Hydrogen Evolution Reaction Catalysis Research Articles

One-pot in-situ hydrothermal synthesis of VSe2/MoSe2 nanocomposite for enhanced hydrogen evolution reaction

Hydrogen is the kind of pure, renewable energy that is required for the world to begin relying less and less on the fossil fuels that it currently consumes. The production of hydrogen by electrocatalytic water splitting is deemed to be preferable to the consumption of fossil fuels for the generation of clean and reliable energy. A catalyst's ability to catalyze is significantly influenced by the number of exposed active sites in the catalyst. Thus, the purpose of the current work is to offer a simple and inexpensive strategy to synthesize a double non-noble metal catalyst of VSe2/MoSe2 via one-step hydrothermal synthesis for catalytic Hydrogen evolution reaction (HER). The strong and unique orientation interaction between the nanosheet-like structure of VSe2 and the nanoflower-like structure of MoSe2 in the nanocomposite significantly improved the electron transfer kinetics. Subsequently, the electrochemical hydrogen production performance of the hybrid VSe2/MoSe2 is enhanced as compared to its constituent materials. Electrochemical characterizations prove that the VSe2/MoSe2 nanocomposite enhances the electrochemical activity performance with the lowest onset potential (330 mV) and a low value of Tafel plot (66 mV/decade) in comparison with sole MoSe2 and VSe2. In addition, the nanocomposite shows a low charge transfer resistance of 65 Ω, which further advocates the HER polarization curve and Tafel plot.

Ru nanoclusters coupling with hierarchical phosphorus and oxygen dual-doped carbon nanotube architectures for effective hydrogen evolution reaction

The construction of an effective catalyst for hydrogen evolution reaction (HER) is a top priority. Herein, we demonstrate ruthenium (Ru) nanoclusters coupled with phosphorus and oxygen dual-doped carbon nanotube (CNT) architecture (Ru-POCA). The increased hydrophilicity and negatively charged surface of CNTs can strongly trap Ru ions. The hierarchical structure is favorable of providing abundant pathways and exposing more active sites for HER. Due to the synergistic effect of the hierarchical structure and modified surface chemistry, Ru-POCA exhibits excellent catalytic HER activity. The overpotential is 22 and 40 mV with a Tafel slope of 28.0 and 27.1 mV dec−1 in 1 M KOH and 0.5 M H2SO4 at 10 mA cm−2. Moreover, Ru-POCA processes good catalytic stability in both acidic and alkaline electrolytes, while the boosted catalytic HER activity is fundamentally studied by density functional theory calculation. This work provides a rational approach to constructing hierarchically structured Ru-CNTs-based catalysts for hydrogen evolution reaction.

Dual-cathode plasma-induced exfoliated WSe2/graphene nanosheet composite mediating an efficient hydrogen evolution reaction

With the goal of obtaining a sustainable earth-abundant electrocatalyst that displays high performance in the hydrogen evolution reaction (HER), herein we describe a new electrochemical process for the simultaneous dual-cathodic plasma-induced exfoliations of two different bulk materials—graphite and WSe2—into two-dimensional (2D) nanosheets for the fabrication of a homogeneous composite. Using this approach, we produced unique structures of graphene nanosheet-engulfed ultrathin WSe2 nanosheets (WSe2@GN), stabilized through van der Waals and π–π interactions between the WSe2 and GNs, possessing various morphologies and featuring large edge and defect densities on the basal plane of the GNs—potential active sites for HER catalysis. This synthesis of WSe2@GN structures not only increased the number of active sites in the semiconducting WSe2 nanosheets but also enhanced their conductivity. Our dual-cathode plasma produced WSe2@GN nanosheet composites exhibited high HER performance, characterized by a low overpotential of 106 mV at a current density of 10 mA cm–2, a Tafel slope of 66 mV dec–1, and long-term stability in 0.5 M H2SO4 (80% activity retention after >48 h); these performance metrics were much better than those of the corresponding WSe2/GN composite, prepared through two sequential single-cathode electrochemical plasma exfoliation and mixed together (233 mV, 111 mV dec–1), and are among the best ever reported for a composite involving WSe2 and GNs, prepared through simultaneous exfoliation at a low temperature and over a short period of time, for the HER. This in situ dual-plasma-exfoliation synthetic approach allows the effective production of homogeneous 2D/2D heterostructures with great potential for various energy-related applications, while being an effective and simple strategy for tuning the morphologies of composites of graphene and transition metal dichalcogenide materials for a broad range of energy applications.

Electrocatalytic Hydrogen Evolution of the Cobalt Triaryl Corroles Bearing Hydroxyl Groups

AbstractThree isomeric A2B‐type new cobalt triaryl corroles bearing hydroxyphenyl substituents have been prepared and well characterized. Their activity and stability in the electrocatalytic hydrogen evolution reaction (HER) have been investigated. The results showed that the hydroxyl position of the phenyl group had significant influence on electrocatalytic HER. The ortho‐hydroxyphenyl substituted cobalt corrole (1) core displays the best HER activity using TsOH proton source, and the turnover frequency (TOF) and catalytic efficiency (C.E) reach 318.68 s−1 and 1.13, respectively. Moreover, a turnover number (TON) of 1447.39 and Faraday efficiency (FE) of 98.7 % have been observed in aqueous medium. The catalytic pathway is via EECEC, EECC or ECEC pathways depending on the acidity of acid proton source (E: electron transfer step, C: chemical step, in this case protonation). The catalytic HER performance of these cobalt corroles follows an order of o‐hydroxyl > p‐hydroxyl > m‐hydroxyl isomer, showing the o‐ and p‐hydroxyl of the phenyl groups are more efficient in accelerating proton relay.

The single-atom catalytic activity of the hydrogen evolution reaction of the experimentally synthesized boridene 2D material: a density functional theory study

Previous theoretical studies have suggested that two-dimensional (2D) MBene materials might display adequate monatomic catalytic activity for the hydrogen evolution reaction (HER). Recently, a study reported the experimental synthesis of a 2D MBene (Mo4/3B2), re-defined as boridene, albeit no effort has been devoted to explore the single-atom catalytic activity for HER of this experimentally synthesized 2D material. Therefore, we herein investigate the single-atom HER performance of the boridene. Interestingly, with Mo defects mixed with single Au and Zn atoms shows excellent hydrogen evolution performance, and the change in the Gibbs free energy ([Formula: see text]) value is close to 0eV, which can even match the performance of Pt-based materials. Through analysis of the charge density difference and density of states, the mechanism affecting the HER performance is explained at the electronic level. This work provides a new direction for the use of the Mo4/3B2 monolayer two-dimensional materials in the field of single-atom catalysis for HER. This study used the DFT calculations in Vienna ab initio simulation package. The GGA-Perdew-Burke-Ernzerhof functional with DFT-D2 correction is used to describe the exchange-correlation interactions. The projection augmented wave is used with the plane wave cutoff of 500eV. The convergences of energy and force are 10-5eV and 0.01eV/Å, respectively. A vacuum layer with a height of 20Å is set in the Z direction. For geometry optimization, self-consistent, and DOS calculations, the k-point grids sampled in Brillouin zones are 3 × 3 × 1, 9 × 9 × 1, and 9 × 9 × 1, respectively. The AIMD simulation is performed in the canonical ensemble (NVT), and the temperature was maintained at 300K by Nosé-Hoover thermostats with a time step of 2.0fs.

A Comparison between Porous to Fully Dense Electrodeposited CuNi Films: Insights on Electrochemical Performance.

Nanostructuring of metals is nowadays considered as a promising strategy towards the development of materials with enhanced electrochemical performance. Porous and fully dense CuNi films were electrodeposited on a Cu plate by electrodeposition in view of their application as electrocatalytic materials for the hydrogen evolution reaction (HER). Porous CuNi film were synthesized using the hydrogen bubble template electrodeposition method in an acidic electrolyte, while fully dense CuNi were electrodeposited from a citrate-sulphate bath with the addition of saccharine as a grain refiner. The prepared films were characterized chemically and morphologically by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). The Rietveld analysis of the XRD data illustrates that both CuNi films have a nanosized crystallite size. Contact angle measurements reveal that the porous CuNi film exhibits remarkable superhydrophobic behavior, and fully dense CuNi film shows hydrophilicity. This is predominately ascribed to the surface roughness of the two films. The HER activity of the two prepared CuNi films were investigated in 1 M KOH solution at room temperature by polarization measurements and electrochemical impedance spectroscopy (EIS) technique. Porous CuNi exhibits an enhanced catalysis for HER with respect to fully dense CuNi. The HER kinetics for porous film is processed by the Volmer-Heyrovsky reaction, whereas the fully dense counterpart is Volmer-limited. This study presents a clear comparison of HER behavior between porous and fully dense CuNi films.

From the single-atom limit to the mixed-metal phase: finding the optimum condition for activating the basal plane of a FePSe3 monolayer towards HER.

Layered ternary transition metal tri-chalcogenides are some of the most promising candidates for hydrogen evolution reaction (HER) because of their ease of synthesis and affordability. However, majority of the materials in this category have HER active sites only at their edges, rendering a large portion of the catalyst useless. In this work, ways for activating the basal planes of one of these materials, namely, FePSe3, are explored. The effects of substitutional transition metal doping and external biaxial tensile strain on the HER activity of the basal plane of a FePSe3 monolayer are studied via first principles electronic structure calculations based on density functional theory. This study reveals that although the basal plane of the pristine material is inactive towards HER (value of H adsorption free energy, ΔGH* = 1.41 eV), 25% Zr, Mo, and Tc doping makes it more active (ΔGH* = 0.25, 0.22 and 0.13 eV, respectively). The effect of reducing the doping concentration, moving to the single-atom limit, on the catalytic activity is studied for Sc, Y, Zr, Mo, Tc and Rh dopants. For Tc, the mixed-metal phase FeTcP2Se6 is also studied. Among the unstrained materials, 25% Tc-doped FePSe3 gives the best result. Significant tunability of HER catalytic activity in the 6.25% Sc doped FePSe3 monolayer via strain engineering is also discovered. An external tensile strain of 5% reduces ΔGH* to ∼0 eV from 1.08 eV in the unstrained material, making this an attractive candidate for HER catalysis. The Volmer-Heyrovsky and Volmer-Tafel pathways are examined for some of the systems. A fascinating correlation between the electronic density of states and HER activity is also observed in most materials.

2D noble metals: growth peculiarities and prospects for hydrogen evolution reaction catalysis.

High-performance electrocatalysts for the hydrogen evolution reaction are of interest in the development of next-generation sustainable hydrogen production systems. Although expensive platinum-group metals have been recognized as the most effective HER catalysts, there is an ongoing requirement for the discovery of cost-effective electrode materials. This paper reveals the prospects of two-dimensional (2D) noble metals, possessing a large surface area and a high density of active sites available for hydrogen proton adsorption, as promising catalytic materials for water splitting. An overview of the synthesis techniques is given. The advantages of wet chemistry approaches for the growth of 2D metals over deposition techniques show the potential for kinetic control that is required as a precondition to prevent isotropic growth. An uncontrolled presence of surfactant-related chemicals on a 2D metal surface is however the main disadvantage of kinetically controlled growth methods, which stimulates the development of surfactant-free synthesis approaches, especially template-assisted 2D metal growth on non-metallic substrates. Recent advances in the growth of 2D metals using a graphenized SiC platform are discussed. The existing works in the field of practical application of 2D noble metals for hydrogen evolution reaction are analyzed. This paper shows the technological viability of the "2D noble metals" concept for designing electrochemical electrodes and their implementation into future hydrogen production systems, thereby providing an inspirational background for further experimental and theoretical studies.

Solvents dramatically influence the atomic composition and catalytic properties of Ti3C2Tx MXenes

Different solvents used as dispersion media in the sample preparation of MXenes provoke changes in the material roughness and dispersibility as well as particle oxidation, resulting in different overpotential values for the catalytic hydrogen evolution reaction.

Organic interlayers boost the activity of MoS2 toward hydrogen evolution by maintaining high 1T/2H phase ratio

1T-MoS2 shows great promise for hydrogen production due to phenomenal performance in catalyzing hydrogen evolution reaction (HER) from water. However, this phase converts to low-active 2H–MoS2 on superambient heating. A series of MoS2 layered compounds (LCs) with guest organic cations was prepared to reveal the effect of cationic organics on the structure stabilization of 1T-MoS2 and thereby on the catalytic performance of this phase in HER. The results showed that LCs provide significantly higher 1T/2H ratio after heating than non-stabilized 1T phase and some of them exhibit excellent thermal durability in the capacity of HER catalysts. The effect is most pronounced for the organics remaining tightly bound to sulfide sheets in a sulfuric acid electrolyte. Cetyltrimethylammonium and bis(dimethylamino)naphthalene perfectly matched this criterion and provided a long-term maintaining of catalyst activity. The data obtained in this study are hoped to offer new approach for rational design of the efficient non-precious 1T-MoS2-based electrocatalysts for hydrogen production.

Microwave induced rapid surface amorphization of metal oxide nanowire into sulfides shell for electronically modulated efficient hydrogen evolution catalyst

Order/dis-order atomic arrangements in crystalline/amorphous structure possess a complementing effect in electro-catalysis as the former one can offer a long-range electronic conducting path, whereas the latter is rich with active sites. However, atomic level interfacing of crystalline and amorphous structures is extremely challenging and thus often end-up with uncontrolled secondary deposition one above another. Here, we are reporting second-scale atomic rearrangement of crystalline oxide nanowire surface into amorphous molybdenum sulfide shell that displayed excellent hydrogen evolution activity. Employing transient heating by microwave-activated graphene filament, surface atoms of molybdenum tungsten oxide (MoWO) nanowire can be arrested in either an amorphous or crystalline state during their sulfidation. Such a nanowire structure consisting of crystalline oxide core-amorphous sulfide shell shows excellent catalytic activity for hydrogen evolution reaction (HER) and exhibits an overpotential of 136 mV at 10 mA cm−2 in the acid electrolyte, which is much lower than the overpotential of parent oxide nanowire (356 mV) and its fully sulfurized crystalline counterpart (163 mV). As-developed catalyst also shows competitive HER performance with excellent alkaline and neutral electrolytes stability, thereby qualifying it as a pH universal catalyst. A detailed density functional theory calculation reveals oxide core promoted activation of multiple sites of amorphous sulfide phase, which provide key insights of electronic modulation via crystalline/amorphous interfacial structure for HER catalysis. This rapid and highly facile technique can lay the platform for the development of generic crystalline/amorphous core-shell nanostructure and their possible use as cost-effective and efficient catalysts.

Facile colloidal synthesis of transition metal (Co, Fe, and Ni)-added Ir-W NPs for HER in acidic electrolyte

Alloying earth-abundant transition metals with iridium, one of the promising catalysts for the hydrogen evolution reaction (HER), is an effective way to reduce the usage of precious metal without sacrificing its catalytic activity. In particular, tungsten and early transition metals (Co, Fe, and Ni) are known to promote the inherent HER activity by modulating surface adsorption energy, but ternary alloy nanoparticles (NPs) of those elements and their universal synthetic strategy are not well-established yet. In this study, we synthesized a new iridium-based ternary alloy NP with tungsten and transition metals of cobalt, iron, and nickel (Ir-M-W, M = Co, Fe, and Ni) through colloidal synthesis. All the constituent metal elements were uniformly distributed over the particle, forming a homogeneous solid solution. The Ir-M-W NPs showed excellent catalytic activity and mass activity toward HER in an acidic electrolyte compared with the binary Ir-W NPs. In particular, the Ir-Co-W NPs exhibited the lowest overpotential (35.82 mV of overpotential to drive −10 mA cm−2) and the highest mass activity (3.98 A/mgIr) with 40 µg cm−2 of catalyst loading. Such superior catalytic HER performance could be achieved by the modulation of the surface electronic structure induced by the inclusion of transition metal.

Recent Advances in Ni‐Based Electrocatalysts for Hydrogen Evolution Reaction

Electrochemical water splitting driven by the electricity generated from renewable energy is an ideal sustainable approach for hydrogen production, where an efficient hydrogen evolution reaction (HER) electrocatalyst is indispensable. Ni‐based electrocatalysts are widely investigated for HER in different electrolytes due to their advantages of high Earth abundance, remarkable corrosion resistance, efficient electrocatalytic performance, and high electrical conductivity. Although several Ni‐based electrocatalysts exhibit catalytic HER performance comparable to that of state‐of‐the‐art Pt catalysts, it is still lack of practical applications possibly due to the duration limitation. In order to promote the next‐step intensive studies and applications, it is important to systematically summarize the progress of Ni‐based electrocatalysts for HER. Accordingly, this review article focuses on the recent advances in Ni‐based electrocatalysts toward HER, which includes the mechanism of the electrocatalytic HER in different electrolytes and progresses in the Ni‐based electrocatalysts toward HER. The preparation approaches, electrocatalytic activities, and overall performance comparison of the Ni‐based electrocatalysts are systemically summarized based on Ni‐based compounds, that is, phosphides, chalcogenides, nitrides, carbides, borides, alloys, and (hydro)oxides. The improvement modulation strategies including morphological/phase control, heterostructures/heterointerfaces, defect engineering, composite engineering, and heteroatom doping are thoroughly discussed. Finally, corresponding challenges and perspectives are proposed.

High-Performance Iridium Thin Films for Water Splitting by CVD Using New Ir(I) Precursors.

Thin films of iridium can be utilized in a wide range of applications and are particularly interesting for catalytic transformations. For the scalable deposition of functional Ir thin films, metalorganic chemical vapor deposition (MOCVD) is the method of choice, for which organometallic precursors that embody a high volatility and thermal stability need to be specifically tailored. Herein, we report the synthesis, analysis, and evaluation of new volatile Ir(I)-1,5-cyclooctadiene complexes bearing all-nitrogen coordinating guanidinate (N,N'-diisopropyl-2-dimethylamido-guanidinate (DPDMG)), amidinate (N,N'-diisopropyl-amidinate (DPAMD)), and formamidinate (N,N'-diisopropyl-formamidinate (DPfAMD)) ligands. The amidinate-based Ir complex [Ir(COD)(DPAMD)] together with O2 was implemented in MOCVD experiments resulting in highly crystalline, dense, and conductive Ir films on a variety of substrate materials. The Ir deposits achieved outstanding electrochemical performance with overpotentials in the range of 50 mV at -10 mA·cm-2 for catalytic hydrogen evolution reaction (HER) in acidic solution. The ability to deposit Ir layers via MOCVD exhibiting promising functional properties is a significant step toward large-scale applications.

Cu/Cu3P@CNT Flexible Film Electrode for Highly Efficient Electrocatalytic Hydrogen Evolution in Alkaline Solution

AbstractGlobal warming and the global energy crisis are worsening owing to the increasing consumption of fossil fuels; thus, there is a need to explore renewable and clean energy sources. The energy conversion efficiency of water electrolysis – an important and feasible means to obtain green hydrogen energy – can be improved by developing catalytic electrode materials with high activity and low overpotential. Herein, low‐temperature phosphating of copper deposited on flexible carbon nanotube (CNT) films was conducted to fabricate high‐performance Cu/Cu3P@CNT electrocatalysts for the hydrogen evolution reaction (HER). The optimal conditions for the preparation of catalytic electrodes were determined by adjusting the electrodeposition time. Owing to the porous CNT framework and the heterostructure of the Cu3P/Cu nanoparticles, the Cu/Cu3P@CNT exhibited long‐term stability and highly efficient catalytic HER in an alkaline electrolyte with an overpotential of only 77 mV at 10 mA cm−2. This study provided a facile strategy for designing inexpensive and efficient electrocatalytic HER systems using highly active transition metal phosphides heterostructures and CNT materials.

A novel 0D/2D/2D hetero-layered nitrogen-doped graphene/MoS2 architecture for catalytic hydrogen evolution reaction

• A hybrid of Ni@N-Gr with exfoliated MoS 2 is reasonably designed and synthesized. • Ni@N-Gr/MoS 2 provides a stimulating insight to form non-precious metal doping onto layered 2D/2D hetero-assembly. • The hybrid has high electronic conductivity and rich exposed active sites. • Ni@N-Gr/MoS 2 exhibits superior and durable electrocatalytic performance in acidic medium. Two-dimensional (2D) layered molybdenum disulfide (MoS 2 ) has emerged as a promising Pt-substituting electrocatalyst for the hydrogen evolution reaction (HER) recently. Despite the ever-growing interest in these MoS 2 based-materials, their catalytic performance is still far from satisfactory due to the nature of the inactive basal plane and poor conductivity. This work presents a novel nickel@nitrogen‐doped graphene@MoS 2 (Ni@N-Gr/MoS 2 ) triad heterostructure for catalytic HER. The as-synthesized Ni@N-Gr/MoS 2 assembly exhibits high intrinsic HER activity with a low overpotential of 270 mV at 10 mA cm −2 , an onset overpotential of 60 mV, a small Tafel slope of 56 mV dec −1 , and robust stability in 0.5 M H 2 SO 4 . The notable activity of the Ni@N-Gr/MoS 2 composites is attributed to two main causes (i) presence of further catalytic active sites due to the incorporation of Ni@N-Gr in the basal plane of MoS 2 (ii) improvement of the conductivity due to nitrogen-doped Gr; where a fast electron transfer takes place from Ni@N-Gr core to MoS 2 during HER. Our novel-hybrid strategy is promising as an electrochemical platform in industrial hydrogen production and hence can stimulate further structural realizations to employ other metal species.

Facile electrochemical preparation of NiFeP submicron-spheres as an efficient electrocatalyst for the electrochemical hydrogenation of 4-nitrophenol at neutral pH

Electrochemical hydrogenation with water as the proton source serves as a clean and sustainable alternative to the removal and upcycling of 4-nitrophenol (4-NP), one of the major toxic pollutants commonly present in industrial wastewater effluents, into value-added chemicals (e.g., 4-aminophenol), but the development of high-performance electrocatalyst for electrochemical hydrogenation of 4-NP (e-NPR) at neutral pH remains challenging. Herein, we represent the facile electrochemical synthesis of micro-structured nickel-iron phosphide (microNiFeP), which acts as an efficient electrocatalyst in catalyzing hydrogen evolution reaction (HER) and e-NPR at neutral pH. The effects of electrosynthetic conditions on the physicochemical and electrocatalytic properties of microNiFeP were investigated and optimized. microNiFeP, prepared with optimized conditions, consisted of submicron-sized spheres (diameter: ∼ 200 nm) with wrinkled surface and exhibited promising HER activity, reaching a turnover frequency (TOF) of 47.18 ± 4.10 h-1 at − 0.2 V vs. reversible hydrogen electrode (RHE) at pH 7 in the absence of 4-NP. In the presence of 4-NP, however, microNiFeP exhibited high e-NPR activity and selectivity against HER. Notably, microNiFeP showed a TOF of 5.12 ± 0.48 h-1 for e-NPR at 0.0 V vs. RHE and enabled conversion of 89.6 ± 3.3 % with a Faradic efficiency of 81.98 ± 5.08 % for 4-aminophenol production after 8-h-controlled-potential electrolysis at − 0.2 V vs. RHE. Mechanistic studies reveal that e-NPR at microNiFeP involved the transfer of 5.3 ∼ 5.8 electrons and may involve the formation of 4,4'-dihydroxyazobenzene as one of the side products.

NiFeOxHy/Ni3Fe interface design via electropassivation for superior catalysis of HER

Herein, a novel tailor-tuned NiFeOxHy/Ni3Fe catalyst is introduced for use as an efficient cathode for the hydrogen evolution reaction (HER). It shows a marked electrocatalytic activity among several NiFe-based catalysts approaching those of noble-metal-based catalysts. That is, low overpotentials of −70 mV and −196 mV are enough to drive 10 mA cm–2 and 100 mA cm–2, respectively, of hydrogen generation in 1.0 M KOH. A foam-like film of Ni3Fe alloy is electrodeposited using the dynamic hydrogen bubble template (DHBT) method and shows a reasonable HER activity by delivering 10 mA cm–2 at −190 mV. Ni3Fe alloy is subsequently electropassivated by controlled repetitive cycling of potential in 1.0 M KOH solution to create NiFeOxHy/Ni3Fe interfaces, so a fast water cleavage occurs during HER boosting the overall rate. The electropassivation treatment is proven a successful technique to develop such metal/oxygenated metal junctions for improved HER kinetics in alkaline media. Various physicochemical analysis techniques such as XRD, SEM, EDS-mapping, ICP-OES, and XPS are employed to characterize the designed catalysts. The proposed NiFeOxHy/Ni3Fe catalyst layer is among the promising earth-abundant catalytic materials used for the electrolytic production of hydrogen in alkaline media.

Hydrogen Evolution Reaction By Metal-Free Poly-Neutral Red Electrocatalyst

While electric power supply by renewable sources such as solar and wind has become viable for their significant cost reduction, its intermittency demands urgent development of large scale storage technologies. Although conversion of electrical energy into storable hydrogen via electrolysis of water is ideal, noble metals and their oxides are used as electrocatalysts for their activities and stabilities. Alternative electrocatalysts out of abundant elements are needed for sustainable technological development.Recently, some metal-free organic conductive polymers with hydrogen-bonding capabilities were found to exhibit high electrocatalytic activities towards hydrogen evolution reaction (HER) [1,2]. We have succeeded electropolymerization of neutral red (NR) resulting in a formation of conductive poly-NR (PNR) that shows a relatively high HER catalytic activity [3]. PNR possesses hydrogen-bonding N atoms as annelated in the phenazine aromatic system as well as those in the amino substituents. Electrochemical analysis combined with in situ spectroscopy as well as DFT calculation was performed to clarify the mechanism and kinetics of HER electrocatalysis by PNR.This films of PNR were obtained by electropolymerization on F-doped SnO2 coated conductive glass (FTO, Asahi Glass) and SIGRATHERM® GFA5 Carbon felt with potential cycling between -0.2 and 1.2 V (vs. Ag/AgCl) at 50 mV s-1 for 50 times in a 5 mM NR - 0.1 M H2SO4 aqueous solution under N2. Polyaniline (PANI) was also obtained by the same method for comparison. The HER catalysis was evaluated by linear sweep voltammetry (LSV) in a 1 M trifluoromethanesulfonic acid (TfOH) under N2. The film samples were characterized by Fourier transform infrared spectroscopy (FT-IR), UV-visible spectroscopy. In-situ UV-visible spectroelectrochemical monitoring of the reaction intermediate was carried out to determine the rate of HER.PNR undergoes a pseudo-reversible reduction that shifts -61.8 mV/pH under acidic conditions and HER takes off right at this point (Fig. 1, a). Coupling of the same number of protons / electrons is expected as the electrochemical stoichometry from the observed Nernstian relationship, namely, resulting in a singly reduced PNR-H or doubly reduced PNR-H2 as depicted in Fig. 1 b. Protonation of N atoms was reasonably expected and also supported by the DFT calculation. The hydrogen atoms stabilized in the reaction intermediate can be associated to release H2 (Tafel mechanism) to complete the cycle of electrocatalysis of HER.Reduction of PNR in fact is associated with a color change. The broad reddish absorption of PNR peaking at around 500 nm attenuates to change the color to a pale yellow by constantly applying -0.15 V vs. RHE to produce PNR-H and/or PNR-H2 state as shown in Fig. 1c. Gradual recovery of the original red PNR was observed under open circuit under N2, associated with the H2 release. Thus, the rate of the spectral change is analyzed to determine the rate of the Tafel process. Pseudo-first order reaction rate law can be applied as proton is abundant and its concentration is constant, so that the rate of HER (r H2) is simply described as the rate of consumption of PNR-H and/or PNR-H2 as, r H2=-dC PNR-H2/dt =-kt (1)The ratio of PNR-H and PNR-H2 as compared to their initial amount (x PNR-red) was defined from the absorbance at 545 nm before and after reduction as, x PNR-red = C PNR-red/C 0-PNR-red = exp (-kt) (2)Good fitting of the experimental data was obtained to yield a pseudo-first order reaction rate constant of 9.98×10-4 s-1 for this rate-limiting step.

Hexagonal NiO nanosheets on Ni-foam as an electrocatalyst for high-performance water splitting application

• NiO electrocatalyst displays excellent electrocatalytic activities. • Low overpotential of 83 mV during hydrogen evolution reaction. • Low overpotential of 380 mV during oxygen evolution reaction. • Low Tafel slope of 209 mV dec −1 during hydrogen evolution reaction. • Low Tafel slope of 299 mV dec −1 during oxygen evolution reaction. This study illustrates the catalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activities of the hexagonal NiO nanosheets on Ni-foam. The NiO electrocatalyst demonstrates the low overpotentials of 83 mV/380 mV and low Tafel slopes of 209 mV dec −1 /299 mV dec −1 during HER/OER, respectively. It also reveals excellent stability, showing a slight change in overpotentials of 116 mV and 430 mV during HER/OER after 2000 CV cycles. Electrochemical impedance spectroscopy spectra show the series resistance of 0.83 Ω/1.04 Ω and charge-transfer resistance of 1.63 Ω/4.70 Ω during HER/OER after 2000 CV cycles.