Microfluidic fuel cells with different types of fuels: A prospective review

Since its first appearance in 2002, microfluidic fuel cell has received great attention in the past two decades, which is mainly targeted at its use in portable electronics. This micro fuel cell technology utilizes microfluidic flows as electrolyte instead of conventional polymer membranes. To date, various fuels have been utilized in it, such as vanadium species, hydrogen, hydrocarbons, hydrogen peroxide, borohydride and nitrogenous materials, each of which has its specific merits and demerits. To optimize its power output and fuel utilization, innovative cell structures and advanced catalysts have been continuously developed for different fuels, with remarkable improvements achieved. The power output can be elevated from several mW cm⁻² to several W cm⁻² at room temperature, while the fuel utilization per single pass can reach 100% by using 3D flow-through electrodes. Also, investigations in recent years have shown that microfluidic fuel cell stacking increases the working voltage. In addition to cells with plastic channel, novel cell designs based on cellulose paper and fabric materials have also been proposed; apart from being lightweight, they are also free from pumping. These innovative cell designs represent a promising route for achieving real applications in areas such as medical diagnostic, wearable healthcare and smart logistics. As for the conventional plastic cells, they are currently less competitive against batteries and other fuel cells because of the extra pumping requirement, which should be resolved in future by developing passive pumps instead. Alternatively, they can be applied in specific circumstances where the extra pumping loss is tolerable.