Assessment of four different cathode materials at different initial pHs using unbuffered catholytes in microbial electrolysis cells

Nickel foam (NF), stainless steel wool (SSW), platinum coated stainless steel mesh (Pt), and molybdenum disulfide coated stainless steel mesh (MoS₂) electrodes have been proposed as catalysts for hydrogen gas production, but previous tests have primarily examined their performance in well buffered solutions. These materials were compared using two-chamber microbial electrolysis cells (MECs), and linear sweep voltammetry (LSV) in unbuffered saline solutions at two different initial pHs (7 and 12). There was generally no appreciable effect of initial pH on production rates or total gas production. NF produced hydrogen gas at a rate of 1.1 m³ H₂/m³·d, which was only slightly less than that using Pt (1.4 m³ H₂/m³·d), but larger than that obtained with SSW (0.52 m³ H₂/m³·d) or MoS₂ (0.67 m³ H₂/m³·d). Overall hydrogen gas recoveries with SSW (29.7 ± 0.5 mL), MoS₂ (28.6 ± 1.3 mL) and NF (32.4 ± 2 mL) were only slightly less than that of Pt (37.9 ± 0.5 mL). Total energy recoveries, based on the gas produced versus electrical energy input, ranged from 0.75 ± 0.02 for Pt, to 0.55 ± 0.02 for SSW. An LSV analysis showed no effect of pH for NF and Pt, but overpotentials were reduced for MoS₂ and SSW by using an initial lower pH. At cathode potentials more negative than −0.85 V (vs Ag/AgCl), NF had lower overpotentials than the MoS₂. These results provide the first assessment of these materials under practical conditions of high pH in unbuffered saline catholytes, and position NF as the most promising inexpensive alternative to Pt.     

Domestic wastewater treatment in parallel with methane production in a microbial electrolysis cell

Microbial electrolysis cells (MECs) have great potential as a technology for wastewater treatment in parallel to energy production. In this study we explore the feasibility of using a low-cost, membraneless MEC for domestic wastewater treatment and methane production in both batch and continuous modes. Low-strength wastewater can be successfully treated by means of an MEC, obtaining significant amounts of methane. The results also suggest that hydrogenotrophic methanogenesis reduce the incidence of homoacetogenic activity, thus improving the overall MEC performance. However, gas production rates are low and important aspects such as methane solubility in water still remain a challenge. Overall, MECs can offer competitive advantages not only for low-strength wastewater treatment but also as an aid to anaerobic methane production by improving the chemical oxygen demand (COD) removal and methane production rates.     

Assessment of biotic and abiotic graphite cathodes for hydrogen production in microbial electrolysis cells

Hydrogen represents a promising clean fuel for future applications. The biocathode of a two-chambered microbial electrolysis cell (biotic MEC) was studied and compared with an abiotic cathode (abiotic MEC) in order to assess the influence of naturally selected microorganisms for hydrogen production in a wide range of cathode potentials (from −400 to −1800 mV vs SHE). Hydrogen production in both MECs increased when cathode potential was decreased. Microorganisms present in the biotic MEC were identified as Hoeflea sp. and Aquiflexum sp. Supplied energy was utilized more efficiently in the biotic MEC than in the abiotic, obtaining higher hydrogen production respect to energy consumption. At −1000 mV biotic MEC produced 0.89 ± 0.10 m³ H₂ d⁻¹ m⁻³NCC (Net Cathodic Compartment) at a minimum operational cost of 3.2 USD kg⁻¹ H₂. This cost is lower than the estimated market value for hydrogen (6 USD kg⁻¹ H₂).     

A monetary comparison of energy recovered from microbial fuel cells and microbial electrolysis cells fed winery or domestic wastewaters

Microbial fuel (MFCs) and electrolysis cells (MECs) can be used to recover energy directly as electricity or hydrogen from organic matter. Organic removal efficiencies and values of the different energy products were compared for MFCs and MECs fed winery or domestic wastewater. TCOD removal (%) and energy recoveries (kWh/kg-COD) were higher for MFCs than MECs with both wastewaters. At a cost of $4.51/kg-H₂ for winery wastewater and $3.01/kg-H₂ for domestic wastewater, the hydrogen produced using MECs cost less than the estimated merchant value of hydrogen ($6/kg-H₂). 16S rRNA clone libraries indicated the predominance of Geobacter species in anodic microbial communities in MECs for both wastewaters, suggesting low current densities were the result of substrate limitations. The results of this study show that energy recovery and organic removal from wastewater are more effective with MFCs than MECs, but that hydrogen production from wastewater fed MECs can be cost effective.     

Electrochemical evaluation of molybdenum disulfide as a catalyst for hydrogen evolution in microbial electrolysis cells

There is great interest in hydrogen evolution in bioelectrochemical systems, such as microbial electrolysis cells (MECs), but these systems require non-optimal near-neutral pH conditions and the use of low-cost, non-precious metal catalysts. Here we show that molybdenum disulfide (MoS₂) composite cathodes have electrochemical performance superior to stainless steel (SS) (currently the most promising low-cost, non-precious metal MEC catalyst) or Pt-based cathodes in phosphate or perchlorate electrolytes, yet they cost ∼4.5 times less than Pt-based composite cathodes. At current densities typical of many MECs (2-5 A/m²), the optimal surface density with MoS₂ particles on carbon cloth was 25 g/m², achieving 31 mV less hydrogen evolution overpotential than similarly constructed Pt cathodes in galvanostatic tests with a phosphate buffer. At higher current densities (8-10 A/m²) the MoS₂ catalyst had 82 mV less hydrogen evolution overpotential than the Pt-based catalyst. MoS₂ composite cathodes performed similarly to Pt cathodes in terms of current densities, hydrogen production rates and COD removal over several batch cycles in MEC reactors. These results show that MoS₂ can be used to substantially reduce the cost of cathodes used in MECs for hydrogen gas production.     

An overview of cathode material and catalysts suitable for generating hydrogen in microbial electrolysis cell

Bio-electrohydrogenesis through Microbial Electrolysis Cell (MEC) is one of the promising technologies for generating hydrogen from wastewater through degradation of organic waste by microbes. While microbial activity occurs at anode, hydrogen gas is evolved at the cathode. Identifying a highly efficient and low cost cathode is very important for practical implication of MEC. In this review, we have summarized the efforts of different research groups to develop different types of efficient and low cost cathodes or cathode catalysts for hydrogen generation. Among all the materials used, stainless steel, Ni alloys Pd nanoparticle decorated cathode are worth mentioning and have very good efficiency. Industrial application of MEC should consider a balance of availability and efficiency of the cathode material.     

Optimization of NiMo catalyst for hydrogen production in microbial electrolysis cells

Non-platinum based cathodes were recently developed by electrodepositing NiMo on carbon cloth, which demonstrated good electrocatalytic activity for hydrogen evolution in microbial electrolysis cells (MECs). To further optimize the electrodeposition condition, the effects of electrolyte bath composition, applied current density, and duration of electrodeposition were systematically investigated in this study. The developed NiMo catalysts were characterized with scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) and evaluated using chronopotentiometry and in MECs. The optimal condition for electrodeposition of NiMo on carbon cloth was determined as: a Mo/Ni mass ratio of 0.65 in electrolyte bath, an applied current density of 50 mA/cm² and electrodeposition duration of 10 min. Under this condition, the NiMo catalyst has a formula of Ni₆MoO₃ with a nodular morphology. The NiMo loading on the carbon cloth was reduced to 1.7 mg/cm² and the performance of MEC with the developed NiMo cathode was comparable to that with Pt cathode with a similar loading. This result indicates that a much lower cathode fabrication cost can be achieved compared to that using Pt catalyst, and thereby significantly enhancing the economic feasibility of the MEC technology.     

Hydrogen production using biocathode single-chamber microbial electrolysis cells fed by molasses wastewater at low temperature

Molasses is by-product from sugar beet process and commonly used as raw material for ethanol production. However, the molasses wastewater possesses high level of chemical oxygen demand (COD), which needs to be properly treated before discharge. In this work, MEC technology, a promising method for hydrogen production from organic waste, was utilized to produce H₂ from molasses wastewater. In this study, the feasibility of operating the MEC at low temperatures was evaluated since the average wastewater temperature in Harbin city is lower than 10 °C. In addition, the feasibility of using biocathode as an alternative to expensive platinum (Pt) as the cathode material was also examined. Both Pt catalyzed MECs and biocathodic MECs were operated at a low temperature of 9 °C. The overall hydrogen recovery of 72.2% (E_ap = 0.6 V) was obtained when the Pt catalyst was used. In contrast, when a cheaper catalyst (biocathode; E_ap = 0.6 V) was used, hydrogen can still be produced but at a lower overall hydrogen recovery of 45.4%. This study demonstrated that hydrogen could be generation from molasses wastewater at a low temperature using a cheaper cathode material (i.e., biocathode).     

Improved performance of porous bio-anodes in microbial electrolysis cells by enhancing mass and charge transport

To create an efficient MEC high current densities and high coulombic efficiencies are required. The aim of this study was to increase current densities and coulombic efficiencies by influencing mass and charge transport in porous electrodes by: (i) introduction of a forced flow through the anode to see the effect of enhanced mass transport of substrate, buffer and protons inside the porous anode and (ii) the use of different concentrations of buffer solution to study the effect of enhanced proton transport near the biofilm. A combination of both strategies led to a high current density of 16.4 A m⁻² and a hydrogen production rate of 5.6 m³ m⁻³ d⁻¹ at an applied voltage of 1 V. This current density is 228% higher than the current density without forced flow and high buffer concentration. Furthermore the combination of the anode and transport resistance was reduced from 36 mΩm² to 20 mΩm². Because of this reduced resistance the coulombic efficiency reached values of over 60% in this continuous system.     

Optimization of high-solid waste activated sludge concentration for hydrogen production in microbial electrolysis cells and microbial community diversity analysis

To enhance hydrogen recovery from high-solid waste activated sludge (WAS), microbial electrolysis cells (MECs) were used as an efficient device. The effects of WAS concentrations were firstly investigated. Optimal concentration for hydrogen production was 7.6 g VSS/L. Maximum hydrogen yields reached to 4.66 ± 1.90 mg-H₂/g VSS and 11.42 ± 2.43 mg-H₂/g VSS for MECs fed with raw WAS (R-WAS) and alkaline-pretreated WAS (A-WAS) respectively, which was much higher than that obtained traditional anaerobic digestion. Moreover, no propionic acid accumulation was achieved at the optimal concentration. Effective sludge reduction was also achieved in MECs feeding with A-WAS. 52.9 ± 1.3% TCOD were removed in A-WAS MECs, meanwhile, protein degradation were 50.4 ± 0.8%. The 454 pyrosequencing analysis of 16S rRNA gene revealed the syntrophic interactions were existed between exoelectrogen Geobacter and fermentative bacteria Petrimonas, which apparently drove the efficient performance of MECs fed with WAS.     

Comparison of cathode catalyst binders for the hydrogen evolution reaction in microbial electrolysis cells

Nafion is commonly used as a catalyst binder in many types of electrochemical cells, but less expensive binders are needed for the cathodes in microbial electrolysis cells (MECs) which are operated in neutral pH buffers, and reverse electrodialysis stacks (RED),which use thermolytic solutions such as ammonium bicarbonate. Six different binders were examined based on differences in ion exchange properties (anionic: Nafion, BPSH20, BPSH40, S-Radel; cationic: Q-Radel; and neutral: Radel, BAEH) and hydrophobicity based on water uptake (0%, Radel; 17–56% for the other binders). BPSH40 had similar performance to Nafion based on steady-state polarization single electrode experiments in a neutral pH phosphate buffer, and slightly better performance in ammonium bicarbonate. Three different Mo-based catalysts were examined as alternatives to Pt, with MoB showing the best performance under steady-state polarization. In MECs, MoB/BPSH40 performed similarly to Pt with Nafion or Radel binders. The main distinguishing feature of the BPSH40 was that it is very hydrophilic, and thus it had a greater water content (56%) than the other binders (0–44%). These results suggest the binders for hydrogen evolution in MECs should be designed to have a high water content without sacrificing ionic or electronic conductivity in the electrode.     

The use of stainless steel and nickel alloys as low-cost cathodes in microbial electrolysis cells

Microbial electrolysis cells (MECs) are used to produce hydrogen gas from the current generated by bacteria, but low-cost alternatives are needed to typical cathode materials (carbon cloth, platinum and Nafion™). Stainless steel A286 was superior to platinum sheet metal in terms of cathodic hydrogen recovery (61% vs. 47%), overall energy recovery (46% vs. 35%), and maximum volumetric hydrogen production rate (1.5 m³ m⁻³ day⁻¹ vs. 0.68 m³ m⁻³ day⁻¹) at an applied voltage of 0.9 V. Nickel 625 was better than other nickel alloys, but it did not perform as well as SS A625. The relative ranking of these materials in MEC tests was in agreement with cyclic voltammetry studies. Performance of the stainless steel and nickel cathodes was further increased, even at a lower applied voltage (0.6 V), by electrodepositing a nickel oxide layer onto the sheet metal (cathodic hydrogen recovery, 52%, overall energy recovery, 48%; maximum volumetric hydrogen production rate, 0.76 m³ m⁻³ day⁻¹). However, performance of the nickel oxide cathodes decreased over time due to a reduction in mechanical stability of the oxides (based on SEM–EDS analysis). These results demonstrate that non-precious metal cathodes can be used in MECs to achieve hydrogen gas production rates better than those obtained with platinum.     

Recovery of flakey cobalt from aqueous Co(II) with simultaneous hydrogen production in microbial electrolysis cells

Flakey cobalt was successfully recovered from aqueous Co(II) with simultaneous hydrogen production in microbial electrolysis cells (MECs). At applied voltages of 0.3–0.5 V, the yields of 0.81 mol Co/mol COD and 1.21 ± 0.03–1.49 ± 0.11 mol H₂/mol COD were achieved while the energy efficiency relative to the electrical input was 22.5 ± 0.1–43.2 ± 0.7% (cobalt) and 170 ± 12–262 ± 7% (hydrogen), and the overall energy efficiency relative to both the electrical input and the energy of the anodic substrate averaged 9.4% (cobalt) and 62.8% (hydrogen). Cathode accumulated flakey crystals were verified as cobalt using both a scanning electron microscope capable of energy dispersive spectroscopy (SEM-EDS) and X-ray diffraction analysis (XRD). Dominant bacteria on the anodes and known as exoelectrogens or recalcitrant substance degraders included Geobacter uraniireducens, Comamonas nitrativorans, uncultured Geobacter sp., Acidovorax caeni, Pseudorhodoferax caeni, and Diaphorobacter nitroreducens. The evidence of influence factors including applied voltage, pH, solution conductivity, temperature and type of buffer can contribute to improving understanding of and optimizing cobalt recovery with simultaneous hydrogen production in MECs.     

Effects of fuel processing methods on industrial scale biogas-fuelled solid oxide fuel cell system for operating in wastewater treatment plants

The performance of three solid oxide fuel cell (SOFC) systems, fuelled by biogas produced through anaerobic digestion (AD) process, for heat and electricity generation in wastewater treatment plants (WWTPs) is studied. Each system has a different fuel processing method to prevent carbon deposition over the anode catalyst under biogas fuelling. Anode gas recirculation (AGR), steam reforming (SR), and partial oxidation (POX) are the methods employed in systems I-III, respectively. A planar SOFC stack used in these systems is based on the anode-supported cells with Ni-YSZ anode, YSZ electrolyte and YSZ-LSM cathode, operated at 800 °C. A computer code has been developed for the simulation of the planar SOFC in cell, stack and system levels and applied for the performance prediction of the SOFC systems. The key operational parameters affecting the performance of the SOFC systems are identified. The effect of these parameters on the electrical and CHP efficiencies, the generated electricity and heat, the total exergy destruction, and the number of cells in SOFC stack of the systems are studied. The results show that among the SOFC systems investigated in this study, the AGR and SR fuel processor-based systems with electrical efficiency of 45.1% and 43%, respectively, are suitable to be applied in WWTPs. If the entire biogas produced in a WWTP is used in the AGR or SR fuel processor-based SOFC system, the electricity and heat required to operate the WWTP can be completely self-supplied and the extra electricity generated can be sold to the electrical grid.     

Hydrogen gas production from electrohydrolysis of industrial wastewater organics by using photovoltaic cells (PVC)

Electrical current generated by a photovoltaic cell (PVC) was supplied to wastewater in a mechanically mixed and sealed reactor using stainless steel electrodes. Hydrogen gas was generated by reaction of protons released from decomposition of organic compounds and electrons provided by electrical current. Gas phase was composed of 75–99% H₂ gas.     

Performance of plug flow microbial fuel cell (PF-MFC) and complete mixing microbial fuel cell (CM-MFC) for wastewater treatment and power generation

Two flow patterns (plug flow (PF) and complete mixing (CM)) of microbial fuel cells (MFCs) with multiple anodes–cathodes were compared in continuous flow mode for wastewater treatment and power generation. The results indicated that PF-MFCs had higher power generation and columbic efficiency (CE) than CM-MFCs, and the power generation varied along with the flow pathway in the PF-MFCs. The gradient of substrate concentrations along the PF-MFCs was the driving force for the power generation. In contrast, the CM-MFCs had higher wastewater removal efficiency than PF-MFCs, but had lower power conversion efficiency and power generation. This work demonstrated that MFC configuration is a key factor for enhancing power generation and wastewater treatment.     

Power generation and organics removal from wastewater using activated carbon nanofiber (ACNF) microbial fuel cells (MFCs)

Carbon-based materials are the most commonly used electrode material for anodes in microbial fuel cell (MFC), but are often limited by their surface areas available for biofilm growth and subsequent electron transfer process. This study investigated the use of activated carbon nanofibers (ACNF) as the anode material to enhance bacterial biofilm growth, and improve MFC performance. Qualitative and quantitative biofilm adhesion analysis indicated that ACNF exhibited better performance over the other commonly used carbon anodes (granular activated carbon (GAC), carbon cloth (CC)). Batch-scale MFC tests showed that MFCs with ACNF and GAC as anodes achieved power densities of 3.50 ± 0.46 W/m³ and 3.09 ± 0.33 W/m³ respectively, while MFCs with CC had a lower power density of 1.10 ± 0.21 W/m³ In addition, the MFCs with ACNF achieved higher contaminant removal efficiency (85 ± 4%) than those of GAC (75 ± 5%) and CC (70 ± 2%). This study demonstrated the distinct advantages of ACNF in terms of biofilm growth and electron transport. ACNF has a potential for higher power generation of MFCs to treat wastewaters.     

Evaluation of stainless steel cathodes and a bicarbonate buffer for hydrogen production in microbial electrolysis cells using a new method for measuring gas production

Microbial electrolysis cells (MECs) are often examined for hydrogen production using non-sustainable phosphate buffered solutions (PBS), although carbonate buffers have been shown to work in other bioelectrochemical systems with a platinum (Pt) catalyst. Stainless steel (SS) has been shown to be an effective catalyst for hydrogen evolution in MECs, but it has not been tested with carbonate buffers. We evaluated the combined using of SS cathodes and a bicarbonate buffer (BBS) at the applied voltages of 0.5, 0.7 and 0.9 V using a new inexpensive method for measuring gas production called the gas bag method (GBM). This method achieved an average error of only 5.0% based on adding known volumes of gas to the bag. Using the GBM, hydrogen production with SS and a BBS was 26.6 ± 1.8 mL which compared well to 26.4 ± 2.8 mL using Pt and BBS, and 26.8 ± 2.5 mL with a Pt cathode and PBS. Electrical energy efficiency was highest with a SS cathode and BBS at 159 ± 17%, compared to 126 ± 14% for the Pt cathode and BBS, and 134 ± 17% for a Pt cathode and PBS. The main disadvantage of the SS was a lower gas production rate of 1.1 ± 0.3 m³ H₂-m⁻³ d⁻¹ with BBS and 1.2 ± 0.3 m³ H₂-m⁻³ d⁻¹ with PBS, compared to 1.7 ± 0.4 m³ H₂-m⁻³ d⁻¹ with Pt and PBS. These results show that the GBM is an effective new method for measuring gas production of anaerobic gas production processes, and that SS and bicarbonate buffers can be used to effectively produce hydrogen in MECs.     

Construction and operation of a microbial fuel cell for electricity generation from wastewater

The increase in the global energy demand every year and the over-consumption of nonrenewable sources of energy has led to the identification and use of renewable and cost effective sources of energy. In this context, wastewater, which contains high levels of easily degradable organic material, has gained importance as a source of electricity generation using a microbial fuel cell.A microbial fuel cell comprising of Pseudomonas sp., mediator, and potassium ferricyanide as the oxidizing agent was developed for generation of electricity using wastewater, as substrate, obtained from wastewater treatment plant. The cells were connected in series with the anodic and cathodic solutions being introduced in batch and continuous modes. A maximum open-circuit potential of 2.2 V was obtained with the anode in batch-fed and cathode in continuous mode of operation. Methylene blue, when used as the mediator was found to produce a higher output from the cell when compared to neutral red. The maximum power output and current density obtained were 979 μW/m² and 1.15 mA/m² respectively. A 10% reduction in COD was observed when the microbial fuel cell was operated using the wastewater as the substrate.