Highly efficient purification of natural coaly graphite via an electrochemical method

Abstract

Due to the fine granularity of CG with the dissemination of non-graphite minerals, highly efficient purification of natural coaly graphite (CG) by low-cost, low-energy consumption and low-pollution methods remains a challenge. In this study, a novel and efficient electrochemical method is demonstrated for purifying natural CG. The electrochemical purification is carried out in a mixed electrolyte containing 0.5 M (NH4)2SO4 and 10% HF with an optimum potential sequence starting at + 2.5 V for a few minutes followed by a short burst of + 5 V. The CG purification mechanism is proposed based on the experimental results. There are three main factors, namely, the type of the graphite anode, the electrolyte and the applied voltage are critical for effectively electrochemical purification. It is interesting to find that the main purified products are graphene mixed with graphite aggregates at a yield of 68%. Raman analysis indicates that the purified CG has fewer structural defects than the natural CG after removing the non-graphite minerals. Moreover, the XPS and FTIR analysis demonstrate that no other functional groups are grafted onto the parent graphite during this purification process. In comparison with the electrolyte only containing HF, the adding of 0.5 M (NH4)2SO4 in the HF electrolyte could reduce the amount of HF by more than 50%, which cuts the cost and alleviates the impact on the environment.