0.025A/cm 2 at 25 °C for 60 min using a three-electrode configuration.Subsequently, the anodes were washed several times with deionized water and were dried at 80 ℃ in a vacuum oven for 6 h.Finally, two samples were prepared: (1) ZnBi sample, using electrodeposition electrolyte containing 2 mM Bi(NO3)3; (2) ZnBi-2 sample, using electrodeposition electrolyte containing 10 mM Bi(NO3)3.After electrodeposition, the thickness of ZnBi and ZnBi-2 samples is 0.63 mm and 0.65 mm, respectively.
Electrodeposition of anodes was carried out at constant current density j=0.025A/cm 2 at 25 °C for 60 min using a three-electrode configuration.Subsequently, the anodes were washed several times with deionized water and were dried at 80 ℃ in a vacuum oven for 6 h.Finally, two samples were prepared: (1) ZnNi sample, using an electrodeposition electrolyte containing 5 mM NiSO4; (2) ZnNi-2 sample, using an electrodeposition electrolyte containing 10 mM NiSO4.After electrodeposition, the thickness of ZnNi and ZnNi-2 samples is 0.46 mm and 0.45 mm, respectively.

Fabrication of MA-V2O5 cathode:
The 1 g urea (Sigma-Aldrich, Ar), 0.75g NaI (Aladdin, 99%), and 0.9 g V2O5 (Sigma-Aldrich, Ar) were dissolved into deionized water (50 mL).Then 500 uL H2SO4 (Beijing Chemical industry, Ar) was added to the solution and continuous stirring followed for 20 min.After that, the above solution was transferred to a 100 ml Teflon autoclave and treated for 3 h at 120 o C.After the autoclave was naturally cooled to room temperature, the product was filtered and washed with alcohol and deionized water.Finally, the green powder was dried at 80 °C for 24 h.The tested MA-V2O5 cathode was fabricated by mixing the MA-V2O5 (70 wt%), acetylene black (20 wt%) (maya, Ar), and polytetrafluoroethylene (10 wt%) (Nbuliv) with some drops of anhydrous ethanol to form a slurry.The slurry was then rolled onto a stainless steel mesh and dried under vacuum at 80 o C for 2 h.

Materials characterization:
The product morphologies were observed by scanning electron microscopy with an energy-dispersive X-ray spectroscopy (EDS) detector (SEM, Nova NanoSEM 450).The microstructures components of electrodes were observed by field emission scanning electron microscopy (FESEM, JEOL JEM-F200), selected-area electron diffraction (SAED), and high-resolution transmission electron microscopy (HRTEM, JED-2300T) at an acceleration voltage of 200 kV, respectively.
The crystal structures and compositions of electrodes were investigated by X-ray diffraction (XRD, Ultima IV Cu K-Alpha radiation).The surface chemical compositions and states of electrodes were measured by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha).The atomic percentages of electrodes were measured by measuring Inductively Coupled Plasma Optical Emission Spectrum (ICP-OES, Agilent 725-OES).

Electrochemical measurements of symmetric and asymmetric cells:
The stripping/plating tests and coulombic efficiency of samples were performed in symmetrical cells using 2 M ZnSO4 solution as the electrolyte on a battery test system (CT2001A, LAND, and CT-4008, Neware).The depth of discharge for Zn-Bi, Zn-Ni alloy and Zn electrode was 28.3%, 35.4% and 29.1% at 5 mA cm -2 , respectively.The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were measured by CHI 760E electrochemical workstation (CHI 760E, Chenhua, Shanghai) using 2 M ZnSO4 solution as the electrolyte.The amount of electrolyte in standard CR2032 coin-type cells is 200 μl.Hydrogen evolution polarization curves and Tafel curves were measured by rotating electrode (Model 636A, Princeton) and CHI 760E electrochemical workstation (CHI 760E, Chenhua, Shanghai) in 0.1 M Na2SO4 solution as the electrolyte.The gas chromatographic (GC2002, Kechuang, Shanghai) and the all-glass automatic online trace gas analysis system were used to monitor the in-situ hydrogen evolution flux (Labsolar 6A, Perfectlight).Specifically, the symmetric battery test was carried out in a closed glass container.The gas produced in the container was sampled by the all-glass automatic online trace gas analysis system, and the amount of hydrogen in the gas was obtained by gas chromatography.

Electrochemical measurements of Zn||V2O5 full batteries:
For the full battery, Zn||V2O5 full batteries were assembled in standard CR2032 coin-type cells using 2 M Zn(CF3SO3)2 (Aladdin, Ar) solution as the electrolyte and Whatman glass fiber (GF/D) as the separator in air atmosphere.The amount of electrolyte in standard CR2032 cointype cells is 200 μl.The typical GCD curves were collected on a Neware battery test system with a potential range of 0.1-1.7 V at room temperature.And we also evaluated the effectiveness and practicability of the alloy anode in Zn-V2O5 pouch batteries.The amount of electrolyte in Zn-V2O5 pouch is 1500 μl.
Calculation methods: The DFT calculations were carried out using the Vienna Ab initio Simulation Package (VASP). [1]The core electrons were modeled with the projector-augmented-wave (PAW) method. [2]The exchange and correlation function of Perdew-Burke-Ernzerhof (PBE) was adopted within the framework of generalized gradient approximation (GGA). [3]The plane-wave basis cut-off energy was set to 400 eV.During the geometrical optimizations, the forces on all the relaxed atoms were less than 0.02 eV/Å.The Zn(001) alloying supercells consisting of 4-layer atomic structures with sizes of 4×4×4 unit cells were adopted in this study.The K-points were set as 5×5×1 according to the Monkhorst-pack method.The alloying element was doped in the top layer of the Zn(001) supercell.For each supercell, the top site of the doping element and hollow site of neighbor Zn atoms were selected to investigate the effects of doping on hydrogen evolution performances.
The 4×4 slab models with X atoms doped on the surface of Zn(001) is constructed, and each model is arranged in the order of Cd, Cr, Pb, In, Pd, Hg, Ag, Bi, Co, Cu, Mn, Ni, Au, Ge, Sn, Ga and Sb atoms doping.The optimized surface doping structure model is shown in Figure S1 to calculate the energy of the system.In our previous work, it has been shown that the 4×4 ZnSn(001) slab system is the most effective in inhibiting hydrogen evolution. [4]In order to study the influence of X atoms on the surface hydrogen evolution behaviors, tow typical sites(X top and Zn hollow) were selected to calculate the Gibbs free energy change ΔGH*.Here, the two typical sites of ZnCd system were taken as an example as shown in Figure S2.
The Gibbs free energy changes ∆  * of H adsorption on top site of doping element and hollow site of neighbor Zn were calculated according to the following equation: [5]   ∆ = ∆  + ∆  − ∆  where ∆  is the difference in zero point energy and ∆  is the entropy between hydrogen adsorption and hydrogen in the gas phase.Here the contributions from the electrode to both ∆  and ∆  are small and could be neglected.As a result, the ∆  and ∆  are obtained by, and   2 0 is the entropy of H2 gas at the standard condition.
The binding energy of H atom adsorption on the surface of the supercell is obtained according to the following equation.
where    is the energy of a H atom adsorbed on the electrode substrate,   is the energy of the substrate and   2 is the energy of gas H2.
Alloy doping also changes the dendrite phenomenon on the zinc electrode surface, which is reflected in its effect on the deposition process of Zn atoms on the Zn(001) surface.The slab model of Zn atom adsorption on ZnX(001) surface is constructed to find the reasonable adsorption position of Zn atom closest to doped at X atoms.The structures of each system are shown in Figure S3.
In addition, the Bader charge transfer between the doping atom and the surrounding Zn atom was calculated to present the chemical interaction activity between the doping atom and Zn atom, and the bond length variation between the doping atom and the surrounding Zn atom was also investigated to demonstrate the local strain induced by the doping atom.

Figure S11 .
Figure S11.Comparison of the cycling performance of Zn, ZnBi and ZnNi in symmetric cells at 2 mA cm -2 for 2 mAh cm -2 .

Figure S12 .
Figure S12.Rate performance of the symmetric cells based on Zn, ZnBi and ZnNi anodes at current densities from 1 to 5 mA cm −2 .

Figure S14 .
Figure S14.The electrochemical impedance spectroscopy of the symmetrical cells (a) before and (b) after 100 cycles at 2 mA cm -2 and 2 mAh cm -2 cutting-off capacity.

Table S1 .
Weight and atomic ratios of Zn and Bi in the Zn-Bi nanoparticle.

Table S2 .
Weight and atomic ratios of Zn and Ni in the Zn-Ni nanoparticle.

Table S3 .
Comparison of the Zn plating/stripping performances in symmetric cells between our Zn-Bi alloy electrode and other reported works