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Over the last decade, as a consequence of the global decarbonization process, the interest towards green hydrogen production has drastically increased. In particular a substantial research effort has focused on the efficient and affordable production of carbon-free hydrogen production processes. In this context, the development of more efficient electrolyzers with low-cost electrode/electrocatalyst materials can play a key role. This work, investigates the fabrication of electrodes of nickel-zinc alloys with nanowires morphology cathode for alkaline electrolyzers. Electrodes are obtained by the simple method of template electrosynthesis that is also inexpensive and easily scalable. Through the analysis of the morphological and chemical composition of nanowires, it was found that the nanowires composition is dependent on the concentration of two metals in the deposition solution. Electrocatalytic tests were performed in 30% w/w potassium hydroxide aqueous solution at room temperature. In order to study the electrodes stability, mid-term galvanostatic test was also carried out. All electrochemical tests show that nanowires with about 44.4% of zinc have the best performances. Particularly, at −50 mAcm−2, these electrodes have an overpotential 50 mV lower than pure Ni nanowire. NiZn nanowires show also a good stability over time without noticeable signs of performance decay.

The exploration of efficient bifunctional electrocatalysts for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) under alkaline conditions is an important way to promote the development of electrolytic water technology. Herein, the reduced graphene oxide-supported MoO42- modified amorphous cobalt metaphosphate cubes (a-Co(PO3)2/MoO4/rGO) as bifunctional OER/HER catalyst is prepared by anion exchange and phosphating, using the Prussian blue analogue (PBA) as a precursor. The resulting composite exhibits the low overpotentials (η) that of 290 and 50 mV for OER and HER in 1.0 M KOH solution at 10 mA cm-2, respectively. The electrochemical test and density functional theory (DFT) results reveal that the MoO42--modified optimizes the adsorption/desorption energy of H* of Co(PO3)2, thus enhance the HER activity. Benefiting from efficient HER and OER performances, an efficient and stable alkaline water electrolysis operation using a-Co(PO3)2/MoO4/rGO used as bifunctional catalyst can be carried out, which can deliver a current density (j) of 20 mA cm−2 at 1.65 V cell voltage and work continuously for 24 h.

Design of the catalyst for efficient water dissociation and hydrogen recombination is paramount in enhancement of the alkaline water electrolysis kinetics. Herein, we reported a delicate hierarchical (VO)2P2O7-Ni2P@NF (VPO-Ni2P@NF) hybrid catalyst that operated efficiently in alkaline media. The VPO and Ni2P respectively act as the water dissociation promoter and the hydrogen recombination center, which synergistically propel water adsorption/dissociation and H intermediates recombination. The resulting synergistic interfaces between VPO and Ni2P are verified to afford the catalyst an outstanding performance for hydrogen evolution reaction in alkaline media with an overpotential of 154 mV at 10 mA cm−2, Tafel slope of 65 mV dec−1, and remarkable durability. Furthermore, the catalyst presents the potential for overall water splitting. This work may shed fresh light on the high-performance electrocatalyst design and the application of VPO on water electrolysis.

A technology for cyclic generation of hydrogen and oxygen using electrodes made of variable valency material that does not need the use of separating ion-exchange membranes is presented. The technological solution enables to fabricate electrolyzers for uninterrupted producing high-pressure hydrogen with reduced energy intensity of the production. The total work for compressing 1 m3 of hydrogen and 0.5 m3 of oxygen has been estimated. Results of investigation of influence of discrete supply of DC current to the electrolysis cell, in order to improve the processes of gas evolution and to simplify the power systems of the electrolysis plant, have been considered. There is also considered an electrolysis installation equipped with a thermosorption compressor in which LaNi5 is used as a hydride-forming compound. The comparative characteristics of the developed electrolyzer and the currently used hydrogen generators are given.

The exploration of highly effective, durable, and affordable electrocatalysts for the oxygen evolution reaction (OER) is vital in developing an alkaline electrolyzer for H2 generation. Although binary transition metal oxides, particularly spinel ferrites with a general chemical formula MFe2O4, are a promising class of candidates for OER, their intrinsically low electrical conductivity has a significant negative impact on their electrochemical performances. Herein, we report a simple one-step method for tailoring carbon spheres (CSs) with CoFe2O4 (CFO) to form a CFO@CSs composite as a cost-effective electrocatalyst. The as-synthesized CFO@CSs features a high specific surface area of 56.94 m2/g and exhibits decently high OER catalytic activity with a relatively small overpotential of 390 mV at 10 mA/cm2, fast kinetics, and high durability in alkaline medium. Moreover, the pairing of the bare Ni mesh electrode as the cathode and the CFO@CSs on Ni mesh as the anode is set up to illustrate its feasibility for the overall alkaline water splitting. This study offers a cost-effective electrocatalyst for OER that can be used instead of precious metals in various renewable energy storage and conversion applications.

Constructing a low cost, and high-efficiency oxygen evolution reaction (OER) electrocatalyst is of great significance for improving the performance of alkaline electrolyzer, which is still suffering from high-energy consumption. Herein, we created a porous iron phosphide and tungsten oxide self-supporting electrocatalyst with oxygen-containing vacancies on foam nickel (Fe2P-WO2.92/NF) through a facile in-situ growth, etching and phosphating strategies. The sequence-controllable strategy will not only generate oxygen vacancies and improve the charge transfer between Fe2P and WO2.92 components, but also improve the catalyst porosity and expose more active sites. Electrochemical studies illustrate that the Fe2P-WO2.92/NF catalyst presents good OER activity with a low overpotential of 267 mV at 100 mA cm−2, a small Tafel slope of 46.3 mV dec−1, high electrical conductivity, and reliable stability at high current density (100 mA cm−2 for over 60 h in 1.0 M KOH solution). Most significantly, the operating cell voltage of Fe2P-WO2.92/NF || Pt/C is as low as 1.90 V at 400 mA cm−2 in alkaline condition, which is one of the lowest reported in the literature. The electrocatalytic mechanism shows that the oxygen vacancies and the synergy between Fe2P and WO2.92 can adjust the electronic structure and provide more reaction sites, thereby synergistically increasing OER activity. This work provides a feasible strategy to fabricate high-efficiency and stable non-noble metal OER electrocatalysts on the engineering interface.

While hydrogen generation by alkaline water electrolysis is a well-established, mature technology and currently the lowest capital cost electrolyser option; polymer electrolyte membrane water electrolysers (PEMWEs) have made major advances in terms of cost, efficiency, and durability, and the installed capacity is growing rapidly. This makes the technology a promising candidate for large-scale hydrogen production, and especially for energy storage in conjunction with renewable energy sources – an application for which PEMWEs offer inherent advantages over alkaline electrolysis. Improvements in PEMWE technology have led to increasingly high operational current densities, which requires adequate mass transport strategies to ensure sufficient supply of reactant and removal of products. This review discusses the current knowledge related to mass transport and its characterisation/diagnosis for PEMWEs, considering the flow channels, liquid-gas diffusion layer, and polymer electrolyte membrane in particular.

Alkaline water electrolysis has the advantage of scalability for industrial-scale mass production of hydrogen; however, it is operated under a lower current density than other methods of water electrolysis because a high overpotential resulting from ion transport limitations will occur at high current density. Bubble dynamics can both prevent ion transport by its existence and accelerate it by bubble-induced flow. In this study, we conduct three-dimensional coupling numerical simulations of two-phase flow and electrochemical phenomena to elucidate the mechanisms by which microscale bubble dynamics influence ion transport and the cell overpotential. We find that the flow induced by rising microbubbles enhances ion transport to the anode and suppresses the cell overpotential. Moreover, bubble atomization further suppresses the overpotential because smaller bubbles approach the anode more closely than larger ones and accelerate ion transport to the anode surface.

Alkaline water electrolysis is the most promising approach for the industrial production of green hydrogen. This study investigates the dynamic operational characteristics of an industrial-scale alkaline electrolyzer with a rated hydrogen production of 50 m3/h. Strategies for system control and equipment improvement in dynamic-mode alkaline electrolytic hydrogen production are discussed. The electrolyzer can operate over a 30%–100% rated power load, thereby facilitating high-purity (>99.5%) H2 production, competitive DC energy efficiency (4.01–4.51 kW h/Nm3 H2, i.e., 73.1%–65.0% LHV), and good gas–liquid fluid balance. A safe H2 content of 2% in O2 (50% LFL) can be guaranteed by adjusting the system pressure. In transient operation, the electrolyzer can realize minute-level power and pressure modulation with high accuracy. The results confirm that the proposed alkaline electrolyzer can absorb highly fluctuating energy output from renewables because of its capability to operate in a dynamic mode.

The motivation of this work is to propose a shared balance of plant (BoP) and power supply (PS) design for industrial scale alkaline electrolyzer plant that has reduced CAPEX with a minimum loss in OPEX for variable load operation. Three important aspects are: a) flowsheet - either shared or individual BoP and PS per stack, b) variable or constant lye flowrate per stack and c) sizing of cooling duty in the lye circulation loop. Steady-state optimization shows that individual BoP per stack (with higher CAPEX) is optimal when the plant is expected to operate at high capacity. For shared BoP and PS, the hydrogen production is higher by 8–12% when operated with variable lye flowrate compared to fixed lye flowrate. Our results further suggest that lye cooling duty should be designed based on the cooling requirements of the degraded electrolyzer stacks at end of life.

The development and optimization of transition metal-based bifunctional electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are critical for high-efficient water splitting. Herein, we propose a carbon dots-oriented strategy for fabricating nitrogen-doped carbon dots/fungus-like CoP microsphere composite on Ni foam (CoP-NCDs/NF). Owing to its unique structure and the strong interactions between carbon dots and CoP, the newly developed CoP-NCDs/NF electrode exhibits significantly enhanced HER and OER bifunctional catalytic activities, with overpotentials as low as 103 and 226 mV at a current density of 10 mA cm−2, respectively. Accordingly, the CoP-NCDs/NF displays excellent overall water splitting performance with a low cell voltage of 1.55 V at 10 mA cm−2 and great stability in a two-electrode alkaline electrolyzer. This study provides an alternative pathway on rationally designing and fabricating carbon dots-based materials for high performance water electrolysis and other energy conversion fields.

Developing high-performing oxygen evolution reaction (OER) electrocatalysts under high-current operation conditions is critical for future commercial applications of alkaline water electrolysis for clean energy generation. Herein, we prepared a three-dimensional (3D) bimetallic oxyhydroxide hybrid grown on a Ni foam (NiFeOOH/NF) prepared by immersing Ni foam (NF) into Fe(NO3)3 solution. In this unique 3D structure, the NiFeOOH/NF hybrid was composed of crystalline Ni(OH)2 and amorphous FeOOH evenly grown on the NF surface. As a bimetallic oxyhydroxide electrocatalyst, the NiFeOOH/NF hybrid exhibited excellent catalytic activity, surpassing not only the other reported Ni–Fe based electrocatalysts, but also the commercial Ir/C catalyst. In situ electrochemical Raman spectroscopy demonstrated the active FeOOH and NiOOH phases involved in the OER process. Profiting from the synergy of Fe and Ni catalytic sites, the NiFeOOH/NF hybrid delivered an outstanding OER performance under challenging industrial conditions in a 10.0 mol∙L−1 KOH electrolyte at 80 °C, requiring potentials as small as 1.47 and 1.51 V to achieve the super-high catalytic current densities of 100 and 500 mA∙cm−2, respectively.

The efficiency of an electrolyzer can be improved by preheating the water consumed, which is generally done by means of solar energy in PVT panels. In this research, the first objective is to determine whether it is possible to preheat the consumed water by using the residual heat given off by the electrolyzer itself fed by a PV array, and if the above is met, the second objective consists of quantify the benefits obtained in the performance of the system. The simulation is carried out over a period of one year, considering the meteorological conditions of the city of Antofagasta, Chile. The results indicate that it is possible to constantly maintain the water temperature consumed by the electrolyzer at its nominal value of 80 °C, since the energy contained in the waste heat is about 30 times higher than this hot water demand. Continuous operation at 80 °C compared to operation at variable temperature achieves an annual increase of 0.22% in hydrogen production and an average of 0.33% in electrolyzer efficiency. Moreover, by considering the thermal energy given off by the electrolyzer as useful output of the system, the overall energy efficiency increases by a relative percentage of 13%.

Collaborative design in both nanoarchitecture and electronic structure is of great significance for cost-effective electrocatalysts towards oxygen evolution reaction (OER). Herein, cactus-like porous cobalt oxide (Co3O4) nanoarchitecture doped with manganese cation and nitrogen anion (N-Mn-Co3O4) was fabricated on the nickel foam by hydrothermal and subsequent N2 plasma treatment. Unique hierarchical structure and surface atomic engineering endow the N-Mn-Co3O4 with rich active sites, abundant oxygen vacancies, enhanced electrical conductivity and rapid ion diffusion. Hence, as electrocatalysts for OER, the N-Mn-Co3O4 exhibits low overpotentials of 302 and 320 mV to drive the current density of 50 and 100 mA cm−2, respectively, and superior stability over 40 h under alkaline environments. More strikingly, when assembling the N-Mn-Co3O4 with Pt/C anode into an alkaline electrolyzer, the system delivers a small voltage of 1.55 V at the current density of 10 mA cm−2 with excellent durability. This work may shed light on design and fabrication of efficient OER electrocatalysts by synergistically tailoring electronic and geometric structures.

Hydrogen is a carbon-free alternative energy source for use in future energy frameworks with the advantages of environment-friendliness and high energy density. Among the numerous hydrogen production techniques, sustainable and high purity of hydrogen can be achieved by water electrolysis. Therefore, developing electrocatalysts for water electrolysis is an emerging field with great importance to the scientific community. On one hand, precious metals are typically used to study the two-half cell reactions, i.e., hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, precious metals (i.e., Pt, Au, Ru, Ag, etc.) as electrocatalysts are expensive and with low availability, which inhibits their practical application. Non-precious metal-based electrocatalysts on the other hand are abundant with low-cost and eco-friendliness and exhibit high electrical conductivity and electrocatalytic performance equivalent to those for noble metals. Thus, these electrocatalysts can replace precious materials in the water electrolysis process. However, considerable research effort must be devoted to the development of these cost-effective and efficient non-precious electrocatalysts. In this review article, we provide key fundamental knowledge of water electrolysis, progress, and challenges of the development of most-studied electrocatalysts in the most desirable electrolytic solutions: alkaline water electrolysis (AWE), solid-oxide electrolysis (SOE), and proton exchange membrane electrolysis (PEME). Lastly, we discuss remaining grand challenges, prospect, and future work with key recommendations that must be done prior to the full commercialization of water electrolysis systems.

High efficiency and energy-saving electrochemical hydrogen production has always been a research challenge, mainly due to the limitation of the sluggish kinetics of anodic reactions. Excellent performance depends largely on the clever design of nano-architectures and smart hybridization of active components. It is also very important to establish the relationship between structure and performance of materials. Form perspectives of chemical composition and nanostructure, we developed a novel heterostructure of Ni3S2@CoMoS4/NiFeOOH coaxial nanorods on NF scaffold, in which the two-dimensional CoMoS4 nanoplates and ultrathin NiFeOOH nanosheets vertically coil around the Ni3S2 nanorods by hydrothermal reaction combined with electrodepostion process. Such hierarchical nanorods can provide the heterointerface with highly open surface, ensuring the maximization of synergistic interaction. This heterostructure results in prominent bifunctional activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline water electrolysis systems, and HER coupled with urea oxidation reaction in urea electrolysis devices. It exhibits very low cell voltages for alkaline water splitting (1.732 V) and urea electrolysis (1.660 V) to afford a current density of 100 mA cm−2 in the two-electrode system as well as excellent long-term stability. Our work provides a new construction for combining various active materials to competitive bifunctional electrocatalysts applied in energy-relative electrochemical devices.

Rational coupling of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysts is extremely important for practical overall water splitting, but it is still challenging to construct such bifunctional heterostructures. Herein, we present a metal–organic framework (MOF)-etching strategy to design free-standing and hierarchical hollow CoS2–MoS2 heteronanosheet arrays for both HER and OER. Resulting from the controllable etching of MOF by MoO42− and in-situ sulfuration, the obtained CoS2–MoS2 possesses abundant heterointerfaces with modulated local charge distribution, which promote water dissociation and rapid electrocatalytic kinetics. Moreover, the two-dimensional hollow array architecture can not only afford rich surface-active sites, but also facilitate the penetration of electrolytes and the release of evolved H2/O2 bubbles. Consequently, the engineered CoS2–MoS2 heterostructure exhibits small overpotentials of 82 mV for HER and 266 mV for OER at 10 mA cm−2. The corresponding alkaline electrolyzer affords a cell voltage of 1.56 V at 10 mA cm−2 to boost overall water splitting, along with robust durability over 24 h, even surpassing the benchmark electrode couple composed of IrO2 and Pt/C. The present work may provide valuable insights for developing MOF-derived heterogeneous electrocatalysts with tailored interface/surface structure for widespread application in catalysis and other energy-related areas.

A three-dimensional (3-D) transient numerical model of an alkaline water electrolysis (AWE) cell with potassium hydroxide solution is developed by rigorously accounting for the hydrogen and oxygen evolution reactions and resulting species and charge transport through various AWE components. First, the AWE model is experimentally validated against a polarization curve corresponding to a wide range of currents as high as 2.0 A·cm−2. In general, the simulation results compare well with the measured data and further reveal the operating characteristics of AWE cells, showing key distributions of solid/electrolyte potentials and multidimensional contours of reactant and product concentrations at various current densities. In particular, the contribution of hydroxide ion (OH−) diffusion to the ohmic losses through porous electrodes and a porous separator are quantitatively examined at low and high electrolyte flow rates. The present full 3-D AWE model provides a basic understanding of the electrochemical and transport phenomena and can be further applied to practical large-scale AWE cell and stack geometries for grid-scale hydrogen production.

The synergistic achievement of low-cost earth-abundant electrocatalysts and high efficiency to meet renewable energy need is highly desirable yet challenging. Here, we developed a simple Ni foam self -templating route for V-doped Ni3S2 nanosheet arrays through in situ formation of metal-organic frameworks (MOFs) combined with subsequent conversion. The as-prepared MOF-V-Ni3S2/NF catalyst delivers outstanding electrocatalytic performance in the alkaline solution, which requires low overpotentials of 118.1 mV @10 mA cm−2 and 268 mV @10 mA cm−2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. The V-doping and MOF-derived 3D hieratical nanostructure play a vital role in the catalytic process, which provides efficient active sites and large surface areas. Furthermore, an alkaline electrolyzer was assembled with two pieces of MOF-V-Ni3S2/NF, which achieves efficient water splitting at 1.58 V @10 mA cm−2. This strategy opens up new channels to synthesize MOF-based bifunctional electrocatalysts toward overall water spitting.

Development of high-efficiency and low-cost electrocatalyst for oxygen evolution reaction (OER) is very important for use at alkaline water electrolysis. Metal-organic frameworks (MOF) provide a rich platform for designing multi-functional materials due to their controllable composition and ultra-high surface area. Herein, we report our findings in the development of amorphous nickel–cobalt bimetal-organic framework nanosheets with crystalline motifs via a simple “ligands hybridization engineering” strategy. These complexes’ ligands contain inorganic ligands (H2O and NO3−) and organic ones, hexamethylenetetramine (HMT). Further, we investigated a series of mixed-metal with multi-ligands materials as OER catalysts to explore their possible advantages and features. It is found that the Ni doping is an effective approach for optimizing the electronic configuration, changing lattice ordering degree, and thus enhancing activities of HMT-based electrocatalysts. Also, the crystalline-amorphous boundaries of various HMT-based electrocatalyst can be easily controlled by simply changing amounts of Ni-precursor added. As a result, the optimized ultrathin (Co, 0.3Ni)-HMT nanosheets can reach a current density of 10 mA cm−2 at low overpotential of 330 mV with a small Tafel slope of 66 mV dec−1. Our findings show that the electronic structure changes induced by Ni doping, 2D nanosheet structure, and MOF frameworks with multi-ligands compositions play critical roles in the enhancement of the kinetically sluggish electrocatalytic OER. The present study emphasizes the importance of ligands and active metals via hybridization for exploring novel efficient electrocatalysts.

The flexible operation of alkaline water electrolyzers enables power-to-x plants to react efficiently to different energy scenarios. In this work, a novel scheduling model for alkaline water electrolysis is formulated as a mixed-integer linear program. The model is constructed by implementing operational states (production, standby, idle) and transitions (cold/full startup, shutdown) as integer variables, while the power loading and hydrogen flowrate are set as continuous variables. The operational characteristics (load range, startup time, ramp rates) are included as model constraints. The proposed model allows finding optimal number of electrolyzers and production schedules when dealing with large data sets of intermittent energy and electricity price. The optimal solution of the case study shows a balance between hydrogen production, energy absorption, and operation and investment costs. The optimal number of electrolyzers to be installed corresponds to 54% of the ones required to absorb the highest energy peak, being capable of loading 89.7% of the available energy during the year of operation, with an overall plant utilization of 93.7% and 764 startup/shutdown cycles evenly distributed among the units.

Hydrogen production via alkaline water electrolysis is of significant interest. However, the additional water dissociation step makes the Volmer step a relatively more sluggish kinetics and consequently leads to a slower reaction rate than that in acidic solution. Herein, we demonstrate an effective strategy that Co(OH)2 can promote the Volmer process by accelerating water dissociation and enhance the electrocatalytic performance of CoP toward alkaline hydrogen evolution reaction. The Co(OH)2 nanoplates are electrochemically induced in-situ generated to form a nanotree-like structure with porous CoP nanowires, endowing the hybrid electrocatalyst with superior charge transportation, more exposed active sites, and enhanced reaction kinetics. This strategy may be extended to other phosphides and chalcogenides and provide insight into the design and fabrication of efficient alkaline HER catalysts.

By increasing demand for hydrogen and oxygen gas for energy and industrial applications, designing a cheap, high-efficiency, and bifunctional electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) seems necessary. For this purpose Ni–Mo–Fe as a bifunctional electrocatalyst was synthesized by one-step electrodeposition. From this electrocatalyst with optimal composition and current density, a small overpotential of 65, 161 mV for delivering 10, 100 mA/cm2 on HER in alkaline media was achieved. As-fabricated electrode exhibited 344,408 mV for delivering 10, 100 mA/cm2 in OER. Furthermore, this electrocatalyst shows high stability and negligible degradation in overpotential for HER and OER under long term stability tests in alkaline media. The notable function of As-fabricated Ni–Mo–Fe is due to the synergism effect between Ni, Mo, and Fe element and binder-free structure. Owing to the high-performance and high-stability of Ni–Mo–Fe electrocatalyst under Hydrogen and Oxygen evolution reactions is a candidate for industrial uses in the alkaline electrolyzer.

Producing green hydrogen by electrolysis is a goal at European and international level. But hydrogen production by water electrolysis is currently more expensive respect the steam methane reforming. Based on our previous studies, extra revenues in order to reduce the production costs could be obtained by considering the co-produced oxygen. Starting from the point of view of a hypothetical enterprise that needs gaseous oxygen for its activity, in order to meet a variety of requests or applications, a financial evaluation of different sizes of RES-based electrolytic plant (100 kW - 10 MW) is proposed. The most relevant result that emerged from our research is the economic sustainability of the investment if the market price of oxygen is at least 3 EUR/kg. In this case, the self-production of oxygen results to be convenient, whatever the size of the electrolyser. This is valid for both the two scenarios assumed, with the exceptions of the smaller electrolyser sizes (

Designing porous active materials and enhancing their contact with conductive substrates is an effective strategy to improve electrolytic water splitting performance of noble metal-free catalysts. Herein, a facile nanostructured electrode, composed of porous Co2P films coated on carbon fiber (CF@P–Co2P), is designed and prepared. The unique three-dimensional interconnected pore structure of Co2P and the close contact between porous Co2P and CF not only increase specific surface areas to expose abundant catalytic sites but also stimulate the transport of electron, mass and gaseous products in catalytic process. Benefit from the reasonable electrode structure, the self-supported CF@P–Co2P electrodes present perfect performance with only needing overpotentials of 107.7/175.5/141.8 mV for hydrogen evolution reaction (HER) in acidic/neutral/alkaline solution and 269.4 mV for oxygen evolution reaction (OER) in alkaline solution to get current density of 20 mA cm−2. In addition, alkaline electrolyzer equipped with CF@P–Co2P bifunctional electrodes only needs a cell voltage of 1.657 V to get water-splitting current density of 20 mA cm−2. Even better, the electrolyzer can continuously electrolyze over 50 h with negligibly decreasing current density and the Faraday efficiency is close to 100% toward both HER and OER.

Water electrolysis is virtually the most renewable and sustainable hydrogen production method. The main challenge in the development of alkaline water electrolysis systems is finding low cost electrode materials with efficient and durable performance. Palladium-nickel alloys are attractive for hydrogen evolution reaction (HER) due to their good catalytic activity, lower cost, and chemical and mechanical stability. On the other hand, composites of metal oxides and carbons have high potential as supports and co-catalysts in electrolysis processes due to their electronic properties and bifunctional mechanisms. In this work, three different composite supports, SnO2-KB600, SnO2-KB300, and SnO2-graphene, are synthesised and decorated with PdNi nanoparticles (NPs). XRD and ICP-OES are used to characterise the produced electrocatalysts. Their activity for HER and stability in 8 M KOH is investigated using linear scan voltammetry and chronoamperometry at temperatures from 25 to 85 °C. Charge transfer coefficients, Tafel slopes, exchange current densities, and activation energies are calculated to assess the electrocatalysts’ efficiency for HER. The results are compared with those obtained with Vulcan-supported PdNi NPs electrocatalyst. Overall, they demonstrate that the three studied SnO2-carbon composites are promising supports for metal NPs, being beneficial for the lifetime of the PdNi-based electrocatalysts, and increasing their electroactivity performance in alkaline HER applications.

Development of Fe-Ni-based electrocatalysts with high efficiency and stability remains a foremost challenge in the research for oxygen evolution reaction (OER) under high-current–density. Herein, a fast reduction strategy is developed for synthesis of strongly coupled crystalline α-Ni(OH)2 with amorphous reduced FeOOH (r-FeOOH) heterostructure grown on Ni foam (r-FeOOH/α-Ni(OH)2/NF). The obtained r-FeOOH/α-Ni(OH)2 with particle sizes around ~ 10 nm is coated orderly on the 3D NF surface in this hybrid. Benefitting from the strong coupling effects between r-FeOOH and α-Ni(OH)2, low potentials of 1.62 and 1.66 V at ultra-high current densities of 1,000 and 1,500 mA cm−2, as well as a robust stability over 10 h at 1,500 mA cm−2 in alkaline electrolyte are achieved in 3D r-FeOOH/α-Ni(OH)2/NF. Such a high OER performance is almost the best among all previously reported Fe-Ni-based OER electrocatalysts. Experimental results revealed that the NiOOH species is the real OER active phase in the 3D r-FeOOH/α-Ni(OH)2/NF. Further, bifunctional 3D r-FeOOH/α-Ni(OH)2 in alkaline electrolyzer delivers low cell voltages of 2.32 and 2.78 V to attain 500 and 1,000 mA cm−2 toward the overall-water-splitting, surpassing the benchmark Pt/C-Ir/C/NF system.

Design of the high-efficiency multifunctional electrocatalysts is highly demanded for many electrochemical energy devices, such as water electrolyzers, metal-air batteries and fuel cells. Here, Rh–Sn bimetallic nanocrystal catalysts supported on single-walled carbon nanotubes (RhxSny/SWNTs) are reported, and the Rh1Sn2/SWNTs electrocatalyst show best multifunctional electrocatalytic performance. Strong electron synergy between Rh and Sn is the source of high catalytic activity.Sn transfers electrons from Rh to Sn, forming a unique positive environment around Rh, which effectively optimizes the binding energy of the reaction intermediate and further improves the catalytic activity. An alkaline electrolyzer using Rh1Sn2/SWNTs catalyst demands an ultra-low cell voltage of 1.56 V at the current density of 10 mA cm−2 for overall water splitting. In addition, Rh1Sn2/SWNTs demonstrates good oxygen reduction reaction (ORR) performance, making it an excellent catalyst for long-life Zn-air batteries. This work provides a facile strategy for the development of multifunctional electrocatalysts for the highly demanded electrochemical energy technologies.

Developing earth-abundant and highly active bifunctional electrocatalysts are critical to advance sustainable hydrogen production via alkaline water electrolysis but still challenging. Herein, heterojunction hybrid of ultrathin molybdenum disulfide (MoS2) nanosheets and non-stoichiometric nickel sulfide (Ni0.96S) is in situ prepared via a facile one-step hydrothermal strategy, followed by annealing at 400 °C for 1 h. Microstructural analysis shows that the hybrid is composed of intimate heterojunction interfaces between Ni0.96S and MoS2 with exposed active edges provided by ultrathin MoS2 nanosheets and rich defects provided by non-stoichiometric Ni0.96S nanocrystals. As expected, it is evaluated as bifunctional electrocatalysts to produce both hydrogen and oxygen via water electrolysis with a hydrogen evolution reaction (HER) overpotential of 104 mV at 10 mA cm−2 and an oxygen evolution reaction (OER) overpotential of 266 mV at 20 mA cm−2 under alkaline conditions, outperforming most current noble-metal-free electrocatalysts. This work provides a simple strategy toward the rational design of novel heterojunction electrocatalysts which would be a promising candidate for electrochemical overall water splitting.

The design and fabrication of highly active, robust and cost-efficient electrocatalysts for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is of great significance towards overall water splitting, but remains challenging as well. Herein, we report, for the first time, heterostructural Co/CeO2/Co2P/CoP@NC dodecahedrons as bifunctional electrocatalyst, in which abundant interfaces are formed between different components. Typical ZIF-67 (ZIF = zeolitic imidazolate framework) dodecahedrons with pre-inserted CeO2 nanowires were selected as precursors to synthesize Co/CeO2/Co2P/CoP@NC via a direct carbonization process followed by phosphidation, simultaneously generating the strong coupled heterojunction interfaces through interactions between CeO2 and CoxP species. Abundant porous structure leads to the exposure of more active sites and the carbon encapsulation of nanodomains sustains the high robustness and conductivity and the synergistic effect between the multi-components heterostructure. Benefiting from the above collective advantages, the Co/CeO2/Co2P/CoP@NC electrocatalysts exhibit small overpotentials of 307 and 195 mV to derive 10 mA cm−2 for OER and HER, respectively. Furthermore, an alkaline electrolyzer assembled by using Co/CeO2/Co2P/CoP@NC as both cathode and anode can achieve a current density of 10 mA cm−2 at a low voltage of 1.76 V and work continuously for over 15 h. This work would provide a rational protocol for fabrication multi-phase interface enriched electrocatalysts toward highly efficient energy conversion.

Hydrogen, as the one of clean energy source, has the advantages of high energy density and carbon-free emission. Water electrolysis is one of the most promising ways to generate hydrogen, but the rather high energy required seriously hinders its widespread applications yet. In this study, we report an alkaline electrolyzer to implement energy-saving H2 generation by coupling cathodic hydrogen evolution reaction (HER) with anodic glucose oxidation reaction (GOR) other than oxygen evolution reaction, in which nickel-doped MoS2 nanoparticles (Ni–MoS2 NPs) has been developed as bifunctional electrocatalyst for HER and GOR. The electrolyzer only requires a cell voltage of 1.67 V to reach an electrolysis current density of 10 mA cm−2, about 270 mV lower than the corresponding value in the traditional electrolyzer. Electrolytic H2 generation with the assistance of biomass derived materials may open a new way for the future sustainable development.

To promote Ni electrode performance during water splitting, a novel coating process, High pressure cold spray, is applied to prepare electrodes from blended Ni + Al powder. By controlling Al fraction, electrodes are obtained with varied microstructure. SEM and EDX are implemented to check the micromorphology of electrodes. Linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) are performed to estimate the effect of Al addition on electrode performance. Resultantly, significant improvement of electrode performance is achieved by increasing the fraction of Al from 10 vol% to 20 vol%. The obtained coatings are found with numerous pores owing to the removal of Al during the activation. By applying electrochemical test, the HER of all samples are dominated by Volmer step, and sample N20A is found with the highest active surface area. Thus, sample N20A exhibits the highest electro-catalytic activity to HER of alkaline water electrolysis.

A series of nanostructure alloys composed of Ni with a non-precious metal (M: Co, Fe, Mn, Cr, Cu and Zn) on the reduced graphene oxide as NiM/rGO are synthesized through a facile solvothermal procedure. Hydrogen evolution reaction (HER) activity of prepared electrocatalysts is evaluated by various electrochemical techniques and performance in alkaline anion exchange membrane (AAEM) electrolyzer. Among the prepared NiM/rGO series, NiCo/rGO electrocatalsyt with Ni/Co mol ratio of 3/1 exhibits the highest HER activity. The measured overpotential at 10 mA cm−2 for a low catalyst loading electrode of NiCo/rGO is as much as 137 mV less than Ni/rGO. The potential of electrolytic cell with Co3O4/rGO and NiCo/rGO electrodes as the anode and cathode, at 0.1 A cm−2 is 1.9 V, lower than that of cell with the same anode but Ni/rGO as the cathode (2.2 V). The differences in NiM/rGO series HER activities are interpreted by various theories such as hypo-hyper-d-electronic combinations, M-H band strength and metals locations in volcano plot. Density functional theory results are also applied along with the experimental data for the explanation of NiM/rGO series HER activity trend. The results confirm the competency of employed procedure for assembling nickel alloys electrodes for alkaline water electrolysis.

In response to the energy crisis, molybdenum-based catalyst has been proposed as a high-performance electrocatalytic material due to its low price and excellent HER performance. However, in contrast with its excellent HER performance, its poor OER performance often limits practical application as a high-performance overall water splitting catalyst. In this study, Prussian blue analogue (PBA) is grown in-situ on molybdenum-based nanosheet spheres by a simple and ingenious method and then subjected to phosphorization. The resulting composite catalyst exhibits highly efficient overall water splitting performance, overpotentials at current densities of 10 mA cm−2 and 100 mA cm−2 for the HER and OER are −61 mV and 268 mV, respectively. Moreover, an alkaline electrolyzer makes up by the catalyst both as positive and negative can reach a cell voltage 1.494 V at 10 mA cm−2 for the overall water splitting. This method has provided a new strategy to effective combine PBA and molybdenum-based catalyst.

Alkaline water electrolysis is one of the easiest methods used to produce hydrogen, offering the advantages of simplicity and low cost. The challenges for the widespread use of water electrolysis are to reduce energy consumption, cost and maintenance and to increase the reliability, durability and safety of the process. The alkaline electrolysis of water has been used for many years to obtain H2 and O2; however, less expensive, more active, endurable and efficient electrodes must be designed. Stainless steel (SS) is considered one of the least expensive electrode materials for alkaline electrolysers, since it is relatively chemically stable and has a low overpotential. Nevertheless, SS anodes do not withstand high concentration alkaline solutions because they undergo a corrosion process. If the electrolyser operates at a voltage of up to 1.6 V, it can generate Fe3O4 and hazardous hexavalent chromium (Cr6+) at the anode. Hexavalent chromium is generated when the chromium-containing stainless steel electrodes undergo an electro-oxidation process. In this work, a low power alkaline electrolyser was designed. In the first step, six electrodes were manufactured of stainless steel. In the second step, a nickel layer with matte or opaque finish was deposited on the SS surfaces to improve their resistance to corrosion and wear. Then, the performance curves and the production of hexavalent chromium were determined. The performance curve after 70 h of operation with nickel-plated electrodes showed an overpotential of 0.5 V at 10 A compared with stainless steel electrodes. Cr6+ was detected in the electrolyte and bubbler water of the system using SS electrodes at values that exceeded the standard (>0.5–1 mg L−1). If the electrolyser used Ni-electrodeposited electrodes, Cr6+ was observed in quantities within the normal range (

Hydrogen is a promising sustainable energy to replace fossil fuels owning to its high specific energy and environmental friendliness. Alkaline water electrolysis has been considered as one of the most prospective technologies for large scale hydrogen production. To boost the sluggish kinetics of hydrogen evolution reaction (HER) in alkaline media, abundant materials have been designed and fabricated. Herein, we summarize the key achievements in the development of layered transition-metal hydroxides [TM(OH)x] for efficient alkaline HER. Based on the structure of TM(OH)x, the mechanism of synergistic effect between TM(OH)x and HER active materials is illuminated firstly. Then, recent progress of TM(OH)x-based HER catalysts to optimize the synergistic effect are categorized as TM(OH)x and active materials, including species, structure, morphology and interaction relationship. Furthermore, TM(OH)x-based overall water splitting electrocatalysts and electrodes are summarized in the design principles for high activity and stability. Finally, some of key challenges for further developments and applications of hydrogen production are proposed.

A commercial FAA3-50 membrane was investigated as a solid polymer electrolyte in an alkaline water electrolysis. An improved chemical treatment based on alkaline KOH solution was carried out. A limited degradation of the functional groups was observed allowing to maintain a good anion conductivity approaching 55 mS cm-1 at 100 °C. Thermal stability up to 200 °C was assessed by thermal analysis. A specific membrane-electrodes assembly based on FAA3-50 anionic membrane and NiMn2O4 anode catalyst was developed and investigated in a single cell for water electrolysis at a moderate temperature (50 °C). Performance stability was assessed by a potential cycling-based durability test for 1000 h by varying the cell potential between 1 and 1.8 V for the FAA3-50 and NiMn2O4 based-MEA. According to this evaluation, both the FAA3-50 membrane and the NiMn2O4 catalyst appear sufficiently stable for electrolysis operation under mild operating temperatures.

Highly efficient and durable non-noble metal-based hydrogen evolution electrocatalysts are critical to advance the production of hydrogen energy via alkaline water electrolysis. Herein, we prepared a novel TiO2@WS2 hybrid via a facile and scalable two-step hydrothermal strategy combined with selective etching. Benefited from acid-etched TiO2 nanobelts with rough surface as substrate, ultrathin WS2 nanosheets nucleated and vertically grew into few layers in the confined configuration with more exposed active edges. Furthermore, the partial incorporation of oxygen in WS2 inherited from the remaining O–W bonds of tungsten precursor enhanced the electrical conductivity of the hybrid. Therefore, TiO2@WS2 hybrid was proved to be efficient and durable electrocatalyst for hydrogen evolution in alkaline medium. Upon optimal conditions, the hybrid only required a small onset overpotential of 95 mV and a low overpotential of 142 mV at 10 mA cm−2, superior to pristine WS2 and TiO2. In addition, better cycling stability during the alkaline HER process was also obtained, indicating its capability in future practical application. The synthesis strategy presents a cost-effective approach to produce efficient WS2-based HER electrocatalyst for electrochemical water splitting.

The development of efficient and non-noble catalyst is of great significance to hydrogen generation techniques. Three surface-oxidized cobalt borides of Co–B–O@CoxB (x = 0.5, 1 and 2) have been synthesized that can functionalize as active catalysts in both alkaline water electrolysis and the hydrolysis of sodium borohydride (NaBH4) solution. It is discovered that oxidation layer and low boron content favor the oxygen evolution reaction (OER) activity of Co–B–O@CoxB in alkaline water electrolysis. And surface-oxidized cobalt boride with low boron content is more active toward hydrolysis of NaBH4 solution. An alkaline electrolyzer fabricated using the optimized electrodes of Co–B–O@CoB2/Ni as cathode and Co–B–O@Co2B/Ni as anode can deliver current density of 10 mA cm−2 at 1.54 V for overall water splitting with satisfactory stability. Meanwhile, Co–B–O@Co2B affords the highest hydrogen generation rate of 3.85 L min−1 g−1 for hydrolysis of NaBH4 at 25 °C.

Highly hydrophilic separators for alkaline water electrolysis were prepared by plasma-initiated grafting of acrylic acid on porous polypropylene (PP) membranes. The membranes were activated in a low-pressure radio-frequency discharge in oxygen and subsequently graft polymerization of acrylic acid was performed in aqueous solution. The membranes were characterized by gravimetric grafting degree (GD), SEM, FTIR, critical wetting surface tension (CWST) test, mechanical strength, and electrolytic conductivity. Moreover, the membranes were applied as separators in alkaline electrolysis cell, and content of hydrogen in the produced oxygen was measured to determine membrane permeability to hydrogen dissolved in the electrolyte. It was observed that increasing GD improves performance of membranes as separators in alkaline electrolysis, although the particular effects on the electrolytic conductivity and hydrogen permeability strongly depend on structure the of initial PP substrate. Ageing test conducted in 30 wt% KOH at 60 °C revealed that although considerable degrafting took place at beginning of the test, the remaining polyacrylic acid provided highly hydrophilic character to membrane for 7000 h of the test.

Contrary to traditional electrolysers which operate continuously at their nominal load, future alkaline electrolysers need to be able to operate over a wide load range due to the variability of renewable electricity supply. We have investigated how the residual ripples from thyristor-based power supplies are influenced by the operating load of the system, and how these ripples affect the efficiency of alkaline electrolysers. For this, a simulation tool was developed which combines a six-pulse bridge thyristor rectifier model with closed-loop current control and semi-empirical electrolysis models. The electrolysis models can simulate the potential response to both direct and high amplitude alternating currents for lab-scale and industrial electrolysers. The electrolysis model of the lab-scale electrolyser was validated with experiments with a square wave current input. The models show that without filters the ripples result in a total system efficiency loss of 1.2–2.5% at full load and of 5.6–10.6% at a part load of 20% depending on the type of electrolyser. The implementation of an optimized L-filter suppresses residual ripples and reduces the efficiency losses to 0.5%–0.8% at full load and to 0.8–1.2% at the minimum load.

How to reduce the bubble coverage of the electrode is one of the key issues in water electrolysis which is related to the reduction of energy consumption. In this work, the magnetized nickel electrode with 100 μm diameter is used as working electrode in hydrogen evolution reaction (HER) during alkaline water electrolysis (1 mol/L, KOH). According to the experimental observation, both of the bubble's diameter and number have decreased at the magnetized surface compared with the unmagnetized one. The B–H loop measurement shows that the residual magnetic field intensity (Br) of the magnetized nickel electrode is up to 0.03 T. It can be found from the numerical simulation result that the residual magnetic field exactly right brings the Lorentz force and magnetohydrodynamic (MHD) convection near the electrode surface, which play the role of helping hydrogen bubble release from the electrode, even if the external magnetic field is absent in experiment. The new finding may develop a new way of utilizing magnetic field in water electrolysis for gas product elimination and energy conservation.

The purpose of this research paper is to describe the characteristics and electrochemical performances of a pressurized alkaline water electrolysis short stack (5-cells, 0.5 kW) operated at 80 °C, from atmospheric pressure up to 100 bars. Expanded grids of metallic nickel covered with specific porous catalytic structures have been used as working electrodes. A polysulfone-based diaphragm with a high ionic conductivity has been specifically designed for operation in pressurized alkaline water electrolysis cells. I–V polarization curves have been recorded at current densities up to 1000 mA/cm2, at temperatures up to 80 °C and under pressures up to 100 bars. The water electrolysis efficiency of this short-stack has been determined. A specific energy consumption of ca. 4.4–4.5 kWh/Nm3 has been obtained in the high current density range. Durability tests have been performed on the short stack over 1000 h. A limited degradation rate

In the present work, a novel electrolyzer concept for alkaline water electrolysis (AEL) with a gas diffusion electrode (GDE) as anode, a conventional immersed porous cathode and a state-of-the-art Zirfon™ separator is presented and compared with a conventional electrolyzer setup. Due to the utilization of a GDE in this configuration, the electrolyte is only circulated through the cathode compartment which greatly simplifies the process. The influence of the catalyst composition and the enhanced electrode surface owing to the three-dimensional porous structure of the GDE are characterized and investigated regarding the electrode performance. Furthermore, process parameters like contact pressure and differential pressure are examined and optimized. The novel process concept with a GDE as anode reveals a similar cell potential compared to a classical electrolysis cell with a Ni/Fe-coated nickel foam anode up to 400 mA cm−2 at 353 K and 32.5 wt% KOH and also exhibits relatively good electrochemical stability over time.

Developing non-precious metal bifunctional electrocatalysts for effective overall water splitting is promising to realize high-efficient renewable energy production. In this work, ternary (NixFey)2P nanoplates arrays on 3D self-supported nickel foams were prepared through a simple coelectrodeposition followed by phosphorization. The synthesized (NixFey)2P nanoplates arrays showed remarkable bifunctional electrocatalytic performances in the KOH electrolyte, with the Tafel slopes of 57.8 mV·dec−1 and 53.6 mV·dec−1, and low overpotentials of 115 mV and 182 mV to reach a current density of 10 mA cm−2 for hydrogen evolution reaction and oxygen evolution reaction, respectively. In addition, the optimized (Ni0.66Fe0.33)2P anode and cathode demonstrated outstanding ability for overall water splitting in the two-electrode alkaline electrolyzer, generating a current density of 10 mA cm−2 at the applied cell voltage of 1.61 V. This benefited by the active surface to expedite reactants absorption, faster electron transport in the solid-liquid interface and reduced charge-transfer resistance due to the bimetallic synergistic effect. This delicate design of bifunctional transition metal phosphides provides a potential approach to realize water reuse and energy regeneration by the full water splitting.

Highly active and durable bifunctional electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) play a pivotal role in overall water splitting. Herein, we demonstrate the construction of interface-strengthened CoP nanosheet array with Co2P nanoparticles as such an electrocatalyst through a facile hydrothermal reaction and the subsequent phosphorization process. The two-dimensional (2D) nanosheets with thickness of ∼55 nm expose a great number of active sites. The surface chemical state indicates that the strongly coupled CoP/Co2P electrocatalysts can adsorb or generate more targeted intermediates (e.g. OH− or OOH*) for both HER/OER. As a result, the CoP/Co2P electrocatalysts exhibit small overpotentials of 68 and 256 mV to drive 10 mA cm−2 for HER and OER, respectively, outperforming most of the recently reported Co-based electrocatalysts. Furthermore, an alkaline electrolyzer assembled by using CoP/Co2P as both cathode and anode can achieve a current density of 10 mA cm−2 at a low voltage of 1.57 V and work continuously for over 58 h. This work provides a feasible structural design for transition metal phosphides electrocatalysts with efficient and stable overall water splitting.

Low-cost and highly efficient electrocatalysts are critical to advance the hydrogen production industry via water electrolysis in alkaline media. Molybdenum disulfide (MoS2) nanosheets as earth-abundant electrocatalyst became a star material due to its unique graphene-like layered structure favoring electron transfer for hydrogen evolution catalysis. However, its electrocatalytic activities greatly hindered by the poor conductivity and the fewer exposure of catalytically active edged sites. Herein, hierarchical ZnS@C@MoS2 core-shell nanostructures were designed by using a bottom-up strategy combined with a simple selective etching. The coexistence of semiconductor ZnS and defective porous carbonaceous shell as the core not only enhanced the electrical conductivity, but also separated and loaded the exfoliated MoS2 nanosheets which effectively proliferated the exposed catalytically active edge sites (Mo and S sites). Upon optimal conditions, hierarchical ZnS@C@MoS2 core-shell nanostructures gave an overpotential of 118 mV at 10 mA/cm2 with a Tafel slope value of 55.4 mV/dec and better cycling stability after 500 cycles during the alkaline HER process, superior to pristine MoS2.