Supercapacitors have attracted interest in energy storage because they have the potential to complement or replace batteries. Here, we report that c ‐type cytochromes, naturally immersed in a living, electrically conductive microbial biofilm, greatly enhance the device capacitance by over two orders of magnitude. We employ genetic engineering, protein unfolding and Nernstian modeling for in vivo demonstration of charge storage capacity of c ‐type cytochromes and perform electrochemical impedance spectroscopy, cyclic voltammetry and charge–discharge cycling to confirm the pseudocapacitive, redox nature of biofilm capacitance. The biofilms also show low self‐discharge and good charge/discharge reversibility. The superior electrochemical performance of the biofilm is related to its high abundance of cytochromes, providing large electron storage capacity, its nanostructured network with metallic‐like conductivity, and its porous architecture with hydrous nature, offering prospects for future low cost and environmentally sustainable energy storage devices. Living supercapacitors: The capacitance of an electrode‐based device can be enhanced 100‐fold using the redox chemistry of c ‐type cytochromes naturally embedded in an electrically conductive network of living bacteria (see picture). This study demonstrates the unique survival strategy by metal‐respiring bacteria when electron acceptors are temporarily unavailable and suggests a novel method for supercapacitive energy storage using self‐renewing microbes
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