Green hydrogen production using Shewanella oneidensis MR-1 bioanode and cuprous oxide-based photocathode

In recent years, many efforts have been made to find alternative renewable energy sources that can ideally replace the use of fossil fuels in all aspects. One of the new emerging energy technologies is the bioelectrochemical system, of which two types are the microbial fuel cell (MFC) and the microbial electrochemical cell (MEC). MFCs can only generate small amounts of energy from waste, such as wastewater and other organic matter. In its photoelectrochemical version, the process is improved to increase the output of low power generation. The purpose of MECs and their photoelectrochemical version (MPECs) is to produce useful chemicals. The aim of this PhD project was to construct a high performance MPEC, consisting of a Shewanella oneidensis biofilm and a cuprous oxide photocathode, which produces hydrogen with a high coulombic efficiency. In this system, under light illumination, the photocathode works synergistically with the bioanode to combine solar energy with the bio-catalysed chemical energy of organic matter. To achieve this goal, an in-depth characterisation of the system was required. Five-layered Cu2O photocathodes have been prepared. A basic electrochemical characterisation, consisting of current versus electrode potential curves, was carried out when exposed to simulated solar light. This electrode reached a photocurrent density production of -5.5 mA cm-2. The onset potential for hydrogen evolution was set at 1 V vs. RHE (~0.1 mA cm-2). Preliminary experiments showed a maximum current several orders of magnitude higher than the bioanode, which was the reason to focus the study on the bioanode with S. oneidensis MR-1. The metabolism of S. oneidensis was studied using different soluble electron acceptors under different conditions. The strain was cultivated in a bioreactor under defined conditions. The concentration of electron donors was measured, and the secretion of metabolites and the use of soluble electron acceptors were monitored during the different stages of cultivation. Under anaerobic respiration with lactate, the molar selectivity for the conversion of lactate to acetate reached 90 ± 8%; acetate was not metabolised by the strain after complete consumption of lactate. The transition from anaerobic to aerobic respiration resulted in an increased lag time for aerobic respiration and acetate was rapidly consumed. S. oneidensis MR-1 biofilm on a solid graphite rod electrode showed a strong dependence between electron transfer kinetics, temperature, mediator concentration and light exposure. Two redox systems were identified, corresponding to direct and mediated electron transfer (DET and MET). The characteristic parameters of DET (jmax, and j0) were quantified after deconvolution of low-scan linear voltammetry, showing that planktonic cells played a role in current generation. The MET was shown to be involved in a fast interfacial electron transfer reaction. A new mechanism for current production involving electron transfer f