Ni Electrocatalysts Research Articles

Ni and Co-based bifunctional electrocatalysts supported on TiO2@C for oxygen evolution and reduction reactions

In the search for affordable, ecofriendly, and sustainable catalysts for energy-related processes, series of Ni and Co-based bifunctional hybrid electrocatalysts supported on TiO2@C were prepared and studied in the oxygen evolution (OER) and oxygen reduction reactions (ORR). The hybrid TiO2@C supports were prepared using three biomass precursors: furfural, chitosan and saccharose. The nature and dispersion of the metallic phases on those supports was evaluated as a function of the biomass-derived precursors. The prepared materials showed electrocatalytic activity towards ORR and OER reaction, being more active for the latter. Despite a lower performance than Pt/C benchmark, these materials are clearly at much lower costs. The long-term stability and performance of the catalysts were also performed. The number of electrons involved in both processes was determined and a mechanism is proposed for the understanding of the mechanism of reactions using the present materials. It can be concluded that the Ni and mainly Co-based electrocatalysts show a remarkable potential for use as alternative catalysts in fuel cells-related reactions.

Exploiting Non-Classical Nucleation for Improved Gas Bubble Mitigation on Nickel Electrocatalysts for the Oxygen Evolution Reaction

The efficient transport of gas poses a significant challenge in the design of high-performance gas evolving electrocatalysts. Inefficiencies due to mass transport limitations become more pronounced at the high current densities required in industrial electrolyzers, largely as a result of gas bubbles that obstruct a portion of the electrochemically active surface area of the anode and/or cathode, while also hindering electrolyte transport to the electrode surfaces. One promising approach to mitigate these sources of inefficiency is to engineer micro-to-nanoscale surface textures that promote the favorable growth and release of these bubbles. A range of structures have been shown to improve electrocatalyst performance under both stagnant and induced convection conditions, particularly in applications for hydrogen production via electrocatalytic water splitting in alkaline water electrolyzers.1 Trends between surface texture and electrocatalyst performance have been observed for a range of features, such as pillars,2,3 ridges,4 dimples,5 “cracked” structures,6 and electrodeposited crystalline structures,7 however substantial work is still needed to optimize the designs of these materials. These textured surfaces can influence bubble evolution in various ways, such as by altering the wetting properties of a surface, or by providing preferential nucleation sites. Previous studies have noted that surface structures that provide good control over bubble nucleation can often be adversely affected by increased bubble retention.3–5 The present study investigates a method of fabricating patterned electrocatalysts that provides a fine, tunable control over the distribution of bubble nucleation sites on the surfaces of planar Ni electrocatalysts for the oxygen evolution reaction (OER). These textured surfaces are designed to take advantage of so-called “non-classical nucleation” or “type IV nucleation”, a scenario in which gas is trapped at specific surface sites with an effective radius larger than the critical radius for classical nucleation, thereby allowing spontaneous bubble growth without being required to overcome an activation energy barrier.8 The separation between these sites on the surface of the OER active material (i.e., Ni) is then varied to obtain the most favorable distribution of these non-classical nucleation sites. Chronoamperometry is used to compare the current densities achieved by these patterned Ni electrodes to that of a planar Ni electrode. High speed videography is used to directly measure how these textures influence processes such as bubble growth and detachment, and to determine how these processes influence performance. A better understanding of these gas evolution processes can help lower the energy requirements for electrolyzers and may have more general implications for the design of energy efficient gas evolving electrocatalysts relevant to a variety of industrially important reactions.

Probing Electrocatalytic Synergy in Graphene/MoS2/Nickel Networks for Water Splitting through a Combined Experimental and Theoretical Lens.

The development of low-cost and active electrocatalysts signifies an important effort toward accelerating economical water electrolysis and overcoming the sluggish hydrogen or oxygen evolution reaction (HER or OER) kinetics. Herein, we report a scalable and rapid synthesis of inexpensive Ni and MoS2 electrocatalysts on N-doped graphene/carbon cloth substrate to address these challenges. Mesoporous N-doped graphene is synthesized by using electrochemical polymerization of polyaniline (PANI), followed by a rapid one-step photothermal pyrolysis process. The N-doped graphene/carbon cloth substrate improves the interconnection between the electrocatalyst and substrate. Consequently, Ni species deposited on an N-doped graphene OER electrocatalyst shows a low Tafel slope value of 35 mV/decade at an overpotential of 130 mV at 10 mA/cm2 current density in 1 M KOH electrolytes. In addition, Ni-doped MoS2 on N-doped graphene HER electrocatalyst shows Tafel slopes of 37 and 42 mV/decade and overpotentials of 159 and 175 mV, respectively, in acidic and alkaline electrolytes at 10 mA/cm2 current density. Both these values are lower than recently reported nonplatinum-group-metal-based OER and HER electrocatalysts. These excellent electrochemical performances are due to the high electrochemical surface area, a porous structure that improves the charge transfer between electrode and electrolytes, and the synergistic effect between the substrate and electrocatalyst. Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations demonstrate that the Ni hydroxide species and Ni-doped MoS2 edge sites serve as active sites for OER and HER, respectively. Finally, we also evaluate the performance of the HER electrocatalyst in commercial alkaline electrolyzers.

Introducing active sites and regulating electron distribution to design Mo and P co-doped Ni for efficient alkaline hydrogen evolution reaction

NiMoP electrocatalysts for hydrogen evolution reaction (HER) are received widespread concern, but the mechanism of Mo/P atoms for improving the HER activity still need to be revealed in details. Herein, we synthesize the Mo/P co-doped Ni (NMP) electrocatalysts, and the alkaline HER performances are tested. The NMP exhibit the excellent activity, durability and stability of material characters. Based on the results of experiments and density functional theory calculations, it is found that the introduced P atoms can regulate the electron redistribution, which can result in enhancing the reaction kinetic, increasing the active sites and charge exchange capacity. Besides, it leads the doped Mo to become the active site in both Volmer and Heyrovsky processes, and Ni bonded to Mo and Ni atoms exhibit the increasing activity. The obtained results display a deeper understanding for the doped metal and non-metallic atoms in enhancing HER activity of Ni, which is of great significant for designing the highly efficient and inexpensive electrocatalysts for producing hydrogen.

Enhancing the Oxygen Evolution Reaction Activity of Sputtered Ni, NiO, and NiNiO Thin Films by Incorporating Fe

AbstractThe effect of Fe‐containing alkaline electrolyte, on the oxygen evolution reaction (OER) activity of Ni electrocatalysts, has long been of interest as Fe increases the OER activity of Ni electrocatalysts. However, controversy exists as to whether it is surface or bulk Fe that is responsible for the increased activity. In this study, magnetron sputtering was employed to sputter Ni, NiO and NiNiO thin film electrocatalysts to study the effect that different concentrations of Fe in the electrolyte have on their OER activities. It was found that increasing concentrations of Fe increasingly enhanced the OER activity of these thin film electrocatalysts until the electrolyte was saturated with Fe. The lowest overpotential achieved is 279 mV (at 10 mA cm−2) for NiNiO cycled in KOH containing 1 mM Fe, with all three thin film electrocatalysts exhibiting overpotentials within the same range after 30 voltammetry cycles in 0.9 ppm Fe and 1 mM Fe. All Tafel slopes are between 36 and 45 mV dec−1 indicating similar kinetics for the samples cycled in different Fe concentrations. Energy‐dispersive X‐ray spectroscopy and X‐ray photoelectron spectroscopy results show that Fe is found in the top layers of the electrocatalysts after cycling.

Significance of slight ambient temperature variation on the electrocatalyst performance toward oxygen evolution reaction

Hundreds of oxygen evolution reaction (OER) electrocatalysts have been developed over the past few decades, and their performances are evaluated and compared at ambient temperature. However, the effect of ambient temperature variation on OER electrocatalyst performance has received less attention, which may play a remarkable role in the electrocatalytic activity. In this work, we systematically investigated the influence of ambient temperature variation on electrocatalyst performance toward OER. The results show that the slight ambient temperature variation has a significant effect on OER catalyst performance based on the changes of overpotential (10 mA cm−2) and Tafel slope. Both remarkable chances are observed on transition metal (Ni) and noble metal (IrO2) electrocatalysts, and the overpotentials decrease around 81 mV with a temperature increase by 20 °C (from 10° to 30°C) for both Ni and IrO2 electrocatalysts with the Tafel slope drops of 36.9 and 29.5 mV dec−1, respectively. A similar trend is also found in the electrochemically active surface (ECSA) normalized performance and the charge transfer resistance. This study demonstrates that reporting the actual operating temperature for OER is not only recommended but also necessary to evaluate and compare electrocatalyst activities from different materials systems properly, and neglecting the ambient temperature variation effect can highly mislead conclusions.

Experimental and computational insight into the role of cobalt and phosphorus in boosting Volmer and Heyrovsky steps of Ni in alkaline hydrogen evolution reaction

Deep insight into the role of doped hetero atoms in boosting the alkaline hydrogen evolution reaction (HER) activity is of significance for designing the low-cost electrocatalysts to produce hydrogen. Herein, we designed, synthesized, and optimized the Co and P co-doped Ni (NCP) electrocatalysts and evaluated the HER performance in alkaline medium. It was found that the NCP exhibited the enhanced hydrogen evolution performance, only need an overpotential of 57.0 mV to afford 10 mA·cm−2. The calculation results of density functional density revealed that the doped P plays an important role in modulating electron distribution of Ni and Co atoms, which reduce the Gibbs free energy of water molecule dissociation at Co sites in Volmer step, and optimize the ab/desorption of hydrogen at Ni site that bonded to P and Ni atoms in Heyrovsky step, respectively. The obtained results are conductive to understanding the electrocatalytic mechanism of alkaline HER.

Porosity engineering of biochar‐based single‐atom Ni catalysts to promote CO2 electroreduction over a broad potential window

AbstractDeveloping electrocatalysts for CO2 electroreduction with high activity and superior selectivity is extremely important and desirable from both academic and industrial perspectives. However, owing to competition with hydrogen evolution, highly efficient CO2 reduction is mostly achieved with high CO selectivity in a narrow potential range, which is incompatible with a large cell voltage required for industrial‐level CO2 reduction. Herein, we report an effective strategy to regulate CO2 reduction performances of single‐atom Ni electrocatalysts over a broad potential window by engineering their pore structures (micropores, mesopores, or hierarchical pores with both micropores and mesopores). It is revealed that hierarchically pores can significantly promote CO2 reduction efficiency of single‐atom Ni electrocatalysts. The hierarchically porous electrocatalyst achieves a maximum CO Faradaic efficiency (FECO) of 97.4% at −1.2 V (vs. RHE) and shows high FECO of >85% over a broad potential window from −0.7 to −1.7 V, much superior to electrocatalysts with other pore structures. More impressively, turnover frequency of the hierarchically porous electrocatalyst increases rapidly with increasing the applied potential and reaches 50,067 h−1 at −1.7 V. Such CO2 electroreduction promotion could be attributed to a synergistic effect of micropores for enhancing CO2 adsorption and mesopores for facilitating rapid release of product bubbles, which significantly improves CO2 reduction and suppresses hydrogen evolution.

The metal ion in single-atom NO reduction electrocatalysts dictates product selectivity

Nitric oxide is a major air pollutant associated with environmental problems, including acid rain, smog, and depletion of the ozone layer. In this issue of Chem Catalysis, Zhang, Li, and co-workers develop mono-dispersed single-atom transition-metal (Fe, Co, and Ni) electrocatalysts, where the metal ion dictates product selectivity, to reduce NO.

Efficient oxygen evolution on spinel MFe2O4 (M = Zn and Ni) electrocatalysts

Efficient oxygen evolution on spinel MFe2O4 (M = Zn and Ni) electrocatalysts

Characterization of paramagnetic states in an organometallic nickel hydrogen evolution electrocatalyst

Significant progress has been made in the bioinorganic modeling of the paramagnetic states believed to be involved in the hydrogen redox chemistry catalyzed by [NiFe] hydrogenase. However, the characterization and isolation of intermediates involved in mononuclear Ni electrocatalysts which are reported to operate through a NiI/III cycle have largely remained elusive. Herein, we report a NiII complex (NCHS2)Ni(OTf)2, where NCHS2 is 3,7-dithia-1(2,6)-pyridina-5(1,3)-benzenacyclooctaphane, that is an efficient electrocatalyst for the hydrogen evolution reaction (HER) with turnover frequencies of ~3,000 s−1 and a overpotential of 670 mV in the presence of trifluoroacetic acid. This electrocatalyst follows a hitherto unobserved HER mechanism involving C-H activation, which manifests as an inverse kinetic isotope effect for the overall hydrogen evolution reaction, and NiI/NiIII intermediates, which have been characterized by EPR spectroscopy. We further validate the possibility of the involvement of NiIII intermediates by the independent synthesis and characterization of organometallic NiIII complexes.

One step fabrication of nanostructured nickel thin films on porous nickel foam for drastic electrocatalytic oxygen evolution

The oxygen evolution reaction (OER) is crucial for sustainable hydrogen production through competitive water electrocatalysis. In this work, nanostructured nickel (Ni) thin films deposited on porous nickel foam (NF) substrate are investigated for improved OER catalysis in alkaline medium. The time-dependent fabrication of Ni thin films is achieved via aerosol-assisted chemical vapor deposition (AACVD), which has shown promising impact on the OER performance. Particularly, the nanoscale Ni@NF electrocatalyst after 60-min of deposition showed outstanding OER properties including minimum overpotential of 285 and 423 mV to reach the current decade (10 mAcm−2) and even higher than 1000 mA/cm2, respectively. The electrode exhibits a small Tafel slope with a value of 70 mV dec−1, a higher TOF, a large electrochemically active surface area, better charge transport performance, and long-term stability over 20 h of continuous operation without significant loss. This OER catalytic activity for pristine Ni electrocatalysts is a significant milestone and is found much better than the benchmark noble metal OER catalysts as well as many other well-known 3D transition metal catalysts. The impressive OER performance is attributed to the synergic effect generated between nanostructured-Ni and porous NF substrate, which enhances the electrical conductivity of the designed catalysts. The low manufacturing cost, robustness, and durability make this catalyst viable in solar energy conversion and storage applications.

Connected design of Ni foam-supported porous Ni and FeOOH/porous Ni electrocatalysts for overall water splitting in alkaline media

The rational construction of advanced non-noble metal-based electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is extremely significant in promoting hydrogen energy from theory to practical application. Herein, we designed a connected preparation route to successfully construct Ni foam-supported porous nickel (labeled as Ni/NF) and FeOOH/porous nickel (labeled as FeOOH/Ni/NF) microstructures. It was found that the as-constructed Ni/NF and FeOOH/Ni/NF integral electrodes separately presented superb HER and OER electrocatalytic activities. In 1 M KOH solution, the Ni/NF and FeOOH/Ni/NF electrodes required overpotentials of 48 mV for HER and 191 mV for OER to deliver the current density of 10 mA cm−2, respectively. At the same time, the chronopotentiometry measurements showed that as-constructed Ni/NF and FeOOH/Ni/NF integral electrodes could continuously work over 180 h at 50 mA cm−2 for HER (Ni/NF) and 300 h at 100 mA cm−2 for OER (FeOOH/Ni/NF), exhibiting excellent long-term stability. In a two-electrodes system employing the Ni/NF as the cathode and the FeOOH/Ni/NF as the anode, more importantly, only a voltage of 1.49 V was required to reach the current density of 10 mA cm−2 with outstanding durability of 200 h at 50 mA cm−2 and high Faradaic efficiency. The present work provides a simple and relied technique to construct connected non-noble metal-based HER and OER electrocatalysts for practical applications.

Recent developments and prospects for engineering first-row transition metal-based catalysts for electrocatalytic NOx− reduction to ammonia

First-row transition metal-based electrocatalysts, including Cu, Fe, Co, Ni, and Ti-based electrocatalysts, for high-efficiency NOx− reduction are reviewed. These electrocatalysts should possess three advantages indicated in the figure above.

Fabrication of Trimetallic Fe-Co-Ni Electrocatalysts for Highly Efficient Oxygen Evolution Reaction.

The development of inexpensive and well-activated water-splitting catalysts is required to reduce the use of conventional fossil fuels. In this study, a trimetallic Fe-Co-Ni catalyst was fabricated using a simple ion electrodeposition method. The metal deposition was performed using cyclic voltammetry, which was more efficient than constant-voltage deposition and significantly increased the stability of the catalyst. The synthesized material presented the morphology of a nanoflower in which the nanosheets were agglomerated. The Fe-Co-Ni catalyst exhibited excellent oxygen evolution reaction (OER) properties because the charge-transfer rate was improved owing to the synergistic effect of the metals. The OER was performed in a 1 M KOH solution using a three-electrode system, and the overpotential was 302 mV at 100 mA/cm2. In addition, the Fe-Co-Ni catalyst exhibited excellent stability in alkaline solution for more than 48 h at 200 mA/cm2. The results show that the method for preparing Fe-Co-Ni significantly improves its catalytic activity, and the resulting material could be used as an economical and efficient catalyst in future.

Investigation of the Structure of Atomically Dispersed NiNx Sites in Ni and N-Doped Carbon Electrocatalysts by 61Ni Mössbauer Spectroscopy and Simulations.

Ni and nitrogen-doped carbons are selective catalysts for CO2 reduction to CO (CO2R), but the hypothesized NiNx active sites are challenging to probe with traditional characterization methods. Here, we synthesize 61Ni-enriched model catalysts, termed 61NiPACN, in order to apply 61Ni Mössbauer spectroscopy using synchrotron radiation (61Ni-SR-MS) to characterize the structure of these atomically dispersed NiNx sites. First, we demonstrate that the CO2R results and standard characterization techniques (SEM, PXRD, XPS, XANES, EXAFS) point to the existence of dispersed Ni active sites. Then, 61Ni-SR-MS reveal significant internal magnetic fields of ∼5.4 T, which is characteristic of paramagnetic, high-spin Ni2+, in the 61NiPACN samples. Finally, theoretical calculations for a variety of Ni-Nx moieties confirm that high-spin Ni2+ is stable in non-planar, tetrahedrally distorted geometries, which results in calculated isotropic hyperfine coupling that is consistent with 61Ni-SR-MS measurements.

PS-b-P4VP block copolymer micelles as a soft template to grow openly porous nickel films for alkaline hydrogen evolution

Highly porous Ni films have been potentiostatically synthesised by micelle-assisted electrodeposition using custom-made PS-b-P4VP block copolymer micelles as a soft template. Two PS-b-P4VP block copolymers with PS/P4VP block ratios of 1:1 and 1:4 were used for the micelle-assisted electrodeposition, resulting in Ni films with large pores of diameters varying from 25 to 600 nm (1:1), and from 10 to 230 nm (1:4). As a result of the interconnected porosity, and hence the drastic increase of the surface-to-volume ratio, the electrocatalytic performance at hydrogen evolution reaction (HER) in alkaline media is significantly improved in comparison to a dense Ni film, and—more importantly—even in comparison to a highly mesoporous Ni film with monodisperse 10 nm wide pores. Most remarkably, it is discovered that the openly porous Ni electrocatalysts not only lead to a simple increase in HER current density, but also to a lower overpotential and a better long-term performance. While the bulk of the films is metallic, Ni(OH)2 is formed on the surfaces of all Ni films during HER. This effect leads to an initial decrease of the catalytic activity, but provides excellent stability in alkaline media. The presented synthesis process for pure Ni may be readily adopted to any other electroplatable metals and alloys.

Electrodeposited NiCu nanoparticles for the borohydride oxidation reaction: Effect of Cu on the activity and stability of Ni upon surface oxidation

Development cost-effective and stable electrocatalysts for the borohydride oxidation reaction (BOR) is mandatory for the deployment of the direct borohydride fuel cells (DBFC). Ni electrocatalysts have recently been shown to outperform noble-metal (Pt, Pd, Au) catalysts at low BOR overpotentials, but their activity largely depends on the state of the Ni surface with metallic Ni demonstrating the highest activity. Here, bimetallic NiCu nanoparticles of different compositions were prepared, which demonstrate high BOR activity comparable to Ni nanoparticles in the metallic state of their surface and allow to overcome in considerable extent the negative issues related to inevitable surface passivation.

Optimization of anion exchange membrane water electrolyzers using ionomer-free electrodes

This work is carried out in the context of the anion exchange membrane water electrolysis (AEMWE) and pursuits to determine the influence of different cell components on the global electrochemical performance. Ionomer-free electrodes consisting of anodic Ni–Fe and cathodic Ni electrocatalysts deposited by magnetron sputtering in an oblique angle deposition configuration were utilized for this study. In addition to the characteristics and equivalent thickness of the electrocatalysts, other factors affecting the efficiency that have been considered in this study encompass the type of gas diffusion layer (GDLs), including carbon paper and stainless-steel fiber paper supports, and several commercial anion exchange membranes. The electrocatalytic performances in both a three-electrode and complete single cell AEMWE set-ups, together with the physico-chemical characterization of the electrodes before and after operation, have served to select the optimum components for the utilized cell configuration. Thus, current densities of 670 mA cm−2, at polarization voltage of 2.2 V, 1.0 M KOH electrolyte and 40 °C were obtained in a membrane electrode assembly. A seven days chronopotentiometry experiment at a fixed current of 400 mA cm−2 demonstrated a noticeable stability of this type of AEMWE cells incorporating ionomer-free electrodes.

Alpha-Nickel Hydroxide Coating of Metallic Nickel for Enhanced Alkaline Hydrogen Evolution.

In this work, alkaline hydrogen evolution reaction (HER) processes of three typical nickel-based electrocatalysts [i. e., Ni, α-Ni(OH)2 , and β-Ni(OH)2 ] were investigated to probe critical factors that determine the activity and durability. The HER activity trend was observed as Ni≫α-Ni(OH)2 >β-Ni(OH)2 , likely attributed to a synergy between metallic Ni and Ni(OH)2 components on the Ni surface and fast water dissociation kinetics on the α-Ni(OH)2 surface. With the HER proceeding, the metallic Ni surface, however, gradually became α-Ni(OH)2 , and α-Ni(OH)2 surface ultimately transformed into β-phase, leading to a dramatic activity decrease of Ni electrodes. Therefore, Ni electrodes were coated with α-Ni(OH)2 nanosheets to slow down the nickel hydroxylation and optimize the surface ratio of Ni(OH)2 to metallic Ni. This simple coating procedure enhanced both activity and durability of Ni electrocatalysts.