Evaluating aqueous zinc electrodeposition and stripping at pyrolytic carbon electrodes with tunable surface functionality

Abstract

Next-generation rechargeable zinc-based batteries require efficient deposition and stripping (D/S) of Zn in mild-pH aqueous electrolyte. With the move away from conversion of bulk Zn electrodes, the properties of D/S substrates, such as surface functionality and electrode architecture, can impact Zn redox processes. To investigate these effects, we adapt planar and 3D pyrolytic carbon (pyC) substrates previously reported for fundamental electroanalytical studies. Thin films of pyC are fabricated by benzene pyrolysis, yielding a low oxygen-content, oleophobic surface that can be further functionalized by plasma oxidation and nitridation or with heteroatom doping by adding precursors such as thiophene to the benzene feed. We evaluate Zn/Zn²⁺ redox at a series of pyC electrodes with varied surface characteristics, cycling in either zinc sulfate (ZnSO₄) or zinc acetate (Zn(OAc)₂) mild-pH aqueous electrolytes of relevance to “zinc-ion” batteries. Characteristics such as wettability, deposition morphology, Zn D/S reversibility, and coulombic efficiency are correlated with pyC surface functionality. Aromatic sulfur doping followed by plasma nitridation (pyC∼S:N) yields the best balance of uniform Zn electrodeposition and effective electrostripping from the substrate: a two-terminal cell charges/discharges at >98% coulombic efficiency for >350 cycles. We then conformally coat macroscale thick fiber-paper electrodes with pyC and plate Zn at 1, 5, and 10 mAh cm⁻². Even at 10 mAh cm⁻², 3D pyC electrodes deposit Zn without a charge-transfer resistance penalty, confirming the viability of such pyC architected electrodes for practical areal capacity.