Tracking bio-hydrogen-mediated production of commodity chemicals from carbon dioxide and renewable electricity

Hydrogen production in microbial electrolysis cells with biocathodes

Electroautotrophic microbes at biocathodes in microbial electrolysis cells (MECs) can catalyze the hydrogen evolution reaction with low energy demand, facilitating long-term stable performance through specific and renewable biocatalysts. However, MECs have not yet reached commercialization due to a lack of understanding of the optimal microbial strains and reactor configurations for achieving high performance. Here, we critically analyze the criteria for the inocula selection, with a focus on the effect of hydrogenase activity and microbe-electrode interactions. We also evaluate the impact of the reactor design and key parameters, such as membrane type, composition, and electrode surface area on internal resistance, mass transport, and pH imbalances within MECs. This analysis paves the way for advancements that could propel biocathode-assisted MECs toward scalable hydrogen gas production.

Sustainable hydrogen production via microalgae: Technological advancements, economic indicators, environmental aspects, challenges, and policy implications

Hydrogen has emerged as an alternative energy source to meet the increasing global energy demand, depleting fossil fuels and environmental issues resulting from fossil fuel consumption. Microalgae-based biomass is gaining attention as a potential source of hydrogen production due to its green energy carrier properties, high energy content, and carbon-free combustion. This review examines the hydrogen production process from microalgae, including the microalgae cultivation technological process for biomass production, and the three main routes of biomass-to-hydrogen production: thermochemical conversion, photo biological conversion, and electrochemical conversion. The current progress of technological options in the three main routes is presented, with the various strains of microalgae and operating conditions of the processes. Furthermore, the economic and environmental perspectives of biomass-to-hydrogen from microalgae are evaluated, and critical operational parameters are used to assess the feasibility of scaling up biohydrogen production for commercial industrial-scale applications. The key finding is the thermochemical conversion process is the most feasible process for biohydrogen production, compared to the pyrolysis process. In the photobiological and electrochemical process, pure hydrogen can be achieved, but further process development is required to enhance the production yield. In addition, the high production cost is the main challenge in biohydrogen production. The cost of biohydrogen production for direct bio photolysis it cost around $7.24 kg-1; for indirect bio photolysis it costs around $7.54 kg-1 and for fermentation, it costs around $7.61 kg-1. Therefore, comprehensive studies and efforts are required to make biohydrogen production from microalgae applications more economical in the future.

Boosting ethanol production rates from carbon dioxide in MES cells under optimal solventogenic conditions

Microbial Electrosynthesis (MES) has been widely applied for acetic acid (HA) production from CO₂ and electricity. Ethanol (EtOH) has a higher market value than HA, and wide application in industry and as a biofuel. However, it has only been obtained sporadically and at low concentrations, probably due to sub-optimal operating conditions. This study aimed at enhancing EtOH productivity in MES cells by jointly optimising key operation parameters, including pH, H₂ and CO₂ partial pressure (pH₂ and pCO₂), and HA concentration, to promote solventogenesis. Two H-type cells were operated in fed-batch mode at -0.8 V vs. SHE with CO₂ as the sole carbon source. A mixed culture, enriched with Clostridium ljungdahlii was used as the biocatalyst. The combination of low pH (<4.5) and pCO₂ (<0.3 atm), along with high HA concentration (about 6 g L^-1) and pH₂ (>3 atm), were mandatory conditions for maintaining an efficient solventogenic culture, dominated by Clostridium sp., capable of high-rate EtOH production. The maximum EtOH production rate was 10.95 g m^-2 d^-1, and a concentration of 5.28 g L^-1 was achieved. Up to 30 % of the electrons and 15.2 % of the carbon provided were directed towards EtOH production, and 28.1 kWh were required for the synthesis of 1 kg of EtOH from CO₂. These results highlight that strict conditions are required for a continuous, reliable, EtOH production in MES cells. Future investigation should focus on improving cell configuration to achieve EtOH production at higher current densities while minimizing the electric energy input.

Review on microbial fuel cells applications, developments and costs

The microbial fuel cell (MFC) technology has attracted significant attention in the last years due to its potential to recover energy in a wastewater treatment. The idea of using an MFC in industry is very attractive as the organic wastes can be converted into energy, reducing the waste disposal costs and the energy needs while increasing the company profit. However, taking aside these promising prospects, the attempts to apply MFCs in large-scale have not been succeeded so far since their lower performance and high costs remains challenging. This review intends to present the main applications of the MFC systems and its developments, particularly the advances on configuration and operating conditions. The diagnostic techniques used to evaluate the MFC performance as well as the different modeling approaches are described. Towards the introduction of the MFC in the market, a cost analysis is also included. The development of low-cost materials and more efficient systems, with high higher power outputs and durability, are crucial towards the application of MFCs in industrial/large scale. This work is a helpful tool for discovering new operation and design regimes.

Let's chat: Communication between electroactive microorganisms

Electroactive microorganisms can exchange electrons with other cells or conductive interfaces in their extracellular environment. This property opens the way to a broad range of practical biotechnological applications, from manufacturing sustainable chemicals via electrosynthesis, to bioenergy, bioelectronics or improved, low-energy demanding wastewater treatments. Besides, electroactive microorganisms play key roles in environmental bioremediation, significantly impacting process efficiencies. This review highlights our present knowledge on microbial interactions promoting the communication between electroactive microorganisms in a biofilm on an electrode in bioelectrochemical systems (BES). Furthermore, the immediate knowledge gaps that must be closed to develop novel technologies will also be acknowledged.

Advances in technological control of greenhouse gas emissions from wastewater in the context of circular economy

This review paper aims to identify the main sources of carbon dioxide (CO₂) emissions from wastewater treatment plants (WWTPs) and highlights the technologies developed for CO₂ capture in this milieu. CO₂ is emitted in all the operational units of conventional WWTPs and even after the disposal of treated effluents and sludges. CO₂ emissions from wastewater can be captured or mitigated by several technologies such as the production of biochar from sludge, the application of constructed wetlands (CWs), the treatment of wastewater in microbial electrochemical processes (microbial electrosynthesis, MES; microbial electrolytic carbon capture, MECC; in microbial carbon capture, MCC), and via microalgal cultivation. Sludge-to-biochar and CW systems showed a high cost-effectiveness in the capture of CO₂, while MES, MECC, MCC technologies, and microalgal cultivation offered efficient capture of CO₂ with associate production of value-added by-products. At the state-of-the-art, these technologies, utilized for carbon capture and utilization from wastewater, require more research for further configuration, development and cost-effectiveness. Moreover, the integration of these technologies has a potential internal rate of return (IRR) that could equate the operation or provide additional revenue to wastewater management. In the context of circular economy, these carbon capture technologies will pave the way for new sustainable concepts of WWTPs, as an essential element for the mitigation of climate change fostering the transition to a decarbonised economy.

Thermophilic bio-electro carbon dioxide recycling harnessing renewable energy surplus

Renewable energies will represent an increasing share of the electricity supply, while flue and gasification-derived gases can be a promising CO₂ feedstock with a heat load. In this study, microbial electrosynthesis of organic compounds from CO₂ at high temperature was proposed as an alternative for valorising energy surplus and decarbonizing the economy. The unremitting fluctuation of renewable energy sources was assessed using two bioreactors at 50 °C, under circumstances of continuous and intermittent power supply (ON-OFF; 8-16 h), simulating an off-grid photovoltaic system. Results highlighted that maximum acetate production rate (43.27 g m^-2 d^-1) and columbic efficiency (98%) were achieved by working with an intermittent energy supply, while current density was reduced three times. This boosted the production of acetate per unit of electricity provided up to 138 g kWh^-1 and reinforced the robustness of the technology by showing resilience to tolerate perturbations and returning to its initial state.

Bacteria coated cathodes as an in-situ hydrogen evolving platform for microbial electrosynthesis

Hydrogen is a key intermediate element in microbial electrosynthesis as a mediator of the reduction of carbon dioxide (CO₂) into added value compounds. In the present work we aimed at studying the biological production of hydrogen in biocathodes operated at − 1.0 V vs. Ag/AgCl, using a highly comparable technology and CO₂ as carbon feedstock. Ten bacterial strains were chosen from genera Rhodobacter, Rhodopseudomonas, Rhodocyclus, Desulfovibrio and Sporomusa, all described as hydrogen producing candidates. Monospecific biofilms were formed on carbon cloth cathodes and hydrogen evolution was constantly monitored using a microsensor. Eight over ten bacteria strains showed electroactivity and H₂ production rates increased significantly (two to eightfold) compared to abiotic conditions for two of them (Desulfovibrio paquesii and Desulfovibrio desulfuricans). D. paquesii DSM 16681 exhibited the highest production rate (45.6 ± 18.8 µM min⁻¹) compared to abiotic conditions (5.5 ± 0.6 µM min⁻¹), although specific production rates (per 16S rRNA copy) were similar to those obtained for other strains. This study demonstrated that many microorganisms are suspected to participate in net hydrogen production but inherent differences among strains do occur, which are relevant for future developments of resilient biofilm coated cathodes as a stable hydrogen production platform in microbial electrosynthesis.

Microbial electron uptake in microbial electrosynthesis: a mini-review

Microbial electron uptake (EU) is the biological capacity of microbes to accept electrons from electroconductive solid materials. EU has been leveraged for sustainable bioproduction strategies via microbial electrosynthesis (MES). MES often involves the reduction of carbon dioxide to multi-carbon molecules, with electrons derived from electrodes in a bioelectrochemical system. EU can be indirect or direct. Indirect EU-based MES uses electron mediators to transfer electrons to microbes. Although an excellent initial strategy, indirect EU requires higher electrical energy. In contrast, the direct supply of cathodic electrons to microbes (direct EU) is more sustainable and energy efficient. Nonetheless, low product formation due to low electron transfer rates during direct EU remains a major challenge. Compared to indirect EU, direct EU is less well-studied perhaps due to the more recent discovery of this microbial capability. This mini-review focuses on the recent advances and challenges of direct EU in relation to MES.

Specific detection of "Clostridium autoethanogenum", Clostridium ljungdahlii and Clostridium carboxidivorans in complex bioreactor samples

The high genetic similarity between some carboxydotrophic bacteria does not allow for the use of common sequencing techniques targeting the 16S rRNA gene for species identification. 16S rRNA sequencing fails to discriminate among Clostridium ljungdahlii and 'Clostridium autoethanogenum', despite this two species exhibit significant differences in CO2 assimilation and alcohol production. In this work we designed PCR primers targeting for the DNA gyrase subunit A (gyrA) and a putative formate/nitrite transporter (fnt) to specifically detect the presence of 'C. autoethanogenum', C. ljungdahlii or Clostridium carboxidivorans. We could confirm the simultaneous presence of C. ljungdahlii and 'C. autoethanogenum' in different bioreactors, and a preference of the latter for high CO2 content.

Zinc: A promising material for electrocatalyst-assisted microbial electrosynthesis of carboxylic acids from carbon dioxide

Microbial electrosynthesis (MES) has been proposed as a sustainable platform to simultaneously achieve wastewater treatment, renewable energy generation and chemicals production. Currently, the CO₂ valorization via MES is restricted by the low production rate, while that via electrochemical reduction is limited by the production of C1 products with high efficiency and selectivity. The electrocatalyst-assisted MES could potentially solve these bottlenecks of both MES and electrochemical reduction technology by increasing the production rate and expanding the product range. Here, four types of metals were evaluated for mixed culture-based, electrocatalyst-assisted MES with the fabrication of electrical-biological hybrid cathodes. Cathodes based on In, Zn, Ti and Cu showed high parallelism at 30 A/m². However, no parallelism was observed at 50 A/m², and only Zn experienced a further increase of the maximum acetic acid production rate (1.23 ± 0.02 g/L/d, 313 ± 5 g/m²/d) and titer (9.2 ± 0.1 g/L), with the highest value of the production rate normalized to the project area of the fiber cathodes. Other volatile fatty acids and ethanol were below 0.5 g/L. Moreover, it was the sharp H₂ generation, which mainly caused the fluctuation of coulombic efficiency. The application of such Zn-based electrical-biological hybrid system shall provide a more efficient route for CO₂ valorization.

[NiFe]-hydrogenases are constitutively expressed in an enriched Methanobacterium sp. population during electromethanogenesis

Electromethanogenesis is the bioreduction of carbon dioxide (CO₂) to methane (CH₄) utilizing an electrode as electron donor. Some studies have reported the active participation of Methanobacterium sp. in electron capturing, although no conclusive results are available. In this study, we aimed at determining short-time changes in the expression levels of [NiFe]-hydrogenases (Eha, Ehb and Mvh), heterodisulfide reductase (Hdr), coenzyme F₄₂₀-reducing [NiFe]-hydrogenase (Frh), and hydrogenase maturation protein (HypD), according to the electron flow in independently connected carbon cloth cathodes poised at– 800 mV vs. standard hydrogen electrode (SHE). Amplicon massive sequencing of cathode biofilm confirmed the presence of an enriched Methanobacterium sp. population (>70% of sequence reads), which remained in an active state (78% of cDNA reads), tagging this archaeon as the main methane producer in the system. Quantitative RT-PCR determinations of ehaB, ehbL, mvhA, hdrA, frhA, and hypD genes resulted in only slight (up to 1.5 fold) changes for four out of six genes analyzed when cells were exposed to open (disconnected) or closed (connected) electric circuit events. The presented results suggested that suspected mechanisms for electron capturing were not regulated at the transcriptional level in Methanobacterium sp. for short time exposures of the cells to connected-disconnected circuits. Additional tests are needed in order to confirm proteins that participate in electron capturing in Methanobacterium sp.

Electron transfer mechanism of biocathode in a bioelectrochemical system coupled with chemical absorption for NO removal

A biocathode with the function of Fe(III)EDTA and Fe(II)EDTA-NO reduction was applied in a microbial electrolysis cell coupled with chemical absorption for NO removal from flue gas. As the mediated electron transfer was excluded by the same electrochemical characterizations of the biocathodes before and after a 48 h continuous operation, the profiles of reduction experiments indicated that direct electron transfer was the main mechanism of Fe(III)EDTA reduction, while Fe(III)EDTA-NO was mainly reduced via Fe(II)-assisted autotrophic denitrification. The microscopy of the biocathode confirmed the existence of pili, which was supposed to be bacterial nanowires for electron transfer. The analysis of microbial community revealed that iron-reducing bacteria, including Escherichia coli, had the possibility of electron uptake from electrode via physical contact. These results first time gave us in-depth understanding of the electron transfer in the multifunctional biocathode and mechanism for further enhancement of the bioreduction processes.

Tracking of Shewanella oneidensis MR-1 biofilm formation of a microbial electrochemical system via differential pulse voltammetry

In this study, the electrochemical properties of a Shewanella oneidensis MR-1 biofilm were investigated using a mini-microbial electrochemical system. The performance of the biofilm was shown, using discharge test and cyclic voltammetry investigations, to improve over time. Differential pulse voltammograms were also acquired to determine the type of extracellular electron transfer that took place and to characterize the structure of the microbial biofilm formed on the electrode of the electrochemical system. These results indicated that extracellular electron transfer via a flavin-like mediator chemical predominated as the biofilm grew. The results, combined with a comparison of the measured current density with the calculated value of a seamless single-layered biofilm, also suggested that S. oneidensis MR-1 formed a multi-layered biofilm on the electrode.

Enrichment of syngas-converting mixed microbial consortia for ethanol production and thermodynamics-based design of enrichment strategies

BACKGROUND

The production of ethanol through the biochemical conversion of syngas, a mixture of H₂, CO and CO₂, has been typically studied using pure cultures. However, mixed microbial consortia may offer a series of benefits such as higher resilience and adaptive capacity, and non-sterile operation, all of which contribute to reducing the utility consumption when compared to pure culture-based processes. This work focuses on the study of strategies for the enrichment of mixed microbial consortia with high ethanologenic potential, investigating the effect of the operational conditions (pH and yeast extract addition) on both the ethanol yield and evolution of the microbial community along the enrichment process. The pH was selected as the main driver of the enrichment as it was expected to be a crucial parameter for the selection of carboxydotrophic bacteria with high ethanologenic potential. Additionally, a thermodynamic analysis of the network of biochemical reactions carried out by syngas-converting microbial consortia was performed and the potential of using thermodynamics as a basis for the selection of operational parameters favoring a specific microbial activity was evaluated.

RESULTS

All enriched consortia were dominated by the genus Clostridium with variable microbial diversity and species composition as a function of the enrichment conditions. The ethanologenic potential of the enriched consortia was observed to increase as the initial pH was lowered, achieving an ethanol yield of 59.2 ± 0.2% of the theoretical maximum in the enrichment at pH 5. On the other hand, yeast extract addition did not affect the ethanol yield, but triggered the production of medium-chain fatty acids and alcohols. The thermodynamic analysis of the occurring biochemical reactions allowed a qualitative prediction of the activity of microbial consortia, thus enabling a more rational design of the enrichment strategies targeting specific activities. Using this approach, an improvement of 22.5% over the maximum ethanol yield previously obtained was achieved, reaching an ethanol yield of 72.4 ± 2.1% of the theoretical maximum by increasing the initial acetate concentration in the fermentation broth.

CONCLUSIONS

This study demonstrated high product selectivity towards ethanol using mixed microbial consortia. The thermodynamic analysis carried out proved to be a valuable tool for interpreting the metabolic network of microbial consortia-driven processes and designing microbial-enrichment strategies targeting specific biotransformations.

Plugging in microbial metabolism for industrial applications

The ability of electric microbes to electrically interact with electrodes is opening up a number of possibilities with industrial applications. Microbes are able to utilise the electrode as an electron source to reduce CO2 for the production of organic compounds directly or produce H2 as a reducing equivalent for partner microbes for the production of commodity chemicals. Electrodes can also allow redox unbalanced fermentation processes to occur through the addition or subtraction of reducing equivalents that remove bottle necks in these pathways. Electrodes are also providing a physical refuge for electric microbes to maintain anaerobic fermenter stability. It can be expected that the role for electric microbes will continued to be expanded as part of industrial applications in the future.