High-Performance Ammonia Electrosynthesis from Nitrate in a NaOH-KOH-H2O Ternary Electrolyte

A glut of dinitrogen-derived ammonia (NH3) over the past century has resulted in a heavily imbalanced nitrogen cycle and consequently, the large-scale accumulation of reactive nitrogen such as nitrates in our ecosystems has led to detrimental environmental issues. Electrocatalytic upcycling of waste nitrogen back into NH3 holds promise in mitigating these environmental impacts and reducing reliance on the energy-intensive Haber-Bosch process. Herein, we report a high-performance electrolyzer using an ultrahigh alkalinity electrolyte, NaOH-KOH-H2O, for low-cost NH3 electrosynthesis. At 3,000 mA/cm2, the device with a Fe-Cu-Ni ternary catalyst achieves an unprecedented faradaic efficiency (FE) of 92.5±1.5 % under a low cell voltage of 3.83 V; whereas at 1,000 mA/cm2, an FE of 96.5±4.8 % under a cell voltage of only 2.40 V was achieved. Techno-economic analysis revealed that our device cuts the levelized cost of ammonia electrosynthesis by ~40 % ($30.68 for Fe-Cu-Ni vs. $48.53 for Ni foam per kmol-NH3). The NaOH-KOH-H2O electrolyte together with the Fe-Cu-Ni ternary catalyst can enable the high-throughput nitrate-to-ammonia applications for affordable and scalable real-world wastewater treatments.

Water Electrolysis toward Elevated Temperature: Advances, Challenges and Frontiers

Since severe global warming and related climate issues have been caused by the extensive utilization of fossil fuels, the vigorous development of renewable resources is needed, and transformation into stable chemical energy is required to overcome the detriment of their fluctuations as energy sources. As an environmentally friendly and efficient energy carrier, hydrogen can be employed in various industries and produced directly by renewable energy (called green hydrogen). Nevertheless, large-scale green hydrogen production by water electrolysis is prohibited by its uncompetitive cost caused by a high specific energy demand and electricity expenses, which can be overcome by enhancing the corresponding thermodynamics and kinetics at elevated working temperatures. In the present review, the effects of temperature variation are primarily introduced from the perspective of electrolysis cells. Following an increasing order of working temperature, multidimensional evaluations considering materials and structures, performance, degradation mechanisms and mitigation strategies as well as electrolysis in stacks and systems are presented based on elevated temperature alkaline electrolysis cells and polymer electrolyte membrane electrolysis cells (ET-AECs and ET-PEMECs), elevated temperature ionic conductors (ET-ICs), protonic ceramic electrolysis cells (PCECs) and solid oxide electrolysis cells (SOECs).

Rational design of NiFe alloys for efficient electrochemical hydrogen evolution reaction: effects of Ni/Fe molar ratios

Developing and designing high-performance and stable NiFe electrodes for efficient hydrogen production are the greatest challenges in electrochemical water splitting. In this work, NiFe alloys with different Ni and Fe contents are prepared by a simple electrodeposition method using different molar ratios of Ni/Fe precursors (Ni/Fe; 1 : 3, 1 : 1 and 3 : 1). The obtained NiFe electrode with a molar ratio of 3 : 1 exhibited better electrocatalytic activity for the HER than the other NiFe electrodes with 1 : 3 and 1 : 1 molar ratios. The NiFe (Ni/Fe, 3 : 1) electrode required an overpotential of 133 mV to reach a current density of 10 mA cm⁻², which was much lower than those of NiFe with molar ratio of 1 : 3 (220 mV), and 1 : 1 (365 mV), respectively. Tafel slope analyses demonstrated that the HER mechanism of NiFe alloy coatings followed the Volmer reaction type. The superior electrocatalytic performance of the NiFe alloy for HER depending on precursor molar ratio of Ni/Fe was attributed to their composition in terms of Ni and Fe content, structure and surface morphology. Specifically, the electrodeposition of the NiFe alloy was obtained in a molar ratio Ni/Fe, 3 : 1, and induced the formation of NiFe layered double hydroxide (LDH) with a nanosheet-array structure. The high electrocatalytic activity of NiFe LDH (Ni/Fe, 3 : 1) confirmed the critical influence of Ni and Fe contents in the alloy resulting in an increase the active surface on the surfaces, which is most likely explained by the higher surface roughness and the low crystallinity structure of NiFe nanosheet-array, supported by ECSA measurement, XRD, SEM and AFM analyses. The present strategy may open an avenue for developing cost-effective, stable and high-performance electrocatalysts as advanced electrodes for large-scale water splitting.

Developing and designing high-performance and stable NiFe electrodes for efficient hydrogen production in alkaline medium.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60–80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)₂ on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm², respectively.

In situ generation of Ni/Fe hydroxide layers by anodic etching of a Ni/Fe film for efficient oxygen evolution reaction

A Ni/Fe hydroxide electrocatalyst was fabricated via a simple and easily controlled method by combining anodic fluoridation and cyclic voltammetry (CV) treatment as an efficient catalyst for the OER.

Reducing the Internal Stress of Fe-Ni Magnetic Film Using the Electrochemical Method

Soft magnetic materials are important functional materials in the electrical engineering, radio, and high-tech fields, but thin and brittle flakes present challenges to the manufacturing industry. In this study, the effect and mechanism of saccharin sodium in reducing the internal stress of Fe-Ni magnetic films were analyzed. The effects of the pH value, temperature, and the concentration of saccharin sodium on the deposition process of Fe-Ni alloys were investigated. The polarization curve of the Fe-Ni alloy deposition process was measured by using a multifunctional electrochemical workstation, and the morphology and crystal structure were measured by a scanning electron microscope (SEM) and X-ray diffraction (XRD). The results show that saccharin sodium significantly reduced the stress of the iron-nickel magnetic film; the mechanism through which the internal stress was reduced is analyzed in this paper. Briefly, the Fe2+ and the amino group of saccharin sodium synthesized a metal complex with positive charge on the surface of the electrode, which prevented the hydrogen ions from approaching the cathode and increased the discharge activation energy of the hydrogen ion, which reduced the hydrogen evolution and improved the internal stress of the coating. This research will help to solve the challenges of producing magnetic film, and promotes the application of new stress-reducing agents.

The real-time investigation of the nickel-iron hydroxide catalyzed oxygen evolution reaction with interdigitated array electrodes

Electrocatalysis of oxygen evolution reaction (OER), one of the most important members in clean and efficient energy conversion, requires increasing studies on reaction process analysis, catalyst investigation and evaluation and so on throughin situexperiments. The bottleneck is the difficulties on clear and precise understanding towards the multi-step reactions with fast reaction rates. Interdigitated array (IDA) electrodes with sensitive responses on the generation, transfer and collection of reaction products are proposed and utilized as a convenient and effective tool toin situmonitor and characterize the reaction thermodynamics and kinetics information. Herein, nickel-iron hydroxide, a promising and novel OER catalyst, is chosen as the candidate to demonstrate the merit of IDA on studying the OER. With the generator-collector mode, the real-time oxygen evolution process is monitored precisely with the IDA collector, distinguished it from the general catalytic current which is normally recorded with conventional electrochemical method. In another word, the actual faradaic efficiency was observed experimentally with IDA electrodes, which is often misled as 100% in many works. The diffusion of the reaction products has been 'seen' as well with the generator-collector mode. This general tool (IDA) may make more contributions on the study of reaction process of all electrocatalytical reactions.

Hybrid Electrochemical Deposition Route for the Facile Nanofabrication of a Cr-Poisoning-Tolerant La(Ni,Fe)O3-δ Cathode for Solid Oxide Fuel Cells

Cr poisoning of cathode materials is one of the main degradation issues hampering the operation of solid oxide fuel cells (SOFCs). To overcome this shortcoming, LaNi_0.6Fe_0.4O_3-δ (LNF) has been developed as an alternative cathode material owing to its superior chemical stability in Cr environments. In this study, we develop a hybrid electrochemical deposition technique to fabricate a nanostructured LNF-gadolinium-doped ceria (GDC) (n-LNF-GDC) cathode with enhanced active reaction sites for the oxygen reduction reaction. For this purpose, Fe and Ni cations are co-deposited onto an electrically conductive carbon nanotube-modified GDC backbone by electroplating, whereas La cations are successively deposited through a chemically assisted electrodeposition method. The proposed method involves a low-temperature (900 °C) calcination step of electrodeposited cations, which avoids the need of fabricating a GDC diffusion barrier layer which is otherwise needed to avoid the formation of insulating phases (e.g., La₂Zr₂O₇) when fabricating by conventional high-temperature (≥1000 °C) sintering. Scanning electron microscopy images reveal a unique nanofibrous structure of n-LNF-GDC, which is believed to play an instrumental role in enhancing the electrochemical characteristics by increasing the active triple-phase boundaries. An anode-supported SOFC with the n-LNF-GDC cathode showed the superior performance of 0.984 W cm^-2 at an intermediate temperature of 750 °C as compared to the power densities of 0.495 and 0.874 W cm^-2 produced by LNF-GDC and state-of-the-art La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF)-GDC composite cathodes fabricated by conventional sintering. A short-term accelerated Cr-poisoning durability test indicated good electrochemical stability of n-LNF-GDC, whereas LSCF exhibited severe degradation. The electrochemically engineered nanostructured n-LNF-GDC can serve as an effective cathode for SOFCs to achieve high performance and long-term durability.

Electrode material studies and cell voltage characteristics of the in situ water electrolysis performed in a pH-neutral electrolyte in bioelectrochemical systems

Hydrogen-oxidizing bacteria (HOB) have been shown to be promising micro-organisms for the reduction of carbon dioxide to a wide range of value-added products in bioelectrochemical systems with in situ water electrolysis of the cultivation medium, also known as a hybrid biological-inorganic systems (HBI). However, scaling up of this process requires overcoming the inherent constraints of the low energy efficiency partly associated with the pH-neutral electrolyte with low conductivity. Most of the research in the field is concentrated on the bacterial cultivation, whereas the analysis and evaluation of the electrode material performance have received little attention in the literature so far. Therefore, in the present work, in situ electrolysis of a pH-neutral medium for HOB cultivation was performed with different combinations of electrode materials. Besides conventional electrode types, electrodes with coatings made of earth-abundant cobalt and a nickel-iron alloy, known for their catalytic activity for the kinetically sluggish oxygen evolution reaction (OER), were prepared and tested as potential substitutes for catalysts made of precious metals. The cultivation of HOB with in situ water electrolysis has been successfully tested in a small scale electrobioreactor in order to support the experimental results. A simplified water electrolysis model was developed and applied to evaluate the current-voltage characteristics of an bioelectrochemical system prototype. Application of the developed model allows quantitative evaluation and comparison of reversible, ohmic, and activation overvoltages of different electrode sets. The modeling results were found to agree well with the experimental data. The developed model and the data gathered can be applied to further investigation, simulation, and optimization of HBI systems.

Enhancement of oxygen evolution performance through synergetic action between NiFe metal core and NiFeOx shell

NiFe/NiFeO_x core/shell electrocatalysts show excellent OER activity by taking advantage of the synergetic effect between the metal core and the oxide/hydroxide shell, i.e. the metal core provides good bulk electron conduction and thus extends the OER active sites to the whole oxide/hydroxide shell, and the shell catalyzes the OER and protects the metal core from oxidation.

Dendritic core-shell nickel-iron-copper metal/metal oxide electrode for efficient electrocatalytic water oxidation

Electrochemical water splitting requires efficient water oxidation catalysts to accelerate the sluggish kinetics of water oxidation reaction. Here, we report a promisingly dendritic core-shell nickel-iron-copper metal/metal oxide electrode, prepared via dealloying with an electrodeposited nickel-iron-copper alloy as a precursor, as the catalyst for water oxidation. The as-prepared core-shell nickel-iron-copper electrode is characterized with porous oxide shells and metallic cores. This tri-metal-based core-shell nickel-iron-copper electrode exhibits a remarkable activity toward water oxidation in alkaline medium with an overpotential of only 180 mV at a current density of 10 mA cm-2. The core-shell NiFeCu electrode exhibits pH-dependent oxygen evolution reaction activity on the reversible hydrogen electrode scale, suggesting that non-concerted proton-electron transfers participate in catalyzing the oxygen evolution reaction. To the best of our knowledge, the as-fabricated core-shell nickel-iron-copper is one of the most promising oxygen evolution catalysts.

Fabrication of Nanoporous Nickel-Iron Hydroxylphosphate Composite as Bifunctional and Reversible Catalyst for Highly Efficient Intermittent Water Splitting

Global-scale application of water-splitting technology for hydrogen fuel production and storage of intermittent renewable energy sources has called for the development of oxygen- and hydrogen-evolution catalysts that are inexpensive, efficient, robust, and can withstand frequent power interruptions and shutdowns. Here, we report the controlled electrodeposition of porous nickel-iron hydroxylphosphate (NiFe-OH-PO₄) nanobelts onto the surface of macroporous nickel foams (NF) as a bifunctional electrocatalyst for efficient whole-cell water electrolysis. The NiFe-OH-PO₄/NF electrode shows both high water oxidation and water reduction catalytic activity in alkaline solutions and is able to deliver current densities of 20 and 800 mA cm^-2 at overpotentials of merely 249 and 326 mV for oxygen-evolution reaction, current densities of 20 and 300 mA cm^-2 at overpotentials of only 135 and 208 mV for hydrogen-evolution reaction. Further, in a two-electrode water electrolytic cell, the bifunctional NiFe-OH-PO₄/NF electrodes can obtain the current densities of 20 and 100 mA cm^-2 at an overall cell potential of only 1.68 and 1.91 V, respectively. Remarkably, the NiFe-OH-PO₄/NF catalyst also represents prolonged stability under both continuous and intermittent electrolysis and can be used for oxygen evolution and hydrogen evolution reversibly without degradation.

A facile conversion of a Ni/Fe coordination polymer to a robust electrocatalyst for the oxygen evolution reaction

A spinel NiFe₂O₄ dominated mixed oxide prepared from Ni hexacyanometallate (NiHCF) was found to be effective for the oxygen evolution reaction (OER) in alkaline solution.

Strongly Coupled FeNi Alloys/NiFe2O4@Carbonitride Layers-Assembled Microboxes for Enhanced Oxygen Evolution Reaction

Hydrogen produced from electrocatalytic water splitting is a promising route due to the sustainable powers derived from the solar and wind energy. However, the sluggish kinetics at the anode for water splitting makes the highly effective and inexpensive electrocatalysts desirable in oxygen evolution reaction (OER) by structure and composition modulations. Metal-organic frameworks (MOFs) have been intensively used as the templates/precursors to synthesize complex hollow structures for various energy-related applications. Herein, an effective and facile template-engaged strategy originated from bimetal MOFs is developed to construct hollow microcubes assembled by interconnected nanopolyhedron, consisting of intimately dominant FeNi alloys coupled with a small NiFe₂O₄ oxide, which was confined within carbonitride outer shell (denoted as FeNi/NiFe₂O₄@NC) via one-step annealing treatment. The optimized FeNi/NiFe₂O₄@NC exhibits excellent electrocatalytic performances toward OER in alkaline media, showing 10 mA·cm^-2 at η = 316 mV, lower Tafel slope (60 mV·dec^-1), and excellent durability without decay after 5000 CV cycles, which also surpasses the IrO₂ catalyst and most of non-noble catalysts in the OER, demonstrating a great perspective. The superior OER performance is ascribed to the hollow interior for fast mass transport, in situ formed strong coupling between FeNi alloys and NiFe₂O₄ for electron transfer, and the protection of carbonitride layers for long stability.

Transition-Metal (Co, Ni, and Fe)-Based Electrocatalysts for the Water Oxidation Reaction

Increasing energy demands and environment awareness have promoted extensive research on the development of alternative energy conversion and storage technologies with high efficiency and environmental friendliness. Among them, water splitting is very appealing, and is receiving more and more attention. The critical challenge of this renewable-energy technology is to expedite the oxygen evolution reaction (OER) because of its slow kinetics and large overpotential. Therefore, developing efficient electrocatalysts with high catalytic activities is of great importance for high-performance water splitting. In the past few years, much effort has been devoted to the development of alternative OER electrocatalysts based on transition-metal elements that are low-cost, highly efficient, and have excellent stability. Here, recent progress on the design, synthesis, and application of OER electrocatalysts based on transition-metal elements, including Co, Ni, and Fe, is summarized, and some invigorating perspectives on the future developments are provided.

Stainless Steel Mesh-Supported NiS Nanosheet Array as Highly Efficient Catalyst for Oxygen Evolution Reaction

Nickel(II) sulfide (NiS) nanosheets with a thickness of 10 nm and a size of 200 nm were facilely grown on stainless steel (SLS) meshes via a one-pot hydrothermal method. This unique construction renders an excellent electrical contact between the porous film of active NiS sheets and the highly conductive substrate, which exhibits a superior catalytic activity toward oxygen evolution reaction (OER). The NiS@SLS electrocatalyst exhibits an unusually low overpotential of 297 mV (i.e., 1.524 V vs RHE) at a current density of 11 mA·cm(-2), and an extra small Tafel slope of only 47 mV·dec(-1) proves an even more competitive performance at high to very high current densities. This performance compares very favorably to other Ni-based catalysts and even to the precious state-of-the-art IrO2 or RuO2 catalyst.

Oxygen evolution catalytic behaviour of Ni doped Mn3O4 in alkaline medium

In this study, the electrocatalytic behavior of Mn₃O₄ was enhanced by non-precious metal doping.

Solution chemistry-based nano-structuring of copper dendrites for efficient use in catalysis and superhydrophobic surfaces

Solution chemistry-based nano-structuring of Cu dendrites is exploited to enhance their efficiency in applications of catalysis and superhydrophobicity.

Modification of Ni–P alloy coatings for better hydrogen production by electrochemical dissolution and TiO2 nanoparticles

This work reports the modification of Ni–P alloy coatings for better hydrogen production by electrochemical dissolution and TiO₂ nanoparticle incorporation.

Ultrathin Two-Dimensional Free-Standing Sandwiched NiFe/C for High-Efficiency Oxygen Evolution Reaction

A NiFe-based compound is considered one of the most promising candidates for the highest oxygen evolution reaction (OER) electrocatalytic activities among all nonprecious metal-based electrocatalysts. In this report, a unique catalyst of free-standing sandwiched NiFe nanoparticles encapsulated by graphene sheets is first devised and fabricated. In this method, we use low-cost, sustainable, and environmentally friendly glucose as a carbon source, ultrathin Fe-doped Ni(OH)2 nanosheets as a precursor, and a sacrificial template. This special nanoarchitecture with a conductive network around active catalysts can accelerate electron transfer and prevent NiFe nanoparticles from aggregation and peeling off during long-time electrochemical reactions, thereby exhibiting an excellent OER activity and stability in basic solutions. In this work, our sandwiched catalyst presents well activities of a low onset of ∼1.44 V (vs RHE) and Tafel slope of ∼30 mV/decade in 1 M KOH at a scan rate of 5 mV/s.

Electrodeposition of hierarchically structured three-dimensional nickel-iron electrodes for efficient oxygen evolution at high current densities

Large-scale industrial application of electrolytic splitting of water has called for the development of oxygen evolution electrodes that are inexpensive, robust and can deliver large current density (>500 mA cm⁻²) at low applied potentials. Here we show that an efficient oxygen electrode can be developed by electrodepositing amorphous mesoporous nickel–iron composite nanosheets directly onto macroporous nickel foam substrates. The as-prepared oxygen electrode exhibits high catalytic activity towards water oxidation in alkaline solutions, which only requires an overpotential of 200 mV to initiate the reaction, and is capable of delivering current densities of 500 and 1,000 mA cm⁻² at overpotentials of 240 and 270 mV, respectively. The electrode also shows prolonged stability against bulk water electrolysis at large current. Collectively, the as-prepared three-dimensional structured electrode is the most efficient oxygen evolution electrode in alkaline electrolytes reported to the best of our knowledge, and can potentially be applied for industrial scale water electrolysis.

Development of efficient and affordable oxygen evolution catalysts is essential for large-scale electrolytic water splitting. Here, the authors report mesoporous nickel–iron composite nanosheets loaded on macroporous nickel foam substrates, and evaluate their electrocatalytic oxygen evolution in basic media.

An array of leaf-like Co3Ni microstructures with ferromagnetic properties, superhydrophobic properties and high catalytic performance in the hydrolysis of ammonia borane

A Cu foil supported array of leaf-like Co₃Ni microstructures simultaneously exhibited ferromagnetic properties, superhydrophobic properties, as well as high catalytic performance in the hydrolysis of ammonia borane.