Earth-Abundant Heterogeneous Water Oxidation Catalysts

Influence of crystalline phase on electrocatalytic behaviour for Sm2-xSrxNiO4-δ (x = 0.4 to 1.0) Ruddlesden Popper based system : A comparative study of bulk and thin electrocatalysts

Here, with the motive of understanding influence of compositional engineering with active site alternation on catalytic behaviour is studied for Ruddlesden Popper based system Sm2-xSrxNiO4-δ. A phase change from orthorhombic...

Emerging Electrocatalysts for Water Oxidation under Near-Neutral CO2 Reduction Conditions

Electrocatalytic CO₂ reduction reaction (CO₂ RR), which produces valuable fuels and chemicals under near-neutral conditions, offers a renewable approach to alleviate the global energy crisis as well as the increasing concerns on climate change. However, to implement this strategy, one of the major challenges, the sluggish kinetics of the paired oxygen evolution reaction (OER) at anode, needs to be surmounted. It is therefore highly desirable to explore high-performance and cost-effective OER electrocatalysts suitable for CO₂ RR conditions, which is very different from those widely investigated under acidic or alkaline conditions. In this review, the ongoing development of OER electrocatalysts under near pH-neutral CO₂ -saturated (bi)carbonate solutions are highlighted and the future opportunities are discussed. It is started with a brief introduction on OER paired with CO₂ RR, the relevant theoretical tools such as density functional theory (DFT) and particularly machine learning (ML), and the operando characterization techniques. Then, there are some detailed discussions of recent progress on the rational design of OER electrocatalysts under CO₂ RR conditions ranging from noble-metal oxides to nonprecious metal phosphides, carbonates, (hydro)oxides, and so on. Finally, a perspective for developing OER electrocatalysts integrated with CO₂ electroreduction is proposed.

NiSe2/FeSe2 heterostructured nanoparticles supported on rGO for efficient water electrolysis

Hybrid architectures composed of NiSe2/FeSe2 heterostructured nanoparticles supported on rGO were synthesized through a facile self-templating strategy, and exhibited excellent electrocatalytic performance for overall water splitting.

Solvent-induced crystal-facet effect of nickel-cobalt layered double hydroxide for highly efficient overall water splitting

Two-dimensional layered materials have been universally acknowledged to be promising candidates for alternative precious metal in the field of catalysis. The crystal-facet effect is currently rare in the field of...

Understanding the factors governing the water oxidation reaction pathway of mononuclear and binuclear cobalt phthalocyanine catalysts

The rational design of efficient catalysts for electrochemical water oxidation highly depends on the understanding of reaction pathways, which still remains a challenge. Herein, mononuclear and binuclear cobalt phthalocyanine (mono-CoPc and bi-CoPc) with a well-defined molecular structure are selected as model electrocatalysts to study the water oxidation mechanism. We found that bi-CoPc on a carbon support (bi-CoPc/carbon) shows an overpotential of 357 mV at 10 mA cm⁻², much lower than that of mono-CoPc/carbon (>450 mV). Kinetic analysis reveals that the rate-determining step (RDS) of the oxygen evolution reaction (OER) over both electrocatalysts is a nucleophilic attack process involving a hydroxy anion (OH⁻). However, the substrate nucleophilically attacked by OH⁻ for bi-CoPc is the phthalocyanine cation-radical species (Co^II–Pc–Pc˙⁺–Co^II–OH) that is formed from the oxidation of the phthalocyanine ring, while cobalt oxidized species (Pc–Co^III–OH) is involved in mono-CoPc as evidenced by the operando UV-vis spectroelectrochemistry technique. DFT calculations show that the reaction barrier for the nucleophilic attack of OH⁻ on Co^II–Pc–Pc˙⁺–Co^II–OH is 1.67 eV, lower than that of mono-CoPc with Pc–Co^III–OH nucleophilically attacked by OH⁻ (1.78 eV). The good agreement between the experimental and theoretical results suggests that bi-CoPc can effectively stabilize the accumulated oxidative charges in the phthalocyanine ring, and is thus bestowed with a higher OER performance.

bi-CoPc can stabilize accumulated oxidative charges in phthalocyanine ring, which leads to the OER proceeding through a nucleophilic attack of OH^- on the phthalocyanine cation-radical species that is formed from the oxidation of phthalocyanine ring.

Mesoporous Mn-Fe oxyhydroxides for oxygen evolution

Development of high-performance and earth-abundant catalysts is imperative for the oxygen evolution reaction (OER), and mesoporous oxyhydroxides show huge potential as advanced catalysts toward OER due to large specific surface...

Iron-catalyzed domino coupling reactions of π-systems

The development of environmentally benign, inexpensive, and earth-abundant metal catalysts is desirable from both an ecological and economic standpoint. Certainly, in the past couple decades, iron has become a key player in the development of sustainable coupling chemistry and has become an indispensable tool in organic synthesis. Over the last ten years, organic chemistry has witnessed substantial improvements in efficient synthesis because of domino reactions. These protocols are more atom-economic, produce less waste, and demand less time compared to a classical stepwise reaction. Although iron-catalyzed domino reactions require a mindset that differs from the more routine noble-metal, homogenous iron catalysis they bear the chance to enable coupling reactions that rival that of noble-metal-catalysis. This review provides an overview of iron-catalyzed domino coupling reactions of π-systems. The classifications and reactivity paradigms examined should assist readers and provide guidance for the design of novel domino reactions.

Sodium Cobalticarborane: A Promising Precatalyst for Oxygen Evolution Reaction

Water splitting is a helpful way of converting renewable electricity into fuel. The oxygen evolution reaction (OER) is a slow reaction that provides low-cost electrons for water reduction reactions. Thus, finding an efficient, low-cost, stable, and environmentally friendly OER catalyst is critical for water splitting. Here, sodium cobalticarborane (1) is introduced as a promising precatalyst for forming an OER cobalt-based catalyst. The cobalt-based catalyst was characterized by several methods and is suggested to be Co(III) (hydr)oxide. Using fluorine-doped tin oxide, glassy carbon, platinum, and gold electrodes, the OER activity of the cobalt-based precatalyst was investigated. The overpotential for the onset of OER in the presence of 1 is 315 mV using fluorine-doped tin oxide electrodes. The onsets of OERs in the presence of 1 using gold, platinum, and glassy carbon electrodes in KOH solutions (1.0 M) turned out to be 275, 284, and 330 mV, respectively. The nanoparticles on the gold electrodes exhibit significant OER activity with a Tafel slope of 63.8 mV/decade and an overpotential at 541 mV for 50 mA/cm². In the case of the glassy carbon electrodes, a Tafel slope of 109.9 mV/decade and an overpotential of 548 mV for 10 mA/cm² is recorded for the catalyst. This paper outlines an interesting approach to synthesize cobalt oxide for OER through a slow decomposition of a precatalyst.

Engineering Lattice Oxygen Activation of Iridium Clusters Stabilized on Amorphous Bimetal Borides Array for Oxygen Evolution Reaction

Developing robust oxygen evolution reaction (OER) catalysts requires significant advances in material design and in-depth understanding for water electrolysis. Herein, we report iridium clusters stabilized surface reconstructed oxyhydroxides on amorphous metal borides array, achieving an ultralow overpotential of 178 mV at 10 mA cm^-2 for OER in alkaline medium. The coupling of iridium clusters induced the formation of high valence cobalt species and Ir-O-Co bridge between iridium and oxyhydroxides at the atomic scale, engineering lattice oxygen activation and non-concerted proton-electron transfer to trigger multiple active sites for intrinsic pH-dependent OER activity. The lattice oxygen oxidation mechanism (LOM) was confirmed by in situ ¹⁸ O isotope labeling mass spectrometry and chemical recognition of negative peroxo-like species. Theoretical simulations reveal that the OER performance on this catalyst is intrinsically dominated by LOM pathway, facilitating the reaction kinetics. This work not only paves an avenue for the rational design of electrocatalysts, but also serves the fundamental insights into the lattice oxygen participation for promising OER application.

Wrinkle facilitated hydrogen evolution reaction of vacancy-defected transition metal dichalcogenide monolayers

Utilizing transition metal dichalcogenides (TMDs) as catalysts in the hydrogen evolution reaction (HER) is a promising prospect for hydrogen production. Here, by first-principles calculations we reveal that the catalytic activities of vacancy-defected TMD MX₂ (M = Mo or W and X = S, Se or Te) monolayers for the HER can be significantly improved by wrinkle engineering. The hydrogen adsorption Gibbs free energies of defected TMDs decrease with decreasing wrinkle length. By appropriately controlling and adjusting the wrinkle size and vacancy number, the hydrogen adsorption Gibbs free energy will be close to zero, allowing the wrinkled TMDs to reach their optimum catalytic capability. The improvement of the catalytic activity of TMDs is mainly attributed to the charge transfer and polarization enhancement of metal atoms at the vacancy sites, which are caused by the coupling effect of vacancy defects and wrinkling deformation induced flexoelectricity. These results provide an attractive route for the application of TMDs in hydrogen production by combining wrinkle engineering and defect engineering.

Promoting the Oxygen Evolution Activity of Perovskite Nickelates through Phase Engineering

Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and cost-effectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO₃ by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO₃ film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO₃ single crystalline film. The obtained current density of 1038 μΑ cm^-2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.

Lattice site-dependent metal leaching in perovskites toward a honeycomb-like water oxidation catalyst

Metal leaching during water oxidation has been typically observed in conjunction with surface reconstruction on perovskite oxide catalysts, but the role of metal leaching at each geometric site has not been distinguished. Here, we manipulate the occurrence and process of surface reconstruction in two model ABO₃ perovskites, i.e., SrSc_0.5Ir_0.5O₃ and SrCo_0.5Ir_0.5O₃, which allow us to evaluate the structure and activity evolution step by step. The occurrence and order of leaching of Sr (A-site) and Sc/Co (B-site) were controlled by tailoring the thermodynamic stability of B-site. Sr leaching from A-site mainly generates more electrochemical surface area for the reaction, and additional leaching of Sc/Co from B-site triggers the formation of a honeycomb-like IrO_xH_y phase with a notable increase in intrinsic activity. A thorough surface reconstruction with dual-site metal leaching induces an activity improvement by approximately two orders of magnitude, which makes the reconstructed SrCo_0.5Ir_0.5O₃ among the best for water oxidation in acid.

The Effect of Interlayer Anion Grafting on Water Oxidation Electrocatalysis: A Comparative Study of Ni- and Co-Based Brucite-Type Layered Hydroxides, Layered Double Hydroxides and Hydroxynitrate Salts

The urge for carbon‐neutral green energy conversion and storage technologies has invoked the resurgence of interest in applying brucite‐type materials as low‐cost oxygen evolution reaction (OER) electrocatalysts in basic media. Transition metal layered hydroxides belonging to the brucite‐type structure family have been shown to display remarkable electrochemical activity. Recent studies on the earth‐abundant Fe³⁺ containing mössbauerite and Fe³⁺ rich Co−Fe layered oxyhydroxide carbonates have suggested that grafted interlayer anions might play a key role in OER catalysis. To probe the effect of such interlayer anion grafting in brucite‐like layered hydroxides, we report here a systematic study on the electrocatalytic performance of three distinct Ni and Co brucite‐type layered structures, namely, (i) brucite‐type M(OH)₂ without any interlayer anions, (ii) LDHs with free interlayer anions, and (iii) hydroxynitrate salts with grafted interlayer anions. The electrochemical results indeed show that grafting has an evident impact on the electronic structure and the observed OER activity. Ni‐ and Co‐hydroxynitrate salts with grafted anions display notably earlier formations of the electrocatalytically active species. Particularly Co‐hydroxynitrate salts exhibit lower overpotentials at 10 mA cm⁻² (η=0.34 V) and medium current densities of 100 mA cm⁻² (η=0.40 V) compared to the corresponding brucite‐type hydroxides and LDH materials.

Structure-Activity Relationship for Di- up to Tetranuclear Macrocyclic Ruthenium Catalysts in Homogeneous Water Oxidation

Two di- and tetranuclear Ru(bda) (bda: 2,2'-bipyridine-6,6'-dicarboxylate) macrocyclic complexes were synthesized and their catalytic activities in chemical and photochemical water oxidation investigated in a comparative manner to our previously reported trinuclear congener. Our studies have shown that the catalytic activities of this homologous series of multinuclear Ru(bda) macrocycles in homogeneous water oxidation are dependent on their size, exhibiting highest efficiencies for the largest tetranuclear catalyst. The turnover frequencies (TOFs) have increased from di- to tetranuclear macrocycles not only per catalyst molecule but more importantly also per Ru unit with TOF of 6 s^-1 to 8.7 s^-1 and 10.5 s^-1 in chemical and 0.6 s^-1 to 3.3 s^-1 and 5.8 s^-1 in photochemical water oxidation per Ru unit, respectively. Thus, for the first time, a clear structure-activity relationship could be established for this novel class of macrocyclic water oxidation catalysts.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Unveiling the boosting of metal organic cage leaching substance on the electrocatalytic oxygen evolution reaction

Catalysts often undergo changes during the process of catalytic reactions, which makes the whole catalytic reaction system complicated and brings about much difficulty for the exploration of catalytic mechanism. Herein, we report that an octahedral metal organic cage (MOC) with stress was directionally transformed into two-dimensional nanoarrays maintaining the structure of precursor and new soluble low-nuclear complexes during the electrocatalytic oxygen evolution reaction (OER). The in-situ generated miscible electrocatalyst exhibits an overpotential as low as 197 mV at 10 mA cm^-2, with a high electrochemical stability up to 5 h. Notably, the miscible catalyst can be used as bifunctional electrocatalyst for OER and hydrogen evolution reaction (HER) and exhibits an ultra-low overpotential of 293 mV, even achieve overall water splitting under the voltage provided by a 1.5 V AA battery. As revealed by density functional theory simulations, the position of SO₄^2- in MOC heterogeneous catalyst is regulated by the soluble low-nuclear complexes to reduce the activation energy of the reaction, leading to an optimization of the OER activity for the reaction system. This work provides a new strategy for the rational design of high-efficiency electrocatalytic system.

Linear Conjugated Polymers for Solar-Driven Hydrogen Peroxide Production: The Importance of Catalyst Stability

Hydrogen peroxide (H2O2) is one of the most important industrial oxidants. In principle, photocatalytic H2O2 synthesis from oxygen and H2O using sunlight could provide a cleaner alternative route to the current anthraquinone process. Recently, conjugated organic materials have been studied as photocatalysts for solar fuels synthesis because they offer synthetic tunability over a large chemical space. Here, we used high-throughput experiments to discover a linear conjugated polymer, poly(3-4-ethynylphenyl)ethynyl)pyridine (DE7), which exhibits efficient photocatalytic H2O2 production from H2O and O2 under visible light illumination for periods of up to 10 h or so. The apparent quantum yield was 8.7% at 420 nm. Mechanistic investigations showed that the H2O2 was produced via the photoinduced stepwise reduction of O2. At longer photolysis times, however, this catalyst decomposed, suggesting a need to focus the photostability of organic photocatalysts, as well as the initial catalytic production rates.

Revealing the Dynamics and Roles of Iron Incorporation in Nickel Hydroxide Water Oxidation Catalysts

The surface of an electrocatalyst undergoes dynamic chemical and structural transformations under electrochemical operating conditions. There is a dynamic exchange of metal cations between the electrocatalyst and electrolyte. Understanding how iron in the electrolyte gets incorporated in the nickel hydroxide electrocatalyst is critical for pinpointing the roles of Fe during water oxidation. Here, we report that iron incorporation and oxygen evolution reaction (OER) are highly coupled, especially at high working potentials. The iron incorporation rate is much higher at OER potentials than that at the OER dormant state (low potentials). At OER potentials, iron incorporation favors electrochemically more reactive edge sites, as visualized by synchrotron X-ray fluorescence microscopy. Using X-ray absorption spectroscopy and density functional theory calculations, we show that Fe incorporation can suppress the oxidation of Ni and enhance the Ni reducibility, leading to improved OER catalytic activity. Our findings provide a holistic approach to understanding and tailoring Fe incorporation dynamics across the electrocatalyst-electrolyte interface, thus controlling catalytic processes.

Enthalpy-Entropy Compensation Effect in Oxidation Reactions by Manganese(IV)-Oxo Porphyrins and Nonheme Iron(IV)-Oxo Models

"Enthalpy-Entropy Compensation Effect" (EECE) is ubiquitous in chemical reactions; however, such an EECE has been rarely explored in biomimetic oxidation reactions. In this study, six manganese(IV)-oxo complexes bearing electron-rich and -deficient porphyrins are synthesized and investigated in various oxidation reactions, such as hydrogen atom transfer (HAT), oxygen atom transfer (OAT), and electron-transfer (ET) reactions. First, all of the six Mn(IV)-oxo porphyrins are highly reactive in the HAT, OAT, and ET reactions. Interestingly, we have observed a reversed reactivity in the HAT and OAT reactions by the electron-rich and -deficient Mn(IV)-oxo porphyrins, depending on reaction temperatures, but not in the ET reactions; the electron-rich Mn(IV)-oxo porphyrins are more reactive than the electron-deficient Mn(IV)-oxo porphyrins at high temperature (e.g., 0 °C), whereas at low temperature (e.g., -60 °C), the electron-deficient Mn(IV)-oxo porphyrins are more reactive than the electron-rich Mn(IV)-oxo porphyrins. Such a reversed reactivity between the electron-rich and -deficient Mn(IV)-oxo porphyrins depending on reaction temperatures is rationalized with EECE; that is, the lower is the activation enthalpy, the more negative is the activation entropy, and vice versa. Interestingly, a unified linear correlation between the activation enthalpies and the activation entropies is observed in the HAT and OAT reactions of the Mn(IV)-oxo porphyrins. Moreover, from the previously reported HAT reactions of nonheme Fe(IV)-oxo complexes, a linear correlation between the activation enthalpies and the activation entropies is also observed. To the best of our knowledge, we report the first detailed mechanistic study of EECE in the oxidation reactions by synthetic high-valent metal-oxo complexes.

Amine-based synthesis of Fe3C nanomaterials: mechanism and impact of synthetic conditions

Iron-based catalysts are a preferred variant of metal catalysts due to the high abundance of iron on earth. Iron carbide has been investigated in recent times as an electrochemical catalyst due to its potential as a great ORR catalyst. Using a unique amine-metal complex anion composite (AMAC) method, iron carbide/nitride nanoparticles (Fe3C and Fe3−x N) were synthesized through varying several reaction parameters. While the synthesis is generally quite robust and can easily afford phase pure Fe3C, it now has been shown that the particle size, morphology, excess carbon, and amount of nitrogen in the resulting nanomaterials can readily be tuned. In addition, it was discovered that Fe2N can be synthesized as an intermediate by stopping the reaction at a lower heating temperature. These nanomaterials were tested for their electrochemical activity in oxygen evolution reactions (OER).

Rare-Earth Elements Can Structurally and Energetically Replace the Calcium in a Synthetic Mn4CaO4-Cluster Mimicking the Oxygen-Evolving Center in Photosynthesis

The oxygen-evolving center (OEC) in photosynthesis is a unique biological Mn₄CaO₅ cluster catalyzing the water-splitting reaction. A great current challenge is to achieve a robust and precise mimic of the OEC in the laboratory. Herein, we report synthetic Mn₄XO₄ clusters (X = calcium, yttrium, gadolinium) that closely resemble the OEC with regard to the main metal-oxide core and peripheral ligands, as well as the oxidation states of the four Mn ions and the redox potential of the cluster. We demonstrate that rare-earth elements can structurally replace the calcium in neutral Mn₄XO₄ clusters. All three Mn₄XO₄ clusters with different redox-inactive metal ions display essentially the same redox properties, challenging the conventional view that the Lewis acidity of the redox-inactive metal ions could modulate the redox potential of the heteronuclear-oxide clusters. The new synthetic rare-earth element-containing Mn₄XO₄ clusters reported here provide robust and structurally well-defined chemical models and shed new light on the design of new water-splitting catalysts in artificial photosynthesis.

Efficient Alkaline Water Oxidation with a Regenerable Nickel Pseudo-Complex

Efficient and robust electrocatalysts are required for the oxygen evolution reaction (OER). Photosystem II-inspired synthetic transition metal complexes have shown promising OER activity in water-poor or mild conditions, yet challenges remain in the improvement of current density and performance stability for practical applications in alkaline electrolytes in contrast to solid-state oxide catalysts. Here, we report that a nickel pseudo-complex (bpy)_zNiO_xH_y (bpy = 2,2'-bipyridine) catalyst, which bridges solid oxide and molecular catalysts, exhibits the highest OER activity among nickel-based catalysts with a turnover frequency of 1.1 s^-1 at an overpotential of 0.30 volts, even outperforming iron-incorporated nickel (oxy)hydroxide under an identical nickel mass load. Benefiting from the strong coordination between bpy and nickel, this (bpy)_zNiO_xH_y catalyst exhibits long-term stability in highly alkaline media at 1.0 mA cm^-2 for over 200 h and at 20 mA cm^-2 for over 60 h. Our findings indicate that dynamically coordinating a small amount of bpy in the catalyst layer efficiently sustains highly active nickel sites for water oxidation, demonstrating a general strategy for improving the activity of transition metal sites with active ligands beyond the incorporation of metal cations to form double-layered hydroxides.

Facile water oxidation by dinuclear mixed-valence CoIII/CoII complexes: the role of coordinated water

Rational design of a catalyst using earth abundant transition metals that can facilitate the smooth O-O bond formation is crucial for developing efficient water oxidation catalysts. The coordination environment around the metal ion of the catalyst plays a pivotal role in this context. We have chosen dinuclear mixed-valence Co^IIICo^II complexes of the general formula of [Co^IIICo^II(LH₂)₂(X)(H₂O)] (X = OAc or Cl) which bear a coordinated water molecule in the primary coordination sphere. We anticipated that the water molecule in the primary sphere can take part in the proton coupled electron transfer (PCET) mechanism which can accelerate the facile formation of the O-O bond under strong alkaline conditions (1 M NaOH). To understand the role of the coordinated water molecule we have generated an analogous complex, [Co^IIICo^II(LH₂)₂(o-vanillin)] (o-vanillin = 2-hydroxy-3-methoxybenzaldehyde), without coordinated water. Interestingly, we have found that the water coordinated complexes show better oxygen evolution reaction (OER) activity and stability.

The Significance of Properly Reporting Turnover Frequency in Electrocatalysis Research

For decades, turnover frequency (TOF) has served as an accurate descriptor of the intrinsic activity of a catalyst, including those in electrocatalytic reactions involving both fuel generation and fuel consumption. Unfortunately, in most of the recent reports in this area, TOF is often not properly reported or not reported at all, in contrast to the overpotentials at a benchmarking current density. The current density is significant in determining the apparent activity, but it is affected by catalyst-centric parasitic reactions, electrolyte-centric competing reactions, and capacitance. Luckily, a properly calculated TOF can precisely give the intrinsic activity free from these phenomena in electrocatalysis. In this Viewpoint we ask: 1) What makes the commonly used activity markers unsuitable for intrinsic activity determination? 2) How can TOF reflect the intrinsic activity? 3) Why is TOF still underused in electrocatalysis? 4) What methods are used in TOF determination? and 5) What is essential in the more accurate calculation of TOF? Finally, the significance of normalizing TOF by Faradaic efficiency (FE) is stressed and we give our views on the development of universal analytical tools to determine the exact number of active sites and real surface area for all kinds of materials.

Activation by oxidation and ligand exchange in a molecular manganese vanadium oxide water oxidation catalyst

Despite their technological importance for water splitting, the reaction mechanisms of most water oxidation catalysts (WOCs) are poorly understood. This paper combines theoretical and experimental methods to reveal mechanistic insights into the reactivity of the highly active molecular manganese vanadium oxide WOC [Mn4V4O17(OAc)3]3- in aqueous acetonitrile solutions. Using density functional theory together with electrochemistry and IR-spectroscopy, we propose a sequential three-step activation mechanism including a one-electron oxidation of the catalyst from [Mn2 3+Mn2 4+] to [Mn3+Mn3 4+], acetate-to-water ligand exchange, and a second one-electron oxidation from [Mn3+Mn3 4+] to [Mn4 4+]. Analysis of several plausible ligand exchange pathways shows that nucleophilic attack of water molecules along the Jahn-Teller axis of the Mn3+ centers leads to significantly lower activation barriers compared with attack at Mn4+ centers. Deprotonation of one water ligand by the leaving acetate group leads to the formation of the activated species [Mn4V4O17(OAc)2(H2O)(OH)]- featuring one H2O and one OH ligand. Redox potentials based on the computed intermediates are in excellent agreement with electrochemical measurements at various solvent compositions. This intricate interplay between redox chemistry and ligand exchange controls the formation of the catalytically active species. These results provide key reactivity information essential to further study bio-inspired molecular WOCs and solid-state manganese oxide catalysts.

A self-healing catalyst for electrocatalytic and photoelectrochemical oxygen evolution in highly alkaline conditions

While self-healing is considered a promising strategy to achieve long-term stability for oxygen evolution reaction (OER) catalysts, this strategy remains a challenge for OER catalysts working in highly alkaline conditions. The self-healing of the OER-active nickel iron layered double hydroxides (NiFe-LDH) has not been successful due to irreversible leaching of Fe catalytic centers. Here, we investigate the introduction of cobalt (Co) into the NiFe-LDH as a promoter for in situ Fe redeposition. An active borate-intercalated NiCoFe-LDH catalyst is synthesized using electrodeposition and shows no degradation after OER tests at 10 mA cm⁻² at pH 14 for 1000 h, demonstrating its self-healing ability under harsh OER conditions. Importantly, the presence of both ferrous ions and borate ions in the electrolyte is found to be crucial to the catalyst’s self-healing. Furthermore, the implementation of this catalyst in photoelectrochemical devices is demonstrated with an integrated silicon photoanode. The self-healing mechanism leads to a self-limiting catalyst thickness, which is ideal for integration with photoelectrodes since redeposition is not accompanied by increased parasitic light absorption.

While self-healing catalysts may survive the harsh environments used for oxygen evolution, understanding how to develop such electrocatalysts remains a challenge. Here, authors find cobalt to promote the self-healing of leached iron centers in borate-intercalated nickel-iron-cobalt oxyhydroxides.

Improved water oxidation with metal oxide catalysts via a regenerable and redox-inactive ZnOxHy overlayer

We report a regenerable and redox-inactive ZnO_xH_y layer that was in situ deposited onto metal oxides MO_z (M = Co, Fe, and Ni) in alkaline media containing [Zn(OH)₄]^2- species during water oxidation. An interface dipole was developed at the MO_z/Zn interface, resulting in a decrease of the OER overpotential. Exemplified by the CoO_z/ZnO_xH_y bilayer structure, it presented a 155 mV lower overpotential to deliver 10 mA cm^-2 and long-term stability relative to the unmodified CoO_z film.

Elucidating the formation and active state of Cu co-catalysts for photocatalytic hydrogen evolution

The design of active and selective co-catalysts constitutes one of the major challenges in developing heterogeneous photocatalysts for energy conversion applications. This work provides a comprehensive insight into thermally induced bottom-up generation and transformation of a series of promising Cu-based co-catalysts. We demonstrate that the volcano-type HER profile as a function of calcination temperature is independent of the type of the Cu precursor but is affected by changes in oxidation state and location of the copper species. Supported by DFT modeling, our data suggest that low temperature (<200 °C) treatments facilitate electronic communication between the Cu species and TiO2, which allows for a more efficient charge utilization and maximum HER rates. In contrast, higher temperatures (>200 °C) do not affect the Cu oxidation state, but induce a gradual, temperature-dependent surface-to-bulk diffusion of Cu, which results in interstitial, tetra-coordinated Cu+ species. The disappearance of Cu from the surface and the introduction of new defect states is associated with a drop in HER performance. This work examines electronic and structural effects that are in control of the photocatalytic activity and can be transferred to other systems for further advancing photocatalysis.

Morphology Engineering of BiVO4 with CoOx Derived from Cobalt-containing Polyoxometalate as Co-catalyst for Oxygen Evolution

Bismuth vanadate (BiVO₄ ) as a metal oxidation semiconductor has stimulated extensive attention in the photocatalytic water splitting field. However, the poor transport ability and easy recombination of charge carriers limit photocatalytic water oxidation activity of pure BiVO₄ . Herein, the photocatalytic activity of BiVO₄ is enhanced via adjusting its morphology and combination co-catalyst. First, the Cu-BiVO₄ was synthesized by copper doping to control the growth of {110} facet of BiVO₄ , which is regarded for the separation of photo-generated charge carriers. Then the CoO_x in-situ generated from K₆ [SiCo^II (H₂ O)W₁₁ O₃₉ ] ⋅ 16H₂ O was photo-deposited on Cu-BiVO₄ surface as co-catalyst to speed up reaction kinetics. Cu-BiVO₄ @CoO_x hybrid catalyst shows highest photocatalytic activity and best stability among all the prepared catalysts. Oxygen evolution is about 34.6 μmol in pH 4 acetic acid buffer under 420 nm LED irradiation, which is nearly 20 times higher than that of pure BiVO₄ . Apparent quantum efficiency (AQE) in 1 h and O₂ yield are 1.83% and 23.1%, respectively. O₂ evolution amount nearly maintains the original value even after 5 cycles.