Hydrovoltaic Properties of Self-Assembled Protein Nanowires

Recently, a new, versatile, and promising source of renewable energy from water has been proposed: hydrovoltaics [1-3]. This emerging technology makes it possible to generate energy directly from the interaction between water vapour and functional nanomaterials. Due to the power output (a few tens of µW/cm²), potential applications are mainly in low-consumption electronics, such as sensors and IoT devices. Although hydrovoltaics is garnering significant attention, the underlying mechanisms remain a topic of debate [4].<br/>In this study, we demonstrate the potential use of self-assembly proteins, <i>i.e.</i> amyloid fibers, as hydrovoltaic biogenerators. These amyloid fibers, composed of protein nanowires with a diameter of 10-20 nm and an average length of several µm, are non-pathological. The hydrovoltaic properties of these bionanowires have been observed in various forms: as a film or as a macro-wire. In the case of macro-wires form, a potential difference forms naturally and spontaneously when the amyloid fibers align and dry. In fact, an open-circuit voltage (OCV) appears when humidity exceeds 40% relative humidity (RH). This OCV is directly correlated to the humidity levels ans increases as the humidity rises. For a single macroscopic wire, an OCV of 1.5 V was recorded at 85% RH.<br/>In the case of the film form, we demonstrated hydrovoltaic properties using protein films placed between two glassy carbon electrodes, taking advantages of a gradient of immobile charges. Electrochemical modifications are required on one of the electrodes (oxidation at the anode) to establish this gradient. Water oxidation at the anode induces chemical changes in the protein side chains, leading to the appearance of an OCV of around 0.4 V at 85% RH and a power output of around 50 μW/cm³. In both cases, water adsorption on the material's surface is crucial and determines the moisture content required to generate electricity.<br/>Our work paves the way for further advancements in the 'fusion' of desirable features, bringing the concept of a true biogenerator for electronics closer to reality.<br/><br/>[1] M. Zhang et al., «An ultrahigh-rate electrochemical capacitor based on solution-processed highly conductive PEDOT:PSS films for AC line-filtering», Energy Environ. Sci., vol. 9, n.o 6, pp. 2005-2010, 2016, doi: 10.1039/C6EE00615A.<br/>[2] L. Wang, W. Zhang, y Y. Deng, «Advances and Challenges for Hydrovoltaic Intelligence», ACS Nano, vol. 17, n.o 15, pp. 14229-14252, ago. 2023, doi: 10.1021/acsnano.3c02043.<br/>[3] J. Xu et al., «Sustainable moisture energy», Nat Rev Mater, feb. 2024, doi: 10.1038/s41578-023-00643-0.<br/>[4] X. Liu et al., «Power generation from ambient humidity using protein nanowires», Nature, vol. 578, n.o 7796, pp. 550-554, feb. 2020, doi: 10.1038/s41586-020-2010-9.