Modelling and optimisation of electrochemical and bioelectrochemical systems for environmental sustainability
Keywords: electrochemical CO2 reduction reaction systems
bioelectrochemical systems
microbial electrosynthesis cells
microbial fuel cells
extracellular electron transfer

The growing depletion of fossil fuels and its associated environmental concerns have drawn significant attention. Recent research is oriented towards finding alternative renewable energies or the technologies used for the valorisation of carbon dioxide (CO2). In this context, bioelectrochemical systems (BESs) and electrochemical CO2 reduction reaction (ECO2RR) systems have emerged and rapidly developed. Yet, their applications face a number of challenges, mostly related to their poor performance and unclear fundamental mechanisms.
Microbial fuel cells (MFCs), as an archetype and the most studied example of BESs, are the one and only technology to directly convert chemical energy to electricity through the microbially catalysed anodic oxidation processes. Nevertheless, the practical application of MFCs is much hindered due to the lower level of the power output. The design of a stable multi-species biofilm offers a perspective for improving the performance of MFCs. A one-dimensional (1D) dynamic numerical model for a ‘fermenter-electrochemically active bacteria (EAB)’ type of biofilm is established and adopted to explore the relationships between input variables and the MFC’s performance (i.e. biofilm composition and current density) with different mechanisms of anodic extracellular electron transfer (AEETs) in various operating modes. The contrasting results between different operating modes are shown to be mainly induced by the different length of the available time and the different manner of the substrate supply. For instance, MR1 proportion decreases with biofilm thickness in the batch mode, while an opposite trend manifests in the continuous mode with both AEET mechanisms. On the other hand, the contrasting results between different AEETs are attributed to the different intrinsic characteristics of the different electron acceptor. It has been shown that a lesser degree of spatial variation in the fractions of the two species, greater overall growth of MR1 in the biofilm, and higher attainable current density in the case of DET. Some counter-intuitive occasions imply the complex interplays among a series of processes within the multi-species biofilm. The optimal operating conditions towards better performance have also been suggested in this work, such as 15 kΩ for the external resistance in batch with MET; 1×10^(-8) and 1×10^(-7) L/s for the flowrate in the continuous MFCs with MET and DET, respectively; and 100 μm for the biofilm thickness with moderate conductivity (i.e. 2.5×10^(-6) S/m) in the continuous MFC with DET.
Microbial electrosynthesis cells (MESs), as another typical type of BESs, have significant potential for transforming CO2 to valuable products through the microbially catalysed cathodic reduction processes without intensive energy input. While the MESs technology is promising, it is still in infancy and its performance (i.e. product yield and production rate) is not satisfactory. As MESs marry microbiological catalysis to electrochemistry through the mechanism of cathodic extracellular electron transfer (CEETs), a deeper insight into CEETs is vital to effectively improve the system’s performance. In accordance with the experimental observations and hypothesis, two types of CEETs are interactively integrated in this work into a 1D dynamic mathematical model. Additionally, this numerical study incorporates the detailed biofilm metabolism, microbial kinetics processes and pH effects. By using this experimentally and quantitatively validated model, the governing CEET mechanism in the simulated system is identified as the ‘direct-conduction’ type; and the impact of several parameters (including applied cathode potentential, initial pH value, specific surface area of the biofilm matrix, biofilm thickness and the specific gas-liquid interfacial area) is analysed. The simulation results have suggested that, for the system modelled in this work, the applied cathode potential of -1.2 V (vs Ag/AgCl) is the optimal value to gain a higher value of acetate production rate and acetate yield simultaneously. The numerical results show that local pH plays an important role in product yield and production rate. A larger value for both the specific surface area of the biofilm matrix and the biofilm thickness is preferable to obtain a higher acetate production rate. Regarding the specific gas-liquid interfacial area, the acetate production rate can be improved when the area increases to ~1 m^(-1). It would become less effective to improve the acetate production rate by further increasing the specific gas-liquid interfacial area (i.e. >1 m^(-1)).
Similar to the MESs technology, ECO2RR is capable of achieving sustainable CO2 valorisation by consuming water and electricity only. Depending on the catalyst type (generally metal catalysts), CO2 can be reduced to different valuable chemicals. However, the lower product yield, which is rooted in the intrinsic problems of CO2 (such as larger mass transfer resistance and lower solubility), prevent ECO2RR from being put into actual use. The integration of a gas diffusion electrode (GDE) into an ECO2RR system could effectively minimise those aforementioned limitations by separating gas and liquid phase. To obtain better performance (i.e. amplified current density and higher product yield), a GDE-based ECO2RR system is modelled and tested to find the optimal design (such as the optimal catalyst loading and porosity) and operating conditions (such as the optimal applied cathode potential). The numerical results show that product yield is improved as the applied cathode potential becomes more negative. Regarding the specific electrical energy consumption, an optimal value of the applied cathode potential of -0.5 V (vs Reversible Hydrogen Electrode, RHE) exists. The simulation results have suggested that a low loading with a sparse arrangement of catalyst particles in the catalyst layer is optimal to gain the maximum product yield, while higher energy efficiencies may prefer a more compact (i.e. thin and dense) catalyst layer. The simulation results also show that GDE is scalable as it retains stable performance when its geometrical surface area varies. This result offers a theoretical basis in the further commercialisation of this strategy.
Overall, the findings from these model-based analyses are instrumental for offering a holistic understanding of respective systems and for furthering these systems towards successful industrial applications.