Hydrogen production with nickel powder cathode catalysts in microbial electrolysis cells

Although platinum is commonly used as catalyst on the cathode in microbial electrolysis cells (MEC), non-precious metal alternatives are needed to reduce costs. Cathodes were constructed using a nickel powder (0.5–1 μm) and their performance was compared to conventional electrodes containing Pt (0.002 μm) in MECs and electrochemical tests. The MEC performance in terms of coulombic efficiency, cathodic, hydrogen and energy recoveries were similar using Ni or Pt cathodes, although the maximum hydrogen production rate (Q) was slightly lower for Ni (Q = 1.2–1.3 m³ H₂/m³/d; 0.6 V applied) than Pt (1.6 m³ H₂/m³/d). Nickel dissolution was minimized by replacing medium in the reactor under anoxic conditions. The stability of the Ni particles was confirmed by examining the cathodes after 12 MEC cycles using scanning electron microscopy and linear sweep voltammetry. Analysis of the anodic communities in these reactors revealed dominant populations of Geobacter sulfurreduces and Pelobacter propionicus. These results demonstrate that nickel powder can be used as a viable alternative to Pt in MECs, allowing large scale production of cathodes with similar performance to systems that use precious metal catalysts.     

Evaluation of low cost cathode materials for treatment of industrial and food processing wastewater using microbial electrolysis cells

Microbial electrolysis cells (MECs) can be used to treat wastewater and produce hydrogen gas, but low cost cathode catalysts are needed to make this approach economical. Molybdenum disulfide (MoS₂) and stainless steel (SS) were evaluated as alternative cathode catalysts to platinum (Pt) in terms of treatment efficiency and energy recovery using actual wastewaters. Two different types of wastewaters were examined, a methanol-rich industrial (IN) wastewater and a food processing (FP) wastewater. The use of the MoS₂ catalyst generally resulted in better performance than the SS cathodes for both wastewaters, although the use of the Pt catalyst provided the best performance in terms of biogas production, current density, and TCOD removal. Overall, the wastewater composition was more of a factor than catalyst type for accomplishing overall treatment. The IN wastewater had higher biogas production rates (0.8–1.8 m³/m³-d), and COD removal rates (1.8–2.8 kg-COD/m³-d) than the FP wastewater. The overall energy recoveries were positive for the IN wastewater (3.1–3.8 kWh/kg-COD removed), while the FP wastewater required a net energy input of −0.7–−1.2 kWh/kg-COD using MoS₂ or Pt cathodes, and −3.1 kWh/kg-COD with SS. These results suggest that MoS₂ is the most suitable alternative to Pt as a cathode catalyst for wastewater treatment using MECs, but that net energy recovery will be highly dependent on the specific wastewater.     

The use and optimization of stainless steel mesh cathodes in microbial electrolysis cells

Microbial electrolysis cells (MECs) provide a high-yield method for producing hydrogen from renewable biomass. One challenge for commercialization of the technology is a low-cost and highly efficient cathode. Stainless steel (SS) is very inexpensive, and cathodes made of this material with high specific surface areas can achieve performance similar to carbon cathodes containing a platinum catalyst in MECs. SS mesh cathodes were examined here as a method to provide a higher surface area material than flat plate electrodes. Cyclic voltammetry tests showed that the electrochemically active surface area of certain sized mesh could be three times larger than a flat sheet. The relative performance of SS mesh in linear sweep voltammetry at low bubble coverages (low current densities) was also consistent with performance on this basis in MEC tests. The best SS mesh size (#60) in MEC tests had a relatively thick wire size (0.02 cm), a medium pore size (0.02 cm), and a specific surface area of 66 m²/m³. An applied voltage of 0.9 V produced a high hydrogen recovery (98 ± 4%) and overall energy efficiency (74 ± 4%), with a hydrogen production rate of 2.1 ± 0.3 m³H₂/m³d (current density of 8.08 A/m², volumetric current density of 188 ± 19 A/m³). These studies show that SS in mesh format shows great promise for the development of lower cost MEC systems for hydrogen production.     

MoS2/NiFeS2 heterostructure as a highly efficient electrocatalyst for overall water splitting at high current densities

Searching for efficient, stable and low-cost nonprecious catalysts for oxygen and hydrogen evolution reactions (OER and HER) is highly desired in overall water splitting (OWS). Herein, presented is a nickel foam (NF)-supported MoS₂/NiFeS₂ heterostructure, as an efficient electrocatalyst for OER, HER and OWS. The MoS₂/NiFeS₂/NF catalyst achieves a 500 mA cm⁻² current density at a small overpotential of 303 mV for OER, and 228 mV for HER. Assembled as an electrolyzer for OWS, such a MoS₂/NiFeS₂/NF heterostructure catalyst shows a quite low cell voltage (≈1.79 V) at 500 mA cm⁻², which is among the best values of current non-noble metal electrocatalysts. Even at the extremely large current density of 1000 mA cm⁻², the MoS₂/NiFeS₂/NF catalyst presents low overpotentials of 314 and 253 mV for OER and HER, respectively. Furthermore, MoS₂/NiFeS₂/NF shows a ceaseless durability over 25 h with almost no change in the cell voltage. The superior catalytic activity and stability at large current densities (>500 mA cm⁻²) far exceed the benchmark RuO₂ and Pt/C catalysts. This work sheds a new light on the development of highly active and stable nonprecious electrocatalysts for industrial water electrolysis.     

Boosting activity and stability by coupling Pt–Ni nanoparticles with La-modified flexible carbon nanofibers for hydrogen evolution reaction

Platinum (Pt) is considered as the most efficient catalyst for hydrogen evolution reaction (HER) with a nearly zero overpotential, but it is limited by the high cost and poor stability. Herein, we report an efficient electrocatalyst of Pt–Ni alloy nanoparticles (NPs) supported on the La-modified flexible carbon nanocomposite fibers (PtNi@La-CNFs) for HER. The rare earth metal oxide in the catalyst has a structure-effect relationship with the carbon fibers to form a flexible fiber membrane. Experimental results show that the macroscopic and microscopic properties of carbon nanocomposite fibers can be optimized by doping La₂O₃, and the Pt–Ni NPs can be anchored effectively. The Pt₁Ni₁@La-CNFs electrocatalyst exhibits a small overpotential of 32 mV to achieve current density of 10 mA cm^?2 with a low Tafel slope of 51 mV dec^?1 in alkaline medium, outperforming that of Pt@La-CNFs and the commercial Pt/C catalyst. This study reveals that the multiple coupling effect of rare earth compound, precious metal, and transition metal in composite catalyst can tailor its the electronic configuration, and results in an enhanced HER performance. This work opens up a novel approach to design high active and low cost Pt-based HER catalysts.     

Multi-shelled CoS2–MoS2 hollow spheres as efficient bifunctional electrocatalysts for overall water splitting

The exploration of highly efficient non-precious electrocatalysts is essential for water splitting devices. Herein, we synthesized CoS₂–MoS₂ multi-shelled hollow spheres (MSHSs) as efficient electrocatalysts both for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) using a Schiff base coordination polymer (CP). Co-CP solid spheres were converted to Co₃O₄ MSHSs by sintering in air. CoS₂–MoS₂ MSHSs were obtained by a solvothermal reaction of Co₃O₄ MSHSs and MoS₄²⁻ anions. CoS₂–MoS₂ MSHSs have a high specific surface area of 73.5 m²g^-1. Due to the synergistic effect between the CoS₂ and MoS₂, the electrode of CoS₂–MoS₂ MSHSs shows low overpotential of 109 mV with Tafel slope of 52.0 mV dec⁻¹ for HER, as well as a low overpotential of 288 mV with Tafel slope of 62.1 mV dec⁻¹ for OER at a current density of 10 mA cm⁻² in alkaline solution. The corresponding two-electrode system needs a potential of 1.61 V (vs. RHE) to obtain anodic current density of 10 mA cm⁻² for OER and maintains excellent stability for 10 h.     

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.     

Advanced catalysts for hydrogen evolution reaction based on MoS2/NiCo2S4 heterostructures in Alkaline Media

There are great challenges to develop and fabricate a high performance, low-cost and stable non-platinum catalyst for hydrogen evolution reaction (HER). In our study, we firstly developed a simple method to successfully fabricate a new MoS₂/NiCo₂S₄ heterostructure by a two-step hydrothermal method, and studied the HER property of MoS₂/NiCo₂S₄, where the as-prepared NiCo-layered double hydroxide (NiCo-LDH) was used as the precursor of NiCo₂S₄. Benefitting from the prominent synergistic effect between NiCo₂S₄ and MoS₂, MoS₂ provided massive catalytic active edge sites, and NiCo₂S₄ enhanced the conductivity of the composite. As a result, the MoS₂/NiCo₂S₄ showed excellent HER catalytic activity, with a current of 10 mA cm⁻² at overpotential of 94 mV for HER and a low Tafel slope of 46 mV dec⁻¹, and good cycling stability in Alkaline Media. As well as, our work offered one promising high active and stable non-platinum catalyst for overall water splitting.     

Hexagonal CoSe2 nanosheets stabilized by nitrogen-doped reduced graphene oxide for efficient hydrogen evolution reaction

CoSe₂ is considered as a promising candidate among non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) due to its intrinsic metallicity and low Gibbs free energy for hydrogen adsorption. Recently, the hexagonal CoSe₂ becoming increasingly popular owing to its chemically favorable basal plane, which provides more active sites, but remains limited by the poor stability. In this study, we design a small-molecule-amine-assisted hydrothermal method to in situ anchor the hexagonal CoSe₂ nanosheets (NSs) on nitrogen-doped reduced graphene oxides (RGO) as an advanced electrode material for HER. Due to the existence of abundant functional groups and high specific surface area of RGO, the hexagonal CoSe₂ NSs could be stably formed on RGO. As a result, only a small overpotential of 172 mV is needed for the optimized sample to drive a current density of 10 mA cm⁻² in 0.5 M H₂SO₄ and the Tafel slope is 35.2 mV dec⁻¹, which is comparable with the state-of-the-art Pt catalyst (32.3 mV dec⁻¹). Therefore, the facile and low-cost method for synthesizing hexagonal TMDs with robust electrical and chemical coupling developed in this work is promising in promoting the large-scale application of non-precious electrocatalysts.     

In-situ synthesis of hollow Co–MoS2 nanocomposites on the carbon nanowire arrays/carbon cloth as high-performance catalyst for hydrogen evolution reaction

The design and synthesis of non-precious metal catalysts that effectively convert water into molecular hydrogen in an acidic environment is essential to reduce the energy loss in the water splitting process. Among them, molybdenum disulfide (MoS₂) is considered as an effective alternative to Pt-based materials due to its excellent structure properties. Here, by using metal-organic framework (MOF) as the precursor, sodium molybdate as molybdenum sources and thiourea as sulfur sources, a hollow structure of Co–MoS₂ electrocatalyst was prepared on highly conductive carbon nanowire arrays/carbon cloth (CA/CC) by hydrothermal reaction. The combination of carbon nanowire arrays and carbon cloth ensures the high conductivity, while the hollow Co–MoS₂ structure promotes the penetration of electrolytes and the release of hydrogen bubbles. The overpotential is only 296 mV when the current density was ~1500 mA/cm², which shows excellent catalytic hydrogen evolution activity of the material. In addition, the three-dimensional hollow structure avoids the use of adhesives between the active material and the self-supporting material, which can improve the stability of the material and provides a new idea to design commercial electrocatalysts.     

Three-dimensional ZnCo/MoS2–Co3S4/NF heterostructure supported on nickel foam as highly efficient catalyst for hydrogen evolution reaction

Exploring inexpensive and earth-abundant electrocatalysts for hydrogen evolution reactions is crucial in electrochemical sustainable chemistry field. In this work, a high-efficiency and inexpensive non-noble metal catalysts as alternatives to hydrogen evolution reaction (HER) was designed by one-step hydrothermal and two-step electrodeposition method. The as-prepared catalyst is composed of the synergistic MoS₂–Co₃S₄ layer decorated by ZnCo layered double hydroxides (ZnCo-LDH), which forms a multi-layer heterostructure (ZnCo/MoS₂–Co₃S₄/NF). The synthesized ZnCo/MoS₂–Co₃S₄/NF exhibits a small overpotential of 31 mV and a low Tafel plot of 53.13 mV dec^?1 at a current density of 10 mA cm^?2, which is close to the HER performance of the overpotential (26 mV) of Pt/C/NF. The synthesized ZnCo/MoS₂–Co₃S₄/NF also has good stability in alkaline solution. The excellent electrochemical performance of ZnCo/MoS₂–Co₃S₄/NF electrode originates from its abundant active sites and good electronic conductivity brought by the multilayer heterostructure. This work provides a simple and feasible way to design alkaline HER electrocatalysts by growing heterostructures on macroscopic substrates.     

Interface engineering of crystalline/amorphous Pt/TeOx nanocapsules for efficient alkaline hydrogen evolution

Hydrogen evolution reaction (HER) is an important process in electrochemical energy technology, and efficient electrocatalysts are of great significance for renewable and sustainable energy conversion. Here, we report a facile hydrothermal and heat treatment process to synthesize a series of Pt-based nanocapsules (NCs) as an effective hydrogen evolution catalyst. The Pt/TeO_x NCs exhibit excellent HER activity in an alkaline medium. The Pt/TeO_x NCs only need the overpotential of 33 mV to achieve the current density of 10 mA cm⁻², and the Tafel slope was as low as 29 mV dec⁻¹, which was even better than that of commercial Pt/C. Detailed experimental characterizations demonstrate that the interface between the crystalline Pt/amorphous TeO_x and the strong electron transfer contribute to alkaline HER activity. This work opens up a new direction for the preparation of efficient catalysts for electrocatalytic reactions or other conversion filed.     

Functionalized 2D materials F–MoS2 and F-g-C3N4 with TiO2 as Composite Electrocatalysts for Electrochemical Hydrogen Evolution

Electrochemical hydrogen evolution is an important research field to produce renewable energy. Nanostructured two dimensional (2D) materials such as g-C₃N₄ and MoS₂ are potential electrocatalysts for hydrogen evolution reaction (HER). The incorporation of semiconducting material into 2D material enhances the hydrogen evolution. Here in, we have developed composite of acid functionalized MoS₂ and g-C₃N₄ with TiO₂ (F–MoS₂/TiO₂, F-g-C₃N₄/TiO₂). The F–MoS₂/TiO₂ composite exhibited excellent electrochemical HER activity with an overpotential of 103 mV Vs RHE at 20 mA/cm² compared to pristine F–MoS₂ of 232 mV, TiO₂ of 455 mV Vs RHE. In addition F-g-C₃N₄/TiO₂ showed high overpotential of 322 mV at 5 mA/cm² than pristine F-g-C₃N₄ and TiO₂ of 433 mV and 448 mV Vs RHE at 2.7 mA/cm² respectively.     

Graphene confined MoS2 particles for accelerated electrocatalytic hydrogen evolution

MoS₂ is a promising noble-metal-free electrocatalyst for the hydrogen evolution reaction. Extensive trials have been carried out to increase its low electrical conductivity and insufficient active sites. Here, a remarkable electrocatalyst for hydrogen evolution is developed based on the in-situ preparation of MoS₂ confined in graphene nanosheets. Graphene effectively controls the growth of MoS₂ and immensely increases the conductivity and structural stability of the composite materials. Remarkably, because of the plentiful active sites, sufficient electrical contact and transport, MoS₂ particles confined in graphene nanosheets exhibit an onset overpotential as small as 32 mV, an overpotential approaching 132 mV at 10 mA cm⁻², and a low Tafel slope of 45 mV dec⁻¹. This work presents a reasonable architecture for practical applications in efficient electrocatalytic H₂ generation.     

Carbon dots modified molybdenum disulfide as a high-efficiency hydrogen evolution electrocatalyst

Molybdenum disulfide (MoS₂) is considered a low-cost material that may replace platinum-based electrocatalysts towards hydrogen evolution reaction (HER). However, the catalytic activity of MoS₂ is limited by the low conductivity and lack of active sites. Here, a biomass carbon dots-molybdenum disulfide (BCDs-MoS₂) composite was synthesized as a HER catalyst by a simple hydrothermal method. The BCDs-MoS₂ catalyst displayed excellent electrocatalytic performance for HER with lower onset overpotential (115 mV), smaller Tafel slope (56.57 mV dec^?1) as well as high cycling stability, which superior to those of homemade MoS₂, commercial MoS₂, and most of the reported MoS₂-based catalysts. According to the characterization results of morphology, surface properties, and valence states of elements, the outstanding catalytic activity of BCDs-MoS₂ is ascribed to its loose structure with a large specific surface area along with abundant edges and defects, and the increase in the amount of S₂^2? and S^2? which possess higher activity due to the addition of the BCDs. This study can afford a new strategy to design high performance HER catalysts.     

Molybdenum/graphene – Based catalyst for hydrogen evolution reaction synthesized by a rapid photothermal method

Catalysis of the hydrogen evolution reaction (HER) is important in the development of an energy economy based on clean hydrogen gas. In this work, we report a new catalyst material for the generation of hydrogen via hydronium reduction. The new material, which consists of MoO₂, sulfur, and graphene, was prepared by co-reduction of molybdenum salt and graphite oxide in air in the presence of focused solar radiation. The potential utility of this material for HER catalysis was evaluated by cyclic and linear-sweep voltammograms and compared against a Pt/C commercial catalyst. The MoO₂/graphene hybrid nanocomposite exhibits a Tafel slope of 47 mV/dec and hydrogen evolution at a potential only ∼120 mV more negative than the standard Pt/Carbon catalyst at 10 mA/cm² current density. The hydrogen gas generated by the catalytic material was measured using gas chromatography. The simple synthesis and low overpotential suggests that this hybrid composite has potential as an HER catalyst.     

Ru–MoS2@PPy hollow nanowire as an ultra-stable catalyst for alkaline hydrogen evolution reaction

Electrocatalytic hydrogen evolution reaction (HER) is one of the green and effective method to produce clean hydrogen energy. However, the development of non-Pt HER catalysts with excellent catalytic activity and long-term stability still remains a great challenge. Herein, a vertically aligned core-shell structure material with hollow polypyrrole (PPy) nanowire as a core and Ru-doped MoS₂ (Ru–MoS₂) nanosheets as a shell is firstly reported as a highly efficient and ultra-stable catalyst for HER in alkaline solutions. Results indicate that Ru–MoS₂@PPy catalyst demands a low overpotential of 37 mV at 10 mA cm^?2. In addition, the overpotential at 100 mA cm^?2 is 157 mV and it is almost unchanged after 40,000 cyclic voltammetry cycles. The existence of PPy core not only ensures the vertical growth of MoS₂ nanosheets to expose more edge sites, but also promotes the rapid transfer of electrons, contributing to the improvement of catalytic activity. More importantly, the strong interface interaction between MoS₂ and PPy prevents the collapse of the vertical structure of MoS₂ sheets in the electrocatalytic process and greatly enhances the stability of catalysts, which offers an effective strategy to design and synthesize the HER catalysts with superior catalytic stability.     

Preparation of ultrathin molybdenum disulfide dispersed on graphene via cobalt doping: A bifunctional catalyst for hydrogen and oxygen evolution reaction

The development of highly active and low-cost catalysts for hydrogen evolution reaction (HER) is significant for the development of clean and renewable energy research. Owing to the low H adsorption free energy, molybdenum disulfide (MoS₂) is regarded as a promising candidate for HER, but it shows low activity for oxygen evolution reaction (OER). Herein, graphene-supported cobalt-doped ultrathin molybdenum disulfide (Co–MoS₂/rGO) was synthesized via a one-pot hydrothermal method. The obtained hybrids modified electrode exhibits a high HER catalytic activity with a low overpotential of 147 mV at the current density of 10 mA cm⁻², a small Tafel slope of 49.5 mV dec⁻¹, as well as good electrochemical stability in acidic electrolyte. Meanwhile, the catalyst shows remarkable OER activity with a low overpotential of 347 mV at 10 mA cm⁻². The superior activity is ascribed not only to the high conductivity originated from the reduced graphene, but also to the synergistic effect between MoS₂ and cobalt.     

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.     

Platinum nanoparticles decorated Nb2CTx MXene as an efficient dual functional catalyst for hydrogen evolution and oxygen reduction reaction

Here, a dual functional Nb₂CT_x@Pt nanocomposite has been synthesized by in situ reduction method. The Pt loading in the composite has been optimized to get minimum overpotential (141 mV at 10 mA/cm²) for hydrogen evolution reaction (HER) along with a promising Tafel slope of 46.3 mV/dec, while Pt/C shows an overpotential and Tafel slope of 104 mV and 32.4 mV/dec, respectively. The Pt mass activity for Nb₂CT_x@Pt3.8 composite at 100 mV overpotential was 3.44 A g^?1 while the Pt mass activity for conventional Pt/C was 0.7 A g^?1, which shows that the activity of Nb₂CT_x@Pt3.8 composite is approximately 5 times higher than Pt/C. In addition, the catalyst was found to be stable for continuous 500 cycles without any binder molecules. The oxygen reduction reaction (ORR) capability of the material was also evaluated and found that the catalyst exhibited a current density of ?4.28 mA/cm² in the diffusion limiting region in comparison with the current density of ?5.82 mA/cm² for Pt/C at 2600 revolutions per minute (RPM). The Pt mass activity of Nb₂CT_x@Pt3.8 composite for ORR is approximately 10 times higher than Pt/C. The Nb₂CT_x@Pt3.8 composite was able to reduce O₂ completely using the 4-electron pathway with very little peroxide production. From these results, the dual functionality of the Nb₂CT_x@Pt3.8 composite for both HER and ORR has been established.