Electrochemical Energy Applications Research Articles

Work Function-Guided Electrocatalyst Design.

The development of high-performance electrocatalysts for energy conversion reactions is crucial for advancing global energy sustainability. The design of catalysts based on their electronic properties (e.g., work function) has gained significant attention recently. Although numerous reviews on electrocatalysis have been provided, no such reports on work function-guided electrocatalyst design are available. Herein, a comprehensive summary of the latest advancements in work function-guided electrocatalyst design for diverse electrochemical energy applications is provided. This includes the development of work function-based catalytic activity descriptors, and the design of both monolithic and heterostructural catalysts. The measurement of work function is first discussed and the applications of work function-based catalytic activity descriptors for various reactions are fully analyzed. Subsequently, the work function-regulated material-electrolyte interfacial electron transfer (IET) is employed for monolithic catalyst design, and methods for regulating the work function and optimizing the catalytic performance of catalysts are discussed. In addition, key strategies for tuning the work function-governed material-material IET in heterostructural catalyst design are examined. Finally, perspectives on work function determination, work function-based activity descriptors, and catalyst design are put forward to guide future research. This work paves the way to the work function-guided rational design of efficient electrocatalysts for sustainable energy applications.

2D Conductive Metal–Organic Frameworks for Electrochemical Energy Application

Two-dimensional conductive metal–organic frameworks (2D c-MOFs) have attracted research attention, benefitting from their unique properties such as superior electronic conductivity, designable topologies, and well-defined catalytic/redox-active sites. These advantages enable 2D c-MOFs as promising candidates in electrochemical energy applications, including supercapacitors, batteries and electrocatalysts. This mini-review mainly highlights recent advancements of 2D c-MOFs in the utilization for electrochemical energy storage, as well as the forward-looking perspective on the future prospects of 2D c-MOFs in the field of electrochemical energy.Table of content:1 Introduction2 Design Principles of 2D c-MOFs3 Synthesis of 2D c-MOFs4 2D c-MOFs for Electrochemical Energy Storage4.1 Supercapacitors4.2 Metallic Batteries4.2.1 Lithium-Ion Batteries4.2.2 Sodium-Ion Batteries4.2.3 Zinc-Ion Batteries4.2.4 Sodium–Iodine Batteries4.2.5 Lithium–Sulfur Batteries4.2.6 Potassium-Ion Batteries5 2D c-MOFs for Electrochemical Energy Conversion6 Conclusions and Outlook

Investigating Electrode-Ionomer Interface Phenomena for Electrochemical Energy Applications.

The endeavor to develop high-performance electrochemical energy applications has underscored the growing importance of comprehending the intricate dynamics within an electrode's structure and their influence on overall performance. This review investigates the complexities of electrode-ionomer interactions, which play a critical role in optimizing electrochemical reactions. Our examination encompasses both microscopic and meso/macro scale functions of ionomers at the electrode-ionomer interface, providing a thorough analysis of how these interactions can either enhance or impede surface reactions. Furthermore, this review explores the broader-scale implications of ionomer distribution within porous electrodes, taking into account factors like ionomer types, electrode ink formulation, and carbon support interactions. We also present and evaluate state-of-the-art techniques for investigating ionomer distribution, including electrochemical methods, imaging, modeling, and analytical techniques. Finally, the performance implications of these phenomena are discussed in the context of energy conversion devices. Through this comprehensive exploration of intricate interactions, this review contributes to the ongoing advancements in the field of energy research, ultimately facilitating the design and development of more efficient and sustainable energy devices.

Impact of Pt(hkl) Electrode Surface Structure on the Electrical Double Layer Capacitance.

The classical theory of the electrical double layer (EDL) does not consider the effects of the electrode surface structure on the EDL properties. Moreover, the best agreement between the traditional EDL theory and experiments has been achieved so far only for a very limited number of ideal systems, such as liquid metal mercury electrodes, for which it is challenging to operate with specific surface structures. In the case of solid electrodes, the predictive power of classical theory is often not acceptable for electrochemical energy applications, e.g., in supercapacitors, due to the effects of surface structure, electrode composition, and complex electrolyte contributions. In this work, we combine ab initio molecular dynamics (AIMD) simulations and electrochemical experiments to elucidate the relationship between the structure of Pt(hkl) surfaces and the double-layer capacitance as a key property of the EDL. Flat, stepped, and kinked Pt single crystal facets in contact with acidic HClO4 media are selected as our model systems. We demonstrate that introducing specific defects, such as steps, can substantially reduce the EDL capacitances close to the potential of zero charge (PZC). Our AIMD simulations reveal that different Pt facets are characterized by different net orientations of the water dipole moment at the interface. That allows us to rationalize the experimentally measured (inverse) volcano-shaped capacitance as a function of the surface step density.

N-Doped Graphene (N-G)/MOF(ZIF-8)-Based/Derived Materials for Electrochemical Energy Applications: Synthesis, Characteristics, and Functionality

In recent years, graphene-type materials originating from metal–organic frameworks (MOFs) or integrated with MOFs have exhibited notable performances across various applications. However, a comprehensive understanding of these complex materials and their functionalities remains obscure. While some studies have reviewed graphene/MOF composites from different perspectives, due to their structural–functional intricacies, it is crucial to conduct more in-depth reviews focusing on specific sets of graphene/MOF composites designed for particular applications. In this review, we thoroughly investigate the syntheses, characteristics, and performances of N-G/MOF(ZIF-8)-based/derived materials employed in electrochemical energy conversion and storage systems. Special attention is given to realizing their fundamental functionalities. The discussions are divided into three segments based on the application of N-G/ZIF-8-based/derived materials as electrode materials for batteries, electrodes for electrochemical capacitors, and electrocatalysts. As electrodes for batteries, N-G/MOF(ZIF-8) materials can mitigate issues like an electrode volume expansion for Li-ion batteries and the ‘shuttle effect’ for Li-S batteries. As electrodes for electrochemical capacitors, these materials can considerably improve the ion transfer rate and electronic conductivity, thereby enhancing the specific capacitance while maintaining the structural stability. Also, it was observed that these materials could occasionally outperform standard platinum-based catalysts for the electrochemical oxygen reduction reaction (ORR). The reported electrochemical performances and structural parameters of these materials were carefully tabulated in uniform units and scales. Through a critical analysis of the present synthesis trends, characteristics, and functionalities of these materials, specific aspects were identified that required further exploration to fully utilize their inherent capabilities.

Single rhodium atom embedded two dimensional MoSi2N4: A promising electrocatalyst for oxygen reduction reaction

It is essential to develop electrocatalysts that exhibit excellent performance in the oxygen reduction reaction (ORR) for electrochemical energy applications. Recently, MoSi2N4 monolayer has been synthesized and attracted widespread interest. Herein, we constructed a series of structures by embedding transition metal (TM = Fe, Co, Ni, Cu, Ru, Rh, Pd, Ir, Pt and Au) atom into MoSi2N4 with N vacancy (VN-MSN), denoted as TM@VN-MSN, and investigated their ORR activity systematically by first-principles calculations. It is found that TM@VN-MSN systems are stable and there is charge transfer between TM atom and MoSi2N4. After screening, Rh@VN-MSN has the best ORR catalytic activity with an overpotential of 0.35 V, even lower than that of classical Pt (111). Furthermore, an implicit solvation model is used to evaluate the solvent effect. The volcano plots and scaling relations are established based on free energies of intermediates, and verified ΔG*OH can use as a descriptor to explore the ORR activity on TM@VN-MSN. Moreover, d-band center theory and crystal orbital Hamilton populations (COHP) are used to reveal the ORR activity origin. This work not only provide a promising method for designing a new type of MoSi2N4-based ORR catalysts, but also expands its application in the field of electrocatalysis.

Poly(arylene alkylene piperidinium)s for Anion Exchange Membrane Water Electrolyzers

Anion exchange membrane (AEM) water electrolyzers have the potential to obviate the need for precious metal catalysts, and are therefore attracting an increasing attention.1 The high performance and durability of the device largely depend on the properties of the AEM with regards to hydroxide conductivity and resistance.2 ,3 The development of viable AEM candidates is continuously advancing. Recently, we reported on a series of poly(arylene alkylene piperidinium)s (PAAs) prepared from a ketone monomer specifically designed for high alkaline stability.4 The PAA AEMs reached high hydroxide conductivities, comparable with the state-of-the-art poly(arylene piperidinium)-based AEMs derived from piperidione, but showed significantly higher chemical stability under harsh alkaline conditions.5 In the present study, we have further developed PAA AEMs by copolymerizations to tune the ion exchange capacity (IEC). An AEM (PpTDMP-2.0) with an IEC of 2.0 mequiv.g−1 reached a high OH− conductivity of 129 mS cm−1 at 80 °C and a controlled water uptake of 80%. This AEM was selected for evaluated in an alkaline water electrolyzer fed with 1 M KOH at 70 °C. Using a novel noble-metal free NiFe layered double hydroxide (LDH) catalyst in the anode, initial results showed a high current density of 4.5 A cm−2 at 2.2 V. The corresponding performance of the state-of the-art Aemion+® membrane was measured for benchmarking, and the PAP AEM demonstrated a lower degradation rate from the beginning of the experiment. In the current presentation, we will further discuss the performance, system stability, and the prospects of PAA AEMs for use in electrolyzers and other electrochemical energy applications. C. Santoro, A. Lavacchi, P. Mustarelli, V. Di Noto, L. Elbaz, et al., ChemSusChem, 2022, 15, e202200027.D. Aili, M. R. Kraglund, S. C. Rajappan, D. Serhiichuk, Y. Xia, V. Deimede, J. Kallitsis, C. Bae, P. Jannasch, D. Henkensmeier, J. O. Jensen, ACS Energy Lett., 2023, 8, 1900–1910.E. J. Park, C. G. Arges, H. Xu and Y. S. Kim, ACS Energy Lett., 2022, 7, 3447-3457.D. Pan, P. M. Bakvand, T. H. Pham and P. Jannasch, J. Mater. Chem. A, 2022, 10, 16478-16489.J. S. Olsson, T. H. Pham and P. Jannasch, Adv. Funct. Mater., 2018, 28, 1702758. Figure 1. (a) Synthetic route and chemical structure of PpTDMP-2.0, (b) polarization curves of PpTDMP-2.0 and Aemion+ (left) after a constant current hold of 1 A cm-2 for 15 h (right). 5 cm² active area, 70 °C, 1 M KOH. Anode: 2.0 mgNiFe-LDH cm-2, Cathode 0.5 mgPt cm-2. Figure 1

Palladium phosphoselenide (PdPSe) – Reduced graphene oxide composite: A tri-functional electrocatalyst and a cathode catalyst for alkaline fuel cells

Developing a new and efficient electrocatalyst is highly desirable for fundamental electrochemical energy applications such as fuel cells. In this work, we have introduced palladium phosphoselenide (PdPSe), a layer-type ternary material, as a tri-functional electrocatalyst in the three most fundamental electrochemical reactions, such as oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and oxygen evolution reaction (OER). To enhance the electrocatalytic activity and stability of the electrode material, reduced graphene oxide (rGO) is incorporated into the system by making a composite (rGO-PdPSe), which effectively reduces the overpotentials and increases the current densities for HER and OER. Moreover, in ORR, it turns into a four-electron pathway with comparable kinetics. Further, the composite material is applied in anion exchange membrane fuel cell application as a cathode catalyst. This composite's facile kinetics and robust performance suggests that ternary phosphoselenides based trifunctional electrocatalysts would be a promising material for large-scale application of clean energy conversion devices.

(Invited) Transforming Metal-Containing Secondary Resources into Functional Materials for Electrochemical Energy Applications

Transforming metal-containing waste into functional materials for electrochemical energy applications presents an innovative and sustainable solution to address the growing issue of waste management and resource conservation. By utilizing end-of-life products, off-specification materials, waste materials, and spent materials as secondary resources, this approach can effectively reduce environmental impacts and promote the development of clean energy technologies.Gas-diffusion electrocrystallization (GDEx) is a novel electrochemical technology that can be used as a step to selectively recover and convert valuable metals from metal-containing waste streams into functional materials for electrochemical energy applications. By leveraging the electrochemical reduction of gases, such as O2 or CO2, for the in-situ production of oxidizing (H2O2) or reducing agents (H2 and CO)—which may also result in the formation of other chemical species, e.g., alkali (OH–) in aqueous media, GDEx facilitates the precipitation of soluble metal precursors. This process results in the formation of metal nanoparticles with controlled compositions, crystal structures, sizes, morphologies, and compositions, tailored for specific electrochemical applications.In this talk, the principles of the GDEx process will be explained and examples will be provided of its applicability in the direct transformation of metals contained in secondary resources for the production of functional materials for electrochemical applications. Specifically, the production of electrocatalysts for methanol oxidation from the recycling of automotive catalysts leachates will be conveyed, as well as the production of active battery materials for Li-ion batteries, and the recovery of lithium from geothermal brines.Gas Diffusion Electrocrystallization (GDEx) is a novel and sustainable approach to transform waste materials into functional materials for electrochemical energy applications. This innovative method enables the recovery and synthesis of valuable metal nanoparticles (NPs) from waste streams, such as end-of-life products and industrial leachates, for use in electrocatalysis, energy storage, and fuel cells.GDEx will be conveyed as a sustainable and scalable technology for the recovery of critical metals, contributing to a circular economy and reducing the environmental impact of mining and refining activities. The opportunity for producing materials with enhanced performance will be portrayed, together with the portent of a customizable and flexible solution which can be integrated with existing recycling chains.AcknowledgementThis research has received funding from the European Union’s Horizon 2020 and Horizon Europe research and innovation programmes under grant agreements 958302 (PEACOC Project), 101069685 (Rhinoceros Project), 101069644 (LICORNE Project), 101058163 (CRM-Geothermal Project), and 101091715 (FIREFLY Project).ReferencesPrato M.R.A., Van Vught V., Chayambuka K., Pozo G., Eggermont S., Fransaer J., Dominguez-Benetton X. (2020) Synthesis of material libraries using gas diffusion electrodes. J. Mater. Chem. A, 8:11674-11686. DOI:10.1039/D0TA00633E.Martinez Mora O., Pozo G., Leon-Fernandez L.F., Fransaer J., Dominguez-Benetton X. (2023) Synthesis of platinum group metal nanoparticles assisted by CO2 reduction and H2 cogeneration at gas-diffusion electrodes. RSC Sustain., 2023, Accepted Manuscript. DOI: 10.1039/D3SU00046J

Nanocellulose as a Sustainable Polymer Electrolyte

Polymer electrolyte membranes in fuel cells and electrolyzers will play a key role in the hydrogen economy. These membranes are generally made of sulfonated fluoropolymers, which have excellent proton conductivity, impressive redox stability, and good gas barrier properties. However, the complex synthetic processes mean that they remain expensive. Their synthesis also involves ecologically damaging perfluorosulfonic acids (PFSAs), otherwise known as “forever chemicals” due to their persistence in the environment. More sustainable and lower cost hydrocarbon electrolytes should therefore be investigated.Cellulose is the most abundant biopolymer in nature, and is derived from plant matter. It is used at vast scale and low cost to make paper and cardboard. Cellulose can be chemically or mechanically treated to break up the fibers to form “nanocellulose”. Nanocellulose is a low-cost nanomaterial which can be formed into thin membranes which are much stronger than conventional cellulosic paper.We propose nanocellulose as an alternative electrolyte to replace sulfonated fluoropolymers in fuel cells. We confirmed that unmodified nanocellulose can conduct protons due to acidic hydroxyl groups. We also showed that nanocellulose membranes present a much better barrier to hydrogen compared to Nafion, and can successfully be used as an electrolyte in fuel cells. However, the current-voltage performance of unmodified nanocellulose was insufficient due to a high membrane resistance.We went on to improve the proton conductivity by introducing sulfonic acid groups and reduced the membrane resistance by using spray deposition to create thinner membranes, resulting in significantly improved fuel cell power density of 160 mW/cm2. We also implemented crosslinking with sulfosuccinic acid to simultaneously improve the ionic conductivity, increase strength, and reduce swelling in water. This work shows that nanocellulose is a promising new class of polymer electrolyte which could significantly reduce the cost and improve sustainability of fuel cell and electrolyser systems.1. Cellulose Nanocrystals Crosslinked with Sulfonic Acid as Sustainable Proton Exchange Membranes for Electrochemical Energy Applications, O. Selyanchyn, T. Bayer, D. Klotz, R. Selyanchyn, K. Sasaki, S. M. Lyth, Membranes 12 (7), 658 (2022)2. Spray Deposition of Sulfonated Cellulose Nanofibers as Proton Conducting Membranes for Fuel Cells, T. Bayer, B. Cunning, R. Selyanchyn, B. Smid, S. Fujikawa, K. Sasaki, S. M. Lyth, Cellulose, 28, 1355–1367 (2021)3. Proton Conductivity in Cellulosic Materials: A Review, O. Selyanchyn, R. Selyanchyn, S. M. Lyth, Frontiers in Energy Research: Fuel Cells, 8, 596164 (2020)4. High Temperature Proton Conductivity in Nanocellulose: Paper Fuel Cells, T. Bayer, R. Selyanchyn, S. Fujikawa, K. Sasaki, and S. M. Lyth, Chemistry of Materials, 28 (13), 4805–4814 (2016)

Recent Progress of Vertical Graphene: Preparation, Structure Engineering, and Emerging Energy Applications.

Vertical graphene (VG) nanosheets have garnered significant attention in the field of electrochemical energy applications, such as supercapacitors, electro-catalysis, and metal-ion batteries. The distinctive structures of VG, including vertically oriented morphology, exposed, and extended edges, and separated few-layer graphene nanosheets, have endowed VG with superior electrode reaction kinetics and mass/electron transportation compared to other graphene-based nanostructures. Therefore, gaining insight into the structure-activity relationship of VG and VG-based materials is crucial for enhancing device performance and expanding their applications in the energy field. In this review, the authors first summarize the fabrication methods of VG structures, including solution-based, and vacuum-based techniques. The study then focuses on structural modulations through plasma-enhanced chemical vapor deposition (PECVD) to tailor defects and morphology, aiming to obtain desirable architectures. Additionally, a comprehensive overview of the applications of VG and VG-based hybrids d in the energy field is provided, considering the arrangement and optimization of their structures. Finally, the challenges and future prospects of VG-based energy-related applications are discussed.

Phosphorous, boron and sulphur-doped silver tungstate-based nanomaterials toward electrochemical methanol oxidation and water splitting energy applications

We employed a one-step precipitation approach to produce B, P, and S-doped Ag2WO4 (AgW) with the aid of a cetyltrimethylammonium bromide template in order to develop novel catalytic materials with components and architectures that tolerate harsh conditions. The P@AgW with the spherical (13.7 nm) shape was found to possess the greatest electrocatalytic activity for the oxidation of methanol in an acidic environment. It provides remarkable performance with an overpotential of 0.58 V (at a current density of 10 mA cm−2 comprised of mass activity of 1172 mA g−1) and considerable long-term reliability. This is attributed to the intense increase in the β-Ag2WO4/α-Ag2WO4 ratio (4:1), the formation of Ag–O–P, the highest electrochemical surface area, and the ease of charge transfer. The B@AgW electrode of polymorphic structure, on the other hand, performed the best for water electrolysis in the basic medium (1.0 M KOH). It produces an OER at overpotentials of 50 mA cm−2 at 0.42 V and 399 mV at 10 mA cm−2. At 10 mA cm−2, the HER overpotential was 0.33 V. Additionally, the B@AgW electrolyzer shows a cell voltage of 1.44 V for overall water splitting, which is lower than that of P@AgW (1.56 V), and it demonstrates great durability for 20 h. The notably higher activity of B@AgW compared to other electrolyzers is due to improved electrical conductivity, transport of mass, more active sites, and reduced particle size (7.2 nm). This work offers a simple way for the large-scale manufacture of electrocatalysts with considerable activity and reliability over time.

Chemical physics of electrochemical energy materials.

Probing the chemistry and materials science of electrochemical energy materials is a central topic in both chemical physics and energy chemistry due to the increasingly important role of energy devices in the current and future energy system and industry. Especially, understanding the chemical physics of electrochemical energy materials is the key to enhance the performance of energy storage and conversion devices such as batteries, fuel cells, electrolyzers, and supercapacitors. This special topic focuses on the fundamental understanding of electrochemical energy applications, including electrochemistry fundamentals, structural dynamics and degradation mechanism of materials, optimization strategies for improving electrochemical performance of energy devices, and emerging simulation and characterization methods applied to advanced energy materials.

Recent advances in MOFs/MXenes composites: Synthesis and their electrochemical energy applications

Recent advances in MOFs/MXenes composites: Synthesis and their electrochemical energy applications

Redistributing the Electronic States of 2D‐Covalent Organic Frameworks for Electrochemical Energy Applications (Adv. Energy Mater. 41/2023)

Redistributing the Electronic States of 2D‐Covalent Organic Frameworks for Electrochemical Energy Applications (Adv. Energy Mater. 41/2023)

Solvothermally designed Pr-MOF/Fe2O3 based nanocomposites for efficient electrocatalytic water splitting

To meet the energy demand of modern civilization, efforts to find renewable, safe, and highly effective fuel generation are still a big challenge. The oxygen evolution reaction (OER) is one of many modern technologies for hydrogen generation, and a number of new electrode materials have been created to increase the effectiveness of O2 evolution. This project utilizes a range of high performance nanomaterials, such as Pr-MOF, Fe2O3, and Pr-MOF/Fe2O3, to carry out the oxygen evolution reaction. This study shows that Pr-MOF/Fe2O3 exhibits exceptional electrocatalytic activity in alkaline solution with 238 mV overpotential at the current density of 10 mA cm−2 and a Tafel slope of 37 mV dec−1 which is much lower when compared to pure Pr-MOF and Fe2O3. The enhanced electrochemical results are due to the higher electrochemical surface area of 237 cm2. This work will lay the foundation for an approach to enhance the crystalline nature of surface-active nanoparticles made from rare earth MOFs for a range of electrochemical energy applications.

Redistributing the Electronic States of 2D‐Covalent Organic Frameworks for Electrochemical Energy Applications

AbstractExploration of significantly active functional materials is highly valued for designing efficient electrochemical energy devices. 2D‐covalent organic frameworks (2D‐COF) have attracted significant interest in the context of important electrochemical applications due to their tunable structural and electronic properties, high thermal/chemical stability, and their carbon‐based motifs. The present review highlights the strategies and properties of modifying 2D‐COF electronically in detail and their prospects in electrochemical devices. The review clearly explains the electronic actuation methods of the COF skeletons by various synthetic and post‐synthetic strategies. The review crucially elucidates the importance of electroactive 2D‐COF as emerging functional materials in practical applications of electrochemical energy conversion devices. The review concludes by summarizing the effective strategies of 2D‐COF in improving electrocatalytic applications and suggests future research scope. The review orients the electroactive 2D‐COFs for critical analysis as advanced functional materials for a sustainable and reliable future.

Two-dimensional van der Waals materials and their mixed low-dimensional hybrids for electrochemical energy applications

Two-dimensional van der Waals materials and their mixed low-dimensional hybrids for electrochemical energy applications

Nickel-constructing interfaces of multiheterostructure on self-supporting electrode for efficient overall urea-water splitting

Water splitting is an environmentally friendly strategy for producing hydrogen but is constrained by the high theoretical potential of the oxygen evolution reaction (OER) at the anode. It is possible to save energy by substituting the OER with the urea oxidation reaction (UOR) with a low theoretical potential. In particular, developing bifunctional catalysts (hydrogen evolution reactions (HER)/UOR) and exploring their catalytic mechanisms and active sites are highly desirable with the rapid development of the hydrogen economy. Herein, the combination of electrodeposition and immersion methods to grow a well-aligned MOF on a graphene-like material (MXene) is reported. Specifically, Ni3S2 species were formed on the hexahedral structure of cobalt-based MOF (CoMOF) by electrodeposition. The resultant M/CM/Ni3S2/NF demonstrates good electrocatalytic capabilities toward the HER and UOR, with overpotentials and potentials of 0.174 and 1.392 V required to reach 100 mA cm−2, respectively. DFT results established that the coupling of MXene, CoMOF, and Ni3S2 can optimize the free energy of hydrogen adsorption on the catalyst surface. Furthermore, in situ XRD revealed that the (111), (2¯ 11) and (220) crystal planes resulted in good stability of the material. This work provides an innovative and novel route for designing high-activity bifunctional catalysts for various electrochemical energy applications.

Hydrous Transition Metal Oxides for Electrochemical Energy and Environmental Applications

Hydrous transition metal oxides (TMOs) are redox-active materials that confine structural water within their bulk, organized in 1D, 2D, or 3D networks. In an electrochemical cell, hydrous TMOs can interact with electrolyte species not only via their outer surface but also via their hydrous inner surface, which can transport electrolyte species to the interior of the material. Many TMOs operating in an aqueous electrochemical environment transform to hydrous TMOs, which then serve as the electrochemically active phase. This review summarizes the physicochemical properties of hydrous TMOs and recent mechanistic insights into their behavior in electrochemical reactions of interest for energy storage, conversion, and environmental applications. Particular focus is placed on first-principles calculations and operando characterization to obtain an atomistic view of their electrochemical mechanisms. Hydrous TMOs represent an important class of energy and environmental materials in aqueous and nonaqueous environments. Further understanding of their interaction with electrolyte species is likely to yield advancements in electrochemical reactivity and kinetics for energy and environmental applications.