Electrochemical Water Splitting Research Articles

Enhanced oxygen evolution reaction using carbon-encapsulated Co-Fe-Al Alloy

Transition metal-based alloy nanostructures have emerged as promising catalysts for electrochemical water splitting, offering a potential solution to the growing demand for clean and renewable energy. Also, research on encapsulation engineering of alloy nanostructures using carbon matrices has recently advanced, presenting a new approach that combines the conductive properties of carbon matrices with an alloy structure to enhance electrochemical performance in water splitting. However, the specific roles of the alloy core, carbon shell, and their synergistic effect in improving electrochemical performance are still not well understood. In this paper, we report on the encapsulation of a ternary Co–Fe–Al alloy nanostructure using a carbon matrix and investigate its electrochemical oxygen evolution reaction (OER). The optimized carbon matrix-encapsulated Co0.3Fe0.2Al0.5 alloy catalyst demonstrated superior electrocatalytic activity for OER (achieving 250 mV at 10 mA cm−2) and long-term stability. Through density functional theory calculations, we also elucidate the crucial role of Al in enhancing the OER catalytic performance of the proposed carbon-encapsulated Co–Fe alloy catalyst. By comparing the free electron concentration and required applied potentials to trigger OER, we provide insights into the beneficial effects of incorporating Al into the Co–Fe alloy catalyst.

CoSex and CoSex/Reduced GO Composites as Electrocatalysts for Water Splitting

Hydrogen stands as an attractive energy source, offering a way to meet increasing energy demands without the reliance on fossil fuels and thus harming the environment. Renewable energy sources such as wind, wave and solar power can be used to generate inexpensive electricity, which we can then use to power water electrolysis.1 The generated hydrogen can then be converted back into electricity by using fuel cells, leaving only water as a by-product, rendering this technology clean and devoid of harmful emissions. The electrolysis cell can be operated at normal room temperatures without the need for high pressure rectors. At present, the practical application of hydrogen production using electrochemical water splitting is hindered by electrocatalyst limitations. Currently, the best electrocatalytic materials for water splitting are Platinum-based for the hydrogen evolution reaction (HER) and Ruthenium-based for the oxygen evolution reaction (OER). These materials are expensive and have limited reserves, 2 and therefore cost-effective alternatives must be developed. Transition metal dichalcogenides (TMDs) are of particular interest due to their tuneable structures. 3,4 Herein we have chosen to investigate CoSe2 as a potential electrocatalyst for overall water splitting (OWS). We describe both the electrochemical and solvothermal synthesis of CoSex. The CoSex was then combined with a carbon substrate to create a total of four electrocatalysts. Reduced graphene oxide (rGO) was employed as the carbon substrate and was synthesised via modified Hummer’s method, drop casted on to a GCE and electrochemically reduced to create a rGO modified GCE. The electrodeposited CoSex/rGO composite (ED-CoSex/rGO) was prepared by immersing a rGO modified GCE in an electrodeposition solution and deposited by holding the electrode at a fixed potential for a period of time (Figure 1). The solvothermal/rGO composite ST-CoSex/rGO was prepared by drop casting the solvothermally prepared CoSex ink on top of a rGO modified GCE. While most of the investigation on these electrocatalysts was done using simple glassy carbon electrode (GCE), the best performing material was then screened on a more porous electrode such as Nickel Foam (NF).Materials were characterised using a combination of HR-SEM, XRD, Raman and XPS, while cyclic voltammetry, linear sweep voltammetry and chronoamperometry were employed to examine catalytic activity of the materials. SEM unveiled distinct morphologies, with the electrochemically formed CoSex appearing as flower-like deposits, while sheet-like morphology was seen with the solvothermal synthesis. The solvothermally produced CoSex was more crystalline, as evidenced from XRD. Despite the morphological differences, both CoSex variants exhibited commendable electrochemical attributes.Materials also showed good stability over a period of 18 hours with the ED-CoSex material showing the best performance over 18 hours. Interestingly, all materials showed improved HER performance when subsequently tested after 18 hours via polarisation curves. All materials exhibited high electrochemical active surface areas (ECSA), stability, and favourable low overpotentials for both the hydrogen evolution reaction (HER) and the Oxygen evolution (OER) in alkaline media, however. It is worth noting that while the introduction of rGO significantly lowers the overpotential of electrodeposited and solvothermally synthesised materials, following chronoamperometry, it appears the beneficial effect of the rGO addition is less evident. These results underscore the potential efficacy of CoSex and CoSex composites in facilitating efficient water splitting for hydrogen production, with the potential to rival the performance of platinum-based electrocatalysts. The significance of this research lies in offering sustainable alternatives to platinum in electrolysers, a crucial step toward establishing a more accessible and eco-friendly hydrogen economy.References(1) Jacobson, M. Z.; Delucchi, M. A.; Bazouin, G.; Bauer, Z. A. F.; Heavey, C. C.; Fisher, E.; Morris, S. B.; Piekutowski, D. J. Y.; Vencill, T. A.; Yeskoo, T. W. 100% Clean and Renewable Wind, Water, and Sunlight (WWS) All-Sector Energy Roadmaps for the 50 United States. Energy Environ Sci 2015, 8 (7). https://doi.org/10.1039/c5ee01283j.(2) Anantharaj, S.; Ede, S. R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent Trends and Perspectives in Electrochemical Water Splitting with an Emphasis on Sulfide, Selenide, and Phosphide Catalysts of Fe, Co, and Ni: A Review. ACS Catalysis. 2016. https://doi.org/10.1021/acscatal.6b02479.(3) Minadakis, M. P.; Tagmatarchis, N. Exfoliated Transition Metal Dichalcogenide-Based Electrocatalysts for Oxygen Evolution Reaction. Advanced Sustainable Systems. 2023. https://doi.org/10.1002/adsu.202300193.(4) Sukanya, R.; da Silva Alves, D. C.; Breslin, C. B. Review—Recent Developments in the Applications of 2D Transition Metal Dichalcogenides as Electrocatalysts in the Generation of Hydrogen for Renewable Energy Conversion. J Electrochem Soc 2022, 169 (6). https://doi.org/10.1149/1945-7111/ac7172. Figure 1

Advancing Catalyst Performance in Electrochemical Water Splitting

The transition to renewable energy, vital for a sustainable future, frequently encounters challenges such as low energy density and intermittency. A promising solution lies in converting renewable energy into chemical bonds, notably hydrogen, through electrochemical water splitting. This process, however, is hindered by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. Consequently, this necessitates the use of rare and expensive iridium-based catalysts. Given that iridium is the only metal resilient enough to withstand the harsh anodic conditions of OER, reliance on it poses significant economic challenges, making water electrolysis less competitive compared to fossil fuel-based technologies.In response to these challenges, our research has two main objectives. First, we aim to understand the often observed trade-off between iridium OER activity and stability. (1) Second, we intent to enhance our understanding of catalyst-support interactions by explore the use of conductive and potentially durable materials as supports for iridium to improve efficiency, a critical yet largely unexplored area.To achieve these goals, we employ advanced techniques, including inductively coupled plasma mass spectrometry coupled with scanning flow cell. This method enables us to perform detailed potential and time-resolved stability analyses.Specifically, we conduct an in-depth investigation of the impact of pH across a range from acidic to alkaline environments. Furthermore, we explore a variety of backing electrodes, investigate accelerated stress tests (AST) parameters, (2) and assessing how catalyst loading and Nafion content affect the performance and stability of iridium catalysts.Our findings provide a deeper understanding of iridium’s stability during OER and highlight the importance of catalyst-support interactions. (3, 4) By addressing the key challenges associated with iridium-based catalysts, this knowledge is crucial for the development of more cost-effective catalysts with minimized use and hence sustainable water electrolysis technologies, marking a step closer to realizing more efficient and economically viable methods for harnessing and storing renewable energy.

Factors Influencing Activity and Electrochemical Stability of Ru-Based Oxygen Evolution Electrocatalysts

A hydrogen-based energy economy is a viable candidate to provide clean power and alleviate carbon emissions produced by hydrocarbon-based fuels. Hydrogen produced by proton exchange membrane water electrolysis (PEMWE) provides a path to clean affordable energy; however, PEMWE costs are still too high for wide-scale market adoption. Electrochemical splitting of water is energetically hindered by the oxygen evolution reaction (OER) which has sluggish kinetics, and the benchmark OER catalyst iridium oxide (IrO2) has high costs and very limited global supply. Ruthenium oxide (RuO2) provides a higher activity and lower cost compared to IrO2; however, under the high potential and highly acidic environment required for OER, RuO2 shows lower stability compared to IrO2. Our prior study demonstrated that Ti-substitution within RuO2 improved electrochemical stability, but also resulted in lower activity.1 Here, we expand our study by exploring the effects of phase, composition, surface structure, electrolyte, and operating voltage on the activity and electrochemical stability of Ru-based OER catalysts to identify key contributing factors towards improving the electrochemical stability. Catalysts materials were synthesized using wet-chemistry or high energy ball milling, and commercial materials were used for comparison and baselining. Structure was determined using X-ray diffraction, scanning electron microscopy, porosimetry measurements, and x-ray photoelectron spectroscopy. We evaluate electrochemical OER activity and stability using rotating disk electrode electrochemistry, coupled with inductive coupled plasma mass spectrometry for analysis of dissolved metals within the electrolyte. We develop structure-activity-electrochemical stability correlations to determine driving factors influencing degradation and provide a pathway to the development of materials with improved catalyst performance and lower cost. References Godínez-Salomón, J. F.; Ospina-Acevedo, F.; Albiter, L.; Bailey, K.O.; Naymik, Z.G.; Mendoza-Cruz, R.; Balbuena, P.B.; Rhodes, C.P., ACS Appl. Nano Mater. 2022, 5, 11752-11775.

Bifunctional Noble-Metal Free Ternary Chalcogenide Electrocatalysts for Water Splitting

With the ever-increasing global energy demand, the development of renewable energy technologies is crucial to fulfilling the energy requirements in a sustainable manner. Hydrogen has emerged as a promising energy carrier that can provide emission-free energy for end-use applications. Among the several pathways of hydrogen production, green hydrogen production by electrochemical water splitting using renewable energy is an emission-free, efficient energy conversion technology. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occur at the cathode and anode, respectively. Water splits into hydrogen and oxygen at the thermodynamic potential of 1.23 V; however, overpotentials are observed experimentally due to the kinetic barriers, which increases the required potential for water splitting. Catalysts lower the required overpotentials, making the process more energy efficient. A significant issue with large-scale hydrogen production by water electrolysis is that the state-of-the-art catalysts for HER and OER are based on precious metals and thus are limited by their high cost and net production capacity. Thus, there is a lot of interest in developing noble-metal-free electrocatalysts based on earth-abundant materials that can efficiently lower the associated overpotentials for the two halves of the water-splitting reaction.A large number of late transition metal-based ternary complex oxides have been investigated for their catalytic performance towards water splitting, where fine-tuning of the properties using compositional engineering has been demonstrated. Binary sulfides, selenides, and phosphides have also been explored as candidates for water splitting. Recently, early transition metal-based ternary complex materials have emerged as a novel class of electronic materials with applications in infrared optics and photovoltaics. However, similar chalcogenide compositions based on late-transition metals remain underexplored.We have synthesized a series of LaMS3 (M = Mn, Fe, Co, Ni) materials by high-temperature sulfurization of corresponding ternary oxides, using carbon disulfide (CS2) as the sulfurizing agent. The LaMS3 compounds are isostructural and crystallize in the hexagonal P 63 space group. When tested for catalytic performance towards water splitting, they demonstrate good activity towards both HER and OER, with LaNiS3 (LNS) being the most active material, and a periodic activity trend is observed, where LNS > LCS >> LFS > LMS. For OER in alkaline electrolyte, LNS demonstrates an overpotential of 373 mV at 10mA/cm2 in 1.0 M KOH, and a low Tafel slope value of 48 mV/dec. The long-term stability was tested using chronopotentiometry measurements, and LNS shows excellent stability, with negligible change in overpotential over several hours. For HER testing in both acidic and alkaline electrolyte, LNS again seems to be the most active material, and a periodic activity trend in the HER activity of LaMS3 materials, similar to the OER activity trend, is observed. LNS has an overpotential of 340 mV in 0.5 M H2SO4 and 362 mV in 1.0 M KOH, indicating that LNS is catalytically active over a wide range of pH levels. LNS also shows excellent long-term stability towards HER in both acidic and alkaline electrolyte. We also compared the activity of LaMS3 materials with their ternary oxide counterparts from which they were synthesized, and the LaMS3 compounds outperform the LaMO3 compounds, demonstrating lower overpotentials and Tafel slope values for both HER and OER, with the HER overpotentials for LaMO3 materials being > 250 mV higher compared to LaMS3, for both acidic and alkaline electrolytes. The catalytic activity of the LaMS3 series towards both HER and OER, along with their excellent long-term stability, makes these materials promising candidates for bifunctional electrocatalysts for water splitting while offering a wide compositional range that allows for further optimization of their performance.

Tuning Oxygen Evolving Catalytic Activity of a Precious Metal-Free Nickel Boride by a Cost-Effective Method

The production of hydrogen (H2) by electrochemical water splitting [Green Hydrogen (Green H2)], if driven by green electricity, has attracted considerable interest as an alternative sustainable energy carrier, e.g., in fuel cells for electricity or power and heat generation [1]. However, the lack of satisfactory and cheap electrocatalysts (ECs) to drive the vital electrochemical reactions of hydrogen evolution (HER) and oxygen evolution (OER) in water splitting is the biggest challenge for this technology.Currently, the most advanced electrocatalysts for the two half-reactions of water electrolysis (HER and OER) are noble metal-based materials [2]. Despite the excellent catalytic properties of these noble metal catalysts, their high cost, and scarcity make their commercial use both uneconomical and impractical in low temperature water electrolysis (LT-WE) systems at industrial scale for green H2 as a clean energy alternative to cheaper fossil fuels[3]. In addition to addressing this issue by the development of precious-material-based with a low-loading of precious metal elements, the development of desirable and high-efficiency electrocatalysts (ECs) based on earth-abundant metal elements materials is paramount. However, their catalytic activity is often limited by poor electron transfer, low conductivity, low stability, and small surface area of these materials. Thus, it is crucial to develop earth-abundant metal elements-based materials that are more cost-effective to reduce the high cost of active catalysts for water electrolysis. Although several low-cost materials have already been designed, the potential required for the practical application of water splitting is still very high compared to the theoretical potential (1.23 V), which means that much energy is consumed, which theoretically could be reduced. Due to their remarkable advanced charge transfer and durability for water oxidation in alkaline media, electrocatalysts derived from metal borides-based materials have received a lot of attention as new high-performance catalysts for water oxidation.[4] These properties stem from the multi-dimensional covalent bonding of the metalloid with the surrounding metal atoms.We present here an approach to tune the OER of transition metal borides (TMBs) using the cost-effective and less energy-consuming method that is nevertheless a highly efficient material for OER (Fig. 1). We have developed a low-cost, rational and easily scalable method to enhance the catalytic activity of nickel boride (Ni–B), which requires an annealing process to optimize the catalytic activity. However, this method may have limited applicability on a large scale as it requires a highly inert environment and high-temperature treatment of Ni–B catalysts. We also elucidate a structure-activity relationship using scale-bridging techniques such as TEM and electrochemical characterization. The developed based on non-noble electrocatalysts are prepared by a simple method of chemical reduction of transition metal elements with boron in aqueous solution. The production is a one-step process and the material does not require any further treatment, yet this material shows comparable activity to the state of the art (precious metal-based materials) for OER in alkaline solution at a selected current density. This improved catalytic performance is further corroborated by the microstructural investigations in the TEM. The nanoparticles obtained have an improved porous structure compared to Ni–B and thus provide more available sites for the surface reactions and hence for the catalytic performance of the material. Furthermore, it is shown that activation enables the morphological and structural changes, while some transition metal elements act as sacrificial elements to provide more accessible and stable sites for oxygen-forming centers. This paves the way for a better understanding of metal boride-derived electrocatalysts for water oxidation, a crucial chemical reaction in water electrolysis and other electrochemical energy technologies. Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 945422. We acknowledge the use of the DFG-funded Micro-and Nanoanalytics Facility (MNaF) at the University of Siegen (INST 221/131-1).

Electronic Structure Investigation of BSCF Catalysts during Electrochemical Water Electrolysis by Using Time Zero Analysis and Advanced Operando X-Ray Absorption Spectroscopy

The rapid increase in global energy demands and environmental awareness regarding the excessive use of fossil fuel has spurred significant academic interest in the development of alternative energy conversion and storage technologies that prioritize both efficiency and environmental sustainability. Among all kinds of renewable energy, hydrogen energy holds the potential to create a carbon free clean society, and a possible solution to the environmental issues.1 Electrochemical water splitting is considered as a greener approach for pure H2, which involved cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). The anodic OER is kinetically sluggish and considered as a rate determining step of the water splitting reaction. Therefore, it is necessary to develop OER catalysts with high activities.Perovskite oxides knows as a promising oxygen electrode catalyst due to their low cost, flexibility, and tailorable properties. However, most perovskite oxides possess poor electrical conductivity at room temperatures, and this limits their oxygen catalytic activity. In our work Ba0.5Sr0.5CoxFe1-xO3-δ (BSCF) catalysts with different cobalt and iron amounts where X=0, 0.2, 0.8, 1 were synthesized and investigated by time zero analysis and flow rate infinitization analysis which are electrochemical testing method that uses pressure to remove generated oxygen gas bubbles and provides accurate activity of the catalyst.2We find the volcano plot of the Fe amount and the OER activities, showing samples with X=0.8 have the best OER activities as Figure 1. Electronic structure of B-Site transition metal elements, oxygen vacancies, crystal structures and electrical conductivity are the key factors for determining the activities of Perovskite catalysts in OER.3Also it is quite important to determine the real active sites during the OER process by using advance operando techniques.Therefore, in order to investigate the in-depth mechanism of OER activities of Perovskite catalysts, the newly designed operando soft X-ray Absorption Spectroscopy (XAS) cells was utilized to analyze the subtle change as shown in Figure 2 on the oxygen K-edge (531 eV - 532 eV). Ba0.5Sr0.5Co0.8Fe0.2O3-δ has the most obvious changes in integral surface area at the O K-edge pre-edge region, which leads to the best activity among other BSCF samples. According to the Energy Dispersive X-ray Spectroscopy and Lattice Oxygen Evolution Mechanism, ions on the A sites (Ba2+,Sr2+) tend to dissolve and ions on the B sites (Co, Fe) are forming hydroxides (BOOH).Keywords: oxygen evolution reaction, BSCF catalysts, Time zero analysis, operando X-ray absorption spectroscopy Acknowledgements This work is based on results obtained from a project (JPNP14021) commissioned by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Reference: (1) Rose, P. K.; Neumann, F. Hydrogen refueling station networks for heavy-duty vehicles in future power systems. Transportation Research Part D: Transport and Environment 2020, 83, 102358.(2) Nagasawa, K.; Matsuura, I.; Kuroda, Y.; Mitsushima, S. A Novel Evaluation Method of Powder Electrocatalyst for Gas Evolution Reaction. Electrochemistry 2022, 90 (1), 017012-017012.(3) Zhu, Y.; Zhou, W.; Shao, Z. Perovskite/carbon composites: applications in oxygen electrocatalysis. Small 2017, 13 (12), 1603793. Figure 1

Montmorillonite Clay-Based Electrocatalyst for Water Splitting Applications

The goal of reaching net-zero emissions by the year 2050 requires a strategic overhaul to be realistically achievable. Hydrogen energy is considered an ideal substitute for fossil fuels, which leave carbon footprints and release hazardous chemicals into the atmosphere. Thus, large-scale commercialization of hydrogen energy could meet future energy demands with zero emissions. Electrochemical water splitting (EWS) is the most sustainable and efficient method to produce hydrogen for the next century. Thus, designing an electrocatalyst with an enhanced surface structure and abundant earth materials makes it a highly adoptable and affordable green energy. Smectite clay minerals are abundant on earth, non-hazardous, and easy to fabricate due to their clay nature. Among the smectites, montmorillonite is a promising candidate for electrocatalysts due to its unique physicochemical properties like expandable layered structure (TOT layers), high cation exchange, water adsorption, and swelling properties. Due to the high cationic exchange capacity of montmorillonite clays (70-110 meq/100g), the structural and interlayer cations can be tuned according to the water adsorption and desorption properties. When we look into the mineral structure, montmorillonite smectites belong to 2:1 phyllosilicate-layered sheets, composed of two tetrahedral silicate sheets (T) and one octahedral sheet (O). In each layer, an octahedral sheet is sandwiched between the two tetrahedral sheets (TOT), which is shown in Figure a. In the tetrahedral sheets, the dominant cation is Si4+, which is often substituted by the trivalent cation Al3+. The isomorphic substitution of Si4+ by Al3+ in the tetrahedral layer raises charge deficiency in the tetrahedral sheet, which is balanced by the absorbed interlayer cations like K+, Na+, and Ca2+ between the water molecules. In octahedral sheets, the common cations are Al3+, Mg2+, Fe3+, or Fe2+, and other cations like Ni2+, Li2+, and Cr2+. When the Mg2+ ions are substituted by Al3+ ions, it gives rise to a positive charge in the octahedral sheet. Depending upon the charges within the tetrahedral and octahedral sheets, the layered structure properties are altered. By considering their cation-rich and expandable properties, we have employed montmorillonite clay materials [Na,Ca)0.3 (Al,Mg)2 Si4O10(OH)2. nH2O] in electrocatalytic water splitting reactions for the first time. In our observation, annealing the montmorillonite at high temperatures such as 300 oC (Mont300/NF), 600 oC (Mont600/NF), and 900 oC (Mont900/NF), reconstruct the surface, and induce structural defects, which enables isomorphic substitution of bivalent (M2+) or trivalent cations (M3+). The annealed samples were prepared as slurry using binder additives (PVDF and NMP solvent) and coated on nickel foam (NF). The electrochemical performance was tested in a three-electrode potentiostat system in 1M KOH electrolyte, where the prepared sample was employed as the working electrode, graphite rod as the counter electrode, and Hg/HgO as the reference electrode. Figure b and c shows the electrochemical performances of the HER and OER reactions. Initially, the Mont600/NF catalysts show lower overpotential of 178 mV and 250 mV at 10 mA/cm2 for HER and OER reactions respectively. The pristine sample (Mont/NF) show much higher overpotential (240 mV for HER and 360 mV for OER). Thus, annealing promotes the formation of dangling bonds by generating cationic and anionic vacancies, which act as active sites for the adsorption of intermediates, which in turn reduces the requirement for higher overpotential. Furthermore, the pristine montmorillonite is mixed with functionalized CNT powder and annealed at the best performing temperature of 600oC (Mont+CNT600/NF). As a result, Mont+CNT600/NF sample shows much higher performance than Mont600/NF by requiring lower overpotential of 168 mV and 220 mV at 10 mA/cm2 for HER and OER reflecting the higher intrinsic kinetics owing to increased electrical conductivity due to CNT which enhances the electron transport properties and the water splitting kinetics. Interestingly, the used clay materials can be treated with organic solutions and can be reused due to their physicochemical retaining properties even after being treated at high temperatures. This would reduce the amount of solid catalyst waste going into landfills. Therefore, montmorillonite clay materials are the new era of catalyst’s material with recyclability due to their self-retaining physicochemical properties, water hydration, and dehydration properties. In this work, we have successfully optimized the montmorillonite properties by annealing at high temperatures, which helps to reconstruct the surface by breaking the OH lattice ions and creating dangling bonds. The formation of dangling bonds increases the rate of water splitting processes by acting as a surface-active site for the adsorption and desorption of H+ and OH- ions. Thus, tuning montmorillonite clays helps in achieving different water splitting properties. Figure 1

Approaching the Multiphase Interface of Ir-Based PEM Electrolyzers Using Operando ‘Tender’ X-Ray AP-XPS

Electrochemical water splitting is a promising pathway for the renewable production of hydrogen as an alternative fuel and chemical feedstock. Yet, the process is plagued by slow kinetics of the essential oxygen evolution reaction at the anode. Further, large-scale hydrogen production from such devices will require reduced platinum group metal (PGM) loadings (often iridium-based anodes) and/or new materials due to factors such as PGM scarcity, cost and loss during operation. In the hopes of enabling progress of rational design for OER catalysts, effective characterization of the electrode-liquid interface is crucial. Synchrotron-based X-ray Photoelectron Spectroscopy (XPS) offers a versatile way to obtain surface chemical and elemental characterization of these materials. The use of X-rays in the ‘tender’ regime (~2-6 keV) allows for deeper probing than the conventional ‘soft’ x-ray (< 2000 eV) set up that is common at synchrotrons and lab-based sources. The photoelectrons generated from tender x-rays have enough energy to penetrate tens of nanometers through low density components such as a layer of liquid or polymeric electrolyte. This opens the door to access the chemistry at the interface of liquid and ionomer electrolyte-electrocatalyst materials. With the use of a fully operational two-electrode system, we are able to study these membrane electrode assemblies under active water splitting conditions.My talk will explain our specific approach, which is often crucial for successful collection of accurate data, highlighting elements of cell design and experimental setup that are helpful for this process. I will also show recent AP-XPS results on iridium-based polymer electrolyte membranes for water splitting. With application of elevated potential, we can correlate the iridium valency and surface species with the appearance of product traces in a mass spectrum. Figure 1

Value-Added Electrolysis Hydrogen Production Via Methanol Oxidation over a Synergetic Pt1-W Dual-Site Catalyst

Electrochemical water splitting is a promising way of generating hydrogen for future sustainable green energy source technology options to replace nonrenewable and environmentally pollutant hydrocarbon energy sources. However, due to its high overpotential in oxygen evolution reaction (OER) and low-value product O2, some candidate anodic reactions with low potential should be proposed to replace OER, such as alcohol oxidation. Unlike fuel cell applications, partial oxidation is preferred to achieve high-value products rather than complete oxidation. For this purpose, we design a synergistic effect Pt1-W site on the dual-doped TiO2 support (Ti0.8W0.2NxOy) to catalyze alkaline methanol oxidation and gain 90% selectivity of formate production. Comparing the case of the Pt nanoparticles (69% selectivity of formaldehyde), the Pt single-atom catalyst controls selectivity and enhances the reactivity and stability attributed to the strong catalyst-support interaction. As a result, this work shows a novel strategy for electrochemical hydrogen production at far lower anodic potential by designing a stable and efficient Pt-based single-atom catalyst.

Metal Supported Metal Oxide Catalysts for Hydrogen Peroxide Production Via Water Splitting

The two-electron water oxidation reaction (WOR) has drawn recent attention as an efficient, simple method of producing on-site hydrogen peroxide through electrochemical water splitting. However, finding electrocatalysts with high activity and stability in the reaction’s harsh oxidizing environment is challenging. This is made especially difficult due to the competing four-electron and one-electron WOR, which necessitate a catalyst material with high selectivity towards the hydrogen peroxide production pathway. Here, we utilize metal/metal oxide coupling interactions to tune metal oxide catalysts to be active for 2 e- WOR via the creation of a Mott-Schottky junction. Metal/metal oxide interactions have been well studied for supported metal catalysts, but their behavior has been less investigated for catalysts where the metal oxide provides the active sites, called inverse catalysts. In this case, the difference in work functions between the metal support and metal oxide semiconductor catalyst drives electron transfer between the two, facilitating electronic interactions which can modulate the binding energy of reaction intermediates at the interface and thus tune the catalytic ability of the metal oxide. In this work, we investigate a variety of metal oxide catalyst and metal support materials and find that a thin layer of indium tin oxide (ITO) supported by a layer of platinum (Pt) has excellent catalytic ability towards 2 e- WOR. The activity, selectivity, and stability of the ITO/Pt catalyst shows significant improvement over the unsupported ITO catalyst and yields hydrogen peroxide production rates greater than those reported in literature using other standard catalysts for this reaction. This method of catalyst engineering also provides a future pathway to create new catalysts for this electrochemical reaction. Figure 1

Engineering of Soft Urea Synthesized Ni3n Nanomaterials for Oxygen Evolution Reaction

Most active electrocatalysts include platinum-group metals (PGMs) and their alloys for electrochemical water splitting, but they are strongly limited by material cost and scarcity. [1] On the other hand, transition metal nitrides overcome this bottleneck and show high electrical conductivity, making them catalytically active for water electrolysis. The high conductivity is based on the incorporation of nitrogen atoms into the unit cell of metal, leading to their d-band contraction with electron density enhancement below Fermi level. [2] For instance, metallic nickel nitride (Ni3N) has been widely reported as a promising electrocatalyst material to accelerate the oxygen evolution reaction (OER) by in-situ forming a nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layer. [3] The Ni3N materials can be prepared by single step soft-urea route to obtain pure crystal phase [4] or a composite with carbon [5], which rises the following two research questions: 1. What are the optimal synthesis parameters of the soft-urea route to control the structure of Ni3N? 2. What is the relationship between structure and OER activity for Ni3N materials?In this work, we used the soft-urea route to prepare Ni3N materials by varying the ratios of NiCl2, urea and ethanol as precursor chemicals. The precursors were made into a gel by magnetic stirring having metal:urea ratio of 1:5, and then heated up to 400 ºC in a tubular furnace with constant flow of argon. The as-prepared Ni3N were then characterized by X-ray diffraction (XRD), elemental analysis (CHN), transmission electron microscopy (TEM) equipped with an energy-dispersive X-ray spectroscopy (EDX) and X-ray absorption spectroscopy (XAS).Based on the quantitative Rietveld refinement, the formation of pristine Ni3N materials is strongly controlled by the annealing temperature. In addition to the Ni3N phase, we observed the formation of the fcc Ni phase (0 – 10 wt.%) as a minor phase. The TEM images show individual particles of below 100 nm in size. Rotating disc electrode (RDE) technique was employed to establish the electrochemical OER activity of Ni3N materials by sweeping the potential from 1.3 to 1.8 VRHE at 10 mV s-1 in Ar-saturated 0.1 M KOH. Polycrystalline Ni and Ni particles are taken as reference materials. We observed an improvement of the OER activity as the overpotential at 10 mA cm-2 (normalized by the geometric surface area) reduced from 379 mV (poly-Ni) to 358 mV (Ni3N). Furthermore, in-situ XAS studies were performed to better understand the nature of the redox reactions between Ni species and the in-situ formation of nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layers before and during the OER as a function of the applied potential. Based on the in-situ XANES data, we correlated the structure of Ni3N/fcc-Ni/carbon composite with observed OER activity under alkaline conditions.To summarize, we were able to prepare a composite of Ni3N/fcc-Ni/carbon material with improved OER activity via soft-urea route. The in-situ formation of the layered nickel (oxy)hydroxide [Ni(OH)2 and NiOOH] layers as the catalytically active OER species were probed by in-situ XANES data.

Dual-Site Bridging Mechanism for Bimetallic Electrochemical Oxygen Evolution.

Heterogeneous dual-site electrocatalysts are emerging cutting-edge materials for efficient electrochemical water splitting. However, the corresponding oxygen evolution reaction (OER) mechanism on these materials is still unclear. Herein, based on a series of in situ spectroscopy experiments and density function theory (DFT) calculations, a new heterogeneous dual-site O-O bridging mechanism (DSBM) is proposed. This mechanism is to elucidate the sequential appearance of dual active sites through in situ construction (hybrid ions undergo reconstruction initially), determine the crucial role of hybrid dual sites in this mechanism (with Ni sites preferentially adsorbing hydroxyls for catalysis followed by proton removal at Fe sites), assess the impact of O-O bond formation on the activation state of water (inducing orderliness of activated water), and investigate the universality (with Co doping in Ni(P4O11)). Under the guidance of this mechanism, with Fe-Ni(P4O11) as pre-catalyst, the in situ formed Fe-Ni(OH)2 electrocatalyst has reached a record-low overpotential of 156.4 mV at current density of 18.0 mA cm-2. Successfully constructed Fe-Ni(P4O11)/Ti uplifting the overall efficacy of the phosphate from moderate to superior, positioning it as an innovative and highly proficient electrocatalyst for OER.

Rapid Synthesis of Ruthenium-Copper Nanocomposites as High-Performance Bifunctional Electrocatalysts for Electrochemical Water Splitting.

Development of high-performance, low-cost catalysts for electrochemical water splitting is key to sustainable hydrogen production. Herein, ultrafast synthesis of carbon-supported ruthenium-copper (RuCu/C) nanocomposites is reported by magnetic induction heating, where the rapid Joule's heating of RuCl3 and CuCl2 at 200A for 10s produces Ru-Cl residues-decorated Ru nanocrystals dispersed on a CuClx scaffold, featuring effective Ru to Cu charge transfer. Among the series, the RuCu/C-3 sample exhibits the best activity in 1m KOH toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with an overpotential of only -23 and +270mV to reach 10mAcm-2, respectively. When RuCu/C-3 is used as bifunctional catalysts for electrochemical water splitting, a low cell voltage of 1.53V is needed to produce 10mAcm-2, markedly better than that with a mixture of commercial Pt/C+RuO2 (1.59V). In situ X-ray absorption spectroscopy measurements show that the bifunctional activity is due to reduction of the Ru-Cl residues at low electrode potentials that enriches metallic Ru and oxidation at high electrode potentials that facilitates the formation of amorphous RuOx. These findings highlight the unique potential of MIH in the ultrafast synthesis of high-performance catalysts for electrochemical water splitting.

Strong electronic coupling between Ni-based MOF and Ni2P enables high-efficiency oxygen evolution reaction for various application scenarios

The high overpotential and sluggish kinetics of oxygen evolution reaction (OER) severely hinder the widespread application of electrochemical water splitting, especially at industrial-level current densities. Herein, we report a novel heterostructural Ni-based metal organic framework (MOF)-Ni2P in-situ grown on iron foam (NiMOF-Ni2P/IF), which shows remarkable OER activity and stability. We demonstrate that the work function difference between NiMOF and Ni2P leads to the strong electron coupling effect by promoting electron transfer at the interface and results in a discernible upward shift of the d-band center of Ni sites for favorable OH− adsorption. The as-fabricated NiMOF-Ni2P/IF exhibits an ultralow overpotential of 330 mV and a small cell voltage of 1.796 V to drive NiMOF-Ni2P/IF‖Pt/C system at 1000 mA cm−2 in 1 M KOH, outperforming the majority of state-of-the-art non-noble-metal-based electrocatalysts. Furthermore, the catalyst shows remarkable performance even under harsh industrial conditions of 40, 60, and 80 °C in 6 M KOH. In addition, NiMOF-Ni2P/IF holds great promise with remarkable stability of 200 h and 600 cycles in simulated seawater and secondary zinc-air batteries (ZABs), respectively. This work offers a rational design strategy for low cost, high activity, and long durability OER electrocatalyst based on in-depth comprehension of the electronic coupling effect.

Fabrication of Ru-doped CuMnBP micro cluster electrocatalyst with high efficiency and stability for electrochemical water splitting application at the industrial-level current density

Electrochemical water splitting has been considered as a key pathway to generate environmentally friendly green hydrogen energy and it is essential to design highly efficient electrocatalysts at affordable cost to facilitate the redox reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this work, a novel micro-clustered Ru/CuMnBP electrocatalyst is introduced, prepared via hydrothermal deposition and soaking-assisted Ru doping approaches on Ni foam substrate. Ru/CuMnBP micro-clusters exhibit relatively low HER/OER turnover overpotentials of 11 mV and 85 mV at 10 mA/cm2 in 1 M KOH. It also demonstrates a low 2-E turnover cell voltage of 1.53 V at 10 mA/cm2 for the overall water-splitting, which is comparable with the benchmark electrodes of Pt/C||RuO2. At a super high-current density of 2000 mA/cm2, the dual functional Ru/CuMnBP demonstrates an exceptionally low 2-E cell voltage of 3.13 V and also exhibits superior stability for over 10 h in 1 M KOH. Excellent electrochemical performances originate from the large electrochemical active surface area with the micro cluster morphology, high intrinsic activity of CuMnBP micro-clusters optimized through component ratio adjustment and the beneficial Ru doping effect, which enhances active site density, conductivity and stability. The usage of Ru in small quantities via the simple soaking doping approach significantly improves electrochemical reaction rates for both HER and OER, making Ru/CuMnBP micro-clusters promising candidates for advanced electrocatalytic applications.

Co–FeOOH thin nanosheets derived from 2D ZIF-L for efficient oxygen evolution reaction

The complicated reaction pathway and slow electron-transfer kinetics of the oxygen evolution reaction (OER) have been identified as a critical bottleneck that restricts the overall performance of electrochemical water splitting. Metal oxyhydroxides have emerged as viable alternative catalysts as compared to the precious metal-based commercial benchmarks. Yet, the activity may be compromised by the instability of the layered structures. Herein, a simple two-step method is developed for the synthesis of Co–FeOOH electrocatalysts by thermal treatment of cobalt-containing zeolitic imidazolate framework (ZIF-L) deposited on the skeleton of NF (nickel foam) in the presence of FeSO4 and urea. Co–FeOOH/NF, the thin nanosheet arrays exhibit a high specific surface area and high hydrophobicity. The addition of Co regulates the electronic environment around the Fe center, which optimizes the adsorption energy of intermediates and enhances the inherent OER activity of active sites. The Co–FeOOH nanoarrays show excellent OER performance and 99.7% Faradaic efficiency (FE). At current densities of 10 and 100 mA cm−2, there is a low overpotential of 208 and 249 mV, respectively at 1 M KOH. This kind of synthesis method will provide a promising approach for the construction of catalysts featuring stable structures for alkaline water oxidation.

Ti3C2 MXene nanosheets integrated cobalt-doped nickel hydroxide heterostructured composite: An efficient electrocatalyst for overall water-splitting

Developing an efficient electrocatalyst for superior electrochemical water splitting (EWS) is crucial for achieving comprehensive hydrogen production. A heterostructured electrocatalyst, free of noble metals, Ti3C2 MXene nanosheet-integrated cobalt-doped nickel hydroxide (NHCoMX) composite was synthesized via a hydrothermal method. The abundant pores in the Ti3C2 MXene nanosheet (MX)–integrated microarchitecture increased the number of active sites and facilitated charge transfer, thus enhancing electrocatalysis. Specifically, the MX-enhanced charge transfer considerably transformed the microelectronic structure of cobalt-doped Ni(OH)2 (NHCo), which promoted its hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Hence, as an EWS catalyst, NHCoMX exhibited an exceptional electrocatalytic activity, demonstrating OER and HER overpotentials of 310 mV and 73 mV, respectively, with low Tafel slopes of 65 mV dec−1 and 85 mV dec−1, respectively; it exhibited a current density of 10 mV cm−2 in 1.0 mol L−1 KOH, representing the closest efficiency to the noble state-of-the-art RuO2 and Pt/C catalyst. Furthermore, the developed electrocatalyst improved the activities of both HER and OER, leading to an overall EWS current density of 10 mA cm−2 at 1.72 V in an alkaline electrolyte with two electrodes. This study describes an efficient heterostructured NHCoMX composite electrocatalyst. It is significantly comparable to the noble state-of-the-art electrocatalysts and can be extended to fabricate resourceful catalysts for large-scale EWS applications.

Spontaneous heterophase atomic structure engineering of NiMo(S)/NiMoP for enhanced overall electrochemical water splitting in neutral media

Hydrogen/oxygen evolution reactions catalyzed by transition metals have sluggish kinetics during prolonged water electrolysis in neutral media. Covering the surface with an amorphous protecting layer can improve the reactivity and stability of these catalysts; however, the rational design of crystalline–amorphous electrocatalysts for efficient and stable water electrolysis in neutral media has been challenging. Here, we present a unique and facile strategy for the in situ synthesis of the heterophase-structured cNiMo(S)/aNiMoP catalyst with a crystalline–amorphous phase boundary. The high-density active sites present on the surface and at the interface of the heterophase is responsible for the excellent electrochemical activity and operational stability of water electrolysis in neutral media. For the overall water splitting reaction, the cNiMo(S)/aNiMoP || cNiMo(S)/aNiMoP system displays a low cell voltage of 1.67 V to achieve a current density of 100 mA cm−2. Moreover, negligible performance degradation was observed during a continuous long-term stability test for 100 h.

Structural optimization: Ternary FeNiZn sulfide nanoparticles anchored on nanosheets to strengthen oxygen evolution reaction

Enhancing the activity of oxygen evolution reaction (OER) is crucial for improving the efficiency of electrochemical water splitting (EWS). The highly adjustable electronic structure and potential high conductivity of transition metal sulfides (TMSs) make them promising candidates for OER electrocatalysts. Herein, we have synthesized trimetallic sulfides Fe2Ni1Zn1–S(II) based on layered double hydroxides (LDHs) precursor employing a facile and efficient two-step solvothermal approach. Benefitting from the multi-metallic synergistic effect and the distinctive interfacial structure, Fe2Ni1Zn1–S(II) shows excellent electrocatalytic OER performance (η10 = 252 mV, Tafel slope = 64.2 mV∙dec−1). Meanwhile, it has outstanding stability (50 h) in 1 M KOH, which is better than RuO2 and many other metal sulfide electrocatalysts. Furthermore, it was found that the structure of the catalyst was still retained after OER test, which further verified its catalytic stability. This work supplies an effective route for the preparation of LDHs derived multi-metallic sulfides.