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

Abstract

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 IrO 2 . 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.

Biosynthesis Parameters Control the Physicochemical and Catalytic Properties of Microbially Supported Pd Nanoparticles

“…Additionally, the Pd loading on cells can control the particle size distribution and extracellular distribution of bio‐Pd. Recently, the continuous step‐wise addition of Pd(II) was shown to offer tight control over bio‐Pd size, [ 38 ] largely avoiding the agglomeration of particles observed in our study from bulk addition of Pd(II). Furthermore, the use of electron shuttles with DMRB, such as riboflavin, can promote the extracellular reduction of Pd(II), resulting in smaller mean particle size.…”

“…Additionally, the Pd loading on cells can control the particle size distribution and extracellular distribution of bio-Pd. Recently, the continuous step-wise addition of Pd(II) was shown to offer tight control over bio-Pd size, [38] largely avoiding the agglomeration of particles observed in our study from bulk addition of Pd(II).…”

“…Acetate-driven bioreduction deposited bio-Pd at the outer membrane (Figure 2E,F), the location of outer membrane ctype cytochromes, which are strongly suggested to play a role in Pd(II) bioreduction in G. sulfurreducens. [38,39] Agglomerates of particles were common at the outer membrane in Acetate -Low Pd (Figure 2E), and no extracellular particles were observed. As for Acetate -High Pd, on the other hand, significant amounts of small (≈2 nm) extracellular particles were observed bound to the EPS protruding from the cell surface (Figure 2F), which significantly decreased the mean particle size compared to Acetate -Low Pd (Table 1).…”

“…[15,37] G. sulfurreducens supplied with acetate is suspected to deposit bio-Pd at the outer membrane, although the precise outer membrane c-type cytochrome(s) responsible have not been identified. [37][38][39] G. sulfurreducens possesses a [NiFe] hydrogenase, Hyb, and a formate dehydrogenase oriented on the periplasmic side of the inner membrane. [40,41] The concentration of hydrogen and formate in bio-Pd synthesis has been shown to affect bio-Pd recovery and size.…”