Electrochemical capacitors: Materials, technologies and performance

Electrochemical capacitor energy storage technologies are of increasing interest because of the demand for rapid and efficient high-power delivery in transportation and industrial applications. The shortcoming of electrochemical capacitors (ECs) has been their low energy density compared to lithium-ion batteries. Much of the research in recent years has focused on increasing the energy density of ECs. This paper is a review of that research. The two primary approaches to increasing the energy density are to increase the maximum voltage of the EC cell and to increase the specific capacitance (F g−1) of its electrodes. Hence this review has focused on the evaluation of the use of nano-structured carbons, metal oxides, and the latest promising pseudocapacitive materials including carbides and nitrides (MXenes) and metal–organic frameworks (MOFs) in the electrodes and, ionic liquid and redox electrolytes towards this goal through the development of advanced electrochemical capacitors. Available test for ECs indicates that the hybrid capacitor approach is likely the best approach to developing ECs with high energy density, high power capability, and long cycle life. A focus of the paper is to examine protocols for evaluating the electrochemical performance and discuss the challenges in developing high-performance cells using different electrochemical energy storage technologies for practical applications. We hope this effort will provide multiple perspectives that can be helpful in addressing advancements from synthesis to development of advanced EC devices.