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Electrochemical conversion of CO2has attracted attention worldwide since this process can convert carbon dioxide to a wide range of value-added chemicals. This reaction required the development of efficient electrocatalysts and fundamental understanding of the reaction kinetics and thermodynamics to overcome the current challenges.

Graphene-related materials are in the forefront of nanomaterial research. One of the most common ways to prepare graphenes is to oxidize graphite (natural or synthetic) to graphite oxide and exfoliate it to graphene oxide with consequent chemical reduction to chemically reduced graphene. Here, we show that both natural and synthetic graphite contain a large amount of metallic impurities that persist in the samples of graphite oxide after the oxidative treatment, and chemically reduced graphene after the chemical reduction. We demonstrate that, despite a substantial elimination during the oxidative treatment of graphite samples, a significant amount of impurities associated to the chemically reduced graphene materials still remain and alter their electrochemical properties dramatically. We propose a method for the purification of graphenes based on thermal treatment at 1,000°C in chlorine atmosphere to reduce the effect of such impurities on the electrochemical properties. Our findings have important implications on the whole field of graphene research.electrochemistry | synthesis G raphene and graphene-derived materials have recently attracted enormous attention from the scientific community because of their extraordinary physical, chemical, and mechanical features (1, 2). Graphene materials can be used in several applications-including electronics (3), composite materials (4, 5), sensing (6), energy storage (7,8), and medicine (9)-with expected or known advantages over conventional materials.In general, there are two routes leading to the production of graphene: (i) a bottom-up approach, consisting of growing single/ bilayered graphene onto a catalytic surface through chemical vapor deposition (CVD) technique (10, 11); and (ii) a top-down approach, starting from graphite to obtain single/few-layered graphene sheets by an exfoliation procedure (12, 13). Because exfoliation in the liquid phase is hardly achieved directly on graphitic materials because of the highly cohesive van der Waals forces between the graphene sheets (14), a chemical treatment is generally performed to oxidize graphite to graphite oxide (GO). The oxidation helps to increase the graphene interlayer distance for an easy exfoliation, which is then followed by the removal of the oxygen functionalities to give single/few-layered graphene (12). The second approach received particularly huge attention because it is suitable for large-scale production of graphene materials and is cost-effective, although the graphene produced presents significant structural defects and lower carrier mobility properties (12). Natural graphite is the preferred starting material for this method of preparation because it is available in great quantities and at a low cost. Alternatively, synthetic graphite is also widely adopted as a starting material. It is important to highlight the differences between these two graphitic materials with particular focus on the content of metallic impurities and possible sources of contamination.Natural graphite is mined using standard ...