Cu foil

First, the NG slurry was prepared by mixing SP-1 graphite powder (TED PELLA, Inc. Prod. No. 61–302, Lot# 042111) and PVDF (10%) (HSV-900) in N-methyl pyrrolidone (Alfa Aesar, 99+%). Then, the slurry was cast on a Cu foil (Ubiq Tech. Inc. LTD, 10 μm) to form a uniform graphite film and dried at 150 °C for 2 h. The Cu foil was then etched by immersing the sample into an iron chloride (Sigma-Aldrich, 97%) solution (0.4 g ml⁻¹) to form a NG film. After which the NG film was rinsed with deionized water to remove the residual FeCl₃, and dried at 70 °C for 3 h to obtain the NG film (2 × 2.3 cm). The specific graphite loading was ∼4 mg cm⁻².

PFA-based Swagelok-type cells were used in this study. The Zn||Cu asymmetric cells were assembled using Cu foil (thickness: 9 μm; diameter: 11 mm) as the positive current collector, Zn metal foil (99.9%; Sigma-Aldrich; thickness: 250 μm; diameter: 10 mm) as the negative, and commercial Whatman glass fiber as the separator (diameter: 12.7 mm). In Zn||Zn symmetric cells, Zn foils were used as both positive and negative electrodes. The full cells with low-loading ZVO or activated carbon (AC) cathodes were assembled with 100 μL electrolyte, using Zn metal foil as the anode. Cu foil with pre-deposited Zn was utilized as the anode in high-loading full cells, where the amount of electrolyte was increased to 200 μL to ensure good electrode infusion. The electrochemical stability window of the hybrid eutectic electrolyte was investigated by linear sweep voltammetry (LSV) at a sweep rate of 1 mV s⁻¹, based on three-electrode cells with Zn as the reference electrode, and Ti as both working and counter electrodes. Chronoamperometry (CA) measurements were conducted at a constant polarization of 5 mV on Zn||Zn symmetric cells. All cells were cycled using a VMP3 potentiostat/galvanostat station (Bio-Logic) at 25 °C.

The electrolytes used in this work were all pre-electrolyzed for 24 h at a constant reducing current of −10 mA in a Nafion membrane-separated (IEM, Nafion 211, Fuel Cell Store) two-compartment cell. The Cu foil (99.998%, Sigma-Aldrich) and a graphite rod were employed as working and counter electrodes, respectively. The pre-electrolyzing can deposit most of the metal impurities in the electrolytes onto the Cu foil.

Copper nitrate hydrate (99.999% Cu(NO₃)₂), sodium hydroxide (10.0 N standardized solution, NaOH), sodium hydroxide (pearl, 97% NaOH), and Cu foil (thickness: 0.025 mm, 99.999%) were purchased from Sigma Aldrich, Saint Louis, MO, USA. Hydrazine solution (35 wt% in H₂O, N₂H₄) was purchased from Alfa Aesar, Heverville, MA, USA. These materials were used for the Cu-NW synthesis. Iron (Fe) nanoparticles (99.5+%, 30 nm, Fe₂O₃ alpha) were used as a catalyst for the CNT growth, and were purchased from RND Korea, Ltd, Gwangmyeong, Korea. A silver nanowire (Ag-NW) solution was purchased from Duksan Hi-Metal (Ulsan, Korea), and graphene oxide (GO) was purchased from Grapheneall, Ltd, Siheung, Korea. Nafion-117 and 20 wt% Nafion solution were purchased from the DuPont Company (Midland, MI, US), and were used as the ionic polymer membrane of the IPMC and the interface material with the Cu-NW, respectively. The cation used for the actuation of the IPMC in the Nafion was 1-Ethyl-3 methylimidazolium trifluoromethylsulfonate (EMIM-Otf, ionic liquid), and this was purchased from Merck KGaA (Darmstadt, Germany). Polyvinylidene difluoride (PVDF) membrane filter paper (pore size: 0.20 μm, diameter: ϕ47 mm) was purchased from Hyundai Micro., Ltd, Seongnam, Korea, and was used for Cu-NW synthesis and fabrication of the GO/Ag-NW paper electrode.