Ultrahigh-mobility microbial cytochrome nanowires generate power from humidity.

Abstract

Mixed electronic-ionic conductors are crucial for various technologies, including harvesting power from humidity in a durable, self-sustainable, manner unrestricted by location or environment1,2. Biological proteins have been proposed as mixed conductors for 50 years3,4. Recently, Geobacter sulfurreducens pili filaments have been claimed to act as nanowires to generate power5,6. Here, we show that the power is generated by G. sulfurreducens-produced cytochrome OmcZ nanowires that show 20,000-fold higher electron conductivity than pili7. Remarkably, nanowires show ultrahigh electron and proton mobility (>0.25 cm2/Vs), owing to directional charge migration through seamlessly-stacked hemes and a charged, hydrogen-bonding surface, respectively. AC impedance spectroscopy and DC conductivity measurements using four-probe van der Pauw and back-gated field-effect-transistor devices reveal that humidity increases carrier mobility by 30,000-fold. Cooling halves the activation energy, thereby accelerating charge transport. Electrochemical measurements identify the voltage and mobilities required to switch pure electronic conduction to mixed conduction for power generation. The high aspect ratio (1:1000) and hydrophilic nanowire surface captures moisture efficiently to reduce oxygen reversibly, generating large potentials (>0.5 V) necessary to sustain high power. Our studies establish a new class of biologically-synthesized, low-cost and high-performance mixed-conductors and identify key design principles for improving power output using highly-tunable electronic and protein structures.