Flexible fuel cells: A prospective review

With the widespread of wearable electronics in healthcare, military and entertainment sectors, flexible power sources have attracted great attention, among which flexible fuel cells are relatively young compared with flexible batteries, supercapacitors and energy harvesters. Fuel cell is well known for its uninterrupted operation, high energy density and instant refueling ability, which is especially advantageous for long-term and outdoor missions. To date, existing flexible fuel cell studies can be classified into three major types based on their electrolyte and catalyst material, namely the flexible polymer electrolyte membrane fuel cell (PEMFC), membraneless fuel cell (MFC) and biofuel cell (BFC). The flexible PEMFC generally employs hydrogen as fuel so that a power density of hundreds of mW cm-2 can be achieved. Relevant research efforts are mainly paid to the replacement of conventional rigid cell components with flexible substitutes. Moreover, novel cell structures such as ultrathin cell and tubular cell have also been proposed. However, the flexible hydrogen storage is still a research gap. The flexible MFC has a much wider choice of fuel such as methanol, ethanol and formate, but the power output is limited to dozens of mW cm-2 due to more sluggish fuel oxidation. To circumvent the demand of pumping, porous materials with capillary action are preferred as cell substrate, such as cellulose paper and cotton thread, which can absorb electrolyte solution passively. Nevertheless, the capillary flow rate is not controllable at the moment. As for the flexible BFC, it is primarily targeted for epidermal applications in order to utilize natural organic materials in human body fluid. Benefited from this, the flexible BFC can have the simplest cell structure of two bioelectrodes only, which can be integrated onto contact lenses, tattoos, clothes, etc. However, the complex organic fuel oxidation as well as the mild electrolyte pH have greatly restricted its power density to μW cm-2 level. In this work, a comprehensive review on existing flexible fuel cell studies is provided, including cell structure, material, performance together with their advantages and disadvantages. Based on this, solid conclusions are made on their development trend and future perspectives are presented as well.