Mechanistic Understanding of Water Oxidation in the Presence of a Copper Complex by In Situ Electrochemical Liquid Transmission Electron Microscopy

Direct in-situ imaging of electrochemical corrosion of Pd-Pt core-shell electrocatalysts

Corrosion of electrocatalysts during electrochemical operations, such as low potential - high potential cyclic swapping, can cause significant performance degradation. However, the electrochemical corrosion dynamics, including structural changes, especially site and composition specific ones, and their correlation with electrochemical processes are hidden due to the insufficient spatial-temporal resolution characterization methods. Using electrochemical liquid cell transmission electron microscopy, we visualize the electrochemical corrosion of Pd@Pt core-shell octahedral nanoparticles towards a Pt nanoframe. The potential-dependent surface reconstruction during multiple continuous in-situ cyclic voltammetry with clear redox peaks is captured, revealing an etching and deposition process of Pd that results in internal Pd atoms being relocated to external surface, followed by subsequent preferential corrosion of Pt (111) terraces rather than the edges or corners, simultaneously capturing the structure evolution also allows to attribute the site-specific Pt and Pd atomic dynamics to individual oxidation and reduction events. This work provides profound insights into the surface reconstruction of nanoparticles during complex electrochemical processes.

Operando Electron Microscopy of Catalysts: The Missing Cornerstone in Heterogeneous Catalysis Research?

Heterogeneous catalysis in thermal gas-phase and electrochemical liquid-phase chemical conversion plays an important role in our modern energy landscape. However, many of the structural features that drive efficient chemical energy conversion are still unknown. These features are, in general, highly distinct on the local scale and lack translational symmetry, and thus, they are difficult to capture without the required spatial and temporal resolution. Correlating these structures to their function will, conversely, allow us to disentangle irrelevant and relevant features, explore the entanglement of different local structures, and provide us with the necessary understanding to tailor novel catalyst systems with improved productivity. This critical review provides a summary of the still immature field of operando electron microscopy for thermal gas-phase and electrochemical liquid-phase reactions. It focuses on the complexity of investigating catalytic reactions and catalysts, progress in the field, and analysis. The forthcoming advances are discussed in view of correlative techniques, artificial intelligence in analysis, and novel reactor designs.

Observing the Evolution of Metal Oxides in Liquids

Metal oxides with diverse compositions and structures have garnered considerable interest from researchers in various reactions, which benefits from transmission electron microscopy (TEM) in determining their morphologies, phase, structural and chemical information. Recent breakthroughs have made liquid-phase TEM a promising imaging platform for tracking the dynamic structure, morphology, and composition evolution of metal oxides in solution under work conditions. Herein, this review introduces the recent advances in liquid cells, especially closed liquid cell chips. Subsequently, the recent progress including particle growth, phase transformation, self-assembly, core-shell nanostructure growth, and chemical etching are introduced. With the late technical advances in TEM and liquid cells, liquid-phase TEM is used to characterize many fundamental processes of metal oxides for CO2 reduction and water-splitting reactions. Finally, the outlook and challenges in this research field are discussed. It is believed this compilation inspires and stimulates more efforts in developing and utilizing in situ liquid-phase TEM for metal oxides at the atomic scale for different applications.

Oxygen-evolution reaction in the presence of cerium(IV) ammonium nitrate and iron (hydr)oxide: old system, new findings

Solar fuel production by photosynthetic systems strongly relies on developing efficient and stable oxygen-evolution catalysts (OECs). Cerium(IV) ammonium nitrate (CAN) has been the most commonly used sacrificial oxidant to investigate OECs. Although many metal oxides have been extensively investigated as OECs in the presence of CAN, mechanistic studies were rarely reported. Herein, first, Fe(III) (hydr)oxide (FeOxHy) was prepared by the reaction of Fe(ClO4)3 and KOH solution and characterized by some methods. Then, changes in Fe oxide in the presence of CAN during the OER were tracked using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, in situ visible spectroscopy, and in situ electron paramagnetic resonance spectroscopy. FeOxHy in the presence of CAN and during the OER converted to γ-Fe2O3 and [Fe(H2O)6]3+, and a small amount of oxygen was formed. A maximum turnover frequency and turnover number of 10-6 s-1 and 1.3 × 10-3 mol(O2)/mol(Fe) (for half an hour) in the presence of CAN (0.20 M) and FeOxHy were observed.

Rhenium-Based Electrocatalysts for Water Splitting

Due to the contamination and global warming problems, it is necessary to search for alternative environmentally friendly energy sources. In this area, hydrogen is a promising alternative. Hydrogen is even more promising, when it is obtained through water electrolysis operated with renewable energy sources. Among the possible devices to perform electrolysis, proton exchange membrane (PEM) electrolyzers appear as the most promising commercial systems for hydrogen production in the coming years. However, their massification is affected by the noble metals used as electrocatalysts in their electrodes, with high commercial value: Pt at the cathode where the hydrogen evolution reaction occurs (HER) and Ru/Ir at the anode where the oxygen evolution reaction (OER) happens. Therefore, to take full advantage of the PEM technology for green H2 production and build up a mature PEM market, it is imperative to search for more abundant, cheaper, and stable catalysts, reaching the highest possible activities at the lowest overpotential with the longest stability under the harsh acidic conditions of a PEM. In the search for new electrocatalysts and considering the predictions of a Trasatti volcano plot, rhenium appears to be a promising candidate for HER in acidic media. At the same time, recent studies provide evidence of its potential as an OER catalyst. However, some of these reports have focused on chemical and photochemical water splitting and have not always considered acidic media. This review summarizes rhenium-based electrocatalysts for water splitting under acidic conditions: i.e., potential candidates as cathode materials. In the various sections, we review the mechanism concepts of electrocatalysis, evaluation methods, and the different rhenium-based materials applied for the HER in acidic media. As rhenium is less common for the OER, we included a section about its use in chemical and photochemical water oxidation and as an electrocatalyst under basic conditions. Finally, concluding remarks and perspectives are given about rhenium for water splitting.

Water Oxidation by a Copper(II) Complex with 6,6'-Dihydroxy-2,2'-Bipyridine Ligand: Challenges and an Alternative Mechanism

Recently, copper(II) complexes have been extensively investigated as oxygen-evolution reaction (OER) catalysts through a water-oxidation reaction. Herein, new findings regarding OER in the presence of a Cu(II) complex with 6,6'-dihydroxy-2,2'-bipyridine ligand are reported. Using scanning electron microscopy, energy dispersive spectrometry, X-ray diffraction, Raman spectroscopy, in situ visible microscopy, in situ visible spectroelectrochemistry, X-ray absorption spectroscopy, and electrochemistry, it is hypothesized that the film formed on the electrode's surface in the presence of this complex causes an appropriated matrix to produce Cu (hydr)oxide. The resulting Cu (hydr)oxide could be a candidate for OER catalysis. The formed film could form Cu (hydr)oxide and stabilize it. Thus, OER activity increases in the presence of this complex.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

Atomic-level flatness on oxygen-free copper surface in lapping and chemical mechanical polishing

Oxygen-free copper (OFC) serves as a core component of high-end manufacturing, and requires high surface quality. It is always a significant challenge to manufacture high-quality atomic-level surfaces. In this study, SiO₂ nanospheres with good dispersibility were prepared and a late-model environmentally friendly chemical mechanical polishing (CMP) slurry was developed. The CMP slurry consists of SiO₂ nanospheres, CeO₂ nanospheres, H₂O₂, NaHCO₃, polyaspartic acid and deionized water. After CMP, the average roughness (Sa) of the OFC wafer reached 0.092 nm with an area of 50 × 50 μm². Atomic-level flatness on the oxygen-free copper surface was acquired, which has never been reported before. Moreover, the mechanical removal mechanism of abrasive particles and the chemical reactions during lapping and CMP are proposed in detail. The thickness and composition of the damaged layer after lapping and polishing were analyzed. The lapping-damaged layer consists of a lattice distortion region, moiré fringes, grain boundary, superlattice and edge dislocations, and the polishing-damaged layer contains a handful of stacking faults with single-layer or multi-layer atoms. The chemical action involves three reactions: oxidation, corrosion and chelation. The processing method and its mechanistic explanation pave the way for the fabrication of high-performance OFC surfaces for use in vacuum, aerospace, military and electronic industries.

Metal-Organic Framework-Based Nanomaterials for Electrocatalytic Oxygen Evolution

Oxygen evolution reaction (OER) is an energy-determined half-reaction for water splitting and many other energy conversion processes, such as rechargeable metal-air batteries and CO₂ reduction, due to its four-electron sluggish process. To reduce the energy consumption and cost of these advanced technologies, various transition metal-based nanomaterials, like metal oxides/hydroxides, nitride, and phosphide are synthesized. Among these, metal-organic framework (MOF)-based materials are considered as the ideal candidate for the fabrication of efficient OER electrocatalysts owing to their unique physicochemical properties. In this review, the fundamental catalytic mechanisms and key evaluation parameters of OER in acidic and alkaline media are presented first. Then, design strategies for MOF-based OER catalysts and research progress in the study of the structure-performance relationship are summarized. Subsequently, the recent research advances of MOF-based OER electrocatalysts in alkaline, acidic, and neutral electrolytes are overviewed. Finally, current challenges and future opportunities are provided under the frame of materials design, theoretical understanding, advanced characterization techniques, and industrial applications.

Homoleptic Ni(II) dithiocarbamate complexes as pre-catalysts for the electrocatalytic oxygen evolution reaction

Four new functionalized Ni(II) dithiocarbamate complexes of the formula [Ni(L_x)₂] (1-4) (L₁ = N-methylthiophene-N-3-pyridylmethyl dithiocarbamate, L₂ = N-methylthiophene-N-4-pyridylmethyl dithiocarbamate, L₃ = N-benzyl-N-3-pyridylmethyl dithiocarbamate, and L₄ = N-benzyl-N-4-pyridylmethyl dithiocarbamate) have been synthesized and characterized by IR, UV-vis, and ¹H and ¹³C{¹H} NMR spectroscopic techniques. The solid-state structure of complex 1 has also been determined by single crystal X-ray crystallography. Single crystal X-ray analysis revealed a monomeric centrosymmetric structure for complex 1 in which two dithiocarbamate ligands are bonded to the Ni(II) metal ion in a S^S chelating mode resulting in a square planar geometry around the nickel center. These complexes are immobilized on activated carbon cloth (CC) and their electrocatalytic performances for the oxygen evolution reaction (OER) have been investigated in aqueous alkaline solution. All the complexes act as pre-catalysts for the OER and undergo electrochemical anodic activation to form Ni(O)OH active catalysts. Spectroscopic and electrochemical characterization revealed the existence of the interface of molecular complex/Ni(O)OH, which acts as the real catalyst for the OER. The active catalyst obtained from complex 2 showed the best OER activity achieving 10 mA cm^-2 current density at an overpotential of 330 mV in 1.0 M aqueous KOH solution.

A copper(II) coordination compound under water-oxidation reaction at neutral conditions: decomposition on the counter electrode

In the context of energy storage, the oxygen-evolution reaction (OER, 2H₂O → O₂ + 4H⁺ + 4e^-) through the water-oxidation reaction is a thermodynamically uphill reaction in overall water splitting. In recent years, copper(II) coordination compounds have been extensively used for the OER. However, challenges remain in finding the mechanism of the OER in the presence of these metal coordination compounds. Herein, the electrochemical OER activity is investigated in the presence of a copper(II) coordination compound at pH ≈ 7. While the investigations on finding true catalysts for the OER are focused on the working electrode, herein, for the first time, the focus is on the decomposition of copper(II) coordination compound (CuL₃, L: 2,2'-bipyridine N,N'-dioxide) during the OER on the counter electrode toward the precipitation of copper(I) oxide and metallic Cu.

Catalysis of the Water Oxidation Reaction in the Presence of Iron and a Copper Foil

The oxygen evolution reaction (OER) can provide electrons for reducing water, carbon dioxide, and ammonia. On the other hand, copper compounds are among the most interesting OER catalysts. In this study, water oxidation of a Cu foil in the presence of K₂FeO₄, a soluble Fe source, under alkaline conditions (pH ≈ 13) is investigated using electrochemical methods, X-ray diffraction, X-ray photoelectron spectroscopy, in situ visible spectroelectrochemistry, Raman spectroscopy, and scanning electron microscopy. After the reaction of the Fe salt with the Cu foil, a remarkable improvement for OER is recorded, which indicates that either the Fe ions on the copper foil directly participate in OER or these ions are critical for activating copper ions on the surface toward OER. Indeed, a remarkable decrease (130 mV) in the overpotential is recorded for the Cu foil in the presence of [FeO₄]^2-. Tafel slopes for the Cu foil in the absence and presence of K₂FeO₄ are 113.2 and 46.4 mV/decade, respectively. X-ray photoelectron spectroscopy shows that there is a strong interaction between Cu(II) and Fe(III) on the surface of the Cu foil. During OER in the presence of Cu(II) (hydr)oxide, Cu(III) is detected. In situ visible spectroelectrochemistry shows that Cu and Fe ions are dynamically active and precipitate on the surface of the counter electrode during cyclic voltammetry (CV). The isotopic experimental data using H₂¹⁸O based on Raman spectroscopy show that there is no change in the lattice oxygen. All of these experiments adopt a new perspective on the role of Fe in OER in the presence of a Cu foil under alkaline conditions.

Strategies for Electrochemically Sustainable H2 Production in Acid

Acidified water electrolysis with fast kinetics is widely regarded as a promising option for producing H₂ . The main challenge of this technique is the difficulty in realizing sustainable H₂ production (SHP) because of the poor stability of most electrode catalysts, especially on the anode side, under strongly acidic and highly polarized electrochemical environments, which leads to surface corrosion and performance degradation. Research efforts focused on tuning the atomic/nano structures of catalysts have been made to address this stability issue, with only limited effectiveness because of inevitable catalyst degradation. A systems approach considering reaction types and system configurations/operations may provide innovative viewpoints and strategies for SHP, although these aspects have been overlooked thus far. This review provides an overview of acidified water electrolysis for systematic investigations of these aspects to achieve SHP. First, the fundamental principles of SHP are discussed. Then, recent advances on design of stable electrode materials are examined, and several new strategies for SHP are proposed, including fabrication of symmetrical heterogeneous electrolysis system and fluid homogeneous electrolysis system, as well as decoupling/hybrid-governed sustainability. Finally, remaining challenges and corresponding opportunities are outlined to stimulate endeavors toward the development of advanced acidified water electrolysis techniques for SHP.

Efficient homogeneous electrochemical water oxidation by a copper(ii) complex with a hexaaza macrotricyclic ligand

A copper complex [CuII(L)](ClO4)2 with a hexaaza macrotricyclic ligand is found to be an efficient homogeneous electrocatalyst for water oxidation with onset overpotential of 480 mV and a turnover frequency of 3.65 s−1.

Homogeneous or heterogeneous electrocatalysis: reinvestigation of a cobalt coordination compound for water oxidation

A cobalt coordination compound with azo-ligand linkers combined with linked bisulfonate moieties has been argued to be an efficient catalyst for the oxygen-evolution reaction (OER) (H.-T. Shi, X.-X. Li, F.-H. Wu and W.-B. Yu, Dalton Trans., 2017, 46, 16321.). In the previously published report, this cobalt compound (compound 1) was believed to display a high turnover frequency (5 s^-1) at η = 720 mV at pH 9. Herein, the OER in the presence of compound 1 is reinvestigated. The nanosized oxide-based particles formed after the OER in the presence of compound 1 were tracked by electrochemical methods, scanning electron microscopy (SEM), energy dispersive spectrometry (EDX), X-ray diffraction studies (XRD), (High-resolution) transmission electron microscopy ((HR)TEM), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). Based on these experiments, it is proposed that a candidate for the true catalyst of the OER in the presence of compound 1 is cobalt oxide. During the OER and using chronoamperometry, the oxidation state of Co ions for the formed Co oxide is (III), but after consecutive CVs the oxidation states of Co ions for the formed Co oxide are (II) and (III). The results shed new light on the role of Co oxide nanoparticles formed in the presence of this Co coordination compound during the OER. Our experimental data also show that for the OER in the presence of a homogeneous (pre)catalyst, careful analyses to find the role of metal oxides are necessary for informed progress. The present findings also might help to find the mechanism of the OER in the presence of coordination compounds.