Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

FeNi2S4-A Potent Bifunctional Efficient Electrocatalyst for the Overall Electrochemical Water Splitting in Alkaline Electrolyte

For a carbon-neutral society, the production of hydrogen as a clean fuel through water electrolysis is currently of great interest. Since water electrolysis is a laborious energetic reaction, it requires high energy to maintain efficient and sustainable production of hydrogen. Catalytic electrodes can reduce the required energy and minimize production costs. In this context, herein, a bifunctional electrocatalyst made from iron nickel sulfide (FeNi2S4 [FNS]) for the overall electrochemical water splitting is introduced. Compared to Fe2NiO4 (FNO), FNS shows a significantly improved performance toward both OER and HER in alkaline electrolytes. At the same time, the FNS electrode exhibits high activity toward the overall electrochemical water splitting, achieving a current density of 10 mA cm-2 at 1.63 V, which is favourable compared to previously published nonprecious electrocatalysts for overall water splitting. The long-term chronopotentiometry test reveals an activation followed by a subsequent stable overall cell potential at around 2.12 V for 20 h at 100 mA cm-2.

Enhanced Hydrogen Evolution Catalysis of Pentlandite due to the Increases in Coordination Number and Sulfur Vacancy during Cubic-Hexagonal Phase Transition

The search for new phases is an important direction in materials science. The phase transition of sulfides results in significant changes in catalytic performance, such as MoS2 and WS2. Cubic pentlandite [cPn, (Fe, Ni)9S8] can be a functional material in batteries, solar cells, and catalytic fields. However, no report about the material properties of other phases of pentlandite exists. In this study, the unit-cell parameters of a new phase of pentlandite, sulfur-vacancy enriched hexagonal pentlandite (hPn), and the phase boundary between cPn and hPn are determined for the first time. Compared to cPn, the hPn shows a high coordination number, more sulfur vacancies, and high conductivity, which result in significantly higher hydrogen evolution performance of hPn than that of cPn and make the non-nano rock catalyst hPn superior to other most known nanosulfide catalysts. The increase of sulfur vacancies during phase transition provides a new approach to designing functional materials.

A Review of Transition Metal Boride, Carbide, Pnictide, and Chalcogenide Water Oxidation Electrocatalysts

Transition metal borides, carbides, pnictides, and chalcogenides (X-ides) have emerged as a class of materials for the oxygen evolution reaction (OER). Because of their high earth abundance, electrical conductivity, and OER performance, these electrocatalysts have the potential to enable the practical application of green energy conversion and storage. Under OER potentials, X-ide electrocatalysts demonstrate various degrees of oxidation resistance due to their differences in chemical composition, crystal structure, and morphology. Depending on their resistance to oxidation, these catalysts will fall into one of three post-OER electrocatalyst categories: fully oxidized oxide/(oxy)hydroxide material, partially oxidized core@shell structure, and unoxidized material. In the past ten years (from 2013 to 2022), over 890 peer-reviewed research papers have focused on X-ide OER electrocatalysts. Previous review papers have provided limited conclusions and have omitted the significance of "catalytically active sites/species/phases" in X-ide OER electrocatalysts. In this review, a comprehensive summary of (i) experimental parameters (e.g., substrates, electrocatalyst loading amounts, geometric overpotentials, Tafel slopes, etc.) and (ii) electrochemical stability tests and post-analyses in X-ide OER electrocatalyst publications from 2013 to 2022 is provided. Both mono and polyanion X-ides are discussed and classified with respect to their material transformation during the OER. Special analytical techniques employed to study X-ide reconstruction are also evaluated. Additionally, future challenges and questions yet to be answered are provided in each section. This review aims to provide researchers with a toolkit to approach X-ide OER electrocatalyst research and to showcase necessary avenues for future investigation.

Operando Scanning Electrochemical Probe Microscopy during Electrocatalysis

Scanning electrochemical probe microscopy (SEPM) techniques can disclose the local electrochemical reactivity of interfaces in single-entity and sub-entity studies. Operando SEPM measurements consist of using a SEPM tip to investigate the performance of electrocatalysts, while the reactivity of the interface is simultaneously modulated. This powerful combination can correlate electrochemical activity with changes in surface properties, e.g., topography and structure, as well as provide insight into reaction mechanisms. The focus of this review is to reveal the recent progress in local SEPM measurements of the catalytic activity of a surface toward the reduction and evolution of O₂ and H₂ and electrochemical conversion of CO₂. The capabilities of SEPMs are showcased, and the possibility of coupling other techniques to SEPMs is presented. Emphasis is given to scanning electrochemical microscopy (SECM), scanning ion conductance microscopy (SICM), electrochemical scanning tunneling microscopy (EC-STM), and scanning electrochemical cell microscopy (SECCM).

Crystal Plane-Related Oxygen-Evolution Activity of Single Hexagonal Co3 O4 Spinel Particles

The electrocatalytic activity for the oxygen evolution reaction in alkaline electrolyte of hexagonal spinel Co₃ O₄ nanoparticles derived using scanning electrochemical cell microscopy (SECCM) is correlated with scanning electron microscopy and atomic force microscopy images of the droplet landing sites. A unique way to deconvolute the intrinsic catalytic activity of individual crystal facets of the hexagonal Co₃ O₄ spinel particle is demonstrated in terms of the turnover frequency (TOF) of surface Co atoms. The top surface exposing 111 crystal planes displayed a thickness-dependent TOF with a TOF of about 100 s^-1 at a potential of 1.8 V vs. RHE and a particle thickness of 100 nm. The edge of the particle exposing (110) planes, however, showed an average TOF of 270±68 s^-1 at 1.8 V vs. RHE and no correlation with particle thickness. The higher atomic density of Co atoms on the edge surface (2.5 times of the top) renders the overall catalytic activity of the edge planes significantly higher than that of the top planes. The use of a free-diffusing Os complex in the alkaline electrolyte revealed the low electrical conductivity through individual particles, which explains the thickness-dependent TOF of the top planes and could be a reason for the low activity of the top (111) planes.

Boosting the overall electrochemical water splitting performance of pentlandites through non-metallic heteroatom incorporation

We report on synthesis of the heterotrimetallic pentlandite-type material Fe3Co3Ni3S8 (FCNS) in presence of suitable phosphorus-(FCNSP) and nitrogen-(FCNSN) donors for the overall electrochemical water splitting. Throughout the experiments, a preferential incorporation of N into the FCNS-lattice is observed whereas the addition of phosphorus generally leads to metal-phosphate-FCNS composites. The obtained FCNSP, FCNSN, and FCNSNP facilitate the oxygen evolution reaction (OER) at 100 mAcm-2 in 1.0M KOH with overpotentials of 479, 440, and 427 mV, respectively, outperforming the benchmark IrO2 (564 mV) and commercial Ni metal powder (>600 mV). Likewise, FCNSN and FCNSNP reveal an improved performance toward the hydrogen evolution reaction (HER) in 0.5M H2SO4, outperforming the pristine FCNS. All materials revealed high stability and morphological robustness during OER and HER. Notably, DFT calculation suggests that N and P doping boost the OER activity of the pristine FCNS, whereas N doping enhances the HER activity.

Controlling Surface Contact, Oxygen Transport, and Pitting of Surface Oxide via Single-Channel Scanning Electrochemical Cell Microscopy

In single-channel scanning electrochemical cell microscopy, the applied potential during the approach of a micropipette to the substrate generates a transient current upon droplet contact with the substrate. Once the transient current exceeds a set threshold, the micropipette is automatically halted. Currently, the effect of the approach potential on the subsequent electrochemical measurements, such as the open-circuit potential and potentiodynamic polarization, is considered to be inconsequential. Herein, we demonstrate that the applied approach potential does impact the extent of probe-to-substrate interaction and subsequent microscale electrochemical measurements on aluminum alloy AA7075-T73.

Revealing the Heterogeneity of Large-Area MoS2 Layers in the Electrocatalytic Hydrogen Evolution Reaction

The electrocatalytic activity concerning the hydrogen evolution reaction (HER) of micrometer-sized MoS2 layers transferred on a glassy carbon surface was evaluated by scanning electrochemical cell microscopy (SECCM) in a high-throughput approach. Multiple areas on single or multiple MoS2 layers were assessed using a hopping mode nanocapillary positioning with a hopping distance of 500 nm and a nanopipette size of around 55 nm. The locally recorded linear sweep voltammograms revealed a high lateral heterogeneity over the MoS2 sheet regarding their HER activity, with currents between -40 and -60 pA recorded at -0.89 V vs. reversible hygrogen electrode over about 4400 different measured areas on the MoS2 sheet. Stacked MoS2 layers did not show different electrocatalytic activity than the single MoS2 sheet, suggesting that the interlayer resistance influences the electrocatalytic activity less than the resistances induced by possible polymer residues or water layers formed between the transferred MoS2 sheet and the glassy carbon electrode.

Unraveling the Structural Sensitivity of CO2 Electroreduction at Facet-Defined Nanocrystals via Correlative Single-Entity and Macroelectrode Measurements

The conversion of CO₂ into value-added products is a compelling way of storing energy derived from intermittent renewable sources and can bring us closer to a closed-loop anthropogenic carbon cycle. The ability to synthesize nanocrystals of well-defined structure and composition has invigorated catalysis science with the promise of nanocrystals that selectively express the most favorable sites for efficient catalysis. The performance of nanocrystal catalysts for the CO₂ reduction reaction (CO₂RR) is typically evaluated with nanocrystal ensembles, which returns an averaged system-level response of complex catalyst-modified electrodes with each nanocrystal likely contributing a different (unknown) amount. Measurements at single nanocrystals, taken in the context of statistical analysis of a population, and comparison to macroscale measurements are necessary to untangle the complexity of the ever-present heterogeneity in nanocrystal catalysts, achieve true structure-property correlation, and potentially identify nanocrystals with outlier performance. Here, we employ environment-controlled scanning electrochemical cell microscopy to isolate and investigate the electrocatalytic CO₂RR response of individual facet-defined gold nanocrystals. Using correlative microscopy approaches, we conclusively demonstrate that {110}-terminated gold rhombohedra possess superior activity and selectivity for CO₂RR compared with {111}-terminated octahedra and high-index {310}-terminated truncated ditetragonal prisms, especially at low overpotentials where electrode kinetics is anticipated to dominate the current response. The methodology framework described here could inform future studies of complex electrocatalytic processes through correlative single-entity and macroscale measurement techniques.

Direct measuring of single-heterogeneous bubble nucleation mediated by surface topology

Heterogeneous bubble nucleation is one of the most fundamental interfacial processes ranging from nature to technology. There is excellent evidence that surface topology is important in directing heterogeneous nucleation; however, deep understanding of the energetics by which nanoscale architectures promote nucleation is still challenging. Herein, we report a direct and quantitative measurement of single-bubble nucleation on a single silica nanoparticle within a microsized droplet using scanning electrochemical cell microscopy. Local gas concentration at nucleation is determined from finite element simulation at the corresponding faradaic current of the peak-featured voltammogram. It is demonstrated that the criteria gas concentration for nucleation first drops and then rises with increasing nanoparticle radius. An optimum nanoparticle radius around 10 nm prominently expedites the nucleation by facilitating the special topological nanoconfinements that consequently catalyze the nucleation. Moreover, the experimental result is corroborated by our theoretical calculations of free energy change based on the classic nucleation theory. This study offers insights into the impact of surface topology on heterogenous nucleation that have not been previously observed.

Trimetallic Pentlandites (Fe,Co,Ni)9S8 for the Electrocatalytical HER in Acidic Media

Recently, pentlandite materials have been shown to exhibit promising properties with respect to the hydrogen evolution reaction (HER). A whole series of trimetallic FeCoNi-pentlandite materials and composites have been synthesized from the elements using high-temperature synthesis and categorized in terms of purity. Furthermore, the electrocatalytic properties regarding the HER were determined and correlated to hydrogen adsorption energies, which were determined by means of density functional theory (DFT) calculations. The relationships between activity and its origin generated in this way help to better understand the pentlandite system and provide meaningful approaches for catalyst synthesis.

Surface Design Strategy of Catalysts for Water Electrolysis

Hydrogen, a new energy carrier that can replace traditional fossil fuels, is seen as one of the most promising clean energy sources. The use of renewable electricity to drive hydrogen production has very broad prospects for addressing energy and environmental problems. Therefore, many researchers favor electrolytic water due to its green and low-cost advantages. The electrolytic water reaction comprises the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). Understanding the OER and HER mechanisms in acidic and alkaline processes contributes to further studying the design of surface regulation of electrolytic water catalysts. The OER and HER catalysts are mainly reviewed for defects, doping, alloying, surface reconstruction, crystal surface structure, and heterostructures. Besides, recent catalysts for overall water splitting are also reviewed. Finally, this review paves the way to the rational design and synthesis of new materials for highly efficient electrocatalysis.

Advanced Spatiotemporal Voltammetric Techniques for Kinetic Analysis and Active Site Determination in the Electrochemical Reduction of CO2

ConspectusElectrochemical reduction of the greenhouse gas CO₂ offers prospects for the sustainable generation of fuels and industrially useful chemicals when powered by renewable electricity. However, this electrochemical process requires the use of highly stable, selective, and active catalysts. The development of such catalysts should be based on a detailed kinetic and mechanistic understanding of the electrochemical CO₂ reduction reaction (eCO₂RR), ideally through the resolution of active catalytic sites in both time (i.e., temporally) and space (i.e., spatially). In this Account, we highlight two advanced spatiotemporal voltammetric techniques for electrocatalytic studies and describe the considerable insights they provide on the eCO₂RR. First, Fourier transformed large-amplitude alternating current voltammetry (FT ac voltammetry), as applied by the Monash Electrochemistry Group, enables the resolution of rapid underlying electron-transfer processes in complex reactions, free from competing processes, such as the background double-layer charging current, slow catalytic reactions, and solvent/electrolyte electrolysis, which often mask conventional voltammetric measurements of the eCO₂RR. Crucially, FT ac voltammetry allows details of the catalytically active sites or the rate-determining step to be revealed under catalytic turnover conditions. This is well illustrated in investigations of the eCO₂RR catalyzed by Bi where formate is the main product. Second, developments in scanning electrochemical cell microscopy (SECCM) by the Warwick Electrochemistry and Interfaces Group provide powerful methods for obtaining high-resolution activity maps and potentiodynamic movies of the heterogeneous surface of a catalyst. For example, by coupling SECCM data with colocated microscopy from electron backscatter diffraction (EBSD) or atomic force microscopy, it is possible to develop compelling correlations of (precatalyst) structure-activity at the nanoscale level. This correlative electrochemical multimicroscopy strategy allows the catalytically more active region of a catalyst, such as the edge plane of two-dimensional materials and the grain boundaries between facets in a polycrystalline metal, to be highlighted. The attributes of SECCM-EBSD are well-illustrated by detailed studies of the eCO₂RR on polycrystalline gold, where carbon monoxide is the main product. Comparing SECCM maps and movies with EBSD images of the same region reveals unambiguously that the eCO₂RR is enhanced at surface-terminating dislocations, which accumulate at grain boundaries and slip bands. Both FT ac voltammetry and SECCM techniques greatly enhance our understanding of the eCO₂RR, significantly boosting the electrochemical toolbox and the information available for the development and testing of theoretical models and rational catalyst design. In the future, it may be possible to further enhance insights provided by both techniques through their integration with in situ and in operando spectroscopy and microscopy methods.

Scanning Electrochemical Cell Microscopy Platform with Local Electrochemical Impedance Spectroscopy

Local electrochemical impedance spectroscopy (LEIS) has been a versatile technology for characterizing local complex electrochemical processes at heterogeneous surfaces. However, further application of this technology is restricted by its poor spatial resolution. In this work, high-spatial-resolution LEIS was realized using scanning electrochemical cell microscopy (SECCM-LEIS). The spatial resolution was proven to be ∼180 nm based on experimental and simulation results. The stability and reliability of this platform were further verified by long-term tests and Kramers-Kronig transformation. With this technology, larger electric double-layer capacitance (C_dl) and smaller interfacial resistance (R_t) were observed at the edges of N-doped reduced graphene oxide, as compared to those at the planar surface, which may be due to the high electrochemical activity at the edges. The established SECCM-LEIS provides a high-spatial approach for study of the interfacial electrochemical behavior of materials, which can contribute to the elucidation of the electrochemical reaction mechanism at material surfaces.

Theoretical insight into surface structures of pentlandite toward hydrogen evolution

Pentlandite (Fe,Ni)₉S₈ is a promising transition-metal catalyst for the hydrogen evolution reaction. However, little is explained about the long activation process that has been observed in experiments, and its facet-dependent hydrogen evolution activity is still theoretically unrevealed. To explain some experimental phenomena and to guide subsequent studies, density functional theory calculations are used to study the main synthetic surfaces: (111) and (311) in this work. The results show that the small metal cube plays an important role in the surface stability, and it is suggested that such cubes remain intact during catalysis. The linking sites serve as a bridge across the metal cubes and are the main catalytic active sites for hydrogen evolution. This is because the metal cubes can tune the electronic structures of the linking sites, and then the free energy of the linking sites is optimized. The (311) surface is a composite surface that consists of (100) and (111) facets and has the profile of a step. A surface conversion between the (311) and (111) facets may occur when the cube layer length increases. Therefore, the active sites can be feasibly engineered by the surface structures, and this could be helpful in further applications of (Fe,Ni)₉S₈.

MoS2 with Controlled Thickness for Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂) has moderate hydrogen adsorption free energy, making it an excellent alternative to replace noble metals as hydrogen evolution reaction (HER) catalysts. The thickness of MoS₂ can affect its energy band structure and interface engineering, which are the avenue way to adjust HER performance. In this work, MoS₂ films with different thicknesses were directly grown on the glassy carbon (GC) substrate by atomic layer deposition (ALD). The thickness of the MoS₂ films can be precisely controlled by regulating the number of ALD cycles. The prepared MoS₂/GC was directly used as the HER catalyst without a binder. The experimental results show that MoS₂ with 200-ALD cycles (the thickness of 14.9 nm) has the best HER performance. Excessive thickness of MoS₂ films not only lead to the aggregation of dense MoS₂ nanosheets, resulting in reduction of active sites, but also lead to the increase of electrical resistance, reducing the electron transfer rate. MoS₂ grown layer by layer on the substrate by ALD technology also significantly improves the bonding force between MoS₂ and the substrate, showing excellent HER stability.

Synergistic effect of S-bridged Fe-Ni group on hydrogen evolution for pentlandite

Pentlandite is reported to exhibit good catalytic activity in hydrogen evolution reaction (HER). Many studies have paid attention to metal catalysis of pentlandite. However, the nonmetal catalysis is not considered for HER. Here, we unravel one probable catalytic mechanism of pentlandite toward HER using density functional theory. In our study models, (001) and (100) surfaces are created because there are three types of S-bridged M-M groups on them. Our study reveals that (Fe-Ni)-S center has a moderate value of Gibbs free energy while the corresponding value for (Fe-Fe)-S or (Ni-Ni)-S center is largely positive or negative. In (Fe-Ni)-S group, Fe and Ni can regulate the antibonding state of S, and then balance adsorption and desorption of proton. In addition, an intrinsic electronic potential difference exists between Fe and Ni in (Fe-Ni)-S group, which may boost the charge transfer. Particularly, (Fe-Ni)-S groups are perpendicular to the surface, and four of them make up one closed loop in the surface. It is suggested that the behaviors of such configuration composed of reaction centers resemble edge sites along the layers of MoS₂ toward HER. This study provides a deep insight into the synergistic effect of S-bridged Fe-Ni groups and enables the modulation of electrocatalytic reaction of pentlandite toward HER.

Visualization and Quantification of Electrochemical H2 Bubble Nucleation at Pt, Au, and MoS2 Substrates

Electrolytic gas evolution is a significant phenomenon in many electrochemical technologies from water splitting, chloralkali process to fuel cells. Although it is known that gas evolution may substantially affect the ohmic resistance and mass transfer, studies focusing on the electrochemistry of individual bubbles are critical but also challenging. Here, we report an approach using scanning electrochemical cell microscopy (SECCM) with a single channel pipet to quantitatively study individual gas bubble nucleation on different electrode substrates, including conventional polycrystalline Pt and Au films, as well as the most interesting two-dimensional semiconductor MoS₂. Due to the confinement effect of the pipet, well-defined peak-shaped voltammetric features associated with single bubble nucleation and growth are consistently observed. From stochastic bubble nucleation measurement and finite element simulation, the surface H₂ concentration corresponding to bubble nucleation is estimated to be ∼218, 137, and 157 mM, with critical nuclei contact angles of ∼156°, ∼161°, and ∼160° at polycrystalline Pt, Au, and MoS₂ substrates, respectively. We further demonstrated the surface faceting at polycrystalline Pt is not specifically correlated with the bubble nucleation behavior.

Bimetallic Phosphides as High-Efficient Electrocatalysts for Hydrogen Generation

Bimetallic transition-metal phosphides are gradually evolving as efficient hydrogen evolution catalysts. In this study, graphene-coated MoP and bimetallic phosphide (MoNiP) nanoparticles (MoP/MoNiP@C) were synthesized via one-step straightforward high-temperature calcination and phosphating process. The precursor was obtained from polyaniline, Ni²⁺ ions, and phosphomolybdic acid hydrate (PMo₁₂) by solvent evaporation. As expected, MoP/MoNiP@C manifests excellent hydrogen evolution activity with a low overpotential of 134 mV at 10 mA cm^-2 and a small Tafel slope of 66 mV dec^-1. Furthermore, MoP/MoNiP@C exhibits satisfactory stability for 24 h in the acid electrolyte. The outstanding catalytic performance can be attributed to the synergistic effect of MoP and MoNiP nanoparticles, the graphene coating protecting MoP and MoNiP from corrosion, as well as an increase in the number of active sites because of porous structures. This work can provide the experimental foundation for the simple synthesis of bimetallic phosphates with remarkable hydrogen evolution performance.

Characterizing photocatalysts for water splitting: from atoms to bulk and from slow to ultrafast processes

This review provides a comprehensive overview on characterisation techniques for light-driven redox-catalysts highlighting spectroscopic, microscopic, electrochemical and spectroelectrochemical approaches.

Recent progress of Ni-Fe layered double hydroxide and beyond towards electrochemical water splitting

The electrochemical water splitting process including the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is considered as one of the most promising methods for high-purity hydrogen production. Ni-Fe based compounds, especially Ni-Fe layered double hydroxide (LDH), have become highly efficient electrocatalysts to expedite the above reactions. During the last decade, great progress has been witnessed in the development of Ni-Fe based electrocatalysts. Diverse regulatory strategies such as morphology modulation, composition control, and defect engineering have been employed to optimize their electrochemical performances for water splitting. In addition, the family of Ni-Fe based compounds has been expanded from LDHs to alloys, sulfides, phosphides and so forth. Deep experimental investigations and theoretical studies have also been carried out to reveal the intrinsic origin of the superior electrocatalytic performances. In this review, we summarise the recent development of Ni-Fe based compounds for electrochemical water splitting with high efficiency. Special focus has been placed on the design principle and synthetic strategies of Ni-Fe based compounds. In the end, remaining challenges and future research directions are briefly discussed.

Insights into the Mo-Doping Effect on the Electrocatalytic Performance of Hierarchical CoxMoyS Nanosheet Arrays for Hydrogen Generation and Urea Oxidation

Energy-efficient, low-cost, and highly durable catalysts for the electrochemical hydrogen evolution reaction (HER) and urea oxidation reaction (UOR) are extremely important for related sustainable energy systems. In the present work, hierarchical coassembled cobalt molybdenum sulfide nanosheets deposited on carbon cloth (CC) were synthesized as catalysts for hydrogen evolution and urea oxidation. By adjusting the doping amount of Mo, 2D nanosheets with different morphologies and compositions (Co_xMo_yS-CC) can be obtained. The as-prepared nanosheet materials with abundant active sites exhibit superior properties on the electrochemical HER and UOR in alkaline medium. Significantly, the Mo-doping concentration and composition of the formed nanosheets have large effects on the electrocatalytic activity. The fabricated nanosheets with optimal Mo doping (Co₃Mo₁S-CC) illustrate the best catalytic properties for the HER in N₂-saturated 1.0 M KOH. A small overpotential (85 mV) is needed to meet the current density of 10 mA/cm². This study indicates that the doping of an appropriate amount of molybdenum into CoS₂ nanosheets can efficiently improve the catalytic performance. Also, the nanosheet catalyst exhibits an extremely high electrocatalytic activity for the UOR, and the electrochemical results indicate that a relatively low cell voltage of 1.50 V is needed to obtain the current density of 10 mA/cm². The present work demonstrates the potential application of CoMoS nanosheets in the energy electrocatalysis area and the insights into performance-boosting through heteroatom doping and optimization of the composition and structure.

Electrocatalysis as the Nexus for Sustainable Renewable Energy: The Gordian Knot of Activity, Stability, and Selectivity

The use of renewable energy by means of electrochemical techniques by converting H2 O, CO2 and N2 into chemical energy sources and raw materials, is the basis for securing a future sustainable "green" energy supply. Some weaknesses and inconsistencies in the practice of determining the electrocatalytic performance, which prevents a rational bottom-up catalyst design, are discussed. Large discrepancies in material properties as well as in electrocatalytic activity and stability become obvious when materials are tested under the conditions of their intended use as opposed to the usual laboratory conditions. They advocate for uniform activity/stability correlations under application-relevant conditions, and the need for a clear representation of electrocatalytic performance by contextualization in terms of functional investigation or progress towards application is emphasized.

Oil-Immersed Scanning Micropipette Contact Method Enabling Long-term Corrosion Mapping

This work reports the development of an oil-immersed scanning micropipette contact method, a variant of the scanning micropipette contact method, where a thin layer of oil wets the investigated substrate. The oil-immersed scanning micropipette contact method significantly increases the droplet stability, allowing for prolonged mapping and the use of highly evaporative saline solutions regardless of ambient humidity levels. This systematic mapping technique was used to conduct a detailed investigation of localized corrosion taking place at the surface of an AA7075-T73 aluminum alloy in a 3.5 wt % NaCl electrolyte solution, which is typically challenging in the conventional scanning micropipette contact method. Maps of corrosion potentials and corrosion currents extracted from potentiodynamic polarization curves showed good correlations with the chemical composition of surface features and known galvanic interactions at the microscale level. This demonstrates the viability of the oil-immersed scanning micropipette contact method and opens up the avenue to mechanistic corrosion investigations at the microscale level using aqueous solutions that are prone to evaporation under noncontrolled humidity levels.

The Role of Phosphate Group in Doped Cobalt Molybdate: Improved Electrocatalytic Hydrogen Evolution Performance

The hydrogen evolution reaction (HER) is a critical process in the electrolysis of water. Recently, much effort has been dedicated to developing low-cost, highly efficient, and stable electrocatalysts. Transition metal phosphides are investigated intensively due to their high electronic conductivity and optimized absorption energy of intermediates in acid electrolytes. However, the low stability of metal phosphide materials in air and during electrocatalytic processes causes a decay of performance and hinders the discovery of specific active sites. The HER in alkaline media is more intricate, which requires further delicate design due to the Volmer steps. In this work, phosphorus-modified monoclinic β-CoMoO4 is developed as a low-cost, efficient, and stable HER electrocatalyst for the electrolysis of water in alkaline media. The optimized catalyst shows a small overpotential of 94 mV to reach a current density of 10 mA cm-2 for the HER with high stability in KOH electrolyte, and an overpotential of 197 mV to reach a current density of 100 mA cm-2. Combined computational and in situ spectroscopic techniques show P is present as a surface phosphate ion; that electron holes localize on the surface ions and both (P-O1-) and Co3+-OH- are prospective surface active sites for the HER.

Confinement Effect of Mesopores: In Situ Synthesis of Cationic Tungsten-Vacancies for a Highly Ordered Mesoporous Tungsten Phosphide Electrocatalyst

Engineering defects in crystalline electrocatalysts is an effective approach to tailor the electronic structure and number of active sites, which are essential for the intrinsic activity of the hydrogen evolution reaction (HER). Unlike previously reported methods, we demonstrate a confinement effect using a mesoporous template for in situ fabrication of cationic W vacancies in as-prepared ordered mesoporous tungsten phosphide (WP) nanostructures by adjusting the nonstoichiometric ratio of the precursor elements. With a plenty of W vacancies and ordered mesoporosity, the as-prepared catalyst WP-Mesop exhibits better catalytic performance than the catalysts without mesopores and/or vacancies. The WP-Mesop shows an ultralow overpotential of 175 mV in acid and 229 mV in alkaline at 100 mA cm^-2 and stability of 48 h without structural collapse in both acid and alkaline media. Meanwhile, density functional theory calculations further reveal that the activation barrier for HER can be lowered by introducing cationic W vacancies. This strategy can be extended to generate cationic defects in other transition metal phosphides to improve their HER activities.

Investigating the Redox Properties of Two-Dimensional MoS2 Using Photoluminescence Spectroelectrochemistry and Scanning Electrochemical Cell Microscopy

Control over photophysical and chemical properties of two-dimensional (2D) transition metal dichalcogenides (TMDs) is the key to advance their applications in next-generation optoelectronics. Although chemical doping and surface modification with plasmonic metals have been reported to tune the photophysical and catalytic properties of 2D TMDs, there have been few reports of tuning optical properties using dynamic electrochemical control of electrode potential. Herein, we report (1) the photoluminescence (PL) enhancement and red-shift in the PL spectrum of 2D MoS₂, synthesized by chemical vapor deposition and subsequent transfer onto an indium tin oxide electrode, upon electrochemical anodization and (2) spatial heterogeneities in its photoelectrochemical (PEC) activities. Spectroelectrochemistry shows that positive electrochemical bias causes an initial ten-fold increase in the PL intensity followed by a quick decrease in the enhancement. The PL enhancement and spectrum red-shift are associated with the decrease in nonradiative decay rates of excitons formed upon electrochemical anodization of 2D MoS₂. Additionally, scanning electrochemical cell microscopy (SECCM) study shows that the 2D MoS₂ crystal is spatially sensitive to PEC oxidation at positive potentials. SECCM also shows a photocurrent increase caused by spatially heterogeneous edge-type defect sites of the crystal.

High-Resolution Electrochemical Mapping of the Hydrogen Evolution Reaction on Transition-Metal Dichalcogenide Nanosheets

High-resolution scanning electrochemical cell microscopy (SECCM) is used to image and quantitatively analyze the hydrogen evolution reaction (HER) catalytically active sites of 1H-MoS₂ nanosheets, MoS₂ , and WS₂ heteronanosheets. Using a 20 nm radius nanopipette and hopping mode scanning, the resolution of SECCM was beyond the optical microscopy limit and visualized a small triangular MoS₂ nanosheet with a side length of ca. 130 nm. The electrochemical cell provides local cyclic voltammograms with a nanoscale spatial resolution for visualizing HER active sites as electrochemical images. The HER activity difference of edge, terrace, and heterojunction of MoS₂ and WS₂ were revealed. The SECCM imaging directly visualized the relationship of HER activity and number of MoS₂ nanosheet layers and unveiled the heterogeneous aging state of MoS₂ nanosheets. SECCM can be used for improving local HER activities by producing sulfur vacancies using electrochemical reaction at the selected region.

Ni3S2 in Situ Grown on Ni Foam Coupled with Nitrogen-Doped Carbon Nanotubes as an Efficient Electrocatalyst for the Hydrogen Evolution Reaction in Alkaline Solution

Searching for a highly efficient electrocatalyst for the hydrogen evolution reaction (HER) in alkaline solution is still challenging. In this work, we report a HER electrocatalyst composed of Ni foam, Ni₃S₂, and nitrogen-doped carbon nanotubes (NF@Ni₃S₂@NCNTs). The good lattice matching between Ni and Ni₃S₂, interstitial doping of C and N atoms, and synergistic effect of NCNTs make NF@Ni₃S₂@NCNTs highly efficient HER electrocatalysts in alkaline solution. NF@Ni₃S₂@NCNTs exhibit an overpotential of 93.89 mV at a current density of 10 mA cm^-2, a Tafel slope of 54 mV dec^-1, and superior stability for HER in 1 M KOH solution. Our findings will promote the reasonable design and synthesis of an efficient electrocatalyst for HER in alkaline electrolytes.

Nanoscale Visualization and Multiscale Electrochemical Analysis of Conductive Polymer Electrodes

Conductive polymers are exceptionally promising for modular electrochemical applications including chemical sensors, bioelectronics, redox-flow batteries, and photoelectrochemical systems due to considerable synthetic tunability and ease of processing. Despite well-established structural heterogeneity in these systems, conventional macroscopic electroanalytical methods-specifically cyclic voltammetry-are typically used as the primary tool for structure-property elucidation. This work presents an alternative correlative multimicroscopy strategy. Data from laboratory and synchrotron-based microspectroscopies, including conducting-atomic force microscopy and synchrotron nanoscale infrared spectroscopy, are combined with potentiodynamic movies of electrochemical fluxes from scanning electrochemical cell microscopy (SECCM) to reveal the relationship between electrode structure and activity. A model conductive polymer electrode system of tailored heterogeneity is investigated, consisting of phase-segregated domains of poly(3-hexylthiophene) (P3HT) surrounded by contiguous regions of insulating poly(methyl methacrylate) (PMMA), representing an ultramicroelectrode array. Isolated domains of P3HT are shown to retain bulk-like chemical and electronic structure when blended with PMMA and possess approximately equivalent electron-transfer rate constants compared to pure P3HT electrodes. The nanoscale electrochemical data are used to model and predict multiscale electrochemical behavior, revealing that macroscopic cyclic voltammograms should be much more kinetically facile than observed experimentally. This indicates that parasitic resistances rather than redox kinetics play a dominant role in macroscopic measurements in these conductive polymer systems. SECCM further demonstrates that the ambient degradation of the P3HT electroactivity within P3HT/PMMA blends is spatially heterogeneous. This work serves as a roadmap for benchmarking the quality of conductive polymer films as electrodes, emphasizing the importance of nanoscale electrochemical measurements in understanding macroscopic properties.

Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media

"Single entity" measurements are central for an improved understanding of the function of nanoparticle-based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF-67)-derived Co-N-doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non-interfering Os complex as free-diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s^-1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects.

Directly Mapping Photoelectrochemical Behavior within Individual Transition Metal Dichalcogenide Nanosheets

Spatial variations in photoelectrochemical reaction rates within individual p-type WSe₂ nanosheets were mapped through the application of scanning electrochemical cell microscopy (SECCM). The simultaneous topographical and electrochemical information provided via SECCM directly revealed how both sheet thickness and the presence of defect structures affect the local rate of photoelectrochemical reactions for both outer sphere and inner sphere redox couples. Sheet thickness was found to play a dramatic role in reaction rates, with onset potentials shifting by as much as 0.5 V over thicknesses of 20-120 nm, attributable to the inability of thin sheets to support independent space charge layers. Step/edge features were found to play a detrimental role for the outer sphere redox couple investigated (Ru(NH₃)₆³⁺ reduction), with taller steps having larger effects on performance. Shorter step features were found to be beneficial for hydrogen evolution, showing a controlled density of defect features is desirable for inner sphere processes. The studies presented here not only provide valuable, quantitative insights into the behavior of transitional metal dichalcogenide materials but also demonstrate the power of applying SECCM to the study of photoelectrochemical systems, particularly those involving two-dimensional (2D) materials.

57 Fe Mössbauer Spectroscopy Characterization of Electrocatalysts

This work addresses the importance of Mössbauer spectroscopy for the characterization of iron-containing electrocatalysts. The most important aspects of electrocatalysis and Mössbauer spectroscopy are summarized. Next, Fe-N-C catalysts and important conclusions made by this technique on preparation, active site identification and degradation are summarized. Furthermore, recent highlights derived for other iron-containing electrocatalysts are summarized.

FexNi9-xS8 (x = 3-6) as potential photocatalysts for solar-driven hydrogen production?

The efficient reduction of protons by non-noble metals under mild conditions is a challenge for our modern society. Nature utilises hydrogenases, enzymatic machineries that comprise iron- and nickel- containing active sites, to perform the conversion of protons to hydrogen. We herein report a straightforward synthetic pathway towards well-defined particles of the bio-inspired material FexNi9-xS8, a structural and functional analogue of hydrogenase metal sulfur clusters. Moreover, the potential of pentlandites to serve as photocatalysts for solar-driven H2-production is assessed for the first time. The FexNi9-xS8 materials are visible light responsive (band gaps between 2.02 and 2.49 eV, depending on the pentlandite's Fe : Ni content) and display a conduction band energy close to the thermodynamic potential for proton reduction. Despite the limited driving force, a modest activity for photocatalytic H2 has been observed. Our observations show the potential for the future development of pentlandites as photocatalysts. This work provides a basis to explore powerful synergies between biomimetic chemistry and material design to unlock novel applications in solar energy conversion.