Transition metal carbides coupled with nitrogen-doped carbon as efficient and stable Bi-functional catalysts for oxygen reduction reaction and hydrogen evolution reaction

Developing non-precious metal catalysts for oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for proton exchange membrane fuel cell (PEMFC), metal-air batteries and water splitting. Here, we report a in-situ simple approach to synthesize ultra-small sized transition metal carbides (TMCs) nanoparticles coupled with nitrogen-doped carbon hybrids (TMCs/NC, including WC/NC, V₈C₇/NC and Mo₂C/NC). The TMCs/NC exhibit excellent ORR and HER performances in acidic electrolyte as bi-functional catalysts. The potential of WC/NC at the current density of 3.0 mA cm^?2 for ORR is 0.814 V (vs. reversible hydrogen electrode (RHE)), which is very close to Pt/C (0.827 V), making it one of the best TMCs based ORR catalysts in acidic electrolyte. Besides, the TMCs/NC exhibit excellent performances toward HER, the Mo₂C/NC only need an overpotential of 80 mV to drive the current density of 10 mA cm^?2, which is very close to Pt/C (37 mV), making it the competitive alternative candidate among the reported non-precious metal HER catalysts.     

Phytic acid-coated titanium as electrocatalyst of hydrogen evolution reaction in alkaline electrolyte

The phytic acid-coated titanium (IP6/Ti) electrode was prepared through a simple drop-drying process, with an aim of improving electrocatalytic activity toward the hydrogen evolution reaction (HER). Scanning electron microscope and X-ray photoelectron spectroscopy showed that the IP6 coated the substrate surface uniformly and completely. Evaluation of the electrode activity was carried out in 1.0 M NaOH by linear polarization, electrochemical impedance spectroscopy (EIS) and chronopotentiometry. The kinetic parameters obtained from Tafel curves reveal that the IP6 coating can enhance the exchange current density of the HER by 489 times compared to the bare Ti, and reduce the HER activation energy by nearly 50%. The EIS data prove that the charge transfer resistance of the HER was considerably reduced due to the IP6 coating, with a decrease in real surface area of the electrode. The catalytic effect of IP6 is due to an improvement in the charge transfer kinetics of the HER. This work indicates that IP6 may be a potent candidate as a catalyst for hydrogen energy production.     

Electrocatalytic properties of porous Ni-Co-WC composite electrode toward hydrogen evolution reaction in acid medium

Porous Ni-Co-(WC)_x ternary composite electrodes were fabricated by means of electrodeposition on a foam Ni substrate. The surface morphology and microstructure of the electrodes were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrocatalytic properties of porous Ni-Co-(WC)_x electrodes for hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ solution at temperatures from 25 to 50 °C were conducted by means of cathodic polarization, electrochemical impedance spectroscopy (EIS), cyclic voltammetry and chronoamperometry (CA). These Ni-Co-WC electrodes are efficient electrocatalysts for HER. Compared with the porous Ni-Co electrode, the porous Ni-Co-(WC)_x electrode exhibited a lower HER overpotential, a lower electrochemical impedance, a lower apparent activation energy and a higher exchange current density. The apparent exchange current density of porous Ni-Co-(WC)_x (x = 10, 20, 30 and 40 g/l) is 2.01, 3.01, 7.8 and 19.91 times of porous Ni-Co electrode, respectively. With the increase of WC concentration and temperature, the apparent exchange current density of HER was enhanced. With the increase of WC concentration and potential, the HER resistance and the activation energy decreased. The Ni-Co-(WC_)x electrode exhibited superior corrosion resistance and stability for HER.     

Electrocatalytic properties for the hydrogen evolution of the electrodeposited Ni–Mo/WC composites

The catalytical activity for the hydrogen evolution reaction (HER) of the electrodeposited Ni–Mo/WC composites is examined in 1 M KOH solution. The structure, surface morphology and surface composition is investigated using the scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The electrocatalytic properties for the HER is evaluated based on the cathodic polarization, electrochemical impedance, cyclic voltammetry and chronopotentiometry methods. The obtained results prove the superior catalytic activity for the HER of Ni–Mo/WC composites to Ni–Mo alloy. The catalytic activity of Ni–Mo/WC electrodes is determined by the presence of WC nanoparticles and Mo content in the metallic matrix. The best electrocatalytic properties are identified for Ni–Mo/WC composite with the highest Mo content and the most oxidized surface among the studied coatings. The impedance results reveal that the observed improvement in the catalytic activity is the consequence of high real surface area and high intrinsic catalytic activity of the composite.     

Facile one-step synthesis of nitrogen-doped carbon sheets supported tungsten carbide nanoparticles electrocatalyst for hydrogen evolution reaction

Exploring non-precious metal catalysts with high activity and stability to replace Pt-based materials is vital for electrochemical water splitting. In this work, a facile one-step method was put forward to synthesize WC/NC composite. Due to the couple effect of KCl/NaCl salt and dicyandiamide, pure WC phase was obtained at 900 °C. Meanwhile, KCl/NaCl salt eliminated the runaway pyrolysis expansion of glucose. Besides contributing to the special surface area, dicyandiamide as N source significantly alleviated W mass loss trigged by KCl/NaCl salt and ensured the appropriate WC content in WC/NC composite. As a HER electrocatalyst in acid media, WC/NC composite exhibits the small overpotential of 156 mV at a current density of 10 mA cm⁻², the low Tafel slope of 64 mV dec⁻¹, as well as the robust stability. This work offers a feasible option to fabricate low-cost and effective transition metal carbide electrocatalysts on a large-scale for hydrogen evolution reaction.     

Bifunctional iron doped CuS catalysts towards highly efficient overall water electrolysis in the alkaline electrolyte

Efficient catalysts towards overall water electrolysis in alkaline electrolytes were highly desirable for the hydrogen production technology. The surface electronic states of copper in CuS nanocrystal catalysts were modified by iron doping through a simple wet-chemical method. The iron-doped CuS catalysts displayed drastically enhanced catalytic activities for overall water electrolysis in the strong alkaline electrolyte of 1 M KOH after a simple cyclic voltammetry activation. The optimized catalytic performance for overall water electrolysis was achieved in the CuFe_0.6S_1.6 catalyst, which exhibited a low overpotential of ?237 mV for HER and 302 mV for OER to reach 10 mA cm^?2. The high activities for overall water electrolysis in CuFe_0.6S_1.6 were induced by the enhanced charge transfer from Cu to S via iron doping, which not only modified the surface electronic state of copper but also enhanced charge transfer during the electrochemical reactions. Moreover, the catalysts displayed satisfying stability for over 20 h at a high current density of 300 mA cm^?2 for both HER and OER, showing great potential for industrial water electrolysis.     

Assessment of the roughness factor effect and the intrinsic catalytic activity for hydrogen evolution reaction on Ni-based electrodeposits

The hydrogen evolution reaction (HER) was studied in 30 wt.% KOH solution at temperatures ranging between 30 and 80 °C on three type of electrodes: (i) rough pure Ni electrodeposits, obtained by applying a large current density; (ii) smooth NiCo electrodeposits; (iii) smooth commercial Ni electrodes. By using steady-state polarization curves and electrochemical impedance spectroscopy (EIS) the surface roughness factor and the intrinsic activities of the catalytic layers were determined. These techniques also permitted us to determine the mechanism and kinetics of the HER on the investigated catalysts. Different AC models were tested and the appropriate one was selected. The overall experimental data indicated that the rough/porous Ni electrode yields the highest electrocatalytic activity in the HER. Nevertheless, when the effect of the surface roughness was taken into consideration, it was demonstrated that alloying Ni with Co results in an increased electrocatalytic activity in the HER when comparing to pure Ni. This is due to an improved intrinsic activity of the material, which was explained on the basis of the synergism among the catalytic properties of Ni (low hydrogen overpotential) and of Co (high hydrogen adsorption).     

Effect of surface carbon on the hydrogen evolution reactivity of tungsten carbide (WC) and Pt-modified WC electrocatalysts

Tungsten carbide (WC) has been previously identified as both an electrocatalyst and a support for several types of electrochemical reactions. The synthesis of WC often leads to excess surface carbon that can greatly affect its electrocatalytic activity. This work will evaluate the effect of surface carbon on WC both as a catalyst and as a support for monolayer (ML) amounts of platinum (Pt). WC thin films with no surface carbon, along with those with 1, 2, 3, or 4 equivalent ML of surface carbon, were synthesized. The hydrogen evolution reaction (HER) activity was used as a probe to test the effect of surface carbon on the electrochemical activity of WC and 1 ML Pt on WC (Pt/WC) using linear sweep voltammogram (LSV) in 0.5 M sulfuric acid. The HER activity of WC was relatively unaffected for very small amounts of surface carbon but decreased when several MLs or more of surface carbon was present. Pt/WC without surface carbon was found to have slightly higher HER activity as compared to Pt deposited on WC with surface carbon.     

Electrocatalytic studies of siloxene sheets encrusted with cobalt chalcogenides (S,Se) for water splitting

Two-dimensional siloxene sheets were superficially coated with cobalt chalcogenides to optimize interfacial properties for broad applications in the field of catalysis. These catalytic composites were investigated for electrochemical water splitting in an alkaline electrolyte medium. The synthesis of siloxene sheet-cobalt chalcogenides composites was confirmed by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, adsorption studies, and X-ray photoelectron spectroscopy analyses. Potentiometric and impedimetric experiments were performed to understand the inherent electrocatalytic activity of the developed catalysts. Variations in the onset potential and overpotential at a constant current density of ±10 mA/cm² for hydrogen and oxygen evolution reactions—HER and OER, respectively—were evaluated with respect to a reversible hydrogen electrode (RHE). The catalysts exhibit superior current and catalytic activity due to interfacial kinetics, retaining lower Tafel slopes of ～30 mV/dec for the OER and HER; they also exhibited improved, long-term stability for 12 h, indicating potential utility in commercial applications.     

Preparation of NiCo-LDH@NiCoV-LDH interconnected nanosheets as high-performance electrocatalysts for overall water splitting

Alkaline water electrolysis is a promising strategy for the production of hydrogen and oxygen. However, developing high-efficiency non-precious electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is still a big challenge. Here, we report a nickel foam-based electrode coated with NiCoV-LDH and NiCo-LDH nanosheets (denoted as NiCo-LDH@NiCoV-LDH/NF) by a two-step method for efficient water splitting performance. The NiCo-LDH@NiCoV-LDH/NF with unique nanosheet-on-nanosheet construction can enlarge the electrochemical active specific surface area greatly, and thus accelerate the charge transfer of electrocatalytic reactions. Besides, the doping of vanadium could also improve the OER performance. The electrode only requires a low overpotential for OER (260 mV at 100 mA cm^?2), and HER (80 mV at 10 mA cm^?2) reactions in 1.0 mol/L KOH solution at room temperature. Furthermore, in the two-electrode water splitting test, a current density of 10 mA cm^?2 was achieved at 1.55 V using 1.0 mol/L KOH solution, with excellent durability of 40 h. This work provided a facile method for developing new bifunctional catalysts.     

Enhancement of hydrogen evolution reaction by Pt nanopillar-array electrode in alkaline media and the effect of nanopillar length on the electrode efficiency

Pt nanopillar-array 3D electrodes with nanopillar length of 150, 450 and 900 nm and nanopillar density of ~10⁹ cm⁻² were fabricated. Their catalytic activity for hydrogen evolution reaction (HER) was evaluated by linear sweep voltammetry and electrochemical impedance spectroscopy. In comparison with straightly electrodeposited black Pt film and forged Pt sheet electrodes, the HER current density has been significantly improved by the nanopillar-array architecture. The overpotential of HER at current density of 10 mA cm-2 at 26 °C is as low as 78 mV, lower than the black Pt film of 107 mV and the Pt sheet of 174 mV. The improvement of HER is ascribed to the low charge transfer resistance of the 3D electrode and the high desorption capability of hydrogen bubbles at the nanotips. Interestingly, the nanopillar-array 3D electrode has an optimal nanopillar length for HER. The mechanisms for the optimal nanopillar length were investigated here.     

Is platinum necessary for efficient hydrogen evolution? – DFT study of metal monolayers on tungsten carbide

In this work WC-supported metal monolayers (Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au) are investigated using Density Functional Theory in order to establish general trends regarding monolayer stability, electronic structure and reactivity. Using calculated hydrogen–metal bond energies and available data on the exchange current densities (j₀) for hydrogen evolution reaction (HER) volcano-type curve is obtained enabling prediction of HER j₀ for the entire series of M_ML/WC systems not considered so far as HER electrocatalysts. Among investigated surfaces, Cu_ML/WC(0001) and Rh_ML/WC(0001) are identified as promising HER electrocatalysts with (i) HER exchange current density matching the one of Pt and (ii) stability in electrochemical environment under HER conditions. Provided results point to a general conclusion that Pt might not be necessary for efficient catalysis of hydrogen electrode reactions – superior catalysts can be obtained by rational design approach with suitable choice of overlayer/support system not involving Pt at all.     

Electrostatic spray catalytic particle coating on carbon electrode for enhancing electrochemical reaction

The uniform coating of catalytic materials on the electrode surface is essential for improving the performance of electrochemical reactions. In this study, the electrostatic spray coating technique is used to deposit Molybdenum disulfide (MoS₂) particles on carbon electrodes and examine the electrochemical hydrogen evolution reaction (HER) performance in a 0.1 M H₂SO₄ electrolyte. The experimental results reveal that the electrostatic spray coating method achieves a more uniform coating MoS₂ catalyst particles on the carbon fiber substrate. It provides good adhesion for electrocatalyst binding on the carbon paper electrodes. During HER analysis, a 25% higher current density is obtained from the electrostatic spray-coated electrodes compared to the manual spray-coated electrode at a potential of 0.95 V. Electrochemical impedance spectroscopy, and electrochemical surface area measurements indicate the superior electrochemical characteristics of the electrostatic spray-coated electrode. The faradaic efficiency of the electrostatic spray-coated electrode is close to 100% at a high applied potential of −1.6 V with a 0.0059 L volume of hydrogen generation. Moreover, chronoamperometric measurements demonstrate the excellent durability of the electrostatic spray-coated carbon electrodes. This study suggests that the facile electrostatic spray method is efficient for coating electrodes with high uniformity and good adhesion for various electrochemical applications.     

Pulse-reverse electrodeposition of Ni–Mo–S nanosheets for energy saving electrochemical hydrogen production assisted by urea oxidation

Electrochemical hydrogen production from water splitting is one of the effective methods for hydrogen production that has recently attracted particular attention. One of the limitations of the electrochemical water splitting method is the slow oxygen evolution reaction (OER), which leads to an increase in overpotential and a decrease in hydrogen production efficiency. Here, Ni–Mo–S ultra-thin nanosheets were synthesized using the pulse reverse electrochemical deposition technique, and then this electrode was used as an electrode material for accelerating hydrogen evolution reaction (HER) and urea oxidation reaction (UOR). Remarkably, the optimized electrode needs only 74 mV to attain the 10 mA cm⁻² current density in HER and require only 1.3 V vs RHE potential in the UOR process. Also, results showed that the replacement of the UOR with the OER process resulted in a significant improvement in the electrochemical production of hydrogen in which for delivering the current density of 10 mA cm⁻² in overall urea electrolysis, only 1.384 V is needed. In addition, outstanding catalytic stability was obtained, after 50 h electrolysis, the voltage variation was negligible. Such outstanding catalytic activity and stability was due to 3-D ultrathin nanosheets, the synergistic effect between elements, and the superhydrophilic/superaerophobic nature of fabricated electrode.     

Facile hydrothermal synthesis of rare earth phosphate for boosting hydrogen evolution reaction

The water electrolysis process has attracted great attention due to the production of high energy density pure hydrogen. However, the involved cell reactions in this process such as hydrogen and oxygen evolution reactions are kinetically sluggish and demands high input energy to accelerate the rate of these reactions. Therefore, the development and application of efficient electrocatalyst is essential for hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER). In the present work, we have successfully synthesized two rare earth phosphates through the hydrothermal route and used as a catalysts towards HER in an acidic medium. The rare earth phosphate PrPO₄ exhibits better catalytic activity than YPO₄ catalyst. The overpotential of PrPO₄, YPO₄ and standard Pt/C were found as 147, 484.3 and 58 mV vs. reversible hydrogen electrode, respectively, to reach current density 10 mA·cm^?2 and corresponding Tafel slopes were found as 107.58, 118.73 and 80.89 mV decade^?1, respectively in 0.5 M H₂SO₄. The catalytic activity of PrPO₄ (472.83 mA·cm^?2) overcome standard Pt/C (179.60 mA·cm^?2) at high overpotential 450 mV vs. reversible hydrogen electrode. The prepared PrPO₄ shows efficient electrocatalytic activity towards HER in acidic medium because it possess high BET surface area, large ECSA value and small charge transfer resistance than YPO₄.     

Catalytic enhancement of hydrogen evolution reaction on copper in the presence of benzylamine

Benzylamine catalytic effect on the cathodic hydrogen evolution in acid aqueous solution has been studied on copper electrode. Experimental results obtained by voltammetric techniques emphasize that the presence of benzylamine in 1 M sulfuric acid solution enhances the hydrogen evolution reaction (HER). In such a solution, benzylamine is protonated and exists as benzylammonium cation. Kinetic parameters of HER have been determined, observing a significant increase of the exchange current in the presence of benzylamine. Also, a decrease of the cathodic transfer coefficient has been noticed. Activation energy for HER has been computed on the basis of the Arrhenius plots. In the presence of benzylamine the activation energy was 33.0 kJ mol⁻¹, whereas without benzylamine a greater value was obtained: 94.6 kJ mol⁻¹. The catalytic effect of benzylamine was confirmed by electrochemical impedance spectroscopy. The considerable diminution of the double layer capacity is a valid proof of the strong adsorption of benzylamine at the Cu - sulfuric acid solution interface, while the reduction of the charge transfer resistance is a symptom of the enhancement of HER. Ab initio molecular modeling of benzylammonium ion showed that, due to a high value of the dipole moment, the molecules are oriented in the most favorable position for the charge transfer reaction, i.e., with the protonated amino group facing toward the electrode.     

Noble fabrication of Ni–Mo cathode for alkaline water electrolysis and alkaline polymer electrolyte water electrolysis

Among the catalysts for hydrogen evolution reaction (HER) in alkaline media, Ni–Mo turns out to be the most active one. Conventional preparations of Ni–Mo electrode involve repeated spraying of dilute solutions of precursors onto the electrode substrate, which is time-consuming and usually results in cracking and brittle electrodes. Here we report a noble fabrication of Ni–Mo electrode for HER. NiMoO₄ powder was synthesized and used as the precursor. After reduction in H₂ at 500 °C, the NiMoO₄ powder layer was converted to a uniform and robust electrode containing metallic Ni and amorphous Mo(IV) oxides. The distribution of Ni and Mo components in this electrode is naturally uniform, which can maximize the interaction between Ni and Mo and benefit the electrocatalysis. The thus-obtained Ni–Mo electrode exhibits a very high catalytic activity toward the HER: the current density reaches 700 mA/cm² at 150 mV overpotential in 5 M KOH solution at 70 °C. This new fabrication method of Ni–Mo electrode is not only suitable for alkaline water electrolysis (AWE), but also applicable to the alkaline polymer electrolyte water electrolysis (APEWE), an emerging technique for efficient production of H₂.     

DFT study of platinum and palladium overlayers on tungsten carbide: Structure and electrocatalytic activity toward hydrogen oxidation/evolution reaction

Platinum and palladium overlayers on W- and C-terminated WC(0001) surface, at coverage ranging from 0.25 ML to 2 ML, have been studied using DFT approach. Strong adhesion of metal monolayers to the WC support, accompanied by a modification of electronic structure, was evidenced. Calculated values of hydrogen binding energy on studied metal overlayers were correlated to available experimentally determined values of exchange current densities of hydrogen oxidation/evolution reaction (HOR/HER), resulting with the volcano curve with both explanatory and predictive power. None of the investigated metal/WC surfaces were found to exceed the Pt(111) surface in terms of catalytic activity toward HER/HOR. The obtained results indicate that WC may efficiently replace the precious metal support in the Pt (Pd) core–shell electrocatalyst for HER/HOR, but no synergism arising due to electronic effects of WC support was evidenced.     

Strategies for improving Co/Ni-based bimetal-organic framework to water splitting

The preparation of high-efficiency, stable, and low-cost oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) electrocatalysts remains a challenge for new energy systems. In this study, three-dimensional (3D) cobalt-nickel bimetal MOFs were used as precursors to synthesize catalysts through thermal decomposition, carbonization, nitriding, oxidation, phosphating, sulfurizing, and selenization, respectively. In 1.0 M KOH electrolyte, the overpotential of Co/Ni-MOFs@Se for OER was 238 mV and the that of Co/Ni-MOFs@P for HER was 194 mV at a current density of 10 mA cm⁻². Based on the excellent OER and HER performances of Co/Ni-MOFs@Se and Co/Ni-MOFs@P, these two materials were further assembled into electrodes for overall water splitting. Results showed that a potential of only 1.59 V was required to provide a current density of 10 mA cm⁻². The electrodes also exhibited long-term durability in a 2000 min stability test without significant changes in the catalytic performances. According to the difference in the doped non-metal elements, an electrode pair with a suitable matching degree was constructed, thereby improving the overall water splitting performance. Thus, the controllable modification of the metal-organic frameworks (MOFs)-derived carbon materials (CMs) effectively improved the materials’ catalytic water splitting performance. It was possible to further develop an efficient, inexpensive, and low-cost assembled electrode pair.     

Novel 3-D urchin-like Ni– Co–W porous nanostructure as efficient bifunctional superhydrophilic electrocatalyst for both hydrogen and oxygen evolution reactions

Fabrication of an electrocatalyst with remarkable electrocatalytic activity for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is important for the production of hydrogen energy. In this study, Ni–Co–W alloy urchin-like nanostructures were fabricated by binder-free and cost-effective electrochemical deposition method at different applied current densities and HER and OER electrocatalytic activity was studied. The results of this study showed that the microstructure and morphology are strongly influenced by the electrochemical deposition parameters and the best electrocatalytic properties are obtained at the electrode created at the 20 mA.cm⁻²applied current density. The optimum electrode requires −66 mV and 264 mV, respectively, for OER and HER reactions for delivering the 10 mA cm⁻² current density. The optimum electrode also showed negligible potential change after 10 h electrolysis at 100 mA cm⁻², which means remarkable electrocatalytic stability. In addition, when this electrode used as a for full water splitting, it required only 1.58 V to create a current density of 10 mA cm⁻². Such excellent electrocatalytic activity and stability can be related to the high electrochemical active surface area, being binder-free, high intrinsic electrocatalytic activity and hydrophilicity. This study introduces a simple and cost-effective method for fabricating of effective electrodes with high electrocatalytic activity.