Alkaline Electrolyte Research Articles

Electrocatalysts with Highly Dispersed Cerium in Nickel Matrix for Hydrogen Evolution in Alkaline Electrolyte

AbstractIt is challenging to develop highly active and durable electrocatalysts for hydrogen evolution reaction (HER) in alkaline water electrolysis. Herein, an electrocatalyst is synthesized with highly dispersed cerium in a nickel matrix, showing an overpotential of 169 mV for HER at a current density of 500 mA cm−2, about 137 mV lower than that observed without cerium. Kinetic analysis results suggest that doping cerium facilitates the evolution of intermediate H for HER, thus accelerating the process of water dissociation. Density functional theory (DFT) calculations also indicate that the introduction of highly dispersed cerium can lower the energy barrier of the kinetically sluggish Volmer step by altering the orbital occupation near the Fermi level, promoting the process of water dissociation. Furthermore, the electrolyzer constructed with the electrocatalyst exhibits a cell voltage as low as 1.67 V at 800 mA cm−2 for 2000 h in 7 м KOH at 80 °C. The robust property makes it an excellent candidate electrocatalyst for practical applications.

Compositional alteration of low dimensional nickel sulfides under hydrogen evolution reaction conditions in alkaline media

Developing cost-effective electrodes for electrochemical water splitting with minimal kinetic losses remains a primary challenge in water electrolysis. Nickel-based dichalcogenides, particularly sulfides, have emerged as promising candidates for the hydrogen evolution reaction (HER). However, the stability of various nickel sulfide (NixSy) phases under cathodic potentials in alkaline electrolytes is subject to debate. In this study, we investigate the HER activity and durability of both 2-dimensional (2D) and 3-dimensional (3D) NixSy nanostructures with diverse chemical compositions. Through cross-correlation analysis of spectroscopic and electrochemical characterization data, we elucidate the transformation behavior of these materials under operating conditions. Our findings reveal that NiS2 tends to undergo conversion to nickel hydroxide (Ni(OH)2), while Ni-rich phases like Ni3S4 and NiS exhibit greater stability. Furthermore, we demonstrate that in 3D nanostructures, the initial formation of a protective hydroxide layer on the surface inhibits further sulfide transformation. This nickel sulfide/hydroxide composite shows an improved HER activity with lower onset potential (−0.25 V), Tafel slope (105 mV s−1), and charge transfer resistance (17.3 Ω).

Cerium-doped Nickel SulfideNanospheres as EfficientCatalysts for Overall Water Splitting.

The development of non-precious metal electrocatalysts with excellent activity and durability for electrochemical water splitting has always been a goal. Transition metal sulfides are attractive electrocatalysts for both hydrogen evolution reaction (HER) and oxygen evolution reaction(OER). In this article, we designed and constructed efficient catalysts with multiple synergistic interactions and synthesized Ce-NiS2@NFnanosphere using a solvothermal method. Ce-NiS2@NF exhibits excellent HER performance, OER performance, and overall water splitting capability in alkaline electrolytes, demonstrating good stability. The addition of Ce influences the activity of the catalysts, attributed to the synergistic interactions creating more active sites and higher intrinsic activity through the introduction of Ce heteroatoms. Additionally, the self-supported conductive substrate promotes electron transfer, enhancing the intrinsic activity and active site density of the catalyst.This study provides an in-depth investigation into structural design and performance enhancement, offering ideas for designing efficient catalysts for overall water electrolysis. This work provides an in-depth study in terms of structural design performance enhancement and provides ideas for designing efficient alkaline bifunctional catalysts. Valuable insights have been provided in elucidating the intrinsic mechanism of the catalytic activity of cerium-doped nickel sulfidenanospheres,thus providing new guidance in the field of energy conversion technology.

Promoting Electrochemical CO2 Reduction to Formate via Sulfur‐Assisted Electrolysis

AbstractElectrochemical CO2 reduction reaction (CO2RR) provides a renewable approach to transform CO2 to produce chemicals and fuels. Unfortunately, it faces the challenges of sluggish CO2 activation and slow water dissociation. This study reports the modification of Bi‐based electrocatalyst by S, which leads to a remarkable enhancement in activity and selectivity during electrochemical CO2 reduction to formate. Based on comprehensive in situ examinations and kinetic evaluations, it is observed that the presence of S species over Bi catalyst can significantly enhance its interaction with K+(H2O)n, facilitating fast dissociation of water molecules to generate protons. Further in situ attenuated total reflectance surface‐enhanced infrared absorption spectroscopy (ATR‐SEIRAS) and in situ Raman spectroscopy measurements reveal that S modification is able to decrease the oxidation state of Bi active site, which can effectively enhance CO2 activation and facilitate HCOO* intermediate formation while suppressing competing hydrogen evolution reaction. Consequently, the S‐modified Bi catalyst achieves impressive electrochemical CO2RR performance, reaching a formate Faradaic efficiency (FEformate) of 91.2% at a formate partial current density of ≈135 mA cm−2 and a potential of −0.8 V versus RHE in an alkaline electrolyte.

Shifting the Oxygen-Evolution Reaction Pathway via Cation Engineering to Activate Lattice Oxygen in Metal-Organic Frameworks.

Metal-organic frameworks (MOFs) as promising electrocatalysts have been widely studied, but their performance is limited by conductivity and coordinating saturation. This study proposes a cationic (V) modification strategy and evaluates its effect on the electrocatalytic performance of CoFe-MOF nanosheet arrays. The optimal V-CoFe-MOF/NF electrocatalyst exhibits excellent oxygen-evolution reaction (OER)/hydrogen-evolution reaction (HER) performance (231 mV at 100 mA cm-2/86 mV at 10 mA cm-2) in alkaline conditions, with its OER durability exceeding 400 h without evident degradation. Furthermore, as a bifunctional electrocatalyst for water splitting, a small cell voltage is achieved (1.60 V at 10 mA cm-2). The practicability of the catalyst is further evaluated by membrane electrode assembly (MEA), showing outstanding activity (1.53 V at 10 mA cm-2) and long-term stability (at 300 mA cm-2). Moreover, our results reveal the apparent reconstruction properties of V-CoFe-MOF/NF in alkaline electrolytes, where the partially dissolved V promotes the formation of more active β-MOOH. The mechanism study shows the OER mechanism shifts to a lattice oxygen oxidation mechanism (LOM) after V doping, which directly avoids complex multistep adsorption mechanism and reduces reaction energy. This study provides a cation mediated strategy for designing efficient electrocatalysts.

New insights into the photoassisted anodic reactions of n-type 4H SiC semiconductors

New insights into the photoassisted anodic reactions of n-type 4H SiC semiconductors

Efficient Decoupled Electrolytic Water Splitting in Acid through Pseudocapacitive TiO2.

Water electrolysis remains a key component in the societal transition to green energy. Membrane electrolyzers are the state-of-the-art technology for water electrolysis, relying on 80°C operation in highly alkaline electrolytes, which is undesirable for many of the myriad end-use cases for electrolytic water splitting. Herein, an alternative water electrolysis process, decoupled electrolysis, is described which performed in mild acidic conditions with excellent efficiencies. Decoupled electrolysis sequentially performs the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER), at the same catalyst. Here, H+ ions generated from the OER are stored through pseudocapacitive (redox) charge storage, and released to drive the HER. Here, decoupled electrolysis is demonstrated using cheap, abundant, TiO2 for the first time. To achieve decoupled acid electrolysis, ultra-small anatase TiO2 particles (4.5nm diameter) are prepared. These ultra-small TiO2 particles supported on a carbon felt electrode show a highly electrochemical surface area with a capacitance of 375Fg-1. When these electrodes are tested for decoupled water splitting an overall energy efficiency of 52.4% is observed, with excellent stability over 3000 cycles of testing. This technology can provide a viable alternative to membrane electrolyzers-eliminating the need for highly alkaline electrolytes and elevated temperatures.

Unraveling Stoichiometry Effect in Nickel‐Tungsten Alloys for Efficient Hydrogen Oxidation Catalysis in Alkaline Electrolytes

Anion‐exchange membrane fuel cells provide the possibility to use platinum group metal‐free catalysts, but the anodic hydrogen oxidation reaction (HOR) suffers from sluggish kinetics and its source is still debated. Here, over nickel‐tungsten (Ni‐W) alloy catalysts, we show that the Ni:W ratio greatly governs the HOR performance in alkaline electrolyte. Experimental and theoretical studies unravel that alloying with W can tune the unpaired electrons in Ni, tailoring the potential of zero charge and the catalytic surface to favor hydroxyl adsorption (OHad). The OHad species coordinately interact with potassium (K+) ions, which break the K+ solvation sheath to leave free water molecules, yielding an improved connectivity of hydrogen‐bond networks. Consequently, the optimal Ni17W3 alloy exhibits alkaline HOR activity superior to the state‐of‐the‐art platinum on carbon (Pt/C) catalyst and operates steadily with negligible decay after 10,000 cycles. Our findings offer new understandings of alloyed HOR catalysts and will guide rational design of next‐generation catalysts for fuel cells.

Unraveling Stoichiometry Effect in Nickel-Tungsten Alloys for Efficient Hydrogen Oxidation Catalysis in Alkaline Electrolytes.

Anion-exchange membrane fuel cells provide the possibility to use platinum group metal-free catalysts, but the anodic hydrogen oxidation reaction (HOR) suffers from sluggish kinetics and its source is still debated. Here, over nickel-tungsten (Ni-W) alloy catalysts, we show that the Ni:W ratio greatly governs the HOR performance in alkaline electrolyte. Experimental and theoretical studies unravel that alloying with W can tune the unpaired electrons in Ni, tailoring the potential of zero charge and the catalytic surface to favor hydroxyl adsorption (OHad). The OHad species coordinately interact with potassium (K+) ions, which break the K+ solvation sheath to leave free water molecules, yielding an improved connectivity of hydrogen-bond networks. Consequently, the optimal Ni17W3 alloy exhibits alkaline HOR activity superior to the state-of-the-art platinum on carbon (Pt/C) catalyst and operates steadily with negligible decay after 10,000 cycles. Our findings offer new understandings of alloyed HOR catalysts and will guide rational design of next-generation catalysts for fuel cells.

Attaining Substantially Enhanced Oxygen Evolution Reaction Rates on Ni Foam Catalysts in a Gas Diffusion Electrode Setup

Water electrolysis plays a central role in the transition to a fossil‐free society, but there are significant challenges to overcome in order to increase its availability on a large scale. Alkaline water electrolysis is a mature and scalable technology, although it has several disadvantages compared to electrolyzers working in acidic environments. In particular, the use of highly alkaline aqueous electrolytes can lead to corrosion, and the achieved current densities are relatively low. This study addresses the latter limitation by introducing a gas diffusion electrode (GDE) setup as a novel development tool that bridges the gap between research and practical applications in commercial devices such as fuel cells and electrolyzers. A high surface area Ni foam catalyst that can sustain exceptional oxygen evolution reaction (OER) current densities of up to 4 A cm−2 in a quasi‐steady‐state within our GDE setup operating in an alkaline environment is presented. The high performance of this Ni‐based benchmark catalyst is attributed to its deposition onto a mesh‐like porous transport layer (PTL) via hydrogen‐templated electrodeposition. This forms a porous foam‐like structure that augments the mass transport of the gaseous reactants at the GDE.

Synthesis of nitrogen and sulfur bi-doped carbon obtained from spent sapelli wood sawdust and polyhydric alcohol with improved catalytic activity for hydrogen evolution reaction

Developing an efficient water electrolysis system is an important solution for on the development of highly active, cost-effective, and stable electrocatalysts. Carbon-based metal-free catalysts are one of the promising candidates for this purpose. Herein, we present a facile strategy to synthesize and characterize nitrogen and sulfur co-doped porous carbon (NS-PC) from spent sapelli wood sawdust and polyhydric alcohol (xylitol) for efficient hydrogen evolution reaction (HER) in alkaline electrolyte. The resultant NS-PC metal-free catalyst exhibits porous structure, high surface areas and enhanced HER performance with lowest Tafel slope of 68.1 mV dec−1. The enhanced performance is due to the high porosity of carbonaceous material and co-existence of active species (N in the forms of graphitic-N, pyridinic-N, pyrrolic-N, and S in the forms of C–S–C) induced by the maximum carbon-carbon bond polarization, charge re-distribution in the carbon matrices with a synergistically HER activity. In addition, this work provides a versatile and effective strategy for synthetizing excellent carbon-based metal free catalysts from the low cost biowaste for large-scale production of hydrogen through HER.

Polyoxometalate coupled polymers induced molybdenum phosphide with large surface area for efficient hydrogen evolution

Developing highly active hydrogen evolution reaction (HER) electrocatalysts possesses great significance for the green renewable energy conversion and storage. Here, through the electrostatic attraction between polyoxometalate (PMo12) and ammonium polyphosphate (APP) and the coordination with polyallylamine hydrochloride (PAH), the precursor was constructed, and then nitrogen-doped graphite carbon layer-coated MoP active particles (MoP@NC) were successfully prepared by high temperature carbonization. For HER, the overpotential of η10 for MoP@NC is 119 and 126 mV in acidic and alkaline electrolyte. Furthermore, in alkaline solution, when the current density reaches a certain degree (≥110 mA cm−2), the hydrogen production activity of the catalysts are the same as that of the industrial Pt/C catalysts, or even have a slight advantage. The synergistic interaction of active particles (MoP) and graphitic carbon layers and high specific surface area ensure the excellent activity of HER. N-doped porous carbon layers also protect MoP@NC working for 24 h. Moreover, the catalysts prepared in the same experimental environment with ammonium molybdate as metal source also showed good HER performance in acidic and alkaline environments. This study opens a general way to develop earth-rich Mo-based phosphide electrocatalysts with high activity for hydrolysis applications.

Self-Assembled Conjugated Coordination Polymer Nanorings: Role of Morphology and Redox Sites for the Alkaline Electrocatalytic Oxygen Evolution Reaction.

Electrocatalytic water splitting provides a sustainable method for storing intermittent energies, such as solar energy and wind, in the form of hydrogen fuel. However, the oxygen evolution reaction (OER), constituting the other half-cell reaction, is often considered the bottleneck in overall water splitting due to its slow kinetics. Therefore, it is crucial to develop efficient, cost-effective, and robust OER catalysts to enhance the water-splitting process. Transition-metal-based coordination polymers (CPs) serve as promising electrocatalysts due to their diverse chemical architectures paired with redox-active metal centers. Despite their potential, the rational use of CPs has faced obstacles including a lack of insights into their catalytic mechanisms, low conductivity, and morphology issues. Consequently, achieving success in this field requires the rational design of ligands and topological networks with the desired electronic structure. This study delves into the design and synthesis of three novel conjugated coordination polymers (CCPs) by leveraging the full conjugation of terpyridine-attached flexible tetraphenylethylene units as electron-rich linkers with various redox-active metal centers [Co(II), Ni(II), and Zn(II)]. The self-assembly process is tuned for each CCP, resulting in two distinct morphologies: nanosheets and nanorings. The electrocatalytic OER performance efficiency is then correlated with factors such as the nanostructure morphology and redox-active metal centers in alkaline electrolytes. Notably, among the three morphologies studied, nanorings for each CCP exhibit a superior OER activity. Co(II)-integrated CCPs demonstrate a higher activity between the redox-active metal centers. Specifically, the Co(II) nanoring morphology displays exceptional catalytic activity for OER, with a lower overpotential of 347 mV at a current density of 10 mA cm-2 and small Tafel slopes of 115 mV dec-1. The long-term durability is demonstrated for at least 24 h at 1.57 V vs RHE during water splitting. This is presumably the first proof that links the importance of nanostructure morphologies to redox-active metal centers in improving the OER activity, and it may have implications for other transdisciplinary energy-related applications.

Substantial Electrocatalytic Oxygen Evolution Performances of Activated Carbon-Decorated Vanadium Pentoxide Nanocomposites

Developing the ecofriendly and high-fidelity electrocatalysts for the oxygen evolution reaction (OER) is essential to foster effective production of environmentally friendly hydrogen. Herein, we fabricated the highly efficient OER electrocatalysts of the activated carbon-decorated vanadium pentoxide (AC-V2O5) nanocomposites using a facile hydrothermal technique. The AC-V2O5 nanocomposites displayed an aggregated structure of the AC nano-sheet-anchored orthorhombic V2O5 nanorods. When performing the OER process in an alkaline electrolyte at 10 mA/cm2, AC-V2O5 exhibited the low overpotential (~230 mV), small Tafel slope (~54 mV/dec), and excellent stability. These substantial OER performances of AC-V2O5 could be ascribed to the synergistic effects from both the electrochemically active V2O5 nanorods and the highly conductive AC nanosheets. The results infer that the AC-V2O5 nanocomposites possess a substantial aptitude as a high-performance OER electrocatalyst for production of the future green energy source—hydrogen.

Unique ping-pong daisy-like catalysts: Efficient overall hydrolysis catalysis driven by vanadium-doped Fe2P and Co3(PO4)2 nanosheet composites

Balancing catalytic activity and durability remains a challenge in the development of high performance, inexpensive and durable electrocatalysts. In this study, fully hydrolyzed composite catalyst V–Fe2P@Co3(PO4)2/NF with ping-pong daisy-like morphology was synthesized using conventional hydrothermal and chemical vapor deposition (CVD) processes. In alkaline electrolyte, V–Fe2P@Co3(PO4)2/NF showed excellent electrochemical performance, achieving a current density of 10 mA cm−2 when the driving overpotentials for the hydrogen production reaction (HER) and oxidation reaction (OER) were 56 mV and 178 mV, respectively. Meanwhile, V–Fe2P@Co3(PO4)2/NF demonstrated good durability in terms of both HER and OER stability. Furthermore, the driving voltage required for the electrolyser assembled with V–Fe2P@Co3(PO4)2/NF as the electrode is lower than that of the RuO2/NF||Pt/C/NF electrode, both of which have driving voltages of 1.503 and 1.562 V (10 mA cm−2), respectively. Meanwhile, it achieves a Faraday efficiency of 95%. This study offers a feasible solution towards synthesizing and applying bifunctional water decomposition electrocatalysts.

Surface modified FeCoZn-ZIF derived polyhedral electrocatalysts for ORR under alkaline and acidic conditions

Electrocatalysts for catalyzing oxygen reduction reactions at variable pH conditions are essential for energy conversion and storage. Herein, Fe@FeCoZn–CN electrocatalysts were prepared with internal and external doping approach. The Fe@FeCoZn–CN electrocatalyst exhibits a desirable ORR performance in both acid and alkaline electrolytes. Specifically, the half-wave potential (E1/2) of Fe@FeCoZn–CN electrocatalyst is 0.890 V which far beyond Pt/C electrocatalyst (0.839 V) in alkaline electrolyte. In addition, Fe@FeCoZn–CN electrocatalyst also exhibits improved ORR activity (E1/2 = 0.801 V) compared to Pt/C (0.797 V) electrocatalysts in acidic electrolytes. What's exciting is that the rechargeable ZABs based on Fe@FeCoZn–CN electrocatalysts exhibit an impressive performance with a favorable open-circuit potential of 1.517 V and a promising specific capacity is 833.7 mA h gzn−1, good charge/discharge cycle stability. The remarkable ORR catalytic performance of Fe@FeCoZn–CN electrocatalysts can be traced to the internal doping of metals which prevent excessive metal aggregation and surface modified which can increase the metal loading to expose more active sites.

Construction of MOF-derived hollow sugar gourd-like Ni-Co-S@NiMoO4•xH2O nanocage arrays for efficient solar-powered overall water splitting

In situ growth of nanoarrays on conductive substrates is an efficient strategy for designing electrocatalysts. However, preparing array with excellent catalytic activities for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) remains a challenge. Here, we construct a novel hollow sugar gourd-like Ni-Co-S@NiMoO4·xH2O/NF nanocage array through MOF derivation strategy for stable solar-driven overall water splitting. Due to the hollow core–shell structure and multi-component advantages, the Ni-Co-S@NiMoO4·xH2O/NF presents high density of active sites and fast charge transfer rate. Therefore, Ni-Co-S@NiMoO4·xH2O/NF exhibits attractive electrocatalytic activity in alkaline electrolytes for HER (90 mV@10 mA cm−2) and OER (208 mV@10 mA cm−2, 245 mV@100 mA cm−2). The corresponding two-electrode electrolytic cell demonstrates small cell voltage (1.515 V @ 10 mA cm−2) and shows durability over 50 h at 10 mA cm−2, exhibiting excellent overall water splitting performance. It is worth noting that the solar-assisted electrolysis cell reaches a high solar-to-hydrogen (STH) conversion rate of 19.4 %. The construction of this unique hollow structure gives a fascinating method for improving the HER and OER performance of transition metal sulfide electrocatalysts.

Ni Nanoparticles Embedded in Multi-Channel Carbon Nanofibers: Self-Supporting Electrodes for Bifunctional Catalysis of Hydrogen and Oxygen Evolution Reactions

Nitrogen-doped carbon nanofibers with multi-channels loaded with nanoscale nickel particles (Ni-MNCNF) were achieved by the combination of coaxial electrospinning and pyrolysis. Which was used as a self-supporting electrode for electrochemical hydrogen and oxygen evolution reactions. The multi-channels of the fiber facilitated the fast mass transfer as well as endowed it with more exposed active sites. In addition, nanoscale Ni crystals were dispersed evenly on the fiber walls due to the confinement effect of carbon materials. Furthermore, carbon nanofibers provided an excellent conductive network, which expedited the charge transfer rate and further enhanced catalytic activity. At a current density of 10mAcm-2, the overpotentials for the hydrogen evolution reaction (HER) were only 65mV and 203mV in alkaline and acidic electrolytes, respectively, and the overpotential for the oxygen evolution reaction (OER) was 193mV in an alkaline electrolyte, promoting its potential industrial applications and reducing the negative impact on the environment. This research presented a strategy for designing self-supporting transition metal-based catalysts with superior activity.

Space confined growth of RuO2/Ru carbon nanorods for highly efficient electrocatalytic hydrogen evolution

Exploring efficient and robust electrocatalysts with low cost for hydrogen evolution reaction (HER) is critical for practical application of electrochemical water splitting. Ru-based platinum group electrocatalysts shows great potential in HER, however, Ru suffer from sluggish kinetics of water dissociation for electrochemical reduction of water to hydrogen in alkaline environments. Here, oxygen atoms are introduced into Ru by simple secondary heat treatment to form Ru–O–C bonds to weak the strength of Ru–H bond. The overpotential of the catalyst is only 19 mV at 10 mA cm−2 and the stability is greatly improved. Theoretical calculations reveal that the Ru–O–C structure formed in RuO2 facilitates the dissociation of H–OH, which is the rate-determining step in alkaline electrolytes, leading to significant improvement in HER activity.

Self-supported Ru-Fe-Ox nanospheres as efficient electrocatalyst to boost overall water-splitting in acid and alkaline media

Developing electrocatalysts with high activity and durability for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) in both acidic and alkaline electrolytes remains challenging. In this study, we synthesize a self-supported ruthenium-iron oxide on carbon cloth (Ru-Fe-Ox/CC) using solvothermal methods followed by air calcination. The morphology of the nanoparticle exposes numerous active sites vital for electrocatalysis. Additionally, the strong electronic interaction between Ru and Fe enhances electrocatalytic kinetics optimization. The porous structure of the carbon cloth matrix facilitates mass transport, improving electrolyte penetration and bubble release. Consequently, Ru-Fe-Ox/CC demonstrates excellent catalytic performance, achieving low overpotentials of 32 mV and 28 mV for HER and 216 mV and 228 mV for OER in acidic and alkaline electrolytes, respectively. Notably, only 1.48 V and 1.46 V are required to reach 10 mA cm−2 for efficient water-splitting in both mediums, exhibiting remarkable stability. This research offers insights into designing versatile, highly efficient catalysts suitable for varied pH conditions.