Advancing GDE-based microbial electrosynthesis: from cell design to downstream processing

Global CO2 emissions produced by human activities are still on the rise, while different international organizations have developed roadmaps to help us achieve a future where CO2 emissions are minimized or even eliminated. However, the development of Carbon Capture and Utilization (CCU) technologies remains a key factor for the successful development of these roadmaps. In this context, microbial electrosynthesis (MES) technology has shown potential as an emerging CCU technology due to its capacity to use biological catalysts with electricity, ideally renewable, to convert CO2 into chemicals under mild operating conditions. This makes it particularly attractive for the manufacture of green chemicals produced through routes different from fossil fuels. The first MES system was reported in literature over a decade ago, and although significant progress has been achieved, most of the research still focuses on the fundamental aspects of the process, such as the mechanisms used by the biocatalyst to utilize the CO2 while improvements on the performance of the overall system are still limited, hindering its application at bigger scales. Understanding how operating factors affect the interactions between the microorganisms and the MES cells containing them can help to develop scalable setups that can help move this technology from lab-scale to pilot-scale. This work aims to further develop a gas diffusion electrode (GDE) based MES system, targeting some of the major limitations of the process: high overpotentials caused by low electrolyte conductivity, mass transport limitations affecting the performance of MES cells, interactions between the cathode and the microorganisms and the lack of a downstream process for the recovery of chemicals produced through MES. The study of a mixed microbial consortium under different NaCl concentrations using H- type cells, helped to identify Acetobacterium sp. as the dominant acetogen responsible for CO2 reduction, and established a Na+ concentration limit of 6 g L-1 as the inhibitory threshold for the non-halophilic community used. The community identified in the H-Type cells was later used as inoculum for a GDE-based three-chamber MES cell, fed with CO2. Despite its high efficiency and production rate obtained in the MES system presented in Chapter 4, the carbon conversion of the system indicated that CO2 availability was suboptimal in the system, laying the groundwork for the optimization of the GDE-MES cell. Cell design is a commonly overlooked variable in MES processes, thus, in Chapter 5, previously developed GDE-MES cell design was compared to a new serpentine design aiming to understand the effects of fluid distribution on the performance of the system. Additionally, biotic experiments were performed to understand the impact that the different fluid distribution can have on the performance of the MES-cells. The CFD model showed that the new design promotes a homogeneous distribution of both electrolyte and CO2 across the cathode, eliminating dead zones and generating hydrodynamic conditions of mixing and homogeneous flow favourable for biofilm formation. These results were validated through ix biotic experiments where the serpentine design demonstrated a higher acetate production. To further evaluate the performance of the design cell, the system was modified to operate in continuous mode, achieving a 30% increase in acetate production rate compared with batch operation and halving the electrical energy consumption per kilogram of acetate produced at a hydraulic retention time (HRT) of 1.08 days, with stable performance maintained over 300 days or operation. After understanding how cell design and fluid distribution affect the performance of GDE- MES systems, the surface GDE surface was modified in Chapter 6 using different conductive polymers in combination with molybdenum oxide (MoO2). Different characterization techniques were applied to evaluate the morphology and electrochemical properties of the cathode, the most promising electrode architecture was selected and introduced to the previously developed serpentine MES cell to study the performance of the system with this new electrode in biotic conditions. After 90 days of operation the modified electrode reached a comparable acetate production rate comparable to the serpentine cell without a modified GDE, which was operated in batch mode for over 100 days and another 125 days in continuous mode before reaching such production. Additionally, the modified electrode showed an increase in hydrogen in the outlet gas, suggesting that the MoO2 particles promote the hydrogen evolution reaction (HER), increasing the amount of reducing equivalents available for CO2 reduction. Another bottleneck affecting MES productivity is the accumulation of products in the catholyte. Conversely, like in most electrochemical technology, product concentration from MES systems is often too low for commercialization. Thus, Chapter 7 evaluates the use of electrodialysis (ED) as a downstream technology for MES. ED experiments were performed in a synthetic MES effluent media to resolve knowledge gaps on process optimization, particularly finding optimum operation ranges for pH and applied current densities, and the most suitable operation mode (continuous and fed batch). Optimal operation within the tested conditions was achieved at pH 5.5, with a current density of 19 mA cm-2, and under fed-batch mode, reaching acetate concentrations of 13.7 g L-1, with ion competition identified as the main limiting factor of the process. Overall, this work shows a series of advancements to key aspects hampering the development of MES technology. The use of more efficient cell designs, in combination with electrode materials that promote hydrogen evolution and microbial growth, as well as the application of combination with energy efficient downstream processes, can serve as basis for further upscaling MES systems. Additionally, the use of computational tools and advanced manufacturing technology, are key to bring this technology closer to industrial application.

Funder

Irish Research Council
Enterprise Ireland
Italian Ministry of University and Research

Publisher

University of Galway

Rights

CC BY-NC-ND

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Collections

University of Galway Theses (PhD Theses)