Biosynthesised nanoparticles as Polymer Electrolyte Membrane Fuel Cell (PEMFC) catalysts

Stephen, Alan J. (2021). Biosynthesised nanoparticles as Polymer Electrolyte Membrane Fuel Cell (PEMFC) catalysts. University of Birmingham. Ph.D.

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Abstract

Polymer Electrolyte Membrane Fuel Cells (PEFMCs) currently utilise platinum group metal (PGM) nanoparticles (NPs) as a catalyst for their hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR). Platinum (Pt) and Palladium (Pd) are the most efficient catalysts for these reactions with platinum nanoparticles on carbon (Pt/C) being used commercially. Biosynthesised nanoparticles have recently emerged as a green alternative to conventional chemical synthesis; Escherichia coli has been shown to synthesise nanoparticles of Pt (E. coli-Pt) and Pd (E. coli-Pd). The HOR and ORR activities of these E. coli-NPs were previously confirmed. This study investigates these E. coli-NPs and their bimetallics (E. coli-Pt:Pd* and E. coli-Pd:Pt*) for direct use as ORR catalysts.

Initially E. coli-Pd, E. coli-Pt, E. coli-Pt:Pd* and E. coli-Pd:Pt* were all synthesised and characterised. E. coli-Pd showed ubiquitous NP synthesis (intracellular NPs and surface bound clusters) whereas E. coli-Pt showed Pt-NPs primarily associated to the surface of the cell. It was shown that this patterning influenced the synthesis of bimetallic E. coli-NPs. E. coli-Pt:Pd* was synthesised by reducing Pt (IV) ions onto E. coli-Pd and E. coli-Pt:Pd* showed similar NP localisation across the cell as E. coli-Pd. Similarly, E. coli-Pd:Pt* was synthesised by reducing Pd (II) ions onto E. coli-Pt and E. coli-Pd:Pt* showed similar NP localisation in surface bound clusters. Unlike E. coli-Pt, however, some intracellular NPs were seen. All E. coli-NPs showed promise for ORR activity with the crystal face (111) being detected via X-ray diffraction (XRD) and selected area electron diffraction (SAED). Both bimetallics are thought to be alloys of Pt and Pd as scanning transmission electron microscopy – energy dispersive X-ray (STEM-EDX) maps showed co-localisation of both metals. This was further evidenced by X-ray photoelectron spectroscopy (XPS) and XRD data. Furthermore, E. coli-Pt:Pd* showed evidence of a “Pt-rich” skin around clusters whereas this was not seen for E. coli-Pd:Pt*.

All E. coli-NPs were tested ex-situ electrochemically for ORR activity. E. coli-Pd:Pt* did not show any discernible electrochemical activity while E. coli-Pt:Pd* (10%:10%) showed better activities than single metal versions with a mass activity (MA) of 2.8 mA/mg relative to a MA of 1.7 mA/mg for E. coli-Pt and E. coli-Pd at 0.3 mA/mg. This increase in activity is attributed to the increased conductivities afforded by Pt:Pd* NP dispersion. However, these results are significantly lower than MA values for commercial Pt/C catalyst at ~ 250 mA/mg. To increase conductivities, and thus MA, multiwalled carbon nanotubes (MWCNTs) were mixed with E. coli-Pt:Pd*. This showed an increase in MA to 8.6 mA/mg relative to 2.8 mA/ mg for untreated E. coli-Pt:Pd*. However, this activity still falls significantly short of commercial Pt/C activities (~250 mA/mg).

A screening life cycle analysis (LCA) was conducted to establish the environmental benefits in biosynthesis of NPs, over using conventional Pt/C, as PEMFC catalysts. However, it showed that relative to E. coli-NPs, commercial Pt/C only had ~2% of the environmental impacts when used as PEMFC catalysts, i.e., using Pt/C was more environmentally friendly than E. coli-NPs. This was attributed to the low MA of E. coli-NPs relative to Pt/C. In using catalysts with low MAs, a greater mass of catalyst had to be used to generate the same power (157.1 mg for E. coli-Pt:Pd* relative to 1.8 mg for Pt/C) resulting in more environmental impacts by their use. An alternative LCA using a hypothetical E. coli-Pt:Pd* catalyst with an MA of ~250 mA/mg (similar to Pt/C) showed fewer environmental impacts. This catalyst only required 1.8 mg to generate the same power and thus, relative to commercial catalyst (Pt/C) it only had ~50% of the environmental impacts.

Thus, this body of work investigated the potential of E. coli-NPs as direct use ORR catalysts. It was concluded that E. coli-NPs showed environmental advantages over commercial catalysts only if their mass activities can be significantly improved.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):