Facile Biological-Based Synthesis of Size-Controlled Palladium Nanoclusters Anchored on the Surface of Geobacter sulfurreducens and Their Application in Electrocatalysis

Metal nanoclusters can be synthesized following the top-down or bottom-up approaches involving multiple chemical and/or physical steps where strong ligands, toxic chemicals, and high temperature and pressure are normally applied. In contrast, biological methods eliminate the use and generation of hazardous substances and do not require the application of high temperature and pressure during the synthesis process. Biological methods based on the extracellular electron transfer (EET) capability of electroactive bacteria (EAB) are considered a promising sustainable route for the synthesis of metal nanoparticles; however, a fine control of the size of the nanoparticles has not been achieved yet. Herein, we report a facile biological-based synthesis method of size-controlled palladium (Pd) nanoclusters using the EET capability of Geobacter sulfurreducens. By controlling the metal precursor concentrations and dosing and incubation times, we synthesized size-controlled Pd nanoclusters (1.0 ± 0.6 to 4.8 ± 1.4 nm) anchored on the surface of G. sulfurreducens. The as-synthesized Pd nanoclusters anchored on the surface of G. sulfurreducens cells (Pd/GS) were tested for their performance as bifunctional electrocatalysts for the overall alkaline water splitting reaction. Despite a very low mass loading of 0.002 mg Pd cm–2, the hybrid material (i.e., Pd/GS) showed exceptionally higher activity for hydrogen evolution reaction (HER) when compared to benchmark 10 wt % Pt/C and Pd/C. Similarly, Pd/GS showed higher activity for oxygen evolution reaction (OER) when compared to Pd/C and comparable OER activity when compared to benchmark IrO2. The findings in this study provide a promising sustainable route for designing size-controlled metal nanoclusters anchored on the surface of EAB as efficient and low-cost electrocatalysts for various applications.