Hydrogen can be used in both power generation systems and direct combustion processes, providing the great advantage of clean combustion. Moreover, H2 is considered a very competitive energy carrier compared to other fuels, thanks to its high net heating value per unit volume. Nowadays, its use as a clean energy source is yet uncommon, while its main use is in ammonia production and hydrogenation of coal and petroleum during hydrocracking of traditional fuels (IEA 2019). However, the good environmental profile of H2 is commonly counteracted by the fact that it is still primarily derived from non-renewable sources, with a high associated energy consumption and related relevant CO2 emissions, posing an urgent need for sustainable production methods.
Several bioprocesses have been investigated over the last decades to produce H2 through sustainable methods (Hallenbeck et al. 2012). Among them, dark fermentation (DF) is considered one of the most promising options. The main reason is that DF averts the major drawbacks of other biological processes (including direct or indirect photolysis and photo-fermentation), related to the intermittent production of H2 and the need of a light source to support the process. Compared to the other biological processes, dark fermentative H2 production has the additional advantages of higher production rate, flexibility of operation under different temperature and pressure conditions, lower net energy input and, noteworthy, applicability to a range of renewable organic sources including organic residues and carbohydrate-based wastewaters (Ghimire et al. 2015; Da Silva Veras et al. 2017; Park et al. 2021).
While DF of organic residues has been widely investigated over the past decades, the major challenges still existing for the process include the poor stability of the biochemical process and the thermodynamic/biochemical limitations to the actual H2 production yield attainable. To this regard, when acetate is the final metabolic product of fermentation, a production of 4 moles of H2 per mole of hexose consumed is expected, which is regarded as an upper threshold for the H2 yield, known as the Thauer limit (Thauer et al.1977). Therefore, of the potential 12 moles of H2 that may be produced by one mole of glucose, only a third can be obtained biochemically. The actual H2 yield can even be lower than the mentioned limit if other more reduced metabolic products (e.g., butyrate, ethanol) are formed or additional competing metabolic pathways occur (e.g. propionic fermentation, homoacetogenesis).
Bioelectrochemical processes (BESs) have been proposed for a variety of applications aimed at improving the performance of biological systems. Their operating principle is based on the ability of specific microorganisms defined as electroactive bacteria (EAB) to interact with solid electrodes by forming a biofilm and catalyze the oxidation of organic matter by generating an electrical potential. Microbial fuel cells (MFCs) are among the most widely investigated BESs, due to their capability of producing an electric power while simultaneously degrading an organic substrate. Generally, in the anodic chamber, where the EAB are attached to dedicated inert electrodes, the oxidation of the organic substances takes place generating CO2 and protons, which migrate into the cathodic chamber through ion exchange membranes. The cathodic chamber is maintained under aerobic conditions, so that in the presence of electrons protons react with oxygen to produce water, resulting in the spontaneous production of electricity, the intensity and flow of which are functions of the construction features of the cell, the substrate characteristics, the inoculum and the operating conditions adopted.
A modified type of MFC, the microbial electrolysis cell (MEC), has been studied since 2005 (Liu et al. 2005b), and its scientific interest has strongly increased in recent years (Santoro et al. 2017). In that case, unlike the MFC, the cathodic chamber is maintained under anaerobic conditions; consequently, protons are reduced to H2, since there are no others electronegative species to intercept electrons. This process requires the supply of an electric current, since the electric potential naturally generated by microorganisms is not enough to reduce H+ to H2. Assuming acetate as a model organic source, the electrode reactions involve oxidation to CO2 at the anode and H+ reduction to H2 at the cathode (see equations 1 and 2). Assuming that the open circuit potential at the anode in an MFC is generally about E0~ – 300 mV (Liu et al. 2005b), and the minimum standard redox potential required for the cathodic reaction of E0= – 410 mV (NHE) at pH 7.0, H2 can theoretically be obtained by applying a > 110 mV circuit voltage (typically 410–300 mV to overcome internal electric resistances). However, the voltage required is significantly lower than that used for conventional water electrolysis (1.21 V at neutral pH, which can increase up to 1.8–2.0 V under alkaline conditions due to electrode overpotential), since the chemical energy extracted from organic substrates oxidized at the anode supplies most of the potential needed.
Anode: CH3COOH + 2H2O →2CO2 + 8e− + 8H+ (1)
Cathode: 8H++ 8e− → 4H2 (2)
Some studies have successfully investigated BESs for exploitation of volatile fatty acids (VFAs) or DF effluents into electricity or H2. Liu et al. (2005b) obtained 2.9 mol H2/mol acetate applying an additional voltage of 0.250 V in a MEC. Through optimization of materials and reactor configuration, Cheng and Logan (2007) achieved H2 yields between 2.0 and 3.9 mol H2/mol acetate at applied voltages of 0.2 to 0.8 V. Chae et al. (2008) showed that H2 production gradually increases as the applied voltage is increased from 0.1 to 1 V, reaching 2.1 mol H2/mol acetate. Liu et al. (2005a) tested power generation from acetate and butyrate in a MFC and observed that acetate is preferred over butyrate as a substrate, producing respectively 506 mW/m2 and 305 mW/m2.
The treatment of a real DF effluent was investigated by Chookaew et al. (2014) using both a MEC and a MFC. A power density of 92 mW/m2 in the MFC was achieved along with 50% COD removal. When treated in the MEC, the same substrate yielded 106 mL H2/g COD. Rivera et al. (2015) evaluated both DF effluent exploitation as a substrate for a MEC. The highest production rate (81 mL H2/L/day) was obtained at a 550 mV voltage and was accompanied by 85% COD removal. Wang et al. (2011) performed a multi-stage process using a DF reactor for cellulose degradation followed by two MFCs that were used as power sources for a subsequent MEC. The MFCs produced a maximum of 0.43 V using the fermentation effluent that induced H2 production in the MEC at a rate of 0.48 m3 H2/m3/d and with a yield of 33.2 mmol H2/g COD removed in the MEC. The authors observed an overall improvement in H2 production for the integrated process by 41% compared with fermentation alone.
The integration of fermentation and electrochemical processes in the same unit has been the focus of specific studies on electro-fermentation (Moscoviz et al. 2016; Schievano et al. 2016; Yu et al. 2018). The fundamental concept is based on driving the fermentation process by modifying the redox potential through polarized electrodes placed in the reactor, which can either supply electrons or act as a sink under certain conditions. This could allow overcoming the metabolic limitations through direct electricity supply to the fermentation medium. Potential inocula include both electroactive and fermentative bacteria that can produce value-added organic acids and alcohols (Xue et al. 2018; Paiano et al. 2019), sometimes with concomitant production of H2 and/or CH4 (Nelabhotla and Dinamarca 2019; Toledo-Alarcón et al. 2019). Electro-fermentation is rapidly gathering attention given the successful results. To date, the study of the process is still in a preliminary stage and future developments include the orientation of the metabolic pathways towards specific end products, the selection of efficient redox mediators, the application to complex substrates and the applicability to suspended biomass configurations.
In the present work, we attempted at developing an innovative BES coupling DF with an electrochemical process, with the multiple aims of enhancing H2 generation, exploiting the fermentation products, produce electricity and provide an internal pH buffering effect. To the best of the authors’ knowledge, the concept behind the proposed process is novel and the BES developed has not been documented in other literature studies so far.