Recent advance in electrochemically activated supercapacitors: Activation mechanisms, electrode materials and prospects

Different from traditional materials synthesis, electrochemical activation technique achieves the dynamic optimization of electrode materials through adjusting the specific surface area/porosity, crystal type, microstructure or composition, further enhancing the electrochemical activity of electrode materials. In recent years, electrochemical activation strategy has been received more attention for boosting the energy storage capability of supercapacitors. However, there is a lack of specific review on the use of electrochemical activation strategy in supercapacitors. To illustrate this novel technique elaborately, this review will offer a comprehensive summary about existing electrochemical activation mechanisms in supercapacitors and corresponding electrode materials. In view of the effect of electrolyte type on reaction mechanism, the existing activation mechanisms can be divided into six types, including optimizing surface layer/porous structure, introducing active groups, intercalation induced phase transition, redox reaction, crystallinity transition, and anion exchange reaction. Based on materials type, existing electrochemical activation materials are classified into six categories: carbon materials, transition metal oxides, hydroxides, sulfides/selenides, phosphates/phosphides and polyanion compounds, and the research progresses about materials design, activation mechanism and enhanced electrochemical performance are highlighted. Finally, the current challenges/problems and prospects in the field of electrochemically-activated supercapacitors are proposed, with the main purpose of providing novel ideas and suggestions for developing self-activated supercapacitors.