Carbon paper

2 M ZnSO₄ electrolyte was prepared by dissolving 0.2 mol ZnSO₄·7H₂O (> 99%, VWR chemicals) into 100 mL deionized water with constant magnetic stirring for 30 min, which is denoted as BE. The designed electrolytes were then prepared by mixing proper amounts of 0.1 mol L⁻¹ NH₃·H₂O with the as-prepared 2 M ZnSO₄ solution to control the concentrations of NH₃·H₂O additives as 0.5, 1, 2, 3, and 5 mM. The optimized concentration of NH₃·H₂O is 1 mM and the corresponding electrolyte is denoted as DE. The MnO₂ cathodes consisted of 70 wt% commercial MnO₂ powder (precipitated active for synthesis, Sigma-Aldrich), 20 wt% acetylene black (battery grade, MTI), and 10 wt% poly(vinylidene fluoride) (PVDF; average Mw ~ 534,000, Sigma-Aldrich) with N-methyl-2-pyrrolidine (NMP; ≥ 99%, Sigma-Aldrich) as the solvent. Carbon paper (hydrophilic type; TORAY) was selected as the current collector. Polished Zinc foils with 1,000 mesh sandy paper (70 μm in thickness; ϕ12 mm circles) were used in Zn symmetric cells, Zn//Cu asymmetric cells, and Zn//MnO₂ full cells.

The HER electrochemical measurements were performed on an electrochemical workstation (CH 760E) in 1M KOH electrolyte using three-electrode system. In a typical process, 5 mg of catalyst and 50 μL Nafion solution (Alfa Aesar, 5 wt.%) were dispersed in 500 μL of ethanol to form a homogeneous ink. Then, 7 μL of the dispersion was loaded onto a glassy carbon electrode (GCE) with 5 mm diameter (loading 0.3 mg cm⁻²) to obtain the working electrode. An HgO/Hg electrode and a graphite electrode were served as the reference electrode and counter electrode, respectively. All of the potentials mentioned later were calibrated to reversible hydrogen electrode (RHE). Linear sweep voltammetry (LSV) with a scan rate of 10 mV s⁻¹ was conducted between−0.6 and 0 V vs. RHE in 1M KOH electrolyte. Electrochemical impedance spectroscopy (EIS) was conducted in the frequency range from 100 kHz to 0.01 Hz with an ac perturbation of 5 mV at the potential of −0.1 V vs. RHE. Cyclic voltammetry (CV) was conducted with scan rate from 20 to 200 mV s⁻¹ ranging from 0.05 to 0.15 V vs. RHE. In order to prevent the mass loss cause by the drastic gas evolution, the stability test was performed using carbon paper (5 × 5 mm, Toray Industries, INC., Japan) as the substrate with catalyst loading of 0.3 mg cm⁻² at an electric potential of −0.08 V vs. RHE for over 50 h.

All reagents and solvents used were of analytical grade. Substrates were described as follows: graphite sheet (thickness: 1.0 mm, bulk density: 1.85 g cm⁻³, supplied from Gaofeng), nickel foam (thickness: 1.5 mm, areal density: 0.035 g cm⁻², Jiayisheng), carbon paper (thickness: 0.20 mm, bulk density: 0.78 g cm⁻³, Toray), titanium sheet (thickness: 0.20 mm, Shuanghua), copper foil (thickness: 0.05 mm, Feintool), indium tin oxide and fluorine-doped tin oxide (thickness: 1.1 mm, Weslsy), and GC sheet (thickness: 1 mm, Aida).