Alkaline HER Research Articles

Modulation of dual centers on cobalt-molybdenum oxides featuring synergistic effect of intermediate activation and radical mediator for electrocatalytic urea splitting

Construction of dual sites to break adsorption-energy scaling limitations offers an effective means of facilitating urea electrolysis for H 2 production, but a fundamental understanding of their synergistic mechanisms remains incomplete. Herein, we report a facile H 2 vapor-assisted strategy for the controllable fabrication of the bimetallic active Co 2 Mo 3 O 8 electrocatalyst for alkaline urea splitting, with an applied voltage of only 1.50 V required to deliver 50 mA cm −2 . Direct spectroscopic evidence and theoretical investigation demonstrate that the Co sites are responsible for the activation of intermediates, whereas the assisting Mo centers are identified as the OH or H radical mediators. Specifically, the synergistic effect of the Co–Mo dual sites was conclusively verified by the in-situ Raman and X­ray photoelectron spectroscopy. Theoretical calculations reveal that the short H-bonding (Mo-HO∙∙∙H–N amine CO–Co) and Co–C–N–Mo configuration formed at the Co–Mo bridge are the key features determining a high reactivity for urea oxidation. The accelerated reaction rate is attributable to the conversion of the endoergic CO 2 desorption to an exoergic reaction. In an alkaline H 2 evolution, Co atoms are the reaction sites for O–H bond cleavage of H 2 O, while Mo sites are considered to be the H 2 -evolving centers. Determination of the individual functionality of synergistic dual centers represents a critical step towards the rational design of highly-efficient electrocatalysts. • We report a facile H 2 vapor-assisted strategy for the fabrication of the Co 2 Mo 3 O 8 catalyst for the alkaline urea splitting. • In situ reconstructed Co 3 O 4 and high-valence Mo (VI) species on the surface are responsible for increased urea oxidation. • The short H-bonding and Co-C-N-Mo configuration are the key features determining a high reactivity for urea oxidation. • In an alkaline HER, Co atom accounts for cleavage of H-OH bond, while Mo site is identified as the H 2 -evolving center.

Eutectic molten salt assisted synthesis of highly defective and flexible ruthenium oxide for efficient overall water splitting

Designing and synthesizing efficient bifunctional electrocatalysts is of great significance for the application of overall water splitting but remains a great challenge. Herein, we synthesized low-crystallinity ruthenium oxides via a facile, eutectic molten salt-assisted method at low temperatures. Specifically, the optimized low-crystallinity RuO2-250 nanosheets with a highly defective structure and better structural flexibility showed both higher acidic OER and alkaline HER activity than commercial counterparts, reaching as low as 210 mV and 52 mV overpotentials at the current density of 10 mA cm−2 respectively. In situ Raman spectroscopy was employed to reveal the structural flexibility on the RuO2 catalyst during HER, which could contribute to its high activity. Furthermore, an electrolyzer with RuO2-250 catalyst requires only 1.57 V and 1.66 V cell voltage to achieve a current density of 10 mA cm−2 for overall water splitting under alkaline and acidic conditions respectively. This work provides a facile strategy for efficient design and preparation of low-crystalline materials and offers a new insight into the kinetic process of the water-splitting on ruthenium oxide.

Recording the Pt-beyond hydrogen production electrocatalysis by dirhodium phosphide with an overpotential of only 4.3 mV in alkaline electrolyte

The innovation for efficient water electrolysis catalysts and the manner for boosting the multistep reaction are highly demanded for hydrogen economy. In this study, P-defected Rh2P nanoparticles have been prepared through pyrolysis of Rh-organic complex under a reductive atmosphere, showing enjoyable HER performances under both acidic and alkaline electrolytes. Especially in alkaline HER, P-defected Rh2P shows the lowest overpotential of 4.3 mV at 10 mA cm−2, outperforming the Pt/C benchmark. First-principle calculations are further conducted to elucidate the rationales behind. The calculations revealed that P-defected Rh2P nanoparticles have the nearest H adsorption sites distance (3.94 Å to 1.98 Å) and lowest activation barrier (1.39 eV to 0.786 eV) compared with Rh2P, promoting the subsequent H2 release. The study provides valuable insights into the rational design of advanced HER electrocatalysts as well as the mechanism comprehension from the neighbored Had distance as an alternative dynamic HER descriptor.

Highly Dispersed Pt Nanoparticles Embedded in N-Doped Porous Carbon for Efficient Hydrogen Evolution.

A novel and facile strategy is presented to synthesize highly dispersed Pt nanoparticles embedded in N-doped porous carbon (Pt@NPC) via carbonization of Zn-containing metal-organic frameworks and chemical replacement of Zn with Pt. The as-prepared Pt@NPC exhibits superior activity and durability towards hydrogen evolution reaction (HER) in comparison with commercial Pt/C catalyst. The excellent HER performance of Pt@NPC can be ascribed to the combined features of catalyst and support material, including high dispersion and ultrathin particle size of Pt, high surface area and nitrogen doping of carbon support, and the strong interaction between metal and support.

Electronic metal\u2013support interaction modulates single-atom platinum catalysis for hydrogen evolution reaction

Tuning metal–support interaction has been considered as an effective approach to modulate the electronic structure and catalytic activity of supported metal catalysts. At the atomic level, the understanding of the structure–activity relationship still remains obscure in heterogeneous catalysis, such as the conversion of water (alkaline) or hydronium ions (acid) to hydrogen (hydrogen evolution reaction, HER). Here, we reveal that the fine control over the oxidation states of single-atom Pt catalysts through electronic metal–support interaction significantly modulates the catalytic activities in either acidic or alkaline HER. Combined with detailed spectroscopic and electrochemical characterizations, the structure–activity relationship is established by correlating the acidic/alkaline HER activity with the average oxidation state of single-atom Pt and the Pt–H/Pt–OH interaction. This study sheds light on the atomic-level mechanistic understanding of acidic and alkaline HER, and further provides guidelines for the rational design of high-performance single-atom catalysts.

Transition Metal/Metal Oxide Interface (Ni–Mo–O/Ni4Mo) Stabilized on N-Doped Carbon Paper for Enhanced Hydrogen Evolution Reaction in Alkaline Conditions

Designing hydrogen evolution reaction catalysts with high performance in alkaline conditions is important for reducing energy consumption during water electrolysis. This study reports an electrodeposition–calcination–electrodeposition strategy to construct the Ni–Mo–O/Ni₄Mo nanointerface on an N-doped carbon support derived from polyaniline on carbon fiber paper. The N-doped carbon support not only stabilizes the metal/metal oxide nanointerface but also generates more active intrinsic sites and electrical conductivity via the metal–N interaction. The obtained Ni–Mo–O/Ni₄Mo@NC displays a low overpotential of only 61 mV at a current density of 10 mA cm–² for alkaline HER in 1 M KOH.

Metal–organic framework-derived Ni doped Co3S4 hierarchical nanosheets as a monolithic electrocatalyst for highly efficient hydrogen evolution reaction in alkaline solution

High density of active sites and promoted mass transfer offered by hierarchical nanosheet structure as well as the good electrical conductivity and modified electronic structure caused by Ni doping collectively enhanced the alkaline HER performance of Ni doped Co 3 S 4 HNS grown in Ti mesh. Hierarchical nanostructure construction and electronic structure engineering are commonly employed to increase the electrocatalytic activity of HER electrocatalysts. Herein, Ni doped Co 3 S 4 hierarchical nanosheets on Ti mesh (Ni doped Co 3 S 4 HNS/TM) were successfully prepared by using metal organic framework (MOF) as precursor which was synthesized under ambient condition. Characterization results confirmed this structure and Ni incorporation into Co 3 S 4 lattice as well as the modified electronic structure of Co 3 S 4 by Ni doping. Alkaline HER performance showed that Ni doped Co 3 S 4 HNS/TM presented outstanding HER activity with 173 mV overpotential at −10 mA·cm −2 , surpassing most of metal sulfide-based electrocatalysts. The hierarchical structure, superior electrical conductivity and electronic structure modulation contributed to the accelerated water dissociation and enhanced intrinsic activity. This work provides a new avenue for synthesizing hierarchical nanostructure and simultaneously tuning the electronic structure to promote HER performance, which has potential application in designing highly efficient and cost-effective HER nanostructured electrocatalyst.

Strategies and Perspectives to Catch the Missing Pieces in Energy-Efficient Hydrogen Evolution Reaction in Alkaline Media.

Transition metal hydroxides (M‐OH) and their heterostructures (X|M‐OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy‐efficient alkaline HER.

Regulation of the adsorption sites of Ni2P by Ru and S co-doping for ultra-efficient alkaline hydrogen evolution

Benefiting from the 3D nanosheets and highly active Ru and S co-doped Ni2P, the prepared electrocatalysts displayed excellent performance for the alkaline HER.

Promoted alkaline hydrogen evolution by an N-doped Pt–Ru single atom alloy

N incorporation induces d orbital tuning and promotes water dissociation on Ru, while single Pt atoms facilitate H desorption. The two effects synergistically enable the excellent alkaline HER performance on an N-doped Pt–Ru single atom alloy.

Oxygen-deficient TiO2 and carbon coupling synergistically boost the activity of Ru nanoparticles for the alkaline hydrogen evolution reaction

Ru nanoparticles/oxygen-deficient TiO2@carbon nanocomposites are efficiently fabricated via one-pot pyrolysis. TiO2-VO and carbon narrow the band gap, optimize ΔGH2O and ΔGH* and synergistically boost the activity of Ru for the alkaline HER.

Electrochemically induced in-situ generated Co(OH)2 nanoplates to promote the Volmer process toward efficient alkaline hydrogen evolution reaction

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.

Single-phase perovskite oxide with super-exchange induced atomic-scale synergistic active centers enables ultrafast hydrogen evolution

The state-of-the-art active HER catalysts in acid media (e.g., Pt) generally lose considerable catalytic performance in alkaline media mainly due to the additional water dissociation step. To address this issue, synergistic hybrid catalysts are always designed by coupling them with metal (hydro)oxides. However, such hybrid systems usually suffer from long reaction path, high cost and complex preparation methods. Here, we discover a single-phase HER catalyst, SrTi0.7Ru0.3O3-δ (STRO) perovskite oxide highlighted with an unusual super-exchange effect, which exhibits excellent HER performance in alkaline media via atomic-scale synergistic active centers. With insights from first-principles calculations, the intrinsically synergistic interplays between multiple active centers in STRO are uncovered to accurately catalyze different elementary steps of alkaline HER; namely, the Ti sites facilitates nearly-barrierless water dissociation, Ru sites function favorably for OH* desorption, and non-metal oxygen sites (i.e., oxygen vacancies/lattice oxygen) promotes optimal H* adsorption and H2 desorption.

Improving plasma sprayed Raney-type nickel\u2013molybdenum electrodes towards high-performance hydrogen evolution in alkaline medium

Rationally designed free-standing and binder-free Raney-type nickel–molybdenum (Ni–Mo) electrodes produced via atmospheric plasma spraying (APS) are developed by correlating APS process parameters with the microstructure of electrodes and their electrochemical performance in alkaline media. The results revealed that the electrode morphology and elemental composition are highly affected by the plasma parameters during the electrode fabrication. It is found that increasing plasma gas flow rate and input plasma power resulted in higher in-flight particle velocities and shorter dwell time, which in result delivered electrodes with much finer structure exhibiting homogeneous distribution of phases, larger quantity of micro pores and suitable content of Ni and Mo. Tafel slope of electrodes decreased with increasing the in-flight particles velocities from 71 to 33 mV dec−1 in 30 wt.% KOH. However, beyond a critical threshold in-flight velocity and temperature of particles, electrodes started to exhibit larger globular pores and consequently reduced catalytic performance and higher Tafel slop of 36 mV dec−1 in 30 wt.% KOH. Despite slightly lower electrochemical performance, the electrodes produced with highest plasma gas flow and energy showed most inter-particle bonded structure as well as highest stability with no measurable degradation over 47 days in operation as HER electrode in 30 wt.% KOH. The Raney-type Ni–Mo electrode fabricated at highest plasma gas flow rate and input plasma power has been tested as HER electrode in alkaline water electrolyzer, which delivered high current densities of 0.72 and 2 A cm−2 at 1.8 and 2.2 V, respectively, representing a novel prime example of HER electrode, which can synergistically catalyze the HER in alkaline electrolyzer. This study shows that sluggish alkaline HER can be circumvented by rational electrode composition and interface engineering.

(Invited) First Principles Studies of Oxygen Cycle Electrocatalysis: Multifunctional Materials and Reactivity Trends

Water splitting to generate O2 and H2 fuel has been a major focus of (photo)electrochemical energy storage and conversion efforts, but many challenges remain. While water oxidation to generate O2 through the oxygen evolution reaction (OER) accounts for the majority of energy loss in this process, water reduction to generate H2 through the hydrogen evolution reaction in alkaline is over two orders of magnitude slower than that in acid. For OER, NiFe layered double hydroxides have attracted significant interest due to their comparable performance with precious metal-based RuO2 and IrO2 catalysts. In spite of extensive study, however, the 3D crystal structure of the active phase under catalytic oxygen evolution reaction conditions remains unclear. The lack of the atomic-scale details of crystal structure makes it challenging to choose appropriate structural models for first principles-based mechanistic studies. Therefore, it is of significant interest to identify these materials’ in-situ crystal structure and, subsequently, determine the intrinsic catalytic mechanism. Similarly, for alkaline HER, ultrathin (oxy)hydroxide films on precious metal substrates possess impressive activity improvement, but the films’ structure and stability are still largely unknown, and the catalytic mechanism remains unclear.In this presentation, we will begin by showing our recent efforts to elucidate the catalytically active phase and OER mechanism on NiFe layered double hydroxides by combining electrochemical measurements, operando experiments, DFT calculations, and ab initio molecular dynamics simulations. Next, for HER, we will introduce the methodologies we have recently developed towards the highly accurate prediction of Pourbaix diagram of transition metal (oxy)hydroxides. Subsequently, using monolayer Ni (oxy)hydroxide films as an example, we will describe a simple scheme to study the structures and the stability of these films on precious metal surfaces. We will show how the ultrathin films can be dramatically stabilized with respect to the corresponding bulk analogs. Then, using the hydrogen evolution reaction as an example, we will demonstrate how these techniques can be applied to understand the steady state, the active phases, and the catalytic mechanism of bi-functional interfaces. We will then demonstrate the extension of the present understanding to real-world catalysts, i.e. precious metal nanoparticles supported on ultrathin transition metal (oxy)hydroxide films. Finally, we will show this understanding can be used to design new bi-functional catalysts with improved performances.If time permits, we will also show our recent work on tunable intrinsic strain in two-dimensional transition metal electrocatalysts for the oxygen reduction reaction.

Mo Doped Amorphous CoSx Porous Leaf-Like Nanostructure on Ti Mesh as Electrocatalyst for Alkaline Hydrogen Production

Reducing water dissociation barrier and then expediting its kinetics are of great significance to enhance alkaline water splitting for hydrogen evolution. Herein, Mo doped CoSx on Ti mesh (defined as Mo doped CoSx/TM) was synthesized by multistep pathways using Co-MOF as precursor. The characterization results confirmed the amorphous structure and porous leaf-like morphology of Mo doped CoSx. The alkaline HER measurement indicated Mo doped CoSx/TM exhibited the enhanced HER performance with overpotential (η) of 230 mV at −10 mA cm−2, lower than that of Co3S4/TM (265 mV) and other samples (>350 mV), and reduced Tafel slope (98.3 mV dec−1), as well as decent stability. The lowest Tafel slope meant the highest water dissociation kinetics and the reaction followed Volmer-Heyrovsky mechanism over Mo doped CoSx/TM. The excellent alkaline HER performance of Mo doped CoSx/TM was due to the increased intrinsic activity caused by cooperative effect between Mo and CoSx, and the porous leaf-like structure, which was in favor of exposing much more accessible active sites and facilitating mass transfer. This work demonstrates the validity of Mo doping in promoting HER performance of cobalt sulfide, which offers a strategy to design highly efficient transition metal chalcogenides for catalysis and energy conversion.

Optimizing electron density of nickel sulfide electrocatalysts through sulfur vacancy engineering for alkaline hydrogen evolution

A novel, rational design for porous S-vacancy nickel sulfide catalysts with remarkable catalytic performance for alkaline HER.

Ni loaded on N-doped carbon encapsulated tungsten oxide nanowires as an alkaline-stable electrocatalyst for water reduction

A hierarchical electrocatalyst, Ni on N-doped carbon shell coated oxygen-vacancy-rich WOx nanowires, exhibits excellent stability and superb alkaline HER activity.

A cascade surface immobilization strategy to access high-density and closely distanced atomic Pt sites for enhancing alkaline hydrogen evolution reaction

A stepwise anchoring synthetic strategy has been developed for the closely-distanced atomic Pt catalysts with superior alkaline HER activity, providing a versatile platform for accessing atomic metal catalysts toward various industrial reactions.

Accelerated alkaline hydrogen evolution on M(OH) x /M-MoPO x (M = Ni, Co, Fe, Mn) electrocatalysts by coupling water dissociation and hydrogen ad-desorption steps.

Developing efficient and cheap electrocatalysts for the alkaline hydrogen evolution reaction is still a big challenge due to the sluggish water dissociation kinetics as well as poor M–Had energetics. Herein, hydroxide modification and element incorporation have been demonstrated to realize a synergistic modulation on a new class of M(OH)x/M-MoPOx catalysts for accelerating water dissociation and hydrogen ad-desorption steps in the HER. Theoretical and experimental results disclosed that in situ modification with hydroxide endowed M(OH)x/M-MoPOx with a strong ability to dissociate water, and meanwhile, oxygen incorporation effectively optimized the M–Had energetics of the NiMoP catalyst. Moreover, the interaction between M(OH)x and M-MoPOx components in M(OH)x/M-MoPOx further enhances their ability to catalyze the two elementary steps in alkaline hydrogen evolution, providing a wide avenue for efficiently catalyzing hydrogen evolution. In general, the optimized Ni(OH)2/NiMoPOx catalyst exhibits excellent alkaline HER activity and durability, superior to the state-of-the-art Pt/C catalyst when the overpotential exceeds 65 mV.