The Cambridge Centre for Advanced Research and Education in Singapore

Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.

an energy carrier from water splitting relies on four elementary reactions, i.e., the hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR). Herein, the central objective is to recommend systematic protocols for activity measurements of these four reactions and benchmark activities for comparison, which is critical to facilitate the research and development of catalysts with high activity and stability. Details for the electrochemical cell setup, measurements, and data analysis used to quantify the kinetics of the HER, HOR, OER, and ORR in acidic and basic solutions are provided, and examples of state-of-the-art specific and mass activity of catalysts to date are given. First, the experimental setup is discussed to provide common guidelines for these reactions, including the cell design, reference electrode selection, counter electrode concerns, and working electrode preparation. Second, experimental protocols, including data collection and processing such as ohmic- and background-correction and catalyst surface area estimation, and practice for testing and comparing different classes of catalysts are recommended. Lastly, the specific and mass activity activities of some state-of-the-art catalysts are benchmarked to facilitate the comparison of catalyst activity for these four reactions across different laboratories.

The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state-of-the-art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure-activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble-metal-based OER catalysts and the current progress of non-noble-metal-based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.

The hydrogen evolution reaction (HER) is a half-cell reaction in water electrolysis for producing hydrogen gas. In industrial water electrolysis, the HER is often conducted in alkaline media to achieve higher stability of the electrode materials. However, the kinetics of the HER in alkaline medium is slow relative to that in acid because of the low concentration of protons in the former. Under the latter conditions, the entire HER process will require additional effort to obtain protons by water dissociation near or on the catalyst surface. Heterostructured catalysts, with fascinating synergistic effects derived from their heterogeneous interfaces, can provide multiple functional sites for the overall reaction process. At present, the activity of the most active known heterostructured catalysts surpasses (platinum-based heterostructures) or approaches (noble-metal-free heterostructures) that of the commercial Pt/C catalyst under alkaline conditions, demonstrating an infusive potential to break through the bottlenecks. This review summarizes the most representative and recent heterostructured HER catalysts for alkaline medium. The basics and principles of the HER under alkaline conditions are first introduced, followed by a discussion of the latest advances in heterostructured catalysts with/without noble-metal-based heterostructures. Special focus is placed on approaches for enhancing the reaction rate by accelerating the Volmer step. This review aims to provide an overview of the current developments in alkaline HER catalysts, as well as the design principles for the future development of heterostructured nano- or micro-sized electrocatalysts.

Abstract The oxygen evolution reaction (OER) is the bottleneck that limits the energy efficiency of water-splitting. The process involves four electrons’ transfer and the generation of triplet state O 2 from singlet state species (OH - or H 2 O). Recently, explicit spin selection was described as a possible way to promote OER in alkaline conditions, but the specific spin-polarized kinetics remains unclear. Here, we report that by using ferromagnetic ordered catalysts as the spin polarizer for spin selection under a constant magnetic field, the OER can be enhanced. However, it does not applicable to non-ferromagnetic catalysts. We found that the spin polarization occurs at the first electron transfer step in OER, where coherent spin exchange happens between the ferromagnetic catalyst and the adsorbed oxygen species with fast kinetics, under the principle of spin angular momentum conservation. In the next three electron transfer steps, as the adsorbed O species adopt fixed spin direction, the OER electrons need to follow the Hund rule and Pauling exclusion principle, thus to carry out spin polarization spontaneously and finally lead to the generation of triplet state O 2 . Here, we showcase spin-polarized kinetics of oxygen evolution reaction, which gives references in the understanding and design of spin-dependent catalysts.

Since it was first discovered, thousands of years ago, silkworm silk has been known to be an abundant biopolymer with a vast range of attractive properties. The utilization of silk fibroin (SF), the main protein of silkworm silk, has not been limited to the textile industry but has been further extended to various high-tech application areas, including biomaterials for drug delivery systems and tissue engineering. The outstanding mechanical properties of SF, including its facile processability, superior biocompatibility, controllable biodegradation, and versatile functionalization have allowed its use for innovative applications. In this review, we describe the structure, composition, general properties, and structure-properties relationship of SF. In addition, the methods used for the fabrication and modification of various materials are briefly addressed. Lastly, recent applications of SF-based materials for small molecule drug delivery, biological drug delivery, gene therapy, wound healing, and bone regeneration are reviewed and our perspectives on future development of these favorable materials are also shared.

Electrocatalytic CO₂ reduction by heterogeneous molecular catalysts is emerging as an important area for CO₂ utilization.

From the start of a synthetic chemist's training, experiments are conducted based on recipes from textbooks and manuscripts that achieve clean reaction outcomes, allowing the scientist to develop practical skills and some chemical intuition. This procedure is often kept long into a researcher's career, as new recipes are developed based on similar reaction protocols, and intuition-guided deviations are conducted through learning from failed experiments. However, when attempting to understand chemical systems of interest, it has been shown that model-based, algorithm-based, and miniaturized high-throughput techniques outperform human chemical intuition and achieve reaction optimization in a much more time- and material-efficient manner; this is covered in detail in this paper. As many synthetic chemists are not exposed to these techniques in undergraduate teaching, this leads to a disproportionate number of scientists that wish to optimize their reactions but are unable to use these methodologies or are simply unaware of their existence. This review highlights the basics, and the cutting-edge, of modern chemical reaction optimization as well as its relation to process scale-up and can thereby serve as a reference for inspired scientists for each of these techniques, detailing several of their respective applications.

Abstract Exploring robust catalysts for water oxidation in acidic electrolyte is challenging due to the limited material choice. Iridium (Ir) is the only active element with a high resistance to the acid corrosion during water electrolysis. However, Ir is rare, and its large-scale application could only be possible if the intrinsic activity of Ir could be greatly enhanced. Here, a pseudo-cubic SrCo 0.9 Ir 0.1 O 3-δ perovskite, containing corner-shared IrO6 octahedrons, is designed. The Ir in the SrCo 0.9 Ir 0.1 O 3-δ catalyst shows an extremely high intrinsic activity as reflected from its high turnover frequency, which is more than two orders of magnitude higher than that of IrO 2 . During the electrochemical cycling, a surface reconstruction, with Sr and Co leaching, over SrCo 0.9 Ir 0.1 O 3-δ occurs. Such reconstructed surface region, likely contains a high amount of structural domains with corner-shared and under-coordinated IrO x octahedrons, is responsible for the observed high activity.

An Al³⁺ intercalation/de-intercalation-enabled dual-band electrochromic smart window featuring simultaneously a high optical modulation, fast response and long cycle life.

Abstract Dual‐band electrochromic smart windows capable of the spectrally selective modulation of visible (VIS) light and near‐infrared (NIR) can regulate solar light and solar heat transmittance to reduce the building energy consumption. The development of these windows is however limited by the number of available dual‐band electrochromic materials. Here, plasmonic oxygen‐deficient TiO 2‐ x nanocrystals (NCs) are discovered to be an effective single‐component dual‐band electrochromic material, and that oxygen‐vacancy creation is more effective than aliovalent substitutional doping to introduce dual‐band properties to TiO 2 NCs. Oxygen vacancies not only confer good near‐infrared (NIR)‐selective modulation, but also improve the Li + diffusion in the TiO 2‐ x host, circumventing the disadvantage of aliovalent substitutional doping with ion diffusion. Consequently optimized TiO 2‐ x NC films are able to modulate the NIR and visible light transmittance independently and effectively in three distinct modes with high optical modulation (95.5% at 633 nm and 90.5% at 1200 nm), fast switching speed, high bistability, and long cycle life. An impressive dual‐band electrochromic performance is also demonstrated in prototype devices. The use of TiO 2‐ x NCs enables the assembled windows to recycle a large fraction of energy consumed in the coloration process (“energy recycling”) to reduce the energy consumption in a round‐trip electrochromic operation.

Abstract Comparing the overpotential required to reach the current density of (per geometric area of an electrode) (η@ ) is a popular method of ranking electrocatalysts for water‐splitting reactions, i.e., the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). However, such methodology is, in essence, not reasonable for studying the intrinsic chemistry difference of electrocatalysts. To have a rational understanding of η@ , here, its historical origin and its limitations on reflecting the intrinsic electrocatalytic activity are discussed. The η@ is a valid practical parameter to assess water‐splitting devices, but it cannot determine whether a particular electrocatalyst is intrinsically active or not.

Electrochemical conversion of CO₂has attracted attention worldwide since this process can convert carbon dioxide to a wide range of value-added chemicals. This reaction required the development of efficient electrocatalysts and fundamental understanding of the reaction kinetics and thermodynamics to overcome the current challenges.

Abstract A rational design for oxygen evolution reaction (OER) catalysts is pivotal to the overall efficiency of water electrolysis. Much work has been devoted to understanding cation leaching and surface reconstruction of very active electrocatalysts, but little on intentionally promoting the surface in a controlled fashion. We now report controllable anodic leaching of Cr in CoCr 2 O 4 by activating the pristine material at high potential, which enables the transformation of inactive spinel CoCr 2 O 4 into a highly active catalyst. The depletion of Cr and consumption of lattice oxygen facilitate surface defects and oxygen vacancies, exposing Co species to reconstruct into active Co oxyhydroxides differ from CoOOH. A novel mechanism with the evolution of tetrahedrally coordinated surface cation into octahedral configuration via non‐concerted proton‐electron transfer is proposed. This work shows the importance of controlled anodic potential in modifying the surface chemistry of electrocatalysts.

Multi-atom cluster catalysts have turned out to be novel heterogeneous catalysts with atomic dispersion for electrochemical energy applications. Beyond a simple combination of single-atom catalysts, they could offer boosted activity as a result of the synergistic effects between adjacent atoms. Meanwhile, the multiple active sites in the catalytic center may render them versatile binding modes toward adsorbates and provide an opportunity for catalyzing complex reactions with diverse products. Herein, a comprehensive review of the recent development of multi-atom cluster catalysts for electrochemical energy applications is provided. Specifically, the origin of synergistic effects in multi-atom cluster catalysts and related modulation methods are illustrated and summarized. The introduction of multi-atom cluster catalysts to circumvent the scaling relationships as well as their potential for developing new descriptors is then discussed. Subsequently, the methods for fabricating multi-atom cluster catalysts and related characterization techniques are reviewed. This is followed by the discussion of their application in key electrochemical reactions such as water splitting, oxygen reduction, and carbon dioxide/monoxide reduction, as well as the real-time techniques for their mechanistic study. Finally, the future challenges and opportunities concerning the improvement of multi-atom cluster catalysts are outlined, which are essential to make such electrocatalysts viable for electrochemical energy conversion.

Origins of solvent-induced enhancement in catalytic reactivity and product selectivity are discussed with computational methods to study them.

To investigate metal oxide surface catalysis, determining an appropriate Hubbard U-correction term is a challenge for the density functional theory (DFT) community and identifying realistic reaction intermediates and their corresponding X-ray photoelectron spectroscopy (XPS) shifts is a challenge for experimental researchers, when these methods are used independently. In this study, using CuO as a model transition metal oxide, we demonstrate that when DFT and XPS are applied synergistically, the determination of the U value and the identification of adsorbate/intermediate species on the surface (and their XPS shifts) can be done simultaneously. The experimental O 1s spectra of the as-synthesized CuO 2D-nanoleaves shows the presence of four different peaks with core level binding energies (CLBEs) of 529.7, 531.4, 533.2, and 534.6 eV. DFT is used to calculate the CLBE shifts for probable adsorbed moieties, in various adsorption configurations, on both, clean and vacancy defect containing surfaces. Comparison of experimental and theoretical CLBEs across the entire U value range of 0–9 eV narrows down the list to only four moieties, namely, O2 in the η1(O) configuration, H2O at the surface oxygen vacancy site, and adsorbed HCO3 and HCO2 (resembling adsorbed HCO3). Finally, the U value of 4–4.5 eV reproduces the experimental CLBE shifts correctly and thus, establishes these experimental XPS spectral peaks to the adsorbates and their geometries. The integrated approach elucidated in this article, results in the identification of adsorbates/intermediates (and their CLBEs) for the experimental XPS spectral analysis and the determination of an appropriate U value concurrently, to study metal oxide surface catalysis.

characterization.

Assessing the impact of digital technologies and artificial intelligence, so-called intelligent cyber-physical systems, on emission reduction in the critical sector of energy provision.

TiO2 has gained tremendous attention as a cutting-edge material for application in photocatalysis. The performance of TiO2 as a photocatalyst depends on various parameters including morphology, surface area, and crystallinity. Although TiO2 has shown good catalytic activity in various catalysis systems, the performance of TiO2 as a photocatalyst is generally limited due to its low conductivity and a wide optical bandgap. Numerous different studies have been devoted to overcome these problems, showing significant improvement in photocatalytic performance. In this study, we summarize the recent progress in the utilization of TiO2 for the photocatalytic hydrogen evolution reaction (HER). Strategies for modulating the properties toward the high photocatalytic activity of TiO2 for HER including structural engineering, compositional engineering, and doping are highlighted and discussed. The advantages and limitations of each modification approach are reviewed. Finally, the remaining obstacles and perspective for the development of TiO2 as photocatalysts toward high efficient HER in the near future are also provided.