Alkaline HER Research Articles

Unveiling the surface hydroxylation and selenite modification of in situ generated nickel for promoting the hydrogen evolution reaction

An electrochemical activation process for Ni–Se–O–H crystal has been found to produce a Ni/Ni(OH)2 heterostructure decorated with a small amount of SeO32−, exhibiting superior alkaline HER activity and durability.

Ruthenium-infused nickel sulphide propelling hydrogen generation via synergistic water dissociation and Volmer step promotion.

The inclusion of ruthenium (Ru) to decorate nickel sulphide (Ru@NiS/Ni foam) resulted in a highly efficient electrocatalyst for the alkaline HER by enhancing water dissociation at the interface and reducing the energy barrier of the Volmer step. This strategic fusion significantly boosts the catalyst's performance in facilitating hydrogen production.

Electrochemically engineered domain: nickel–hydroxide/nickel nitride composite for alkaline HER electrocatalysis

The impact of in situ electrooxidation in a Ni3N electrocatalyst on its hydrogen evolution activity and electrochemical stability was investigated in alkaline media.

Ultrathin core–shell Au@RuNi nanowires for superior electrocatalytic hydrogen evolution

Ultrathin core–shell Au@RuNi nanowires have been synthesized via the epitaxial growth of uniform fcc-structured RuNi alloy layer on the surface of ultrathin Au NWs, demonstrating remarkable alkaline HER performance and outstanding stability.

A self-phosphorized carbon-based monolithic chainmail electrode for high-current-density and durable alkaline water splitting

A monolithic chainmail electrode is developed by embedding self-phosphorized carbon-coated Co2P nanoparticles within a porous carbon membrane, exhibiting excellent activity and durability for both alkaline HER and OER at high current densities.

Blocking Metal Nanocluster Growth through Ligand Coordination and Subsequent Polymerization: The Case of Ruthenium Nanoclusters as Robust Hydrogen Evolution Electrocatalysts.

Metal nanoclusters providing maximized atomic surface exposure offer outstanding hydrogen evolutionactivities but their stability is compromised as they are prone to grow and agglomerate. Herein, a possibility of blocking metal ion diffusion at the core of cluster growth and aggregation to produce highly active Ru nanoclusters supported on an N, S co-doped carbon matrix (Ru/NSC) is demonstrated. To stabilize the nanocluster dispersion, Ru species are initially coordinated through multiple Ru─N bonds with N-rich 4'-(4-aminophenyl)-2,2:6',2''-terpyridine (TPY-NH2) ligands that are subsequently polymerized using a Schiff base. After the pyrolysis of the hybrid composite, highly dispersed ultrafine Ru nanoclusters with an average size of 1.55nm are obtained. The optimized Ru/NSC displays minimal overpotentials and high turnover frequencies, as well as robust durability both in alkaline and acidic electrolytes. Besides, outstanding mass activities of 3.85Amg-1 Ru at 50mV, i.e., 16fold higher than 20wt.% Pt/C are reached. Density functional theory calculations rationalize the outstanding performance by revealing that the low d-band center of Ru/NSC allows the desorption of *H intermediates, thereby enhancing the alkaline HER activity. Overall, this work provides a feasible approach to engineering cost-effective and robust electrocatalysts based on carbon-supported transition metal nanoclusters for future energy technologies.

Activity and Stability of High-Surface-Area Nickel-Based Catalysts for the Alkaline Hydrogen Evolution Reaction Under Industrially Relevant Conditions

Alkaline water electrolysis (AWE) is at a stage of high technological maturity, and modern pressurized designs have mitigated the weaknesses of now-outdated atmospheric electrolyzers. For instance, pressurized systems now deliver high hydrogen outputs and can efficiently adjust to rapid changes in power loads. These improvements were, in part, made possible through significant reductions to internal ohmic losses coming from improved gas separators, shorter distances between electrodes, and increased operating temperatures.From an industrial point of view, the critical parameter to optimize is the full stack voltage. The stack voltage is tightly bound to the individual cell voltage and can thus be lowered by employing more efficient electrocatalysts. However, reducing the cell voltage by raising the temperature of operation—thereby improving both reaction kinetics and the conductivity of the KOH electrolyte—can be of similar or even higher importance (e.g., ca. 4.5 mV/˚C, Figure 1) [1]. Therefore, stability under operation at elevated temperatures must be considered when evaluating novel hydrogen and oxygen evolution reaction catalysts for AWE. Else, a novel catalyst risks becoming the bottleneck in the pursuit of higher operating temperatures, limiting its applicability for large-scale electrolysis [2]. Many assessments of novel AWE catalysts are still carried out at room temperature in diluted electrolytes and at low current densities [3]. In the harsher conditions of an electrolyzer, catalysts experience higher dissolution rates and are subject to higher mechanical stress from, for instance, the aggressive bubble evolution. This gap hinders the transfer of catalyst developments from academia to industrial research laboratories and, ultimately, to practical applications.In this work, we attempted to narrow that gap by conducting electrochemical measurements under more application-oriented conditions (i.e., 80-120 °C and 8-11 M KOH). We evaluated the HER activity and the robustness of high-surface-area electrodes produced through controlled leaching of bimetallic nickel-based alloys. A parametric study of the overpotential and robustness as a function of electrochemically active surface area was enabled by varying the synthesis parameters of a Raney-type preparation. Direct coupling to renewable energy sources requires the electrolyzer to frequently and rapidly shut off and on. This intermittent operation induces additional stress on the electrodes as the catalysts transition through different redox states, which is often overlooked in AWE electrocatalyst research. To track redox, compositional, and morphological changes, we used ex situ X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy. At the same time, the dissolution and redeposition of ionic species were monitored using inductively coupled plasma mass spectrometry. By matching the characterization data with the evolution of the overpotential until complete deactivation, we gained an understanding of the degradation mechanisms in high-surface-area nickel-based electrodes. These results shed light on the challenges and prospects of this promising class of electrocatalysts for alkaline HER under industrial conditions.Acknowledgement:This work is partly funded by the Innovation Fund Denmark (IFD) under File No. 1044-00162B.

Molten-salt mediated homogenous Mo-doped CoP for enhanced anion exchange membrane water electrolysis

Membrane electrode water electrolyzer is the most promising industrial large-scale hydrogen production energy equipment, which is used to promote the conversion of old and new energy and the realization of low carbon goals. However, there is still a lack of efficient and robust alkaline HER electrocatalysts for tightly assembled, bubble-dense and high-current density anion exchange membrane (AEM) electrolyzers. Herein, we developed an effective salt-melting homogenization synthesis strategy for the preparation of cobalt foam (CF)-supported uniform Mo-doped chrysanthemum-like CoP electrodes (M-Mo-CoP/CF). Alkaline AEM electrolytic cells driven by M-Mo-CoP/CF have ideal water splitting properties (1000 mA/cm−2 at 1.80 Vcell) and high current tolerance. In addition, detailed experimental and theoretical calculation results show that the M-Mo-CoP/CF prepared by this strategy has excellent metal-like conductivity and optimized H* adsorption free energy. In conclusion, this work strongly confirms the application potential of salt-melting assisted homogeneous doping strategy in the preparation of high-performance hydrogen evolution catalytic materials.

Interstitial boron-doped Ni4Mo nanoarray catalyst boosts stable hydrogen evolution reaction

Designing a strategy to make the intrinsic activity of NiMo-based catalysts while being able to achieve long-term stable hydrogen production at ampere-level current densities is important for their development in AEM electrolyzers. Herein, interstitial boron atom-doped Ni4Mo is designed to obtain the catalyst with excellent alkaline HER properties. B4.7-Ni4Mo/NF exhibits a small overpotential of 135.2 mV at 1000 mA cm−2 and a high turnover frequency (TOF) value of 6.14 s−1 at 100 mV in alkaline HER, which very close to that of Pt/C (6.86 s−1). The catalyst requires only 1.73 V to achieve a current density of 1.0 A cm−2 in AEM electrolyzer at 60 °C and is stable for 800 h. Experimental and theoretical calculations reveal that interstitial boron doping not only optimizes the electronic structure to optimize H* adsorption and promote the water dissociation but also inhibits the dissolution of Mo by forming boron-metal bond.

Fast and in situ releasing of active species confined in two-dimensional layers for superior overall electrochemical water splitting

Realizing bifunctional catalysts for the HER and OER in a unified environment bears the promising goal of reducing the cost and complexity of electrolyzer devices. Dual atoms configuration intrinsically active to hydrogen/oxygen evolution is a feasible way to achieve that target. In this work, Co, Ir active species were selected to be inserted into the interlayer space of layered V2O5, forming a two-dimensional confined structure that significantly improves the atomic efficiency. During the electrochemical water splitting in alkaline medium, the V-O framework that encapsulated the active species in layered V2O5 was collapsed and disrupted due to its acidic nature. The HER/OER active Co, Ir were readily released and exposed to the reactant. In situ structure evolution was monitored and elaborately analyzed by operando Raman, TEM and XPS characterizations. Further, DFT calculation disclosed that the Co atom was covalently stabilized by the nearest oxygen that leads to electron-depleted Co. The stabilization of Ir was achieved by forming both a covalent bond with oxygen and a metallic bond with sublayer V. This, however, resulted in a changed electron distribution in Ir, leading to better binding with oxygenate intermediates and improved OER activity. As for the alkaline HER, the potential determining step (PDS) of IrCo@V2O5 is predominately determined as the OH desorption, regardless of the H evolution pathway of Volmer-Tafel or Volmer-Heyrovsky. The bifunctional IrCo@V2O5 acted like a “Trojan Horse” and achieved an ultrahigh mass activity of 848 A g-1Co,Ir for the OER and 461 A g-1Co,Ir for the HER. This work provides new insight into creating highly effective electrocatalysts via the in situ releasing methodology.

Ce3+-Induced metal vacancies engineering of NiSe2 with needle-like structure for alkaline hydrogen evolution

The weak water dissociation ability and hydrogen adsorption of bulk NiSe2 inhibits their HER performance under alkaline conditions. Most work focused on tuning the electron structure and activity of Se sites, the role of Ni atoms in HER process is commonly ignored. In this work, we developed a Ce3+ induced strategy to introduce Ni cationic vacancies (VNi) on NiSe2 and simultaneously regulate its morphology to form hierarchical needle-like structure. The unique nanostructure provides more exposed active sites and facilitates mass and charge transfer. After the introduction of VNi, Ni2+ (t2g6eg2) in NiSe2 can be partially converted to Ni3+ (t2g6eg1) with optimal eg1 electron configuration, which improves the water dissociation barrier and facilitate the fast electron transfer during HER process. The Ni3+ also increases the charge density around the Se atom to improve H adsorption energy on Se sites, thus improving the alkaline HER activity.

Significantly Enhanced Energy-Saving H2 Production Coupled with Urea Oxidation by Low- and Non-Pt Anchored on NiS-Based Conductive Nanofibers.

Rational designing electrocatalysts is of great significance for realizing high-efficiency H2 production in the water splitting process. Generally, reducing the usage of precious metals and developing low-potential nucleophiles oxidation reaction to replace anodic oxygen evolution reaction (OER) are efficient strategies to promote H2 generation. Here, NiS-coated nickel-carbon nanofibers (NiS@Ni-CNFs) are prepared for low-content Pt deposition (Pt-NiS@Ni-CNFs) to attain the alkaline HER catalyst. Due to the reconfiguration of NiS phase and synergistic effect between Pt and nickel sulfides, the Pt-NiS@Ni-CNFs catalyst shows a high mass activity of 2.74-fold of benchmark Pt/C sample. In addition, the NiS@Ni-CNFs catalyst performs a superior urea oxidation reaction (UOR) activity with the potential of 1.366V versus reversible hydrogen electrode (RHE) at 10mA cm-2 , which demonstrates the great potential in the replacement of OER. Thus, a urea-assisted water splitting electrolyzer of Pt-NiS@Ni-CNFs (cathode)||NiS@Ni-CNFs (anode) is constructed to exhibit small voltages of 1.44 and 1.65V to reach 10 and 100mA cm-2 , which is much lower than its overall water splitting process, and presents a 6.5-fold hydrogen production rate enhancement. This work offers great opportunity to design new catalysts toward urea-assisted water splitting with significantly promoted hydrogen productivity and reduced energy consumption.

Cobalt Nanocube Electrocatalysts Comprising Sub-Nanometer Oxide-Phosphide Hetero-Interface for Highly Efficient Water Electrolysis

Hydrogen production from electrochemical water splitting is known as an eco-friendly energy conversion process, which has drawn attention for a few decades. Typically, the water electrolysis is divided into PEM and AEM electrolysis where acidic and alkaline electrolyte are used, respectively. In case of PEM water electrolysis, the acidic water splitting reveals critical drawback of poor counterpart OER efficiency. For this reason, AEM water electrolysis in alkaline condition has huge advantages in high efficiency of full cell owing to the lower overpotential for oxygen evolving anode, and accessibility of using transition metal-based materials for each anode and cathode. However, alkaline HER is 2-3 orders of magnitude slower than acidic HER due to the lack of proton nearby the surface.Thus, it is necessary to find highly efficient alkaline HER electrocatalysts for the future commercialization of AEM electrolyzer.Herein, by phosphidation of 10 nm Co3O4 nanocube (CO) loaded on nickel foam (NF) substrate, unique heterojunction interface of CoP and Co3O4 within 10 nanometers were realized, which converged both the excellent electrocatalytic performance of CoP and high stability of Co3O4. Unique sub-nanometer oxide-phosphide heterojunction was discovered via inverse fast fourier transform (IFFT), which confirmed the phase transition alongside the particle, while maintaining its nanocube morphology. X-ray photoelectron spectroscopy (XPS) was brought out to further confirm the co-exsistence of cobalt oxide and phosphide materials with respect to the chemical structure.Experimental investigations revealed the excellent water splitting activity of the hybrid-phosphidated Co3O4 (H-PCO) catalyst, only requiring an overpotential of 162, 171 and 395 mV to reach 100 mA cm-2, each for acidic HER, alkaline HER, and alkaline OER, respectively. H-PCO/PNF showed excellent stability over 20 hours under various working conditions, which was superior to that of conventionally fabricated CoP catalysts. Such outstanding alkaline water electrolysis performance and stability stems from the evolution of new active sites are expected at the interface because the interface has different chemical and electronic structures compared to bulk materials. In addition, the scalability of the electrocatalyst was demonstrated where a 34.56 cm-2 catalyst foam was uniformly fabricated As a result, all 15 points of as-prepared H-PCO electrode showed negligible distribution of the overpotentials at 100 mA cm-2, and the large unit-cell scale measurement showed remarkably improved HER performance compared to bare Ni foam. Applicability of the electrocatalyst was demonstrated by the construction of unbiased PV-EC water splitting system, where a high STH efficiency of 11.5% over 100 hours was achieved with the help of Si solar cells.In conclusion, we developed water splitting electrode containing hybrid interface of cobalt oxide and phosphide in sub-10 nm cube structure. The hybrid structure of H-PCO comprising sub-nanometer hetero-interface between cobalt oxide and phosphide evoluted synergetic effect that greatly improved either HER performance, pH indenpendency, or long-term stability, which outperformed single CO and CoP electrodes. We believe our rationally designed hetero-interfacial electrocatalyst can outperform previously established electrocatalysts via its high efficiency, versatility, and scalability.

Amorphous nickel oxide electrodes with high-current-density electrocatalytic performance for hydrogen evolution

The widespread use of unsustainable fossil fuels has caused serious damage to the ecological environment. The development of sustainable clean energy is urgent. Producing hydrogen by water electrolysis can effectively solve fossil energy shortage and environmental pollution issues. The preparation of inexpensive, efficient, and stable electrocatalytic hydrogen evolution electrode materials is the key to achieving the application of electrocatalytic hydrogen evolution. Herein, the amorphous nickel oxide/nickel foam (NiOx/NF) hydrogen evolution electrode was prepared by a simple one-step hydrothermal method with hydrogen peroxide as an oxidant. The amorphous leaf array structure of NiOx/NF electrode can enrich the surface composition and expose more catalytic active sites. Further, hydrogen peroxide etching can optimize the electronic structure of NiOx, and accelerate the electron migration. Thus, NiOx/NF displays excellent alkaline hydrogen evolution activity with the low over potentials of 70, 318 and 361 mV at 10, 500 and 1000 mA cm−2, respectively, leading to high performance in alkaline HER than those of reported Ni-based electrodes and even surpasses commercial Pt/C at high current density. Our work provides a new approach for developing inexpensive electrocatalysts with excellent alkaline hydrogen evolution activity and stability.

Supported Ruthenium Single-Atom and Clustered Catalysts Outperform Benchmark Pt for Alkaline Hydrogen Evolution.

Guaranteeing satisfactory catalytic behavior while ensuring high metal utilization has become the problem that needs to be addressed when designing noble metal based catalysts for electrochemical reactions. Here, the well-dispersed ruthenium (Ru) based clusters with adjacent Ru single atoms (SAs) on layered sodium cobalt oxide (Ru/NC) is demonstrated to be a superb electrocatalyst for alkaline HER. The Ru/NC catalyst manifests an activity increase by a factor of two relative to the commercial Pt/C. Operando characterizations in conjunction with density functional theory (DFT) simulations uncover the origin of the superior activity and establish a structure-performance relationship, that is, under HER condition, the real active species are Ru SAs and metallic Ru clusters supported on the NC substrate. The excellent alkaline HER activity of the Ru/NC catalyst can be understood by a spatially decoupled water dissociation and hydrogen desorption mechanism, where the NC substrate accelerates the water dissociation rate, and the generated H intermediates would then migrate to the Ru SAs or clusters and recombine to have H2 evolution. More importantly, comparing the two forms of Ru sites, it is the Ru cluster that dominates the HER activity. This article is protected by copyright. All rights reserved.

Amorphous MoOx with High Oxophilicity Interfaced with PtMo Alloy Nanoparticles Boosts Anti-CO Hydrogen Electrocatalysis.

Advancing electrocatalysts for alkaline hydrogen oxidation/evolution reaction (HOR/HER) is essential for anion exchange membrane-based devices. The state-of-the-art Pt-based electrocatalysts for alkaline HOR suffer from low intrinsic activities and severe CO poisoning due to the challenge of simultaneously optimizing surface adsorption toward different adsorbates. Herein, this challenge is overcome by tuning an atomic MoOx layer with high oxophilicity onto PtMo nanoparticles (NPs) with optimized Had , OHad , and COad adsorption for boosting anti-CO-poisoning hydrogen-cycle electrocatalysis in alkaline media. For alkaline HOR, this catalyst exhibits high kinetics and an exchange current density of 3.19mAµgPt -1 at 50mV versus reversible hydrogen electrode and 0.83mAcmPt -2 , 10.3- and 3.8-fold higher than those of commercial Pt/C, respectively. For alkaline HER, it achieves an unprecedented overpotential of 37mV at 10mAcm-2 . Experimental and theoretical studies show that the orchestrated electronic and oxophilic regulation of the PtMo/MoOx interface NPs simultaneously optimizes Had and OHad adsorption for boosting alkaline hydrogen electrocatalysis, whereas reactive oxygen from the amorphous MoOx atomic layer lowers the CO oxidation reaction barrier, leading to superior anti-poisoning ability even at 100ppm CO.

Enhanced CO Functionality on Carbon Papers Ensures Lowering Nucleation Delay of ALD for Ru towards Unprecedented Alkaline HER Activity.

The success in lowering the nucleation delay for Atomic Layer Deposition (ALD) of Ru on carbon surfaces is mitigated by constructive pretreatments resulting enhancement of CO functionality. Treatment of the carbon papers (CP) allowed Ru species deposition for minimum number of ALD cycles (25 cycles) with good conformality. The development of electrocatalysts from single atoms to nanoparticles (NPs) on conductive supports with low metal loadings, thus improving performance, is essential in electrocatalysis. For alkaline hydrogen evolution reaction, ALD decorated CPs with Ru exhibit low onset potentials of ≈4.7mV versus reversable hydrogen electrode (RHE) (at 10mAcm-2 ) and a high turnover frequency of 1.92 H2 s-1 at 30mV versus RHE. The Ru decorated CPs show comparable to higher catalytic activity than of Platinum (Pt) decorated CP also developed by ALD. The current representation of unfamiliar catalytic activities of Ru active centers developed by ALD, pave a bright and sustainable path for energy conversion reactions.

Breaking Volmer step limitation on NiO/PtNi heterojunctions via ATO-induced charge injection and interfacial modulation

Driven by the persisting poor understanding of the slow kinetics of hydrogen evolution (HER) on Pt-based alloys in alkaline media, it is necessary and challenging to explore the clear relationship between heterogeneous interface components and activity. Herein, we develop an exquisite triphasic interfacial structure featuring the Sb-doped SnO2 (ATO)-supported NiO/PtNi heterojunction with controllable heterogeneous interface. The strong electron-donating effect of the ATO carrier and the electronic coupling between PtNi and appropriate NiO components effectively optimize the adsorption of H2O* and dissociation of H*, especially the adsorption-dissociation of H2O on the NiO site (Volmer step), thus accelerating the overall alkaline HER kinetics. The resulting catalyst delivers high alkaline HER performances with a low Tafel slope (only 25.5 mV dec−1) and an ultra-long stability of 140 h operation. Our findings provide new insights into breaking the water dissociation limit of Pt-based catalysts by rational design of heterogeneous interfaces.

Bio-Inspired FeMo2S4 Microspheres as Bifunctional Electrocatalysts for Boosting Hydrogen Oxidation/Evolution Reactions in Alkaline Solution.

Developing robust and effectual nonprecious electrocatalysts for the bifunctional hydrogen oxidation and evolution reactions (HOR and HER) in alkaline electrolyte is of critical significance for the realization of future hydrogen economy but challenging. Herein, this work demonstrates a new routine for the preparation of bio-inspired FeMo2S4 microspheres via the one-step sulfuration of Keplerate-type polyoxometalate {Mo72Fe30}. The bio-inspired FeMo2S4 microspheres feature potential-abundant structural defects and atomically precise iron doping and act as an effective bifunctional electrocatalyst for hydrogen oxidation/reduction reactions. The FeMo2S4 catalyst presents an impressive alkaline HOR activity compared to FeS2 and MoS2 with the high mass activity of 1.85 mA·mg-1 and high specific activity as well as excellent tolerance to carbon monoxide poisoning. Meanwhile, FeMo2S4 electrocatalyst also displayed prominent alkaline HER activity with a low overpotential of 78 mV at a current density of 10 mA·cm-2 and robust long-term durableness. Density functional theory (DFT) calculations indicate that the bio-inspired FeMo2S4 with a unique electron structure possesses the optimal hydrogen adsorption energy and enhanced adsorption of hydroxyl intermediates, which accelerates the potential-determining Volmer step, thus promoting the HOR and HER performance. This work provides a new pathway for designing efficient noble-metal-free electrocatalysts for the hydrogen economy.

Electron Delocalization of Au Nanoclusters Triggered by Fe Single Atoms Boosts Alkaline Overall Water Splitting.

The rational design and in-depth understanding of the structure-activity relationship (SAR) of hydrogen and oxygen evolution reaction (HER and OER) bifunctional electrocatalysts are vital to decreasing the energy consumption of hydrogen production by electrochemical water splitting. Herein, we report an inducing electron delocalization method where Fe single atoms as inducers are used to regulate the electron structure of Au nanoclusters by the M-Nx-C substrate to acquire satisfactory intrinsic HER activity. Meanwhile, Fe single atoms also serve as efficient OER active sites to construct bifunctional electrocatalysts. On account of the strong synergistic effect between Au nanoclusters and Fe single atoms, the hybrid catalyst Au-Fe1NC/NF performs an outstanding alkaline HER and OER activity. Only 35.6 mV, 246 mV, and 1.52 V are needed to reach 10 mA cm-2 for alkaline HER, OER, and two-electrode electrolytic cells, respectively. In addition, the bifunctional electrocatalysts also display excellent electrochemical stability. DFT calculations demonstrate that the strong synergistic effect can enhance the O-H bond activation ability of Au nanoclusters and upshift the d-band center of the Fe single atom to promote alkaline electrocatalytic water splitting. The strong synergistic effect is proven to arise from the electron delocalization of Au nanoclusters triggered by Fe single atoms.