Giuliano C. Premier

BACKGROUND: The concept of carbon dioxide (CO₂) conversion to formate has attracted increasing interest in recent years and various small‐scale studies are present in the literature. However, upscaling of electrochemical CO₂ reduction comes with many challenges and there are very few reports available on it. In this study, we present a scalable three‐chamber reactor system for electrochemical CO₂ reduction to formate, a precursor suitable for the production of fuels, pharmaceuticals and fertilizers and its extraction as pure formic acid by electrodialysis.RESULTS: The reactor produced 11.7 g L^–1 formic acid in 6 h, i.e. 1.95 g L^–1 h^–1 at −1.8 V applied potential, 5 mol L^–1 KOH as an electrolyte, GDE (gas diffusion electrode) cathode with SnO₂ catalyst and Nafion™ 200 membrane. The maximum Faradaic efficiency achieved was 38%. In addition, recovery of the formate is equally important as its production for use as feedstock to form chemicals. We therefore also investigated the extraction of formic acid through conventional electrodialysis (CED) and bipolar membrane electrodialysis (BMED). The formic acid was extracted with 88% recovery using CED and 46% with BMED. Furthermore, BMED resulted in recovery of >95% K⁺ as base and 12 L pure CO₂ for possible recycling to the electrochemical cell.CONCLUSION: We consider this study to provide essential empirical evidence on factors influencing the scale‐up and subsequent performance of a liquid electrolyte‐based electrochemical CO₂ reduction reaction (CO₂RR) system to formate and its extraction at scale. However, optimized systems and operating strategies still need further investigation, and constituent materials, particularly in terms of membranes and cathode catalyst, need to be developed. © 2021 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).

Mixed microbial culture (MMC) electrofermentation (EF) represents a promising tool to drive metabolic pathways toward the production of a specific compound. Here, the MMC-EF process has been exploited to obtain butyric acid in simplified membrane-less reactors operated by applying a difference of potential between two low-cost graphite electrodes. Ten values of voltage difference, from −0.60 V to −1.5 V, have been tested and compared with the experiment under open circuit potential (OCP). In all the tested conditions, an enhancement in the production rate of butyric acid (from a synthetic mixture of glucose, acetate, and ethanol) was observed, ranging from 1.3- to 2.7-fold relative to the OCP. Smaller enhancements in the production rate resulted in higher values of the calculated specific energy consumption. However, at all applied voltages, a low flow of current was detected in the one-chamber reactors, accounting for an average value of approximately −100 µA. These results hold a substantial potential with respect to the scalability of the electrofermentation technology, since they pinpoint the possibility to control MMC-based bioprocesses by simply inserting polarized electrodes into traditional fermenters.

The electrogenic biofilm and the bio-electrode interface are the key biocatalytic components in bioelectrochemical systems (BES) and can have a large impact on cell performance. This study used four different anodic carbons to investigate electrogenic biofilm development to determine the influence of charge accumulation and biofilm growth on system performance and how biofilm structure may mitigate against pH perturbations. Power production was highest (1.40 W/m³) using carbon felt, but significant power was also produced when felt carbon was open-circuit acclimated in a control reactor (0.95 W/m³). The influence of carbon material on electrogenic biofilm development was determined by measuring the level of biofilm growth, using sequencing to identify the microbial populations and confocal microscopy to understand the spatial locations of key microbial groups. Geobacter spp. were found to be enriched in closed-circuit operation and these were in close association with the carbon anode, but these were not observed in the open-circuit controls. Electrochemical analysis also demonstrated that the highest mid-point anode potentials were close to values reported for cytochromes from Geobacter sulfurreductans. Biofilm development was greatest in felt anodes (closed-circuit acclimated 1209 ng/μL DNA), and this facilitated the highest pseudo-capacitive values due to the presence of redox-active species, and this was associated with higher levels of power production and also served to mitigate against the effects of low-pH operation. Supporting carbon anode structures are key to electrogenic biofilm development and associated system performance and are also capable of protecting electrochemically active bacteria from the effects of environmental perturbations.

3-Hydroxypropionic acid (3-HP) is an important platform chemical which can be produced biologically from glycerol. Klebsiella pneumoniae is an ideal biocatalyst for 3-HP because it can grow well on glycerol and naturally synthesize the essential coenzyme B12. On the other hand, if higher yields and titers of 3-HP are to be achieved, the sustained regeneration of NAD+ under anaerobic conditions, where coenzyme B12 is synthesized sustainably, is required. In this study, recombinant K. pneumoniae L17 overexpressing aldehyde dehydrogenase (AldH) was developed and cultured in a bioelectrochemical system (BES) with the application of an electrical potential to the anode using a chronoamperometric method (+0.5 V vs. Ag/AgCl). The BES operation resulted in 1.7-fold enhancement of 3-HP production compared to the control without the applied potential. The intracellular NADH/NAD+ ratio was significantly lower when the L17 cells were grown under an electric potential. The interaction between the electrode and overexpressed AldH was enhanced by electron shuttling mediated by HNQ (2-hydroxy-1,4-naphthoquinone). Enhanced 3-HP production by the BES was achieved using recombinant K. pneumoniae L17. The quinone-based electron transference between the electrode and L17 was investigated by respiratory uncoupler experiments. This study provides a novel strategy to control the intracellular redox states to enhance the yield and titer of 3-HP production as well as other bioconversion processes.

An ultra-low voltage customized DC-DC booster circuit was developed using a LTC3108 converter, and used continuously on a flat-plate microbial fuel cell (FPM) system. The boost converter successfully stepped up the microbial fuel cell (MFC) voltage from ~0.5 V to 3.3 and 5.0 V of outputs. The designed circuit and system displayed the dynamic variations of the source FPM as well as the output voltage through the designed three connection points within the booster circuit. The source MFC voltage was interrelated with the booster circuit and its performance, and it adapted to the set points of the booster dynamically. The maximum output power density of the MFC with the DC-DC booster circuit was 8.16 W/m³ compared to the maximum source FPM input power of 14.27 W/m³ at 100 Ω, showing a conversion efficiency of 26–57%, but with a 10-fold higher output than that of the source voltage. The combined LTC3108 with FPM supplied power for electronic devices using synthetic and real domestic wastewater. This report presents a promising strategy for utilizing the electrical energy produced from MFCs, and expands the applicability of bioelectrochemical systems with an improved energy efficiency of the present wastewater treatment system.