In-situ studies of biomolecular assisted Li-air batteries and studies of biobased redox mediators and ORR catalysts

PhD project strategic theme: Bioscience for renewable resources and clean growth

Li-air batteries (LABs) have gained significant attention lately due to their comparable energy densities to fossil fuels, with the added advantage of being greener than Lithium-ion batteries, as they contain no transition metals. Typical non-aqueous LABs rely on an oxygen reduction reaction (ORR) to discharge. Redox mediators (RMs) are often used to assist in the electron transfer between the cathode surface and O2, lowering the battery overpotential and controlling the rate of formation of the discharge products. Redox-active biomolecules, which play vital roles in energy assimilating processes in nature and often bind to enzymes as cofactors, are often used as RM in LABs. For instance, Quinones, such as DBBQ, Vitamin K2 or Enzyme Q10 have been tested as RMs with varying success. ORR biocatalysts, such as heme (haemoglobin's co-factor), and enzymes, such as laccase from Tramtes versicolor (LacTv), can be used to further increase the energy efficiency in LABs by providing stable intermediates to the formation/decomposition of the discharge product. The activity of biological ORR catalysts depends on the media where the reaction takes place, which affects their ability to solvate oxygen and binding energetics. This is particularly important since LABS operate in organic-based electrolytes, in contrast to the aqueous solutions of biological systems. The rate and properties of discharge product will be influenced by the pH and electron transfer rate in the cell, which in turn depends on the electrolyte, rate and type of catalyst/RM used.

Currently, mechanistic studies of the activity of bio-based RM/ORR catalysts in battery set-ups is very limited. In order to establish a structure-function relationship that would help the development of biomolecular-assisted LABs, novel in-situ characterization techniques are required. The aim of this project is to develop in-situ X-ray Diffraction and Nuclear Magnetic Resonance Spectroscopies with which a comprehensive catalogue of biomolecular ORR catalysts can be studied. With these novel tools, we aim to develop optimized LABs based on sustainably sourced biomolecules by screening a library of potential ORR catalysts and RMs such as porphyrins, flavins, phenazines, phthalocyanins and enzymes to which these co-factors are bonded such as cytochrome c oxidase (CcO), monodehydroascorbate reductase (MDAR) or NADH oxidase. The differences of oxygen binding characteristics between soluble co-factors and enzymes grafted onto the electrodes in the battery setup and the effect to the overall performance will also be studied. These will allow us to study important properties regulating the activity of redox mediators, such as diffusivity, equilibrium redox potential, state of charge, and degradation pathways during battery operation. Studying redox active biomolecules with catalytic functions opens up a new path high performance RMs and ORR catalysts made of sustainable, cost-effective materials such as recyclable bio-waste (e.g. blood waste or natural enzymes). Both of these aspects tie in well with the BBSRC strategic theme Bioscience for renewable resources and clean growth, which aims to transform a range of industries by reducing the reliance on fossil fuels and thereby help to meet international climate change targets.