Electrocatalytic Properties of Quasi-2D Oxides LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, and Zn) for Hydrogen and Oxygen Evolution Reactions.

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Efficient bifunctional electrocatalyst for water splitting is essential for replacing fossil-fuel energy sources with clean energy-dense hydrogen fuel (142 MJ/kg). Efficient electrocatalyst can be obtained by either increasing active site density or specific activity on individual active sites. The active site densities can be increased through roughening the potential energy surface or exposing the facets which has higher active site densities. The specific activity can be increased through modulation of strain or charge densities on active sites which can be achieved through introduction of dopants, defects or stabilization of “non-native phases” that are all the other crystalline and amorphous states that differ in terms of discrete translational symmetry in the sub-surface region from the “native” phase (or bulk ground-state). While for a given composition, there is a unique native state for a given set of thermodynamic condition while, there can be many non-native structures having different bond-angles, bond-distances and surface atom densities from the native phase, leading to different electrocatalytic properties. In this context, polymorphic engineering via stabilizing ‘non-native phase’ offers a potential approach for improving both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts and its activity. The beneficial effect of polymorphic engineering with regards to bifunctional electrochemical OER and HER is demonstrated by first principle calculation by taking CoSe2 as a model electrocatalyst which has marcasite (Space Group-58) and pyrite (Space Group-205) as the native (N) and non-native (NN) structures, respectively. The first principle computations predict pyrite (NN) structure of CoSe2 would have better electrochemical activity towards OER and HER than its marcasite (N) counterpart which is confirmed through experimental results in literature too. Though the co-ordination number of Co remains same in both the structures, the co-ordination symmetry surrounding Co atom varies. This results in differential charge distribution in constituting Co- and Se-atoms consequently resulting in variable density of state (DOS) near Fermi level (Figure 1) thereby affecting the binding energies (BE) of reaction intermediates of OER and HER. Pyrite (NN) phase of CoSe2 has a greater electron density near Fermi Level in comparison to its marcasite (N) counterpart due to differential co-ordination symmetry. A greater electron density near Fermi-level is indicative of lower work function and consequently lower polarization resistance during water splitting. A greater electron density near Fermi level is contributed by Co-3d orbitals which is the common active site for both OER and HER. The greater electron density and lower work function in Pyrite (NN) results in stronger metal-hydrogen BE (0.03 eV) resulting in lower overpotential of HER. Hydrogen adsorption on Se sites occurs only at higher HER overpotential due to weak Se-hydrogen BE (0.59 eV). This results in observation of twin Tafel slopes during HER on CoSe2 electrocatalyst as the potential determination step (PDS) switches from Volmer to Heyrovsky step with participation of Se during HER. The lower work function and higher electron density near Fermi level in Pyrite (NN) structure results in weaker metal-oxygen bond thereby promoting multi-electron OER activity. The OER intermediates (-OH, -O, -OOH) has a higher BE over Co- than Se-sites. The transformation of Oads à HOOads on Co-sites of CoSe2 (001) structure is the potential determination step with an onset potential of 1.66 V (vs RHE).The desorption of O2 from Se site is found to be the potential-determination-step (PDS) for OER (η=0.79 V). Furthermore, pristine CoSe2 acts as a precursor for OER which undergoes dissolution to form a surface Co-O structure which has a greater activity than pristine pyrite CoSe2 surfaces (η=0.31 V). This energetics is more favourable for pyrite (NN) structure than marcasite (N) structure for dissolution process to form surface Co-O structure due to stronger Co-Se bonds present in the latter case. Furthermore, point-defects which can aid both OER and HER, can be more easily formed in pyrite (NN) structure than marcasite (N) structure due to the aforementioned reason. The present study underlines the importance of stabilization of non-native structures which has a great potential to produce higher electrocatalytic activity thus providing greater options in search of better water splitting electrocatalyst. Figure 1

NiCuCoS3 chalcogenide as an efficient electrocatalyst for hydrogen and oxygen evolution

Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne

As a replacement for renewable energy sources, an earth-abundant electrocatalyst for water splitting is effectively explored. In this work, Ni9S8 and cobalt-doped Ni9S8 nanostructures are fabricated on carbon cloth using the hydrothermal technique. The developed electrocatalysts are characterized through various techniques, for example, powder X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy , the Brunauer–Emmett–Teller method, and inductively coupled plasma atomic emission spectroscopy. Tuning of cobalt doping is performed to obtain the best optimized ratio of Co/Ni for electrocatalytic activity. All the developed materials are used for a water splitting reaction in an alkaline electrolyzer, and Co0.05Ni8.95S8 is an optimized material for both hydrogen and oxygen evolution. The electrocatalyst Co0.05Ni8.95S8 only requires −0.151 V versus RHE (reversible hydrogen electrode) to obtain a 10 mA/cm2 current density in the hydrogen evolution reaction (HER), and in the oxygen evolution reaction (OER), it requires 1.557 V versus RHE to generate a 30 mA/cm2 current density. The corresponding Tafel slope values for the HER and OER are 125 and 49.8 mV/dec, respectively, obtained by using Co0.05Ni8.95S8 electrocatalysts in 1.0 M KOH solution. The stability of Co0.05Ni8.95S8 is also checked, and it is stable for up to 60 and 80 h for the HER and OER, respectively. The cell voltage of 1.89 V is required to generate a 10 mA/cm2 current density for the overall water splitting reaction. The electrocatalyst is also used for the HER and OER in a wide pH range for practical applicability. The overall experimental findings were verified by theoretical calculations, which state that the higher metallic nature of Co-doped Ni9S8 facilitates efficient electrocatalytic activity. The optimum Gibbs free energy and hydrogen and oxygen coverage calculations also prove that the optimized Co0.05Ni8.95S8 electrocatalyst exhibits the best HER and OER activity. Therefore, this work provides a robust electrocatalyst for the electrocatalytic water splitting reaction.

Sustainable energy development can no longer be met by fossil fuels alone. Hence, electrochemical water splitting containing oxygen and hydrogen evolution reactions is appealing as a clean energy pathway. As respects the water splitting efficiency which is largely determined by the selectivity, durability, and intrinsic activity of the electrocatalysts, one of the most challenging questions when studying these materials is “which category of electrocatalysts will show the best performance in this issue?” The best electrocatalysts for water splitting still come from noble metals. Although these materials show particularly good efficiency, but due to the scarce resources their massive use is limited. Therefore, the noble metal‐free materials due to their stability, efficiency, abundance and variety of reaction sites were introduced as an interesting candidate for electrochemical water splitting reactions. In this review, based on the important above‐mentioned points, our attention was focused on key categories based on transition metals (TMs), metal organic framework derived (MOF‐derived), carbon‐based hybrids, graphitic carbon nitride (g‐C3N4) hybrids, and bio‐assisted electrocatalysts. These compounds have shown significant activity and stability for broad electrocatalysis applications in water splitting reactions and displaying remarkable potential to replace with noble metal‐based catalysts. This comprehensive review identifies rational strategies for designing and synthesizing high‐performance novel noble metal‐free electrocatalysts for water splitting.

Water electrolysis is an electrolysis reaction of water, and it is a technology that can produce a large amount of hydrogen environmentally friendly. The purposes of water electrolysis are responding to current hydrogen demand and storing electric energy produced from renewable energy such as solar or wind power to solve their intermittency. Water electrolysis reaction is divided into two half reactions; hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The theoretical equilibrium potential of the water electrolysis is 1.23 V, but the electrolysis reaction of the actual water requires much higher voltage due to the slow kinetics of oxygen and hydrogen reaction. Therefore, in order to increase hydrogen production efficiency, it is necessary to develop a catalyst having high activity and low cost for OER and HER. At present, transition metal based materials such as iron (Fe), cobalt (Co), and nickel (Ni) are mainly studied as high efficiency and low cost catalysts for oxygen and hydrogen evolution reactions for alkaline water electrolysis. Those transition metals are being studied in the form of various compounds such as oxide, carbide, nitride and phosphide. Among various type of compounds, transition metal phosphides (TMP) have drawn much attention for use in the alkaline water electrolysis system. These TMP catalysts have relatively good catalytic activity and stability for both OER and HER, and have high electrical conductivity compared to oxide based catalyst. In addition, TMP catalysts can be synthesized using non-toxic precursors as compared to nitride and carbide based catalysts. However, since most of TMP catalysts requires several hours to days for synthesis process, including high temperature heat treatment processes, the efficiency problem has not been solved. In this study, we have developed a method for producing nickel-phosphorus electrodes for high efficiency oxygen and hydrogen evolution reaction by fast-simple electrodeposition method. The synthesis procedure of the Ni-P electrodes ends in a few seconds at room temperature. High cathodic current density was applied to promote the generation of hydrogen bubbles and the deposition of Ni-P, thereby obtaining a highly porous structure. Hydrogen bubbles played an important role in forming the porous Ni–P layer by acting as self-dynamic templates. The surface area of highly porous Ni-P (HP Ni-P) was 50 times larger than that of the flat Ni electrode. The HP Ni-P exhibits remarkable electro-catalytic activity and stability towards the HER and OER in alkaline solutions. For the HER, overpotential at 10 mA/cm2 was 180 mV, which is slightly higher than that of Pt/C, but HP Ni-P required a lower overpotential than that of Pt/C at the high current region. For the OER, HP Ni-P shows higher catalytic activity than IrO2 at the entire current region; overpotential at 10mA/cm2 was 287 mV. The durability of HP Ni-P was also superior to the precious metal-based catalysts (Pt/C, IrO2) and Ni electrode in the long-term chronopotentiometry test. This result is ascribed to increase in surface area by hydrogen bubble generation and electronic structure change of Ni. Electronic structure change may lead to change in adsorption energy of intermediate adsorbates such as H*, OH*, O* and OOH*, which are participated in the steps of HER and OER, and can promote the kinetics of reactions in alkaline media. Figure 1

Study on density functional theory of MFe2O4 (M=Co, Ni, Cu) for electrocatalytic hydrogen and oxygen evolution reaction

Electrochemical water splitting is a promising method for the renewable production of high-purity hydrogen via the hydrogen evolution reaction (HER). Ni-Fe layered double hydroxides (Ni-Fe LDHs) are highly efficient materials for mediating the oxygen evolution reaction (OER), a half-reaction for water splitting at the anode, but LDHs typically display poor HER performance. Here, we report the preparation of self-organized Ag@NiFe layered double hydroxide core-shell electrodes on Ni foam (Ag@NiFe/NF) prepared by galvanic etching for mediating both the HER and OER (bifunctional water-splitting electrocatalysis). This synthetic strategy allowed for the preparation of organized hierarchical architectures which displayed improved the electrochemical performance by tuning the electronic structure of the catalyst and increasing the surface area utilization. X-ray photoelectron spectroscopy (XPS) and theoretical calculations revealed that electron transfer from the Ni-Fe LDH to Ag influenced the adsorption of the reaction intermediates leading to enhanced catalytic activity. The Ag@NiFe/NF electrode displayed overpotentials as low as 180 and 80 mV for oxygen and hydrogen evolution, respectively, at a current density of 10 mA cm-2, and improvements in the specific activity by ∼5× and ∼1.5× for the oxygen and hydrogen evolution reaction, respectively, compared to benchmark NiFe hydroxide materials. Additionally, an integrated water-splitting electrolyzer electrode can be driven by an AA battery.

Splitting water to hydrogen and oxygen through hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is basis to feed the fuel cell. OER is a thermodynamically uphill process as it requires higher potential to form O2 molecule. The best known catalysts to split water with low overpotential are the oxides of Ir and Ru. As these catalysts are precious metals there has been intensive research to substitute them with earth-abundant resources. The new catalyst should fulfill the following: (i) the catalyst to split water has to use minimal energy (measured as applied voltage and referred to as overpotential); (ii) should comprise of earth abundant elements; (iii) should be stable; (iv) should be scalable.1 Transition metal oxides have been recently identified as high-efficiency catalysts for oxygen and/or hydrogen evolution reaction. Apart from the oxides there has been reports of sulfide, selenide and nitride based catalysts.2 Although transition metal phosphides has recently gathered attention as HER catalysts,3 very less metal phosphides have been reported as OER electrocatalyst. In this presentation, ultra-small iron phosphide nanoparticles as well nanoparticle composite with reduced graphene oxide (FeP-rGO) has been reported as efficient electrocatalysts for OER under alkaline conditions. FeP nanoparticles require an overpotential of 290 mV @ 10 mAcm-2. It is well known that rGO is a good conductor and a support material for the catalysts. Hence, mixing of FeP nanoparticles with rGO has improved the catalytic efficiency further by reducing the overpotential to 260 mV to achieve 10 mAcm-2 current density4. The FeP nanoparticles and FeP-rGO composite showed the lowest overpotential at 10 mA/cm2 that has observed among the pnictide making these the most-efficient phosphide based OER catalyst till date. The hybrid catalyst showed a low Tafel slope of 50.8 mV/dec. The stability of the catalyst was excellent with 4h of continuous oxygen evolution and the catalytic activity was retained with a similar overpotential @ 10 mAcm-2. The synthesis of hybrid catalysts, detailed characterization, and electrochemical studies including liner scan voltammetry (LSV), choronoamperometry will be discussed in detail.

Semiconductors are the workhorse material of solar cells for the conversion of solar energy into electrical current. Similarly, catalysts play a major role in the electrochemical water splitting, providing efficient shuttles for the exchange of charge carriers between the solid and electrolyte. Simultaneous use of semiconductors as catalysts is under an ongoing research effort bringing together these different research fields toward a common target of renewable hydrogen production. In this Review, we summarize the common concepts from photovoltaics and electrocatalysis, highlighting the available literature on the catalytic properties of semiconductors toward hydrogen evolution and oxygen evolution reactions. The overpotential, photovoltage, and Tafel slope are considered as the main figures of merit for the water splitting reactions. The surveyed literature presents a broad span of Tafel slopes highlighting the complexity of the photoelectrode processes and rate-determining steps of the water splitting reactions. This work aims to provide arguments and suggestions for investigations of the common catalytic properties of semiconductors during the quest for the efficient photoelectrodes.

Comparing electrocatalytic hydrogen and oxygen evolution activities of first-row transition metal complexes with similar coordination environments

Developing a single electrocatalyst that can facilitate both rechargeable aqueous metal-air batteries and water splitting has become a crucial focus in renewable-energy technologies. This necessitates addressing the three distinct electrocatalytic reactions: the electrochemical oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Despite significant efforts, the creation of a alkaline medium based trifunctional catalyst with high activity at a low cost has proven to be a considerable challenge. Currently, Pt and its alloys have been considered as the most active catalysts for the ORR and HER, whereas noble-metal oxides such as IrO2 and RuO2 are considered as the golden standards of OER catalyst. However, noble-metals based catalyst have been suffered from their high cost, limited reserves in the Earth’s crust, and poor electrocatalytic stability. Moreover, these noble metals face challenges in simultaneously exhibiting trifunctional catalytic activity for ORR, OER, and HER. In recent study, Transiton metal sulfides(TMSs) have great attraction as trifunctional catalyts due to their eqrth abudance, tunnable band structure, and crystal structure. especially, one of widely used TMCs is MoS2 because of their Pt-like high catalytic activity for HER as well as thermodynamic electrochemical stability and rich catalytic sites in the planar nature. However, the most stable 2H (hexagonal) phase MoS2 suffers from poor electrical conductivity, low wettability, and aggregation indced reducing active catalytic sites and resulting high resistance. 2H MoS2 also shows poor activity for OER and ORR, thereby hampering its practical applications as trifunctional catalyst. To overcome this threshold, various strategies have been conducted to improve their electrochemical charateristics, such as defect engineering, heterojunction foramtion, phase transform and integrating porosity control. However, commonly considered method to synthesis requires complex multi-steps with high temperature, vaccum system, explosive gas, and toxic etchant.In this study, we successfully synthesized the homogeneous growth of a Co-based nanometer-scale metal-organic framework (MOF) on graphene oxide at room temperature. Furthermore, a facile one-pot solvothermal method was employed to synthesize Co-MoSx/Graphene, which consists of a hollow, heterogeneous bimetallic sulfide (Co3S4/MoS2 with Co-S-Mo bonding) within a sandwiched graphene/MoS2 layer, demonstrating superior trifunctional activity and stability. Incorporating a conductive graphene layer between MoS2 layers is an effective strategy for not only realizing high electrical conduction to MoS2 layers, but also increasing MoS2 interlayer spacing for high ion accessibility. Besides, a variety of techniques, including Cs-corrected scanning transmission electron microscope (Cs-STEM), X-ray diffraction(XRD), and X-ray absorption spectroscopy (XAS), are used to confirm the atomic configurations of Co-MoSx/Graphene structure and morphologies. Also, Raman, Fourier-transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) were conducted to investigating the binding structure and chemical states. Furthermore, The internal electric field (IEF) within heterojunction, which induced from the differing electron density of the bimetallic species and the sandwiched graphene are contribute not only electron density structure optimization for enhancing reaction kinetics but also accelerating electron-hole exchange. The IEF in the microporous-heterostructure accelerates the diffusion of reaction intermediate with sufficient mass transport and facilitates a Graphene/MoS2-to-Co-MoSx pathway for enhancing redox kinetics of sluggish OER and ORR. Consequently, the OER and ORR-inactive MoS2, HER, and ORR-inactive Co3S4, along with less catalytically effective graphene, demonstrate outstanding performance when combined in the bimetallic sulfide based highly active heterojunctional structure. To investigate the electrochemical catalytic properties of Co-MoSx/Graphene, Rotating disk electrode(RDE) was used with three electrode measurment. The presence of MoS2/Graphene on Co3S4 and Co-MoSx bonding species in Co-MoSx/Graphene enhances the alkaline electrochemical catalytic activity by reducing overpotential and Tafel slopes(220 mV, 110 mV dec-1 in HER and 320 mV, 55.8 mV dec-1 in OER) under 1M KOH solution. Moreover, the ORR performance was evaluated by using Koutecky-Levich (K-L) Plot with different rotating speeds under 0.1M KOH. The electron transfer number (n) is closed theoretical value of 4.0 also shows outstanding performance of the onset potential, half-wave potential and kinetic current density (0.88 V, 0.67 V, and 10.4 mA cm-2), which is comparable that of Pt/C (0.92 V, 0.8 V, and 10.3 mA cm-2). Furthermore, Co-MoSx/G based rechargeable Zinc-Air battery achieve over 85% of theoretical zinc utilization efficiency and 1.4 times higher power density than a Pt/C + RuO2 air cathode based system. We believe that this work could provide a rational strategy for achieve trifunctional electrocatalyst and high performance self-powered hydrogen production system.This research was supported by the National Research Foundation of Korea (2022M3H4A1A04096482, RS-2023-00229679) funded by the Ministry of Science and ICT.

Among water splitting techniques, electrochemical water splitting is enhanced using efficient catalysts to complete hydrogen evolution (HER) and oxygen evolution (OER) reactions. However, when it comes to commercial level processing to create water electrolyzers including AEM and PEM electrolyzers, these catalysts mostly in powder state require to be immobilized onto a current collector using a suitable polymeric binder. This coating process is very important to maintain the catalyst strength, reducing the interfacial resistance between catalyst and current collector etc. However, peeling off of the catalysts and thereby catalyst aggregation is often confronted during long term operation causing large decrease in electrolyzer performance. In this scenario, self-supported catalysts which are directly grown or developed on conductive substrates or forming free-standing films are identified as a solution to overcome this problem while progressing to realize efficient water electrolyzers. Some major advantages of the self-supported catalysts include; direct use of catalysts as anode/cathode electrodes, excellent synergistic effect between the catalyst and substrate, reduced peeling off catalysts and more importantly greater charge transfer between catalyst layer and current collector. In the present work, we developed self-supporting multi metal catalysts over nickel foam which can be used as electrode materials for integrating water electrolyzers capable of high performance and durability in alkaline conditions. Oxygen evolution reaction (OER) studies under half cell conditions in 1 M KOH using the developed self-supported catalysts involving Fe and Ru over nickel foam displayed an over potential of 185 mV at 10 mA cm-2, while 111 mV was for observed during hydrogen evolution reaction (HER). The presentation will include water splitting performance data using the processed catalysts under alkaline conditions and also the detailed electrochemical and spectroscopic results during and post OER/HER. Further, the synergistic interactions among the metal species, creation of active species/sites and changes in electron charge transfer leading to the excellent activity and stability will also be discussed. Fig 1. HER and OER CV profiles in half cell conditions using the self-supported catalysts References Zhang et al, Homogeneously dispersed multimetal oxygen-evolving catalysts, Science 2016, 352,333-337 Kwon, H. Han, S. Choi, K. Park, S. Jo, U. Paik, T. Song, Current Status of Self-Supported Catalysts for Robust and Efficient Water Splitting for Commercial Electrolyzer, ChemCatChem 2019, 11, 5898–59 Miyanishi, T. Yamaguchi, Highly conductive mechanically robust high M wpolyfluorene anion exchange membrane for alkaline fuel cell and water electrolysis application, Polym. Chem. 2020, DOI: 10.1039/D0PY00334DA. Miller, K. Bouzek, J. Hnat, S. Loos, C. I. Bernacker, T. Weißgarber, L. Rontzsch, J. Meier-Haack, Green hydrogen from anion exchange membrane water electrolysis: a review of recent developments in critical materials and operating conditions, Sustainable Energy Fuels, 2020,4, 2114-2133 Acknowledgements: This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO), Japan Figure 1

Ultrathin 2D metal-organic framework nanosheet arrays to boost the overall efficiency of water splitting