Metal-free Hydrogen Evolution Reaction Research Articles

Precious Metal Free Hydrogen Evolution Catalyst Design and Application.

The quest to identify precious metal free hydrogen evolution reaction catalysts has received unprecedented attention in the past decade. In this Review, we focus our attention to recent developments in precious metal free hydrogen evolution reactions in acidic and alkaline electrolyte owing to their relevance to commercial and near-commercial low-temperature electrolyzers. We provide a detailed review and critical analysis of catalyst activity and stability performance measurements and metrics commonly deployed in the literature, as well as review best practices for experimental measurements (both in half-cell three-electrode configurations and in two-electrode device testing). In particular, we discuss the transition from laboratory-scale hydrogen evolution reaction (HER) catalyst measurements to those in single cells, which is a critical aspect crucial for scaling up from laboratory to industrial settings but often overlooked. Furthermore, we review the numerous catalyst design strategies deployed across the precious metal free HER literature. Subsequently, we showcase some of the most commonly investigated families of precious metal free HER catalysts; molybdenum disulfide-based, transition metal phosphides, and transition metal carbides for acidic electrolyte; nickel molybdenum and transition metal phosphides for alkaline. This includes a comprehensive analysis comparing the HER activity between several families of materials highlighting the recent stagnation with regards to enhancing the intrinsic activity of precious metal free hydrogen evolution reaction catalysts. Finally, we summarize future directions and provide recommendations for the field in this area of electrocatalysis.

Low-temperature synthesized Mo2C and novel Mo2C–MnO2 heterostructure for highly efficient hydrogen evolution reaction and high-performance capacitors

Here, a facile and cost-effective way is proposed to synthesize Mo2C nanoparticles and Mo2C/MnO2 heterostructures by utilizing laboratory waste. The Mo2C nanoparticles are well encapsulated by graphitic carbon layers, which provide high cyclic and structural stability to Mo2C. The electrochemical hydrogen evolution reaction (HER) activity and capacitive properties of these samples are studied in detail. Mo2C synthesized at 500 °C exhibits higher HER activity with a Tafel slope of 79 mV dec−1. To further improve the HER activity of Mo2C nanoparticles, the heterostructure of Mo2C and MnO2 is developed. The optimized Mo2C/MnO2 (2:1) heterostructure delivers a current density of 10 mA cm−2 at a low overpotential of 58.3 mV and shows a very low Tafel slope of 36 mV dec−1. It exhibited long-term cyclic stability up to 5000 cyclic voltammetry (CV) cycles. This paper describes a novel and effective method for fabricating noble metal-free HER electrocatalysts utilizing laboratory waste as a carbon precursor.

Dual-phase MoS2/MXene/CNT ternary nanohybrids for efficient electrocatalytic hydrogen evolution

Two-dimensional (2D) molybdenum disulfide (MoS2) has been recognized as a potential substitution of platinum (Pt) for electrochemical hydrogen evolution reaction (HER). However, the broad adoption of MoS2 is hindered by its limited number of active sites and low inherent electrical conductivity. In this work, we employed a one-step solvothermal synthesis technique to construct a ternary hybrid structure consisting of dual-phase MoS2, titanium carbide (Ti3C2) MXene, and carbon nanotubes (CNTs), and demonstrated synergistic effects for active site exposure, surface area enlargement, and electrical conductivity improvement of the catalyst. The dual-phase MoS2 (DP-MoS2) is directly formed on the MXene with CNTs acting as crosslinks between 2D islands. The existence of edge-enriched metallic phase MoS2, the conductive backbone of MXene along with the crosslink function of CNTs clearly improves the overall HER performance of the ternary nanocomposite. Moreover, the integration of MoS2 with MXene not only increases the interlayer distance of the 2D layers but also partially suppresses the MXene oxidation and the 2D layer restacking, leading to good catalytic stability. As a result, an overpotential of 169 mV and a low Tafel slope of 51 mV/dec was successfully achieved. This work paves a way for 2D-based electrocatalyst engineering and sheds light on the development of the next-generation noble metal-free HER electrocatalysts.

Co3W intermetallic compound as an efficient hydrogen evolution electrocatalyst for water splitting and electrocoagulation in non-acidic media

The hydrogen evolution reaction (HER) is the cathodic reaction of water electrolysis and electrocoagulation. The development of precious metal-free HER catalyst with high performance is important to decrease the energy consumption and capital cost. Due to the adjustable electronic structure and composition, intermetallic compounds are promising candidates to act as efficient HER catalysts. Herein, Co3W intermetallic compound is synthesized by reducing CoWO4 with H2, during which CoWO4 is transformed into a composite in the form of Co3W interconnected with W. The Co3W/W composite requires a low overpotential of 78 mV to reach the current density of 10 mA cm−2 in 1 M KOH. Moreover, the electrocoagulation device assembled by Co3W/W composite and iron plate only needs 0.2 V to initiate H2 production in 0.5 M Na2SO4; meanwhile, high removal ability for multiple water pollutants is also achieved. As the active species, Co3W presents low water dissociation energy barrier and proper adsorption strength of hydrogen atom, both of which are beneficial to HER kinetics. This work demonstrates a promising HER electrocatalyst that can be applied in non-acidic media for water splitting and electrocoagulation.

Heterostructured CoP·CoMoP nanocages as advanced electrocatalysts for efficient hydrogen evolution over a wide pH range

A sustainable and environmental-friendly method to produce hydrogen with high purity is the electrochemical water splitting, but its commercialization is challenged due to lack of cost-effective electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range. Herein, a series of CoP·xCoMoP heterostructured nanocages (NCs) were prepared via a dissolution-regrowth and subsequent phosphorization process using metal-organic frameworks (MOFs) as template. The three-dimensional (3D) architecture of CoP·xCoMoP is constituted by the heterostructured nanosheets composed with CoP and CoMoP phase. These noble-metal-free earth-abundant transition metal phosphide (TMP) catalysts show a pH-universal HER activity with high efficiency. Under the optimal atom ratio of Co and Mo (6:5), CoP·5CoMoP NC catalysts can deliver a current density of 10 mA cm−2 at the overpotential of 72 mV with a Tafel slope of 60.3 mV dec−1 in 1.0 M KOH solution. The same current output requires overpotential of 44 mV in 0.5 M H2SO4 solution and 151 mV in1.0 M phosphate buffered solution (PBS), respectively. The superior HER activity of CoP·5CoMoP NC catalysts can be comparable to or even better than most of noble metal-free HER electrocatalysts reported recently. In addition, CoP·5CoMoP NC catalysts also show a fairly high HER stability over a wide pH range, and their HER activity can be well kept without significant loss for long-term electrolysis. The 3D CoP·5CoMoP heterostructured catalysts hold promise as efficient and low-cost catalysts for water splitting devices over a wide pH range.

Nanostructured NiCo2S4@NiCo2O4-reduced graphene oxide as an efficient hydrogen evolution electrocatalyst in alkaline electrolyte

Metal-organic frameworks (MOFs), serving as precursors or templated to construct nanomaterials, which have gained great attentions in the field of electrocatalysis. However, their applications still remain some challenges due to poor conductivity and easy agglomeration. In this work, the MOFs-derived NiCo2S4@NiCo2O4 deposited on reduced Graphene Oxide (rGO) surface is designed by using a facile hydrothermal procedure. Attribute to the enlarged active surface area of the nanostructure and the strong synergistic effect between NiCo2S4 and NiCo2O4, as well as the excellent conductivity of rGO. The NiCo2S4@NiCo2O4-rGO catalyst displays ultrahigh hydrogen evolution reaction (HER) property and excellent stability, only need an overpotential of η10 = 95 mV to attain 10 mA cm−2 and deliver a small Tafel slope of b = 52 mV dec−1 in 1 M KOH. This work can provide a window to construct and develop new noble metal-free HER catalysts base on Ni-MOFs served as precursors.

Cathodic pre-polarization studies on the carbon felt/KOH interface: An efficient metal-free electrocatalyst for hydrogen generation

The recent decades have witnessed a booming research effort on the development of highly active electrode materials in electrocatalysis with the aim to replace the commonly used noble metal-based ones. Although non-noble metal electrodes can fulfill some requirements in terms of performance and price, their stability in alkaline media is generally weak. Additionally, from the environmental view point, the use of metals can cause water contamination. Therefore, the preparation of metal-free electrocatalysts represents an appealing approach to overcome the above-mentioned shortcuts. In this work, we used a simple electrochemical activation process, chronoamperometry measurements conducted in 1 M KOH at various overpotentials (−2, −2.5, −3, and −4 V vs. RHE) and pre-polarization periods (0.5, 2, 4, and 12 h), of commercially available and cheap carbon felt (CF) electrode to achieve good performance for hydrogen evolution reaction (HER) in alkaline media. Under optimized operating conditions (12 h of chronoamperometry measurement acquired at an overpotential of −4 V vs. RHE), the CF electrode exhibited an overpotential of 182 mV at 10 mA/cm 2 , a value surpassing that recorded for the most efficacious metal-free HER electrocatalysts published in the literature. The two CF samples exposed to 12 h of cathodic activation at −2.5 and −3.0 V vs. RHE recorded respectively Tafel slope ( b c ) values of 110 and 126 mV dec −1 . These b c values are close to the standard b c value of 120 mV dec −1 , suggesting the Volmer step as the main step controlling the HER. Increasing the applied cathodic potential to −4.0 V vs. RHE further decreased the b c value to 90 mV dec −1 , referring to increased accessible active catalytic sites for the HER. The cathodically activated CF surface's morphology and composition were assessed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) examinations. Raman spectroscopy was also conducted to assess the influence of the cathodic activation process on the structural defects/disorder of the CF surface. The formation of oxygen-containing groups on the CF surface after cathodic activation, as evidenced from XPS analysis, is believed to enhance water adsorption and act as active sites for hydrogen generation. The technique developed herein is simple and straightforward and can be easily implemented for almost any type of electrode.

Protein denaturation induced electrocatalytic hydrogen evolution

In recent years, there has been an increasing interest in designing economically viable metal-free carbon-supported catalysts for hydrogen-based energy technologies. Here we propose carbon nanotubes (CNTs) covalently functionalized with peptides or proteins as effective catalysts for electrochemical hydrogen evolution reaction (HER). We demonstrate that the HER activity of pristine CNTs can be greatly enhanced via simple functionalization by electrically insulating macromolecules. The roles of the conductive residues and the nitrogen content of the proteins, as well as the importance of their covalent connectivity with the CNTs, have been established. Moreover, we have observed a multifold enhancement in the HER current density due to electrochemical protein denaturation induced by the acidic medium used for HER. We have provided mechanistic insights into the HER activity of different metal-free peptides and proteins. Our work opens up new avenues in carbon-based metal-free HER catalysis, where, by careful selection of natural proteins, both the onset potential and the current density of the HER can be augmented.

Nitrogen doped carbon fibers derived from carbonization of electrospun polyacrylonitrile as efficient metal-free HER electrocatalyst

Development of durable and efficient electrocatalyst for hydrogen evolution reaction (HER) is significantly important for forwarding the commercialization of water splitting technology. In this work, we report a facile synthesis of nitrogen doped carbon fibers derived from the carbonization of the electron-spun polyacrylonitrile (PAN) membrane at 800 °C (NCFs-800) as efficient and stable metal-free electrocatalyst for HER catalysis in both acidic and alkaline mediums. Ascribing to the homogenous nitrogen dopants in electrocatalyst, NCFs-800 requires only 114.3 mV and 198.6 mV vs. RHE to achieve current density of 10 mA cm−2 in 0.5 M H2SO4 and 1 M KOH electrolytes, respectively. Moreover, the HER activity is well maintained after 2000 potential cycles indicating that NCFs-800 possesses high durability in both acidic and alkaline conditions due to the fibrous structure with high corrosion resistance. Our study offers new strategy to synthesize stable and efficient metal-free electrocatalyst, which could be extended to other heteroatom doped carbon electrocatalyst.

Recent Advances in Electrocatalytic Hydrogen Evolution Using Nanoparticles.

Hydrogen fuel is considered as the cleanest renewable resource and the primary alternative to fossil fuels for future energy supply. Sustainable hydrogen generation is the major prerequisite to realize future hydrogen economy. The electrocatalytic hydrogen evolution reaction (HER), as the vital step of water electrolysis to H2 production, has been the subject of extensive study over the past decades. In this comprehensive review, we first summarize the fundamentals of HER and review the recent state-of-the-art advances in the low-cost and high-performance catalysts based on noble and non-noble metals, as well as metal-free HER electrocatalysts. We systemically discuss the insights into the relationship among the catalytic activity, morphology, structure, composition, and synthetic method. Strategies for developing an effective catalyst, including increasing the intrinsic activity of active sites and/or increasing the number of active sites, are summarized and highlighted. Finally, the challenges, perspectives, and research directions of HER electrocatalysis are featured.

Carbon materials for use in the electrocatalytic hydrogen evolution reaction

As an important clean energy carrier with a high combustion heat value, hydrogen is of great importance for addressing the challenges caused by an increasingly severe energy shortage and environmental pollution. Compared with traditional hydrogen production from fossil fuel (e.g. natural gas, coal) by steam reforming, the electrocatalytic production of hydrogen by water splitting is a renewable and eco-friendly technique. Currently, the widely used catalyst for the hydrogen evolution reaction (HER) is a noble metal (e.g. Pt), which has very limited availability and a high cost. Therefore, it is crucial to develop low-cost and highly effective alternative HER electrocatalysts. In this review, we summarize the use of carbon materials for noble metal-free HER catalysts, in which they play roles including as an active catalyst and a matrix component. By heteroatom doping, the intrinsic activities of carbon materials are optimized and improved, and carbon-based metal-free catalysts are produced. As a matrix, the main roles of the carbon materials are as conductive substrates, highly-dispersed support materials and corrosion resistant layers. The catalytic activity of carbon-based catalysts is improved by a synergistic effect between a co-catalyst and the carbon skeleton. The HER performance of some reported carbon-based catalysts are even close to that of Pt. We summarize the strategies for tailoring the HER performance of carbon-based electrocatalysts and the prospects for future research on developing highly efficient and low-cost catalysts are discussed.

Nitrogen-self doped activated carbon nanosheets derived from peanut shells for enhanced hydrogen evolution reaction

Abstract Herein, we demonstrate a facile synthesis of peanut shell-derived activated carbon nanosheets using a facile thermal technique aided by chemical activation process for improved hydrogen evolution reaction (HER). The obtained activated carbon nanosheets show a multilayer sheet-like morphology with a highly porous nature and possess a very high surface area of ~2338 m2 g−1. The physiochemical analysis reveals that the catalytically active sites are associated with the prepared activated carbon nanosheets. The as-prepared activated carbon material employs as a potential metal-free electrocatalyst for H2 production under acidic electrolyte, exhibiting superior electrocatalytic performance with a less negative onset potential of 0.08 V Vs. RHE with low overpotential (η) response, small Tafel slope, and long-term stability. The enhanced HER activity of the developed activated carbon is attributed to high BET specific surface area, high porous nature and two-dimensional structure, employing as an encouraging impeccable metal-free HER catalyst. For the first time, we have successfully analysed and reporting an excellent electrocatalytic activity towards H2 production by activated carbon derived from peanut shell biomass.

Mo2C based electrocatalyst with nitrogen doped three-dimensional mesoporous carbon as matrix, synthesis and HER activity study

H2 is clean power source, which can be obtained from hydrogen evolution reaction (HER) through electrocatalysis directly. To obtain highly effective electrocatalyst with low price for HER, small Mo2C nanoparticles with dimension about 4–6 nm were loaded on rice paper derived nitrogen doped 3D mesoporous carbon matrix with ammonium heptamolybdate as precursor after calcination. The unique 3D mesoporous carbon matrix donates the electrocatalyst large surface as well as high conductivity, which enhances HER activity in great extent. In 0.5 M H2SO4, to obtain the current with density 10 mA cm−2, the electrocatalyst only requires overpotential of 155 mV with Tafel slope merely 73 mV·dec−1. In 1.0 M KOH, these values decrease to 78 mV and 64 mV·dec−1. Moreover, this electrocatalyst also exhibits outstanding stability and its activity keeps stable after 2000 runs 10 h long-time tests. In 0.5 M H2SO4, the H2 production rate of this electrocatalyst achieves 456 μmol h−1. The faradaic efficiency this electrocatalyst arrives nearly 100% in both acidic and basic conditions. This work introduces a new and efficient strategy to fabricate noble metal free HER electrocatalyst with cheap material as precursor. The simple preparation procedure facilitates the application of this electrocatalyst in HER.

Exfoliated MoS2 with porous graphene nanosheets for enhanced electrochemical hydrogen evolution

Porous graphene (P-rGO) was synthesized from graphene oxide (GO) via a one-pot calcination method with CO2 as an activation agent at 800 °C. Due to the special porous structure, the surface area of P-rGO can be increased to ∼759 m2/g. The P-rGO was then used as a support to incorporate with chemical exfoliated molybdenum disulfide (MoS2) for the fabrication of MoS2/P-rGO composite. Compared to bulk MoS2, the exfoliated MoS2 is in the 1T phase with a metallic property and smaller charge transfer resistance, thus has a better activity in electrochemical hydrogen evolution reaction (HER). The HER activity of 1T MoS2 could be further increased after the combination with P-rGO. The overpotential of 1T MoS2/P-rGO was only ∼130 mV vs. RHE, and the corresponding Tafel slope was ∼75 mV Dec−1. The special porous structure and good electric conductivity of P-rGO decrease the charge transfer resistance of the composite without sheltering too many active sites of MoS2, thus leading to the enhanced HER activity. As an efficient noble metal free HER catalyst, the 1T MoS2/P-rGO has great potential for large-scale hydrogen production.

Simple routes for the improvement of hydrogen evolution activity of Ni-Mo catalysts: From sol-gel derived powder catalysts to graphene supported co-electrodeposits

Development of noble metal-free catalysts for hydrogen production is one of the cores of the sustainable energy economy. Here we present results of systematic analysis of catalytic activity of Ni-Mo alloy powders in alkaline media towards hydrogen evolution reaction (HER). Catalysts were prepared in a wide concentration range (from Ni0.2Mo0.8 to Ni0.9Mo0.1), and resulted with a volcano shaped activity-composition relationship, with maximum catalytic activity achieved for the powder with nominal composition Ni0.6Mo0.4. Improved HER activity is ascribed to reduced deactivation by hydride formation and adequate hydrogen-surface energetics on Ni-Mo catalysts. In the second part, we demonstrate a novel method for electrochemical formation of NiMo@rGO composites. Prepared composite electrodes show improved electrocatalytic activity compared to both pure Ni and Ni@rGO electrodes. Activity was observed to depend on the deposition time and is contributed by two factors: (i) formation of Ni-Mo system and (ii) formation of an interfacial region with rGO. We expect that the provided activity-composition relationship in combination with novel electrochemical NiMo-rGO composite formation procedure will provide a route for the development of new highly efficient noble metal-free HER electrocatalysts.

(0D/3D) MoS2 on porous graphene as catalysts for enhanced electrochemical hydrogen evolution

A new composite material consisting of 0D MoS2 nanodots and 3D MoS2 nano-flowers grown on porous reduced graphene oxide (P-rGO) was synthesized via a two-step process. The P-rGO with a surface area of 759 m2 g−1 was obtained by CO2 activation of reduced graphene oxide (rGO) at 800 °C. MoS2 was then grown on the P-rGO under hydrothermal conditions. Compared to the nonactivated rGO, P-rGO has functional pores for deposition of MoS2 nanodots and less charge transfer resistance, which can provide more active sites for hydrogen generation, thus leading to the improved activity of (0D/3D) MoS2/P-rGO in hydrogen evolution reaction (HER). The overpotential of (0D/3D) MoS2/P-rGO was only ∼150 mV vs. RHE, and the corresponding Tafel slope was ∼56 mV Dec−1, which is comparable to most of the present MoS2/Graphene HER catalysts. The (0D/3D) MoS2/P-rGO exhibits as an efficient noble metal free HER catalyst, and has great potential for the electrochemical hydrogen production.

Hierarchically interconnected nitrogen-doped carbon nanosheets for an efficient hydrogen evolution reaction.

Exploring low-cost and efficient electrocatalysts based on earth-abundant elements for the hydrogen evolution reaction (HER) is of great importance for the development of clean and renewable energy. In this work, we report a facile self-foaming strategy for synthesis of hierarchically interconnected nitrogen-doped carbon nanosheets (NCNS). The doping N species within the 3D interconnected carbon network affords rich active sites for the HER and facilitates fast charge transfer. As a result, the NCNS exhibit excellent catalytic activity with an onset potential of -65 mV, and a Tafel slope of 81 mV dec-1 with robust stability over 10 h in acidic media. Further analyses suggest that the graphitic N species in the NCNS contribute to their catalytic activity. Such a high catalytic performance makes the NCNS a promising metal-free HER electrocatalyst for practical hydrogen production.

Three-dimensional N-doped, plasma-etched graphene: Highly active metal-free catalyst for hydrogen evolution reaction

Exploring highly active, durable and low-cost catalysts toward hydrogen evolution reaction (HER) holds a key to clean energy technologies. Heteroatoms-doped graphene-based materials are emerging as the most promising metal-free HER catalysts, but their catalytic activity remains largely unexplored. Herein, by rationally engineering the macroscopic architecture of graphene and its chemical/defective structures, we developed a three-dimensional (3D), N-doped, plasma-etched graphene (3DNG-P) as a highly active metal-free HER catalyst. The obtained 3DNG-P combined the merits of freestanding 3D porous architecture, high-level N-doping and plasma-induced enriched defects, resulting in a highly enhanced HER activity with a low overpotential of 128mV at 10mAcm−2 in acidic medium. Furthermore, the 3DNG-P displayed a favorable HER activity and stability over a wide pH range. The present study thus provides a new methodology for the design of graphene-based metal-free catalysts with high HER performance.

A highly efficient and stable biphasic nanocrystalline Ni–Mo–N catalyst for hydrogen evolution in both acidic and alkaline electrolytes

The hydrogen evolution reaction (HER) is of critical importance for sustainable hydrogen production from water electrolysis. In this work, we report a highly efficient and stable noble metal free HER catalyst, which is composed of homogeneously distributed metallic Ni and NiMo4N5 nanocrystals. The biphasic nanocrystalline Ni–Mo–N catalyst shows very low overpotential (53mV in 0.5M H2SO4 and 43mV in 1M KOH, at current density 20mA/cm2) and good stability for HER in both acidic and alkaline electrolytes, which is a promising low cost alternative for platinum based HER catalysts.

Constructing holey graphene monoliths via supramolecular assembly: Enriching nitrogen heteroatoms up to the theoretical limit for hydrogen evolution reaction

Carbocatalysts with low cost and high stability are highly desirable to substitute expensive Pt-based materials for electrocatalysis. High conductivity, highly porous structure and high concentration of dopants are essential for efficient metal-free carbon-based electrocatalysts. Herein, a facile method was utilized to fabricate nitrogen-rich holey graphene monoliths (Nr-HGM) from cheap molecules and ammonium sulfate with the nitrogen content close to the upper limit of nitrogen-doped carbons. Our sample showed relatively high electrochemical catalytic activity and stability for hydrogen evolution reaction (HER) in both acid and base electrolytes, surpassing state-of-the-art metal-free HER catalysts in the literature.