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.     

Hydrogen production in single-chamber tubular microbial electrolysis cells using non-precious-metal catalysts

Platinum has excellent catalytic capabilities and is commonly used as cathode catalyst in microbial electrolysis cells (MECs). Its high cost, however, limits the practical applications of MECs. In this study, precious-metal-free cathodes were developed by electrodepositing NiMo and NiW on a carbon-fiber-weaved cloth material and evaluated in electrochemical cells and tubular MECs with cloth electrode assemblies (CEA). While similar performances were observed in electrochemical cells, NiMo cathode exhibited better performances than NiW cathode in MECs. At an applied voltage of 0.6 V, the MECs with NiMo cathode accomplished a hydrogen production rate of 2.0 m³/day/m³ at current density of 270 A/m³ (12 A/m²), which was 33% higher than that of the NiW MECs and slightly lower than that of the MECs with Pt catalyst (2.3 m³/day/m³). At an applied voltage of 0.4 V, the energy efficiencies based on the electrical energy input reached 240% for the NiMo MECs. These results demonstrated the great potential of using carbon cloth with Ni-alloy catalysts as a cathode material for MECs. The enhanced MEC performances also demonstrate the scale-up potential of the CEA structure, which can significantly reduce the electrode spacing and lower the internal resistance of MECs, thus increasing the hydrogen production rate.     

Multi-electrode continuous flow microbial electrolysis cell for biogas production from acetate

Most microbial electrolysis cells (MECs) contain only a single set of electrodes. In order to examine the scalability of a multiple-electrode design, we constructed a 2.5 L MEC containing 8 separate electrode pairs made of graphite fiber brush anodes pre-acclimated for current generation using acetate, and 304 stainless steel mesh cathodes (64 m²/m³). Under continuous flow conditions and a one day hydraulic retention time, the maximum current was 181 mA (1.18 A/m², cathode surface area; 74 A/m³) within three days of operation. The maximum hydrogen production (day 3) was 0.53 L/L-d, reaching an energy efficiency relative to electrical energy input of η_E = 144%. Current production remained relatively steady (days 3–18), but the gas composition dramatically shifted over time. By day 16, there was little H₂ gas recovered and methane production increased from 0.049 L/L-d (day 3) to 0.118 L/L-d. When considering the energy value of both hydrogen and methane, efficiency relative to electrical input remained above 100% until near the end of the experiment (day 17) when only methane gas was being produced. Our results show that MECs can be scaled up primarily based on cathode surface area, but that hydrogen can be completely consumed in a continuous flow system unless methanogens can be completely eliminated from the system.     

High hydrogen production from glycerol or glucose by electrohydrogenesis using microbial electrolysis cells

The use of glycerol for hydrogen gas production was examined via electrohydrogenesis using microbial electrolysis cells (MECs). A hydrogen yield of 3.9 mol-H₂/mol was obtained using glycerol, which is higher than that possible by fermentation, at relatively high rates of 2.0 ± 0.4 m³/m³ d (E_ap = 0.9 V). Under the same conditions, hydrogen was produced from glucose at a yield of 7.2 mol-H₂/mol and a rate of 1.9 ± 0.3 m³/m³ d. Glycerol was completely removed within 6 h, with 56% of the electrons in intermediates (primarily 1,3-propanediol), with the balance converted to current, intracellular storage products or biomass. Glucose was removed within 5 h, but intermediates (mainly propionate) accounted for only 19% of the electrons. Hydrogen was also produced using the glycerol byproduct of biodiesel fuel production at a rate of 0.41 ± 0.1 m³/m³ d. These results demonstrate that electrohydrogenesis is an effective method for producing hydrogen from either pure glycerol or glycerol byproducts of biodiesel fuel 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.     

Ni foam cathode enables high volumetric H2 production in a microbial electrolysis cell

Valuable, “green” H₂ can be produced with a microbial electrolysis cell (MEC). To achieve a high volumetric production rate of high purity H₂, a continuous flow MEC with an anion exchange membrane, a flow through bioanode and a flow through Ni foam cathode was constructed. At an electrical energy input of 2.6 kWh m⁻³ H₂ (applied cell voltage: 1.00 V), this MEC was able to produce over 50 m³ H₂ m⁻³ MEC d⁻¹ (22.8 ± 0.1 A m⁻²). The MEC had a low cathode overpotential compared to an MEC with Pt-based cathode, because of the high specific surface area of Ni foam (128 m² m⁻² projected area). The MEC performance however, decreased during 32 days of operation due to an increase in anode and cathode overpotentials. Scaling likely caused the increase in anode overpotential, but it remained unclear what caused the increase in cathode overpotential.     

Copper-phthalocyanine and nickel nanoparticles as novel cathode catalysts in microbial fuel cells

In this study, four different catalysts (i.e., carbon black, nickel nanoparticle (Ni)/C, Phthalocyanine/C and copper-phthalocyanine/C), were tested in a two-chamber Microbial Fuel Cell (MFC) and their performances were compared with Pt as the common cathode catalyst in MFC. The characterization of catalysts was done by TEM, XPS and EDX and their electrochemical characteristics were compared by cyclic voltammetry (CV) and Linear Sweep Voltammetry (LSV). The results proved that copper phthalocyanine and nickel nanoparticles are potential alternatives catalyst for Pt. Even copper-phthalocyanine generated power is almost the same as Pt. The CV and LSV results reported high electrochemical activity of these catalysts. The maximum power density and coulombic efficiency was achieved by copper-phthalocyanine/C as 118.2 mW/m² and 29.3%.     

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.     

Cathodic hydrogen recovery and methane conversion using Pt coating 3D nickel foam instead of Pt-carbon cloth in microbial electrolysis cells

A cheap but efficient electrode material is required to explore and apply to microbial electrolysis cell (MEC) with high hydrogen evolution reaction (HER) efficiency and low over-potential loss. Pt coating carbon cloth (Pt/CC) was one of the most efficient catalyst for hydrogen production in current lab research, but it is difficult to be applied in practice because of expensive cost and week strength from the base material (carbon cloth). Thus a cheap and effective supporting base material is worth to evaluate on hydrogen recovery and loss to methane for the MEC future application. In this study, nickel foam (NF) was used as an alternative to expensive carbon cloth, and NF coated with Pt (Pt/NF) was applied and evaluated through catalytic performance, hydrogen production efficiency and economic assessment in comparison with Pt/CC. The Pt/NF showed a competitive HER performance to Pt/CC. The highest hydrogen yield was reached 0.71 ± 0.03 m³/m³·d by Pt/NF under 0.8 V, which exceeded 6%, 10% over Pt/CC and NF, respectively. The energy efficiency relative to the electrical energy input was 127% for Pt/NF and 123%, 110% for Pt/CC and NF, respectively. For fifteen cycles, the methane content of Pt/NF got the lowest due to its higher hydrogen evolution activity. The economic analysis showed a 56% reduction when using Pt/NF as supporting base in place of carbon cloth to achieve similar performance. The linear sweep voltammetry (LSV) showed the possibility to further reduce input voltage in a long term operation.     

Hydrogen production from glycerin by steam reforming over nickel catalysts

Increasing biodiesel production has resulted in a glut of glycerin that has led to a precipitous drop in market prices. In this study, the use of glycerin as a biorenewable substrate for hydrogen production, using a steam reforming process, has been evaluated. Production of hydrogen from glycerin is environmentally friendly because it adds value to this byproduct generated from biodiesel plants. The study focuses on nickel-based catalysts with MgO, CeO₂, and TiO₂ supports. Catalysts were characterized with thermogravimetric analysis and X-ray diffraction techniques. Maximum hydrogen yield was obtained at 650 °C with MgO supported catalysts, which corresponds to 4 mol of H₂ out of 7 mol of stoichiometric maximum.     

Nickel powder blended activated carbon cathodes for hydrogen production in microbial electrolysis cells

Although pure Ni catalysts can achieve a hydrogen production rate similar to Pt in microbial electrolysis cells (MECs), a reduction in the amount of Ni used is needed to reduce the cost. In this study, nickel powder (pNi) was blended with activated carbon (AC) to reduce the mass of Ni used, while improving catalytic activity for the hydrogen evolution reaction (HER) by increasing the active surface area. Ni powder blended AC cathodes (AC-pNi) were fabricated at different nickel powder loadings (4.8, 19, 46 mg/cm² with AC and 77 mg/cm² without AC as control). AC-pNi4.8 (Ni loading: 4.8 mg/cm²) produced higher hydrogen production rates (0.38 ± 0.04 L-H₂/L-d) than pNi77 (0.28 ± 0.02 L-H₂/L-d) with a 16 times less Ni loading. Cathodic hydrogen recovery of using the AC-pNi4.8 (98 ± 5%) was also higher than pNi77 (82 ± 4%), indicating catalytic activities were improved by AC blending. Nickel dissolution into the catholyte after completion of each cycle was negligible for AC-pNi4.8 (<0.2 mg/L), while Ni dissolution was detected for pNi77 (5–10 mg/L). These results indicate that AC blending with Ni powder can improve hydrogen production in MECs while minimizing the amount of Ni in the cathode.     

Hydrogen production from the gasification of lignin with nickel catalysts in supercritical water

Hydrogen production from the gasification of lignin with Ni/MgO catalysts in supercritical water was conducted using stainless steel tube bomb reactor. Ni/MgO catalysts were prepared by impregnation method and were calcined at 773–1173 K in air for 8 h. The results of characterization for reduced Ni/MgO catalysts showed that Ni metal and NiO–MgO phase are formed after the reduction of calcined catalyst by _H2${\mathrm{H}}_2$. Furthermore, Ni metal surface area, which was calculated by CO chemical adsorption technique, decreased with increase in calcination temperatures. It was found that the carbon yield of gas products was increased with increase in Ni metal surface area except 10 wt% Ni/MgO (773 K) catalyst. Thus, it can be supposed that there is an optimal Ni particle size for the gasification of lignin in supercritical water. It should be noted that 10 wt% Ni/MgO (873 K) catalyst showed the best catalytic performance (carbon yield 30%) under reaction condition tested. It was concluded that Ni/MgO catalyst is a promising system for the gasification of lignin in supercritical water.     

Performance of metal alloys as hydrogen evolution reaction catalysts in a microbial electrolysis cell

H₂ can be produced from organic matter with a microbial electrolysis cell (MEC). To decrease the energy input and increase the H₂ production rate of an MEC, a catalyst is used at the cathode. Platinum is an effective catalyst, but its high costs stimulate searching for alternatives, such as non-noble metal alloys. This study demonstrates that copper sheet coated with nickel-molybdenum, nickel-iron-molybdenum or cobalt-molybdenum alloys have a higher catalytic activity for the hydrogen evolution reaction than nickel cathodes, measured near neutral pH. However, the catalytic activity cannot be fully exploited near neutral pH because of mass transport limitation. The catalytic activity is best exploited at alkaline pH where mass transport is not limiting. This was demonstrated in an MEC with a cobalt-molybdenum coated cathode and anion exchange membrane, which produced 50 m³ H₂ m⁻³ MEC d⁻¹ (at standard temperature and pressure) at an electricity input of 2.5 kWh m⁻³ H₂.     

Hydrogen production in a microbial electrolysis cell with nickel-based gas diffusion cathodes

Gas diffusion cathodes with Ni alloy and Ni catalysts manufactured by chemical deposition were tested for H₂ production in a microbial electrolysis cell (MEC). In a continuous flow MEC, multi-component cathodes containing Ni, Mo, Cr, and Fe, at a total catalyst load of 1 mg cm⁻² on carbon support demonstrated stable H₂ production at rates of with only 5% methane in the gas stream. Furthermore, a Ni-only gas diffusion cathode, with a Ni load of 0.6 mg cm⁻², demonstrated a H₂ production rate of . Overall, H₂ production was found to be proportional to the Ni load implying that inexpensive gas diffusion cathodes prepared by chemical deposition of Ni can be successfully used for continuous production of H₂ in a MEC.     

Hydrogen production in microbial electrolysis cells: Choice of catholyte

A catholyte is a key factor to hydrogen production in microbial electrolysis cells (MECs). Among the four groups of catholytes investigated in this study, a 100 mM phosphate buffer solution (PBS) resulted in the highest hydrogen production rate of 0.237 ± 0.031 m³H₂/m³/d, followed by 0.171 ± 0.012 m³H₂/m³/d with a 134 mM NaCl solution and 0.171 ± 0.004 m³H₂/m³/d with the acidified water adjusted with sulfuric acid. The MEC with all catholytes achieved good organic removal efficiency, but the removal rate varied following the trend of the hydrogen production rate. The reuse of the catholyte for an extended period led to a decreasing hydrogen production rate, affected by the elevated pH. The cost of both the acidified water and the NaCl solution was much lower than the PBS, and therefore, they could be a better choice as an MEC catholyte with further consideration of cost reduction and chemical reuse/disposal.     

Hydrogen and/or syngas production by combined steam and dry reforming of methane on nickel catalysts

Two alumina supported Ni catalysts with pore sizes of 5.4 nm and 9 nm were synthetized, characterized and tested in the Combined Steam and Dry Reforming of Methane (CSDRM) for the production of hydrogen rich gases or syngas. The reaction mixture was designed to simulate the composition of real clean biogas, the addition of water being made in order to have molar ratios of H₂O:CO₂ corresponding to 2.5:1, 7.5:1 and 12.5:1. Structural and functional characterization of catalysts revealed that Ni/Al₂O₃ with larger pore size shows better characteristics: higher surface area, lower Ni crystallite sizes, higher proportion of stronger catalytic sites for hydrogen adsorption, and higher capacity to adsorb CO₂. At all studied temperatures, for a CH₄:CO₂:H₂O molar ratio of 1:0.48:1.2, a (H₂+CO) mixture with H₂:CO ratio around 2.5 is obtained. For the production of hydrogen rich gases, the optimum conditions are: CH₄:CO₂:H₂O = 1:0.48:6.1 and 600 °C. No catalyst deactivation was observed after 24 h time on stream for both studied catalysts, and no carbon deposition was revealed on the used catalysts surface regardless the reaction conditions.     

Biohydrogen production from sugar industry effluents using nickel based electrode materials in microbial electrolysis cell

Biohydrogen production from sugar industry effluents in a dual chamber microbial electrolysis cell (MEC) was investigated in this study. The MEC reactor was operated with different effluents as a substrate from cane sugar and raw sugar reprocessing units of sugar industry. The biohydrogen production was investigated using different cathode materials of Nickel plate, Nickel foam, Stainless Steel mesh. The performance of MEC was tested based on the production of hydrogen, coloumbic efficiency, hydrogen recovery and COD removal efficiency respectively. The MEC hydrogen productions revealed that cane sugar effluent was more effective as compared to raw sugar effluent. The experimental results showed that at an applied voltage of 1.0 V, Ni-foam exhibited maximum hydrogen production of 1.59 and 1.43 mmol/L/D in cane sugar and raw sugar effluents respectively, which was about twice than SS-mesh and 1.2 times Ni-plate. This study shows that Ni-foam is one of the potential candidate as low cost electrode for improving hydrogen production in MEC technology with the treatment of industrial effluents.     

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.     

Fe-Cu coated nickel mesh usage as cathode catalyst for hydrogen evolution reaction

Nickel mesh electrodes were used as the working electrode. Iron and copper were electrochemically deposited on the nickel mesh in different amounts. When electrochemical coatings had been carried out, different currents were passed from the circuit at different times and coatings were accumulated at constant load. The prepared electrodes called as FexCux, FexCu3x and FexCu9x and these electrodes have been used for hydrogen evolution reaction (HER). The surface morphologies were investigated by scanning electron microscopy. The HER activity is assessed by recording cathodic current–potential curves, cyclic voltammetry, electrochemical impedance spectroscopy. The results show that FexCu9x catalysts have a compact and porous structure as well as good electrocatalytic activity for the HER in alkaline media.