Optimizing Ni/Co ratios in NiCo-LDH for enhanced hydrogen and oxygen evolution reactions in overall water splitting

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

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Designing cost-effective and highly efficient electrocatalysts for water splitting is a significant challenge. We have systematically investigated a series of quasi-2D oxides, LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, Zn), to enhance the electrocatalytic properties of the two half-reactions of water-splitting, namely oxygen and hydrogen evolution reactions (OER and HER). The four materials are isostructural, as confirmed by Rietveld refinements with X-ray diffraction. The oxygen contents and metal valence states were determined by iodometric titrations and X-ray photoelectron spectroscopy. Electrical conductivity measurements in a wide range of temperatures revealed semiconducting behavior for all four materials. Electrocatalytic properties were studied for both half-reactions of water-splitting, namely, oxygen-evolution and hydrogen-evolution reactions (OER and HER). For the four materials, the trends in both OER and HER were the same, which also matched the trend in electrical conductivities. Among them, LaSrMn0.5Co0.5O4 showed the best bifunctional electrocatalytic activity for both OER and HER, which may be attributed to its higher electrical conductivity and favorable electron configuration.

Developing efficient, nanostructured electrocatalysts with desired compositions and structures is of great significance for improving the efficiency of water splitting toward hydrogen production. In this regard, metal organic framework (MOF) derived nanoarrays have attracted great attention as promising electrocatalysts because of their diverse compositions and adjustable structures. This presentation summarizes our recent work on the design and fabrication of MOF-derived nanosheet arrays toward enhanced catalytic activity for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) as well as overall water splitting.First, heterostructured inter-doped ruthenium-cobalt oxide ((Ru-Co)Ox) hollow nanosheet arrays were prepared on carbon cloth for efficient overall water splitting. Benefiting from the desirable compositional and structural advantages of more exposed active sites, optimized electronic structure, and interfacial synergy effect, the (Ru-Co)Ox nanoarrays exhibited outstanding performance as a bifunctional catalyst. Particularly, they showed a remarkable hydrogen evolution reaction (HER) activity with an overpotential of 44.1 mV at 10 mA cm–2 and a small Tafel slope of 23.5 mV dec–1, as well as an excellent oxygen evolution reaction (OER) activity with an overpotential of 171.2 mV at 10 mA cm−2. As a result, a very low cell voltage of 1.488 V was needed at 10 mA cm–2 for alkaline overall water splitting.Second, Mo-doped ruthenium–cobalt oxide (Mo-RuCoOx) nanosheet arrays were produced for high-efficiency water splitting through combining electronic and vacancy engineering. The unique Mo-RuCoOx nanosheet arrays were able to act as a high-performance bifunctional electrocatalyst toward both HER and OER. Theoretical calculations and experimental results reveal that the incorporation of Ru and Mo can effectively tune the electronic structure, and the controllable Mo dissolution coupling with the oxygen vacancy generation during surface reconstruction is able to optimize the adsorption energy of hydrogen/oxygen intermediates, thus greatly accelerating the kinetics for both HER and OER. As a result, the Mo-RuCoOx nanoarrays exhibit remarkably low overpotentials of 41 mV and 156 mV at 10 mA cm–2 for HER and OER in 1 M KOH, respectively. Furthermore, the two-electrode electrolyzer assembled by the Mo-RuCoOx nanoarrays requires a cell voltage as low as 1.457 V to achieve 10 mA cm–2 for alkaline overall water splitting.Third, hollow nanosheet arrays assembled by ultrafine ruthenium-cobalt phosphide nanocrystals were fabricated toward exceptional pH-universal hydrogen evolution. The development of high-efficiency electrocatalysts for pH-universal HER is promising for constructing feasible water splitting systems at all pH values, but it remains challenging. A facile approach toward hollow nanosheet arrays assembled by ultrafine ruthenium-cobalt phosphide (Ru-CoxP) nanocrystals was developed through conversion from the MOF template. The synergic effects of optimized electronic structure, increased active sites, and rapid charge/mass transfer endowed the Ru-CoxP nanoarrays with outstanding electrocatalytic performance toward pH-universal HER. While exhibiting remarkably low overpotentials of 34.6 mV and 22.7 mV at 10 mA cm–2 in 1 M KOH and 0.5 M H2SO4, respectively, the Ru-CoxP nanoarrays showed an extremely low overpotential of 21.6 mV at 10 mA cm–2 in 1 M phosphate buffer solution (PBS). Furthermore, they were able to stably drive a Pt-free neutral electrolyzer for overall water splitting at 10 mA cm−2 with a cell voltage as low as 1.557 V.

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.

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.

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

NiCuCoS3 chalcogenide as an efficient electrocatalyst for hydrogen and oxygen evolution

With increasing global demand for energy, rapid depletion of fossil fuels and intensification of environmental concerns, exploring clean and sustainable energy carriers to replace fossil fuel is becoming critical. Among the various alternatives, hydrogen has been intensively regarded as a promising energy carrier to fulfill the increasing energy demand due to its large energy density per unit mass and eco-friendly production possibilities. However, hydrogen does not exist in molecular structure in nature, and it is essential to obtain efficient and sustainable H2 production technologies. Alkaline water electrolysis is an effective, clean and sustainable process to produce high-quality hydrogen. In this process, highly active electrocatalysts for the hydrogen evolution reaction (HER) are required to accelerate the sluggish kinetics and lower the overpotentials (η) for efficient hydrogen evolution. To date, a noble metal, platinum (Pt), is the state-of-art electrocatalyst for HER. However, exploration of alternative electrocatalysts with low cost and excellent electrocatalytic activity is of vital importance to realize large-scale hydrogen production through water electrolysis. Generally, an electrochemically active catalyst should have an optimal hydrogen adsorption free energy to allow efficient catalytic hydrogen adsorption/desorption. In alkaline solution, dissociation of water onto the electrocatalyst determines the overall HER efficiency. This thesis focuses on rational design and synthesis of different earth-abundant electrocatalysts for electrocatalytic HER in alkaline media. Through facile anion or cation doping strategies, electrocatalysts with abundant accessible active sites, enhanced electronic conductivity and accelerated HER kinetics have been systematically fabricated, characterized and evaluated. First, an efficient HER electrocatalyst in alkaline media was fabricated by incorporating sulfur atoms into a cobalt (hydro)oxide crystal structure. The resultant catalyst exhibits a remarkably enhanced HER activity with a low-overpotential of 119 mV at 10 mA/cm2 and an excellent durability. The results suggest that cobalt hydroxide benefits water adsorption and cleavage, while the negatively charged sulfur ligands facilitate hydrogen adsorption and desorption on the surface of electrocatalysts, leading to significantly promoted Volmer and Heyrovsky steps for HER in alkaline media. Second, exploring bifunctional electrocatalysts which can simultaneously accelerate the HER and oxygen evolution reaction (OER) activities plays a key role in alkaline water splitting. Here, sulfur atoms were incorporated into the mixed transition metal hydroxide with high OER performance to render excellent HER activity. The enhanced catalytic activity towards HER was confirmed by a synergistic effect between the retained metal hydroxide host and the incorporated sulfur atoms. In addition, the full water splitting electrolyzer equipped with fabricated bifunctional electrocatalysts as anode and cathode materials exhibited remarkable overall water splitting performance comparable to that with benchmark Pt and RuO2 electrocatalysts. The S/Se co-doped Co3O4 nanosheets on carbon cloth were fabricated by a facile room temperature chalcogen atom incorporation methodology and were applied as the electrocatalyst for HER in alkaline media. The sulfur and selenium atoms were homogeneously distributed on the surface by forming Co-S or Co-Se bonds which play a key role in the structural change in electrochemical activation. The obtained electrocatalysts demonstrated remarkably improved HER activity compared to that of the original Co3O4. Finally, molybdenum doped cobalt hydroxide was fabricated with significantly accelerated HER kinetics. The introduced Mo sites not only effectively facilitate water dissociation process and desorption of the OHads intermediates, but also simultaneously optimize the hydrogen adsorption free energy. Therefore, the in situ-generated Mo-doped amorphous cobalt hydroxide exhibited a remarkable HER performance in alkaline media with an overpotential of only -80 mV at a current density of 10 mA/cm2. This thesis innovatively explores strategies to improve the catalytic activity towards HER of metal (hydro)oxide in alkaline media. The surface foreign atom doping was demonstrated to manipulate the surface structure of catalysts, thus not only improving the water dissociation processes, but also facilitating the hydrogen adsorption/desorption on the catalysts. The demonstrated facile and effective strategies could be adopted for the fabrication of cost-effective and highly active catalysts for other important chemical reactions for energy conversion applications.

Highly efficient photocatalysts of the type M x O y @MIL‐125(Ti) (M x O y = MnO 2 , Fe 2 O 3 , Co 3 O 4 , NiO and CuO) have been prepared by in‐situ incorporation of pre synthesized metal oxide nanoparticles into MIL‐125(Ti) through hydrothermal method. The synthesized samples have been characterized by Powder X‐ray diffraction, Raman spectroscopy, Scanning Electron Microscopy, Energy Dispersive X‐ray Spectrometry, Elemental Mapping, UV‐Vis spectrophotometry andN 2 ‐adsorption Isotherm. The photoelectrochemical properties for oxygen evolution reaction and hydrogen evolution reaction have been studied by Cyclic voltammetry and Linear sweep voltammetry analysis in 2 M aq. KOH electrolyte and stability for constant current generation of these samples is observed by Chronoamperometric measurements. These studies indicate that metal oxide nanoparticles have been successfully incorporated into MIL‐125(Ti), which enhance the efficient absorption of visible light and improve the oxygen evolution and hydrogen evolution reaction activity during water splitting. Furthermore, from linear sweep voltammetry results at 1mVs −1 scan rate it is observed that M x O y @MIL‐125(Ti) samples have lower onset potential and higher current density as compared to pure MIL‐125(Ti). CuO@MIL‐125(Ti)/NF exhibited highest current density, lowest onset potential and better oxygen evolution reaction and hydrogen evolution reaction activity as compared to all other synthesized samples.

Open AccessCCS ChemistryRESEARCH ARTICLE25 May 2022Controlled Growth Interface of Charge Transfer Salts of Nickel-7,7,8,8-Tetracyanoquinodimethane on Surface of Graphdiyne Yuxin Liu, Yang Gao, Feng He, Yurui Xue and Yuliang Li Yuxin Liu Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Yang Gao Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Feng He Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 , Yurui Xue *Corresponding authors: E-mail Address: [email protected]; E-mail Address: [email protected] Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 Science Center for Material Creation and Energy Conversion, School of Chemistry and Chemical Engineering, Institute of Frontier and Interdisciplinary Science, Shandong University, Jinan 250100 and Yuliang Li *Corresponding authors: E-mail Address: [email protected]; E-mail Address: [email protected] Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190 University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202005 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Here we report an in situ assembly growth method that controls the growth of NiTCNQ on the surface of graphdiyne (GDY). The catalytic system of donor–acceptor–donor (GDY/TCNQ/Ni) structure with multiple charge transfer (CT) was achieved by controlling the growth of NiTCNQ on the surface of GDY. Significantly, a controlled double layer interface of GDY/TCNQ/Ni was formed. This system implemented simultaneously the two elements we expected (1) an incomplete CT, and (2) the infinite distribution of active sites originating from highly asymmetric surface charge distribution. The high conductivity and typical semiconductor characteristics of the catalyst endows it with high catalytic activity. We found that an electrolytic cell consisting of the CT salt as a catalyst provided a 1.40 V ultra-small cell voltage up to 10 mA cm−2 and the outer GDY film effectively prevented the corrosion of the catalyst. Our study is the first to introduce CT complexes to a novel catalytic material platform for high selectivity of catalysts, and undoubtedly demonstrates the high selectivity, stability, and activity of such catalytic systems, which provides a new space for the development of novel conceptual catalysts. Download figure Download PowerPoint Introduction The sustainable hydrogen (H2) production through electrocatalytic overall water splitting (OWS) provides a promising method to overcome the global energy crisis and environmental problems, which is of great significance for industrial and scientific progress.1,2 In terms of hydrogen conversion, precious metal-based materials (e.g., Ru or Ir-based materials for the oxygen evolution reaction (OER); Pt-based materials for the hydrogen evolution reaction (HER)) remain the benchmark catalysts with high catalytic performances, but they are not suitable for large-scale commercialization due to their high cost, low abundance, and insufficient stability. At present, the high cost of precious metals has seriously restricted the development of this field. Transition metal-based materials, such as metal hydroxides, oxides, nitrides, phosphides, carbides, and sulfides, have been extensively studied for anodic OER and cathodic HER.3–6 However, the low catalytic activity of these systems results in sluggish kinetics of OER and HER. In addition, there is no stability advantage of these catalysts. Furthermore, to meet the requirement of industrial alkaline water splitting for hydrogen production, the catalysts should produce large current densities at low cell voltages (e.g., ∼200–400 mA cm−2 at 1.8–2.4 V) and be synthesized from earth-abundant elements to minimize the H2 production cost.7–10 Therefore, to develop catalysts with excellent comprehensive performance, we need to find a new way to produce electrocatalysts with low cost, high activity, and high stability. The unique and excellent structures and properties of graphdiyne (GDY) has triggered extensive investigations on its fundamental properties and potential applications in various research fields, such as catalysis, energy conversion and storage, optical and electrical devices, and so on.11–26 As a new sp- and sp2-cohybridized two-dimensional carbon material, GDY shows large porous structure, high intrinsic activity, excellent electron transfer ability, high electronic conductivity, and high stability.27–29 GDY is an ideal carbon material for synthesizing high-performance catalysts with determined structures and abundant active sites, and provides the opportunities for clearly understanding the relationship of structure and performance. GDY can be grown on arbitrary substrates at low temperatures and ambient pressures, which protects the parent structure and the active site structure from destruction and generates new active sites. 7,7,8,8-Tetracyanoquinodimethane (TCNQ) is a well-known electron acceptor with high electron affinity, which easily reacts with metal atoms to form charge transfer (CT) complexes with high conductivity, high carrier density, and special electron transport properties.30 Metal-TCNQs show excellent electrical and optical properties, and unique and adjustable electron and high CT ability. CT complexes where the CT amount is not an integer is called incomplete CT; it is different from traditional inorganic complexes with an integer CT. Because of their high conductivity and semiconductor characteristics, they have attracted more and more interest in many research fields.31–34 However, until now, research on CT complexes mainly focused on controlled growth, photoelectric devices. and semiconductor properties, while basic and applied research in the field of catalysis remain silent. Recently, we took NiTCNQ–GDY as an example and explored the basis and application of NiTCNQ–GDY as an emerging catalyst in hydrogen energy conversion. In addition, we deeply understand the GDY plays the key role in the generation of efficient catalysts, and find its introduction overcomes important issues such as solvent corrosion, structural instability caused by light or electricity, and poor conductivity.35,36 In this work, we report the successful synthesis of a donor–acceptor–donor (GDY/TCNQ/–Ni) structure with multiple CT by in situ growing GDY layers on the surface of NiTCNQ nanowires (GDY–NiTCNQ). Experimental results demonstrated the incomplete CT and the infinite distribution of active sites originating from highly asymmetric surface charge distribution endow the catalyst with high conductivity, typical semiconductor characteristics, high catalytic activity, and excellent intrinsic properties. These unique and fascinating properties endow GDY–NiTCNQ with excellent catalytic activities toward OER, HER, and OWS. For instance, GDY–NiTCNQ exhibited small overpotentials of 218 and 61 mV at 10 mA cm−2 for OER and HER in 1.0 M KOH. When used as an electrolytic cell, it can drive 10 mA cm−2 at a low cell voltage of 1.40 V. This work provides a new direction toward the development of cost-effective and high-performance electrocatalysts. Experimental Methods Material TCNQ was purchased from Tokyo Chemical Industry (TCI, Shanghai, China). Acetonitrile and methanol were obtained from Concord Technology (Tianjin) Co., Ltd. Nickel foams (NFs) were treated with dilute hydrochloric acid, rinsed with deionized water, and dried with N2 flow before use. Deionized water was purified with a Millipore system. All chemicals were analytical grade and used without further purification unless otherwise specified. Preparation of NiTCNQ in acetonitrile A piece of freshly pretreated nickel film (3 cm × 1.5 cm) was immersed in 10 mM TCNQ acetonitrile solution for 10 min followed by the addition of 3% deionized water at room temperature. After reaction for 6 h, the material was removed from the solution, washed carefully with acetonitrile and water, and dried under the protection of N2. Preparation of GDY on NiTCNQ (GDY–NiTCNQ) A piece of NiTCNQ was immersed in 50 mL acetone solution of hexaethynylbenzene (HEB) at 50 °C under Ar atmosphere protected from light. After reaction for 12 h, the resulting GDY–NiTCNQ was washed with acetone, deionized water, and acetone, followed by drying at 50 °C in a vacuum oven. Characterization The morphology of the material was characterized by scanning electron microscopy (SEM; Hitachi, S 4800; Institute of Chemistry, Chinese Academy of Sciences), transmission electron microscopy (TEM, a JEOL JEM-2100F field-emission high-resolution transmission electron microscope at an accelerating voltage of 200 kV; Institute of Chemistry, Chinese Academy of Sciences), and high-resolution TEM (HRTEM). The energy-dispersive spectroscopy (EDS) mapping analysis was also obtained to characterize the elemental composition of the samples. X-ray diffraction (XRD) measurements were carried out on a PANalytical high-resolution XRD system (EMPYREAN) using Cu Kα radiation (λ = 1.540598 mm). X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific ECSALab 250Xi instrument with monochromatic Al Kα X-ray radiation (Institute of Chemistry, Chinese Academy of Sciences). Electrochemical measurements All electrochemical tests were carried out using an electrochemical workstation (CHI 760E) through a typical three-electrode system, in which the as-prepared samples, graphite rods, and saturated calomel electrode were used as the working electrode, the counter electrode, and the reference electrode, respectively. All the electrolytes used in the experiments were bubbled with Ar for 2 h before use. Linear sweep voltammetry (LSV) measurements were carried out at a scan rate of 2 mV s−1 in 1.0 M KOH solution. Cyclic voltammetry (CV) measurements were carried out at a scan rate of 100 mV s−1. Electrochemical impedance spectroscopy (EIS) was obtained at the frequency range of 100 KHz to 0.1 Hz with a sampling rate of 12 points per decade, and the data obtained was fitted. All potentials were converted to the reversible hydrogen electrode (RHE) according to ERHE = ESCE + E0SCE + 0.059 × pH. X-ray absorption fine structure measurements The X-ray absorption fine structure (XAFS) spectra (Ni K-edge) were collected at 1W1B station in Beijing Synchrotron Radiation Facility (BSRF). The storage rings of BSRF were operated at 2.5 GeV with an average current of 250 mA. Using a Si(111) double-crystal monochromator, the data collection was carried out in transmission/fluorescence mode using an ionization chamber. All spectra were collected in ambient conditions. XAFS analysis and results The acquired extended XAFS (EXAFS) data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages. The k3-weighted EXAFS spectra were obtained by subtracting the post-edge background from the overall absorption and then normalizing with respect to the edge-jump step. Subsequently, k3-weighted χ(k) data of Ni K-edge were Fourier transformed to real (R) space using a Hanning windows (dk = 1.0 Å−1) to separate the EXAFS contributions from different coordination shells. To obtain the quantitative structural parameters around central atoms, least–squares curve parameter fitting was performed using the ARTEMIS module of the IFEFFIT software packages. Results and Discussion As shown in Figures 1a and 1b, a two-step in situ growth strategy was used to selectively synthesize the GDY–NiTCNQ catalysts with D–A–D (GDY/TCNQ/Ni) structure, including the first in situ growth of NiTCNQ nanorods and the subsequent growth of the GDY film on the outer layer of the NiTCNQ nanorods that leads to the formation of GDY–NiTCNQ nanowires. Typically, with the dropwise addition of water into the acetonitrile solution of TCNQ, the Ni ions released from the substrate and then interacted with TCNQ forming the A–D (TCNQ–Ni) structure. The NiTCNQ assembled into highly ordered NiTCNQ-nanorods array on the surface of NF.37 The NiTCNQ was then immersed into the acetone solution, after which the HEB precursor was added to grow GDY films on the NiTCNQ surface with the morphology gradually changing from nanorod to nanowire. During this process, the D–A–D (GDY/TCNQ/Ni) structure was formed. Figure 1 | Schematic representation of the controlled synthesis route to the catalytic system of donor–acceptor–donor (GDY/TCNQ/Ni) structures through in situ growth of (a) NiTCNQ nanorods and (b) GDY–NiTCNQ. Download figure Download PowerPoint Figure 2a shows that the color of the samples changed from gray for NF to dark green for NiTCNQ and black for GDY–NiTCNQ during the synthetic process. SEM images clearly show that the smooth surface of 3D NF (Figures 2b–2d) was covered by a film of NiTCNQ nanorods (average diameter, ∼210 nm; Figures 2e–2h). As shown in Supporting Information Figure S1, the NiTCNQ has a flat surface and a fringe lattice spacing of 0.254 nm. The electrostatic potential (ESP) of NiTCNQ (Figure 2h) confirms the interactions between Ni ions and TCNQ with evident CT from Ni to TCNQ, which also demonstrates the formation of the A–D structure. With the growing of GDY on the NiTCNQ surface, the morphology of the nanorods of NiTCNQ changed to nanowires for GDY–NiTCNQ (Figures 2i–2l). As revealed by HRTEM images (Figures 2l and 2m), an obvious shell layer was observed outside of the NiTCNQ core, confirming the successful growth of GDY on the surface of NiTCNQ (Figure 2m). GDY–NiTCNQ exhibits a rough and porous surface, which is beneficial for enlarging the active surface area and the number of active sites. The lattice spacing of the core structure of GDY–NiTCNQ is 0.254 nm ( Supporting Information Figure S2), in accordance with the NiTCNQ, indicating the structure of NiTCNQ was well maintained during the GDY growth process. Scanning TEM (STEM) and EDS mapping images (Figures 2n–2q and Supporting Information Table S1) confirm the presence of carbon, nitrogen, and nickel elements in the GDY–NiTCNQ sample without any impurities. Ni and N are densely distributed in the inner space and sparsely distributed in the edge, and C is evenly distributed throughout the whole material. These results demonstrated the formation of the double layer interface of GDY/TCNQ/Ni, and the GDY film on the outer layer can effectively prevent the catalyst corrosion, which guarantees the long-term stability of the catalyst. Figure 2 | Morphological characterizations of GDY–NiTCNQ and NiTCNQ. (a) Photographs of (i) NF, (ii) NiTCNQ, and (iii) GDY–NiTCNQ. SEM images of (b–d) NF, (e–g) NiTCNQ, and (i–k) GDY–NiTCNQ samples. (h) Electrostatic potential of NiTCNQ. TEM images of (l and m) GDY–NiTCNQ. Elemental mapping images (n–p) of C, Ni, and N atoms in the GDY–NiTCNQ. Download figure Download PowerPoint XRD and XPS measurements were performed to characterize the compositions and structures of the samples. As shown in Figure 3a, the intensity of the peaks for GDY–NiTCNQ decreased as compared with that of NiTCNQ, which might be due to the growth of GDY on NiTCNQ. The XPS survey spectra confirm the presence of Ni, C, and N elements in accordance with the EDS result ( Supporting Information Figures S3 and S4). Compared with pure NiTCNQ (Figure 3b), the C 1s XPS spectra of GDY–NiTCNQ shifted to higher binding energies (BE) by 0.75 eV, revealing the CT from GDY to NiTCNQ. The appearance of an sp-C peak in the C 1s spectra of GDY–NiTCNQ (Figure 3c) indicates the successful growth of GDY on NiTCNQ. Moreover, an independent new peak at 291.96 eV originating from the π–π* shakeup satellite implies the interactions between NiTCNQ and GDY.7 In the Ni 2p region (Figure 3d), the peaks for GDY–NiTCNQ decreased by 1.23 eV as compared with that of NiTCNQ ( Supporting Information Figure S5), revealing the remarkable CT between NiTCNQ and GDY. For GDY–NiTCNQ, the peaks for Ni2+ 2p3/2 and Ni2+ 2p1/2 appear at 856.39 and 873.72 eV, and Ni3+ 2p3/2 and Ni3+ 2p1/2 appear at 858.30 and 875.58 eV, respectively, indicating the presence of the mixed valence states of nickel species. The N 1s (Figure 3e) and O 1s (Figure 3f) peaks of GDY–NiTCNQ located at higher BE than those of pristine NiTCNQ further reveal the presence of electron-transfer in the GDY–NiTCNQ sample.38,39 The Raman ( Supporting Information Figure S6) and Fourier transform infrared (FTIR) ( Supporting Information Figure S7) measurements of the GDY layer were conducted. As shown in Supporting Information Figure S6, pure NiTCNQ spectra features several sharp characteristic peaks. GDY–NiTCNQ exhibits characteristic peaks corresponding to the D-band, G-band, and the vibration of conjugated diine links (2189.8 and 1926.2 cm−1), while the characteristic peaks of NiTCNQ diminished. Moreover, as shown in Supporting Information Figure S7, the FTIR shows the characteristic peak corresponding to GDY, as compared with that of pure NiTCNQ. These results demonstrated the successful growth of GDY on the surface of NiTCNQ. Figure 3 | Structural characterization of GDY–NiTCNQ. (a) XRD spectra of NiTCNQ and GDY-NiTCNQ. XPS spectra for GDY–NiTCNQ and NiTCNQ of (b and c) C 1s, (d) Ni 2p, (e) N 1s, and (f) O 1s. The inset figure in (b) represents the deformation charge density of NiTCNQ. (g) The normalized Co K-edge XANES spectra and (h) the first derivative XANES of GDY–NiTCNQ along with the references. (i) EXAFS spectra of GDY–NiTCNQ at the Co K-edge. Download figure Download PowerPoint Ni K-edge X-ray absorption near-edge structure (XANES) for the samples was performed. Figure 3g shows that the GDY–NiTCNQ exhibits the higher edge energy position than those of NiCl2 and NiO, suggesting the average valence state of nickel in GDY–NiTCNQ is larger than Ni2+. This also implies the mixed valence state of nickel in GDY–NiTCNQ (Figure 3h). The Fourier transforms of the Ni K-edge EXAFS spectrum of GDY–NiTCNQ exhibited a dominant peak at 1.6 Å, which could be assigned to the nearest shell coordination of the Ni–N bond (Figure 3i). The weak peak at 2.5 Å might be attributed to the aggregation of a small number of nickel atoms. The fitted Ni–N coordination number for GDY–NiTCNQ was 4.9 with an average Ni–N bond distance of 2.05 Å ( Supporting Information Table S2), smaller than that of hexa-coordinated NiTCNQ,40 which might be ascribed to the strong interactions between the GDY and NiTCNQ species. The obviously enhanced CT ability at the double layer interfaces of GDY/TCNQ/Ni was expected to endow the catalyst with incomplete CT and the infinite distribution of active sites originating from highly asymmetric surface charge distribution. The significantly increased number of active sites would further accelerate CT and lower energy barriers of the reaction. These are all beneficial to improve the catalytic activity of the catalysts. The OER catalytic performances of the samples were studied using a three-electrode system by testing LSV in O2-saturated 1.0 M KOH solution at the scan rate of 2 mV s−1. All polarization curves were corrected by the iR-compensation. NiTCNQ, NF, and RuO2 were also tested in the same conditions for reference. As shown in Figure 4a, the anodic peak appearing in the range of 1.3–1.4 V versus RHE referred to the nickel oxidation from Ni(II) to Ni(III), and the following peak represented the OER catalytic activity. GDY–NiTCNQ showed the best OER activity with the smallest overpotential of 218 mV at 10 mA cm−2 compared with pure NiTCNQ (343 mV) and commercial RuO2 (282 mV). Moreover, GDY–NiTCNQ exhibited a lower of mV than NiTCNQ mV NF mV and RuO2 mV indicating it the reaction kinetics for OER evolution (Figure For further of the OER activity, the frequency was At the overpotential of GDY–NiTCNQ a higher of s−1 than NiTCNQ NF and of the electrocatalysts ( Supporting Information Table Figure | OER and HER (a) curves of NiTCNQ, NF and RuO2 for OER at the scan rate of 2 mV s−1. (b) of the samples for curves of GDY–NiTCNQ obtained before and after OER (d) O 1s XPS spectra of GDY–NiTCNQ after OER (e) Ni 2p XPS spectra and (f) the of Ni2+ of GDY–NiTCNQ obtained after OER (g) curves of NiTCNQ, NF, and for HER at the scan rate of 2 mV s−1. (h) of the materials for HER. (i) curves of GDY–NiTCNQ obtained before and after HER O 1s XPS Ni 2p XPS and the of Ni2+ in GDY–NiTCNQ samples. Download figure Download PowerPoint stability of the catalyst is of great significance for As shown in Supporting Information Figure GDY–NiTCNQ maintained its catalytic activity for h under at mA revealing its long-term stability ( Supporting Information Figure GDY–NiTCNQ also a in catalytic activity during the tests for (Figure The of the GDY–NiTCNQ catalysts after the stability characterized by SEM ( Supporting Information Figure and TEM ( Supporting Information Figure showed no and the nanorod morphology was well the high stability of the as-prepared catalysts. situ XPS tests were on GDY–NiTCNQ to the structural evolution during the The O 1s XPS spectra showed two new peaks at and eV lattice oxygen in metal after the tests (Figure was also found that the position of the Ni 2p peak shifted to lower BE and the of the Ni2+ increased gradually during the catalysis (Figures and the of Ni2+ at these results confirm the generation of the was with the of Ni2+ in OER which a key role in the oxygen The HER catalytic activity of GDY–NiTCNQ was further tested in 1.0 M KOH solution. Figure shows the LSV curves of GDY–NiTCNQ exhibited a small overpotential of 61 mV at 10 mA which was smaller than NiTCNQ mV) and NF and to the catalyst mV). The corresponding of GDY–NiTCNQ, NiTCNQ, NF, and were and mV (Figure The for GDY–NiTCNQ, NiTCNQ, and NF were and respectively, at the overpotential of 100 All results the obvious of HER activity. Moreover, there was a in current density after indicating the high stability (Figure The of the GDY–NiTCNQ catalysts after the HER tests ( Supporting Information Figure for Supporting Information Figure for were well the high stability of the as-prepared catalysts. To reveal the of the catalytic activity, the in situ XPS measurements were conducted. As shown in Figure new peaks corresponding to the were observed at eV in the O 1s XPS spectra as compared with the as-prepared catalysts. The of Ni2+ decreased as the HER (Figures and the of state of nickel which might be the of of HER activity. For the further of the activity of GDY–NiTCNQ, were to the CT The parameters obtained were fitted with solution CT and absorption to the (Figure GDY–NiTCNQ exhibited the of and compared with NiTCNQ = = and NF = = which the conductivity and transfer process. The electrochemical active surface area was also through layer under various scan of (Figure and Supporting Information Figure GDY–NiTCNQ showed a larger of cm−2 than NiTCNQ and NF indicating the larger surface area and higher surface assigned to the introduction of GDY. on OER and HER performance, GDY–NiTCNQ was used as and in a for OWS. The catalytic system exhibited high with a low cell voltage of 1.40 V to 10 mA lower compared with those V) (Figure and the catalysts, including [email protected] V at 10 mA [email V at 10 mA V at 10 mA and so on (Figure and Supporting Information Table S4). During the of H2 and were from the and (Figure The catalytic in in basic demonstrates the promising in applications of GDY–NiTCNQ. Figure | Electrochemical properties and performance. (a) of samples in 1.0 M KOH solution. for and the fitting is (b) The at V of scan for samples. of the (d) activities of GDY–NiTCNQ and electrocatalysts. (e) The water in a working Download figure Download PowerPoint We report an in situ assembly growth method that controls the growth of NiTCNQ on the surface of GDY, to the formation of a unique D–A–D (GDY/TCNQ/Ni) structure with multiple CT. In such catalytic systems, the amount of CT between and acceptor is that incomplete CT between and surface charge distribution the number of active sites and intrinsic activity of the catalytic system, a new for the activity of the catalytic system through incomplete CT between the The results show that the by incomplete CT demonstrates excellent electrical conductivity and typical semiconductor characteristics, to the high catalytic activity of such a catalytic system in OER, HER, and OWS. We found that an electrolytic cell consisting of the CT salt as a catalyst provided a 1.40 V ultra-small cell voltage up to 10 mA is that the outer GDY film effectively prevented the corrosion of the which is to its long-term stability. This work provides a new way to selectively novel catalysts with high selectivity, activity, and stability. Supporting Information Supporting Information is the for the of Figure and Table of is no of interest to Information This research was by a from the and of the Science of and and the of the Chinese Academy of

Hydrogen (H2) is a promising energy source to replace fossil fuels and the key to solving current energy and environment problems. Hydrogen production from water splitting via photocatalysis or electrolysis is considered to be an economical and environmentally friendly approach to convert clean energy into chemical fuels. The main difficulty of water splitting is the lack of low-cost, stable and efficient catalysts. The works presented in this thesis are focused on the development of highly efficient noble metal free catalysts for both hydrogen evolution and oxygen evolution reactions via photocatalytic and electrocatalytic water splitting. In these multi-step processes, more than one component are generally required to accomplish light absorption and charge separation (for photocatalytic reaction), charge carrier transportation and surface redox reaction. The overall efficiency of the process is strongly affected by the interplay among the components as well as the interfacial properties. Therefore, the overall aim of this thesis works is to assemble appropriate functional components with engineered interfaces to achieve remarkably enhanced photocatalytic and electrocatalytic performances for water splitting. In the first part of the research works, MoP particulates were synthesized and dispersed into nanosized particles using probe sonicator. The MoP nanoparticle as a co-catalyst exhibits 4 times of improved photocatalytic hydrogen evolution reaction (HER) activity compared to the bulk form due to the small particle size with increased surface area and better integration with the semiconductor light absorber, CdS quantum dots (QDs). The nanosized dimension of CdS QDs facilitate the charge migration from bulk to surface where holes are consumed by lactic acid. More importantly, the good dispersion of CdS QDs in solution allows them to be trapped in the cluster of MoP nanoparticles. An intimate interface between CdS QDs and MoP is thus formed, which is favorable for the efficient charge transfer from CdS QDs to MoP. Besides, the metallic property and good HER activity of MoP lead to efficient and stable H2 evolution. Next, to further reduce the particle size of metal phosphides and improve the interface between light absorber and co-catalysts, metal oxide (ZnO) was introduced as a low-cost metal oxide support to disperse and stabilize CoP on its surface. The interface between metal oxides and CdS QDs is formed via electrostatic interactions since ZnO is positively charged whereas CdS QDs is negatively charged. Besides, the band structure alignment between ZnO and CdS QDs facilitate the charge transfer from CdS QDs to ZnO, which was further transferred to CoP. The excellent HER activity of CoP and the engineered interface result in the highly efficient and stable H2 production under visible light irradiation. Apparent quantum efficiency of this system can reach as high as 66% at 420 nm and no activity loss is observed for this system after 144 h photocatalytic reaction. The third part of the research work is focused on the development of a noble metal free HER electrocatalyst that has an activity close to that of Pt. CoNA/PDA (NA: Nitrilotriacetic acid; PDA: Polydopamine) core/shell nanowires were first synthesized by coating PDA on the surface of CoNA nanowires (NWs). N, P co-doped carbon nanotube is obtained through phosphidation of CoNA/PDA NWs with subsequent pyrolysis in N2 atmosphere. CoNA NWs decomposed to Co nanoparticles wrapped by several graphene layers. CoP is formed at the cobalt/carbon interface. After activation, the wrapped nanoparticles become accessible and less stable Co nanoparticles are removed by acidic solution. The CoP nanoparticles stabilized by NCoP bonding are exposed which exhibit a high HER activity and stability. Lastly, research efforts of this thesis work were also spent to tackle the other half of the water splitting reaction, oxygen evolution reaction (OER), since the sluggish kinetics of OER is the bottleneck of the overall performance of water splitting. In this part of the work, a promising OER catalyst, Ni-Fe layered double hydroxide (LDH) was chosen and its intrinsic high OER activity was harnessed by blending ultra-fine NiFe-LDH nanocrystals with conductive carbon. The NiFe-LDH/C hybrid was fabricated by a novel one-pot solvothermal method using molecule precursors of metal cations and organic ligand. The resultant NiFe-LDH/C nanosheet consists of poorly crystalized NiFe-LDH (< 5 nm) interconnected with N doped carbon nanodomains. The in situ formation of both components leads to a self-confined growth and fine blending of NiFe-LDH nanocrystals and carbon domains. Such a unique structure results in improved electrical conductivity, increased active sites and enhanced electrochemical active surface area. In addition, the strong interaction between metal centers and carbon leads to the local electronic structure modification of metal centers. These factors contribute together to the development of a highly efficient and stable NiFe-LDH based OER catalyst. In summary, the research efforts in this thesis were spent on designing efficient and noble metal free photocatalysts and electrocatalysts for water splitting reactions. In particular, engineering suitable interfaces is a key focus. Detailed materials characterization and structural analyses were carried out to understand the key factors contributing to the high performances of the catalysts. Through such efforts, several promising transition-metal based catalysts have been developed with high efficiency for HER and OER reactions. It is believed that the findings from this work would contribute to the advancement of the energy research field and the development of practical catalysts for water splitting utilizing solar energy directly or electricity from clean energy.

The detrimental impacts of climate change coupled with increasing global energy demand has resulted in a significant research effort to develop clean technologies for energy generation, conversion, storage, distribution as well as the removal of CO2 from various industrial sectors. Undoubtedly, electrocatalysis will play a major role in each of these aspirations, which is reflected in the topics covered in this Special Issue. The contributions included here range from the more mature areas of fuel-cell-relevant reactions and electrochemical water splitting to rapidly emerging reactions such as CO2 reduction and nitrogen conversion to ammonia with further mechanistic insights provided by new experimental techniques and computational studies. Electrocatalytic reactions are at the heart of fuel-cell technology and therefore understanding and improving the efficiency of these reactions remains a highly active area of research. This is reflected in this Special Issue by the work that encompasses many aspects of fuel cells including reactions at the anode and cathode, dissolution of the catalyst, the role of the catalyst support, and understanding the dynamics between the electrodes in a fuel cell. Özaslan and co-workers investigate the role of the capping agent on Pt nanocubes and how it influences both the structural stability of the catalyst and ORR performance. Sandbeck, Cherevko et al. also investigate Pt and determine that dissolution occurs to a different extent on different Pt single-crystal basal planes and polycrystalline Pt. However, not only is corrosion of the catalyst an issue but Maillard and co-workers demonstrate that corrosion of the carbon catalyst support used in proton exchange membrane fuel cells (PEMFCs) is also problematic and involves a Pt-catalyzed decarboxylation mechanism which leads to CO and CO2 evolution. Kunze-Liebhäuser and co-workers demonstrate that the high stability of zirconium oxycarbide lends it well to anodic reactions such as alcohol or CO oxidation. Tremiliosi-Filho et al. show that the electrocatalytic oxidation of ethanol on disordered Pt(111) surfaces is highly influenced by the presence of defects on the surface and provides insights into the operation of real catalysts. Varela and co-workers gain new insights into fuel cell operation by inserting an external reference electrode in a direct formic acid fuel cell (DFAFC) and direct methanol fuel cell (DMFC) under stationary and oscillatory conditions. Electrolysis of water coupled to renewable energy sources is a promising method to produce green hydrogen with zero emissions. The past decade has witnessed remarkable progress in the understanding of both the cathodic hydrogen evolution reaction (HER) and the anodic oxygen evolution reaction (OER) for water electrolyzers. This Special Issue reflects the ongoing efforts in: 1) elucidation of structure–activity–stability relations, 2) investigations of the role of the interface structure and the support, 3) combining electrochemical methods with in situ advanced characterization, and 4) studying the electrocatalytic HER and OER using both model extended surfaces and nanoparticles. Model studies on single-crystalline surfaces are essential to gain detailed knowledge on the relations between the electrode structure and the electrocatalytic properties. Feliu and co-workers combine cyclic voltammetry, in situ spectroscopy, and laser-induced temperature jump technique to investigate the interfacial properties of Ni-modified Pt(111) surfaces in phosphate electrolyte for the HER. Arenz, Aschauer and co-workers also use Pt single-crystalline surfaces to establish the structure-sensitivity of the OER in acidic electrolyte by combining experimental work and theoretical calculations. The slow kinetics of the OER causes significant overpotentials in water electrolyzers. To improve the efficiency of water electrolysis, it is necessary to design and develop more active and stable OER electrocatalysts. Zhao and co-workers review the field of OER electrocatalysis with a special focus on multimetallic-based catalysts to improve the OER performance in both acidic and alkaline electrolytes. In acidic media, where polymer electrolyte membrane electrolyzers operate, catalysts based on Ir are required. Escudero-Escribano and co-workers show that both the composition and concentration of the acidic electrolyte play an important role in the performance of Ir nanoparticles for the OER. In alkaline electrolyzers, earth-abundant materials are typically used as OER catalysts. Cao, Zheng, and co-workers present highly active and stable hollow nanocubes based on Co–Fe hydroxides for OER in alkaline media. Abu Sayeed and O'Mullane present an electrodeposition method to fabricate bimetallic Co–Fe oxides for both the HER and OER. Boettcher and co-workers report an electrochemical study on the structure–activity relations for Fe (oxy)hydroxides on different metallic supports, showing how the activity can be significantly tuned by the substrate–electrocatalyst interactions. Schuhmann, Andronescu, and co-workers report a method to improve the OER activity of Ni–Fe (oxy)hydroxides by enhancing their electrical conductivity. Arenz, Delgado, and co-workers report an accelerated durability test for high-surface-area Ni-based (oxy)hydroxides for OER in alkaline media. Katayama and co-workers present a method to fabricate Cu-deposited catalysts for the OER in alkaline media. Finally, the OER is also a very relevant reaction in batteries. Risch and co-workers combine electrochemical methods with X-ray absorption to investigate the electrocatalytic OER on LiMn2O4 in LiOH electrolytes. Electrocatalysis encompasses a wide variety of chemical transformations that are not solely limited to fuel-cell-relevant or water-splitting reactions, which are currently of intense interest. Kortlever and co-workers have reviewed another reaction gaining significant attention, namely electrochemical CO2 conversion into fuels and valuable chemicals. In particular, they focus on the effect of the electrolyte employed in the electrocatalytic reaction and its influence on efficiency and selectivity. Herranz et al. also study the electrochemical CO2 reduction reaction using a thin-film Cu2O electrode; they investigate the oxidation state of the catalyst via post mortem analysis with XPS, where complete reduction of the surface to Cu was found; their work has implications for previous work on rough Cu2O electrodes. Scherson and co-workers investigate hydroxylamine oxidation on polycrystalline electrodes and determine that N2 is formed which is dependent on the pH and applied potential. Symes et al. investigate the effect of ultrasound on the electro-oxidation of sulfate solutions to generate useful and powerful oxidants like persulfate and find that at low sulfate concentration and low current density, the use of ultrasound results in a lower concentration of this oxidant. Stimming and co-workers investigate the V(II)/V(III) and V(IV)/V(V) redox reactions employed in redox flow batteries; by developing a method to accurately assess the electrochemically active surface area of the working electrodes they conclude that in disagreement to the received wisdom that porous carbon does not catalyze vanadium redox chemistry. The development of in situ or operando techniques is a key research area that is continuously being developed to gain a better understanding of the mechanisms of electrocatalytic reactions. This is particularly important when considering the validation of theoretical predictions, which is described below, and covered in this Special Issue. Kibler et al. have used in situ scanning tunneling microscopy (STM) to study the adsorption of unreactive acetate on Au(111) surfaces where a phase transition within an adsorbed adlayer is observed, providing key information on the role of reactive adsorbates such as formate on electrocatalytic reactions. Cuesta and co-workers review the area of in situ infrared spectroscopy identifying key theoretical aspects of the technique and highlight recent uses in studying the electrochemical CO2 reduction reaction for the detection of reaction intermediates. Horch et al. demonstrate the integration of ultra-high-vacuum equipment with an electrochemical cell as a way of producing complex surface structures not attainable by regular electrochemical methods. They study increased step density on Pt(111), Cu(111), and Pt/Cu(111) electrodes by STM and the effect on their electrochemical behavior in acidic and alkaline electrolytes. Computational electrocatalysis is a rapidly emerging field that is employed to gain a more fundamental mechanistic understanding of quite complex reactions such as those covered in this Special Issue. This field not only provides support for experimental observation but is being used to predict the activity of electrocatalysts not yet synthesized in the laboratory. Malek and co-workers provide a perspective where they critically assess the use of artificial-intelligence-driven modelling and computational approaches for such a task and take CO2 conversion as a test case. In addition, Tang and Jiang used first-principles density functional theory (DFT) to predict that Ti, Sc, and Fe dimer clusters supported on phosphorene constitute promising electrocatalysts for N2 reduction to NH3. Baletto et al. developed a multi-scale approach to study the catalytic properties of MgO(100) supported Pt nanoparticles for the ORR, where reconstruction of the interface layer is predicted to increase activity. A major challenge, however, in the application of theoretical models is the incorporation of the electrochemical interface and the electrolyte into the simulation while keeping computational times manageable. Rossmeisl and co-workers have used ab initio methods to construct a thermodynamically realistic interface to present simulated cyclic voltammograms of Cu basal plane electrodes that are validated by comparison to experimental data over a large pH range, which therefore provides an atomistic understanding of the interfacial structure of Cu electrodes. Calle-Vallejo et al. tackle the challenge of modelling the role of solvation and its influence on the adsorption energy of species at surfaces. They evaluate the influence of van der Waal interactions on the solvation of *OH adsorbed on alloys of Pt. Chan and co-workers present a hybrid continuum/ab initio method where they introduce a capacitor model for the relationship between the reaction energetics and the potential and charge. This results in an order of magnitude reduction in computational costs to determine electrochemical reaction energetics. To conclude, the topics outlined in this Special Issue highlight the beneficial impact that electrocatalysis can play in developing a cleaner and more sustainable society. The future of this field is indeed bright and brings together not only the expertise of electrochemists but material scientists, theoreticians, engineers, and surface scientists. The outcome is the continuous development of new materials with enhanced performance underpinned by the greater understanding of reaction mechanisms via integration of electrochemical systems with sophisticated in situ techniques and validation with increasingly realistic simulation environments. Professor Anthony O'Mullane received his PhD degree (2001) from University College Cork (Ireland) and completed postdoctoral fellowships at Technische Universitat Darmstadt (Germany), the University of Warwick (UK), and Monash University (Australia). He previously held a position (2008) at RMIT University (Australia) until moving to Queensland University of Technology (QUT) in 2013. He is a Fellow of the Royal Society of Chemistry and Fellow of the Royal Australian Chemical Institute (FRACI). He is the immediate past-Chair of the Electrochemistry Division of the RACI and served as vice chair of the Physical Electrochemistry Division of the International Society of Electrochemistry. His research interests are the electrochemical synthesis and characterization of nanostructured materials; electrocatalysis (water splitting, fuel-cell-relevant reactions); catalysis (water remediation); room-temperature liquid metals; Li-metal-based batteries; and the application of electrochemical methods to various aspects of physical, chemical, and biological science. He has published over 170 journal articles in these areas. María Escudero-Escribano is an assistant professor at the University of Copenhagen (Denmark) since 2017. She received her PhD in Chemistry from the Autonomous University of Madrid (Spain) in 2011. She completed postdoctoral fellowships at the Technical University of Denmark and Stanford University (US). At the University of Copenhagen, María leads the Nanoelectrocatalysis Group, which investigates tailored electrochemical interfaces for sustainable energy conversion and production of renewable fuels and chemicals. She is the Chair of the Danish Electrochemical Society since 2018 and holds a Villum Young Investigator Grant from the Villum Foundation. María has received numerous awards in recognition of her early-career achievements, including the European Young Chemist Award 2016 (Gold Medal, 35-year-old level), the Energy Technology Division Young Investigator Award 2018 from the Electrochemical Society, the Princess of Girona Scientific Research Award 2018, the Young Researchers Award 2019 from the Spanish Royal Society of Chemistry, and the Clara Immerwahr Award 2019. Ifan Stephens is Senior Lecturer at the Department of Materials at Imperial College London. Prior to his appointment to Imperial in 2017, he was at the Department of Physics at the Technical University of Denmark (DTU); he was first employed as a postdoctoral researcher, then as assistant professor, and finally as associate professor and leader of the Electrocatalysis Group there. In 2015, Massachusetts Institute of Technology (MIT) appointed Ifan as the Peabody Visiting Associate Professor. He taught and conducted research at the Department of Mechanical Engineering at MIT for a whole semester. Ifan′s research aims to enable the large-scale electrochemical conversion of renewable energy to fuels and valuable chemicals and vice versa. Such processes will be critical in order to allow the increased uptake of renewable energy. Ifan has published 66 papers on topics including oxygen reduction, oxygen evolution, CO2 reduction and N2 reduction. Ifan′s research on H2O2 electrosynthesis led to the establishment of the spinout HPNow, which he co-founded. Katharina Krischer is a Professor of Physics at the Technical University of Munich (TUM), Germany. She is also a member of the Catalysis Research Center of TUM and serves on editorial boards of several journals on electrochemistry or nonlinear sciences. She did her Ph.D. at the Fritz-Haber-Institut, Berlin, in the group of Prof. Ertl. After postdoctoral training at Princeton University, USA, she returned as a group leader to the Fritz-Haber-Institut, and completed her habilitation in 1998. In 2002 she moved to Munich to take on her current position. Her research interests cover two broad topics, electrochemistry and nonlinear dynamics. She works on photoelectrochemistry, solar fuels, and semiconductor electrochemistry as well as on nonlinear phenomena during electrochemical reactions. Furthermore, she has a strong interest in theory, bridging the gap between physico-chemical continuum models describing self-organization phenomena at the solid-liquid interface and normal form approaches and abstract mathematical models. She has coauthored about 130 publications in peer-reviewed journals and a text book on "Physics of Energy Conversion". She was elected a fellow of the International Society of Electrochemistry and is a member of the German Physical Society (DPG) and the Society of German Chemists (GDCh).

Developing cheap, highly efficient and stable electrocatalysts for both oxygen and hydrogen evolution reactions (OER and HER) is extremely meaningful to realize large-scale implementation of water splitting technology. Herein, we report the phase and composition controlled synthesis of cobalt sulfide (CoSx) hollow nanospheres (HNSs) and their catalytic efficiencies for hydrogen and oxygen evolution reactions in alkaline media. Three CoSx compounds, i.e., Co9S8, Co3S4, and CoS2 HNSs, were precisely synthesized by simply adjusting the molar ratio of carbon disulfide to cobalt acetate using a facile solution-based strategy. Electrochemical results reveal that the as-prepared CoS2 HNSs exhibit superior OER and HER catalytic performance to Co9S8 and Co3S4 HNSs in 1.0 M KOH, with overpotentials of 290 mV for the OER and 193 mV for the HER at 10 mA cm-2, and the corresponding Tafel slopes of 57 and 100 mV dec-1, respectively. In addition, the CoS2 HNSs exhibit remarkable long-term catalytic durability, which is even superior to precious metal catalysts of RuO2 and Pt/C. Moreover, an alkaline electrolyzer assembled using CoS2 HNSs as both anode and cathode materials can achieve 10 mA cm-2 at a low cell voltage of 1.54 V at 60 °C with a faradaic efficiency of 100% for overall water splitting. Further analysis demonstrates that the surface morphology, crystallographic structure and coordination environment of Con+ active sites in combination determine the HER/OER activities in the synthesized binary CoSx series, which would provide insight into the rational design of transition metal chalcogenides for highly efficient hydrogen and oxygen-involved electrocatalysis.

Electrocatalytic water splitting is a promising approach for obtaining clean hydrogen energy. In this work, novel molybdate@carbon paper composite electrocatalysts (CoxFe10-xMoO@CP), displaying outstanding electrocatalytic capabilities, were deriving from anchoring cobalt/iron molybdate materials onto the surface of carbon paper fibers. By adjusting the cobalt-to-iron ratio, the composite (Co5Fe5MoO@CP), with the optimal molar proportion (Co/Fe = 5/5), exhibited a distinctive nanoflower morphology (50-100 nm), which provided a significant number of active sites for electrocatalytic reactions, and showed the strongest electrocatalytic potency for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Specifically, the overpotentials for HER and OER were 123.6 and 245 mV at 10 mA·cm-2, with a Tafel slope of 78.3 and 92.2 mV·dec-1, respectively. The hydrogen and oxygen evolution reactions remained favorable and stable over 35 days and 2 weeks of cyclic voltammetry cycles. In a two-electrode system, efficient overall water splitting was achieved at a cell voltage of 1.60 V. Under high alkaline concentration and temperature conditions, the Co5Fe5MoO@CP composite still maintained excellent HER and OER catalytic activity and stability, indicating its satisfactory potential for industrial applications. Density functional theory (DFT) analysis revealed that the promoted hydrogen evolution capability derived from the synergistic catalytic effect of iron and cobalt atoms within the molecule, while cobalt atoms functioned as the catalytic core for the oxygen evolution process. This work provides a novel strategy towards high-efficiency electrocatalysts to significantly accelerate the overall water splitting.