Chemically tuning graphene via anodic exfoliation for enhanced performance in aqueous zinc-based electrochemical energy storage applications

Graphene materials are attractive for use in novel aqueous electrochemical energy storage devices, including aqueous zinc-ion batteries (AZIBs) and hybrid capacitors (AZICs). Ideally, such materials should be readily accessible by eco-friendly routes and possess physicochemical features beneficial for the intended application. Here, we propose an anodic exfoliation strategy, using a proper combination of common salts/bases as the aqueous electrolyte, for the preparation of highly oxidized graphenes with some control over the populations of their oxygen groups and retention of electrical conductivity (∼102–103 S m-1). As an active cathode material for AZIC cells, the hydrophilic, carboxyl-enriched anodic graphene processed into a compact film outperformed reduced graphene oxides derived from common routes (e.g., Hummers method), in terms of capacity and rate capability. Furthermore, the performance of this new anodic graphene was enhanced by combining it with a multifunctional biomolecule (flavin mononucleotide), which promoted the cathode wettability and provided extra capacity due to its redox-active center, particularly at higher currents. The carboxyl-enriched graphene was also shown to act as an effective coating layer for the protection of the zinc metal electrode in AZICs/AZIBs, extending its cycle life for longer than is usually attained with carbon-based protective coatings.