Solid-Solutions as Supports and Robust Photocatalysts and Electrocatalysts: A Review

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

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

We present a controlled fabrication of selective ultrathin metal–organic framework (MOF) nanosheets as preassembling platforms, yolk–shell structured with a few-layered N-doped carbon (NC) shell-en...

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.

Hydrogen is widely regarded as an environmentally-friendly and practical alternative energy to reduce reliance on fossil fuels. Also, hydrogen has the outstanding advantages of high energy storage density and sustainable reaction products. The most promising technology to produce hydrogen from abundant renewable sources is water electrolysis. Electrochemical water splitting involves two reaction systems, namely, an anodic oxygen evolution reaction (OER) and a cathodic hydrogen evolution reaction (HER). In general, typical HER catalysts with good activity in acidic solutions, such as Pt, exhibit poor activity in OER. Therefore, it is important to study bifunctional catalyst with high activity, efficiency and stability in acidic solutions.In this presentation, we demonstrate the synthesis of AuRu alloy nanofibers as bifunctional electrocatalysts for water splitting. The Au/RuO2 nanocomposites are synthesized via electrospinning and calcination. After the subsequent reduction process, the AuRu alloy nanofibers are formed. The reduction process of Au/RuO2 nanocomposites to AuRu alloy nanofibers are investigated and the physical properties of AuRu alloy nanofibers in comparison to those of pure Au and Ru are analyzed with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The morphologies of the Au/RuO2 nanocomposites and AuRu alloy nanofibers are confirmed with field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The electrocatalytic activity of AuRu alloy nanofibers are studied toward both OER and HER in Ar-saturated 0.5 M H2SO4 aqueous solution with rotating disk electrode (RDE) voltammetry. The Tafel slopes which represent kinetics of the reaction for OER and HER are obtained based on the corresponding RDE curves. In summary, this study presents the AuRu alloy nanofibers as a better bifunctional electrocatalyst for overall water splitting than Au/RuO2, RuO2, Ru, Pt/C, Ir/C, Au/C and Ru/C.This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT& Future Planning (NRF-2017R1A2A2A14001137).

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

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.

Although electricity feedstock currently dominates hydrogen production costs in commercial electrolysis, capital cost will become a significant factor as electrochemical water splitting is directly coupled with low-cost power sources. [1,2] To minimize device costs and reach cost targets, reducing catalyst loading in proton exchange membrane-based (PEM) electrolysis will be necessary and efforts to evaluate and improve upon catalyst performance and durability will become critical. Beyond PEM-based systems, anion exchange membrane-based (AEM) electrolyzers can also reduce capital cost, with the high pH enabling non-platinum group metal (PGM) catalysts and improving the durability of system components.In these areas, efforts at the National Renewable Energy Laboratory have included developing and evaluating electrolyzer catalysts, establishing baseline performance and durability, and linking ex- and in-situ testing for commercial nanoparticles and novel materials. In PEM electrolysis, ink compositions and coating methodologies were evaluated in rotating disk electrodes, establishing best practices for screening catalysts in the oxygen evolution reaction. [3] Test factors were considered, including working electrode dissolution and catalyst delamination, and electrochemical surface area measurements adapted to differentiate between site quantity and quality. While half- and single-cell performances and durabilities do not match, higher activities translated to kinetic improvements in membrane electrode assemblies and half-cell testing was a reasonable tool for assessing relative differences between material sets. Projecting differences at the device level, however, required segregating materials based on surface composition and oxide content, and some caution is needed when using rotating disk electrodes to assess catalysts for electrolysis applications. In membrane electrode assemblies, baseline durability was evaluated when accounting for low catalyst loading and intermittent load profiles. How catalyst layers are incorporated has a significant impact on device performance and durability, and efforts have been made to translate improvements from spray coating to roll to roll coating and manufacturing appropriate processes. Various catalyst and membrane combinations have been used to assess that ability of component development and system control to limit performance loss with extended operation.In AEM electrolysis, baselines and best practices have been established in rotating disk electrodes for catalysts in the oxygen and hydrogen evolution reactions. Test factors, including electrolyte purity, conditioning protocols, and counter electrode choices were found to significantly impact measurements, and some care is needed to avoid under- or over-estimating kinetic improvements. Novel catalysts were developed for the hydrogen and oxygen evolution reactions, including low- and non-PGMs, sulfides, and metal organic frameworks, where activity improvements ex-situ generally translated to membrane electrode assemblies. [4] In in-situ testing, ionomers were varied with standard catalysts to assess the role of catalyst-ionomer interactions and supporting electrolytes in device performance.[1] H2 at Scale: Deeply Decarbonizing our Energy System. Presented at Annual Merit Review, U.S. Department of Energy; Washington, DC, June 6−10, 2016. https://www.hydrogen.energy.gov/pdfs/review16/2016_amr_h2_at_scale.pdf.[2] Denholm, P.; O’Connell, M.; Brinkman, G.; Jorgenson, J. Overgeneration from Solar Energy in California: A Field Guide to the Duck Chart; Vol. NREL/TP-6A20-65023; National Renewable Energy Laboratory: Golden, CO, 2015. http://www.nrel.gov/docs/fy16osti/65023.pdf.[3] S. M. Alia and G. C. Anderson, J. Electrochem. Soc., 166, F282 (2019).[4] S. Ghoshal, S. Zaccarine, G. C. Anderson, M. B. Martinez, K. E. Hurst, S. Pylypenko, B. S. Pivovar and S. M. Alia, ACS Applied Energy Materials, 2, 5568 (2019).

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.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Trace Iron-Doped Nickel-Cobalt selenide with rich heterointerfaces for efficient overall water splitting at high current densities

Theoretical and experimental investigations of Co-Cu bimetallic alloys-incorporated carbon nanowires as an efficient bi-functional electrocatalyst for water splitting

NiCuCoS3 chalcogenide as an efficient electrocatalyst for hydrogen and oxygen evolution

MoS2–CoS2 heteronanosheet arrays coated on porous carbon microtube textile for overall water splitting

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.