Mechanisms of pH-dependent activity for water oxidation to molecular oxygen by MnO2 electrocatalysts

Microkinetic Model to Rationalize the Lifetime of Electrocatalysis: Trade-off between Activity and Stability

Electrocatalysts which can operate for several years are required to produce hydrogen and commodity chemicals in an environmentally friendly manner. However, designing materials with long operational lifetimes is challenging, due to the lack of a conceptual framework to predict catalytic lifetimes quantitatively. Here, we report a microkinetic equation which quantifies the lifetime of an electrocatalyst undergoing dissolution. This equation was obtained by taking advantage of the fact that catalysis is much faster than deactivation, which allows the ordinary differential equations to be solved via the quasi steady-state approximation. All chemical reactions were modeled as irreversible, first-order elementary reactions. Under this assumption, the catalytic rate correlates linearly with the deactivation rate, leading to a trade-off relationship between activity and stability. Our model was supported by the correlation between theoretical and experimental lifetimes (r2 = 0.86) of a manganese oxide electrocatalyst during the oxygen evolution reaction.

Deconvoluting Surface and Bulk Charge Storage Processes in Redox-Active Oxides by Integrating Electrochemical and Optical Insights

Redox-active transition metal oxides (TMOs) play crucial roles in diverse energy storage and conversion technologies, such as batteries and pseudocapacitors. These materials show intricate electrochemical charge storage processes, encompassing both bulk ion-intercalation, typical of battery electrodes, and pseudocapacitive-like behavior localized near the surfaces. However, understanding the underlying mechanisms of charge storage in redox-active TMOs is challenging due to the coexistence of these behaviors. In this study, we propose an integrated approach that combines operando electrochemical and optical techniques to disentangle the contributions of bulk and surface phenomena. Using birnessite δ-MnO2-x as a model system, we account for surface pseudocapacitive-like layers and employ a refined model that incorporates both surface reactions and bulk chemical diffusion. This methodology allows us to extract essential kinetic parameters, establishing a fundamental framework for unraveling surface and bulk electrochemical processes. This advancement provides a valuable tool for the rational design of energy storage devices, enhancing our ability to tailor these materials for specific applications.

Electronic Structure Modification of MnO2 Nanosheet Arrays with Enhanced Water Oxidation Activity and Stability by Nitrogen Plasma

The strategic design of catalysts for the oxygen evolution reaction (OER) is crucial in tackling the substantial energy demands associated with hydrogen production in electrolytic water splitting. Despite extensive research on birnessite (δ-MnO2) manganese oxides to enhance catalytic activity by modulating Mn3+ species, the ongoing challenge is to simultaneously stabilize Mn3+ while improving overall activity. Herein, oxygen (O) vacancies and nitrogen (N) doping have been simultaneously introduced into the MnO2 through a simple nitrogen plasma approach, resulting in efficient OER performance. The optimized N-MnO2v electrocatalyst exhibits outstanding OER activity in alkaline electrolyte, reducing the overpotential by nearly 160 mV compared to pure pristine MnO2 (from 476 to 312 mV) at 10 mA cm-2, and a small Tafel slope of 89 mV dec-1. Moreover, it demonstrates excellent durability over a 122 h stability test. The introduction of O vacancies and incorporation of N not only fine-tune the electronic structure of MnO2, increasing the Mn3+ content to enhance overall activity, but also play a crucial role in stabilizing Mn3+, thereby leading to exceptional stability over time. Subsequently, density functional theory calculations validate the optimized electronic structure of MnO2 achieved through the two engineering methods, effectively lowering the intermediate adsorption free energy barrier. Our synergistic approach, utilizing nitrogen plasma treatment, opens a pathway to concurrently enhance the activity and stability of OER electrocatalysts, applicable not only to Mn-based but also to other transition metal oxides.

Seawater electrolysis for fuels and chemicals production: fundamentals, achievements, and perspectives

Seawater electrolysis for the production of fuels and chemicals involved in onshore and offshore plants powered by renewable energies offers a promising avenue and unique advantages for energy and environmental sustainability. Nevertheless, seawater electrolysis presents long-term challenges and issues, such as complex composition, potential side reactions, deposition of and poisoning by microorganisms and metal ions, as well as corrosion, thus hindering the rapid development of seawater electrolysis technology. This review focuses on the production of value-added fuels (hydrogen and beyond) and fine chemicals through seawater electrolysis, as a promising step towards sustainable energy development and carbon neutrality. The principle of seawater electrolysis and related challenges are first introduced, and the redox reaction mechanisms of fuels and chemicals are summarized. Strategies for operating anodes and cathodes including the development and application of chloride- and impurity-resistant electrocatalysts/membranes are reviewed. We comprehensively summarize the production of fuels and chemicals (hydrogen, carbon monoxide, sulfur, ammonia, etc.) at the cathode and anode via seawater electrolysis, and propose other potential strategies for co-producing fine chemicals, even sophisticated and electronic chemicals. Seawater electrolysis can drive the oxidation and upgrading of industrial pollutants or natural organics into value-added chemicals or degrade them into harmless substances, which would be meaningful for environmental protection. Finally, the perspective and prospects are outlined to address the challenges and expand the application of seawater electrolysis.

Innovative Insights into Water-Oxidation Mechanism: Investigating Birnessite's Reaction with Cerium(IV) Ammonium Nitrate

Developing Mn-based water-oxidation reaction (WOR) catalysts is key for renewable energy storage, utilizing Mn's abundance, cost-effectiveness, and natural role. Cerium(IV) ammonium nitrate (CAN) has been widely utilized as a sacrificial oxidant in the exploration of WOR catalysts. In this study, advanced techniques, such as X-ray absorption spectroscopy (XAS), in situ Raman spectroscopy, and in situ electron paramagnetic resonance (EPR), to delve into the WOR facilitated by CAN and birnessite were employed. XANES analysis has demonstrated that the average oxidation states (AOSs) of Mn in birnessite, a birnessite/CAN mixture, and in the birnessite/CAN mixture postwater addition are 3.7, 3.8, and 3.9, respectively. In situ Raman spectroscopy performed in the presence of birnessite and CAN revealed a distinct peak at 784 cm-1, which is attributed to Mn(IV)═O. A shift of this peak to 769 cm-1 in H218O confirms its association with Mn(IV)═O. No change in this peak was observed in D2O, further supporting the notion that it is linked to Mn(IV)═O rather than Mn-OH (D). Furthermore, EPR spectroscopy shows the presence of Mn(IV). It is suggested that the WOR mechanism initiates with the oxidation of birnessite by CAN, which enhances the concentration of Mn(IV) sites in the birnessite structure. Under acidic conditions, birnessite, enriched in Mn(IV), facilitates oxygen evolution and subsequently transitions into a form with reduced Mn(IV) levels. This process highlights the critical function of the Mn (hydr)oxide structure, similar to its role in the water-oxidizing complex of Photosystem II, where it serves as charge storage for oxidizing equivalents from CAN, paving the way for a four-electron reaction that drives the WOR.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

The effect of crystal structure of MnO2 electrode on DMAC removal: degradation performance, mechanism, and application evaluation

The crystal structure has a significant impact on the electrochemical properties of electrode material, and thus influences the electrocatalytic activity of the electrode. In this work, α-, β-, and γ-MnO2 electrodes were fabricated and applied for investigating the effect of crystal structure on electro-oxidation treatment of N,N-dimethylacetamide (DMAC) containing wastewater. The prepared MnO2 electrodes were characterized by scanning electron microscopy and X-ray diffraction, suggesting that different crystal structures of MnO2 electrodes with the same morphology of stacking-needle structure were successfully prepared. The electrochemical performances, including removal efficiencies of DMAC, chemical oxygen demand (COD) and total nitrogen (TN), and energy consumption, were compared between different MnO2 electrodes. Results indicated that β-MnO2 electrode presented the excellent electrochemical activity, and could remove 93% DMAC, 62% COD, and 78.9% TN, which was much higher than that of α- and γ-MnO2; moreover, energy consumptions of 11.3, 9.7, and 10.5 kWh/m3 were calculated for α-, β-, and γ-MnO2, respectively. Additionally, the oxidation mechanism of the MnO2 electrodes was presented, indicating that DMAC was mainly oxidized by hydroxyl radical through reactions of hydroxylation, demethylation, and deamination, and electrode characteristics of specific surface area, oxygen evolution potential, and hydroxyl radical production were the key factors for degrading DMAC on MnO2 electrodes. Finally, an actual DMAC containing wastewater was applied for testing the electrochemical performance of the three electrodes, and β-MnO2 electrode was verified as the suitable electrode for potential application which achieved removal efficiencies of 100%, 64.5%, and 73% for DMAC, COD, and TN, respectively, after system optimization.

Human-Machine Collaboration for Accelerated Discovery of Promising Oxygen Evolution Electrocatalysts with On-Demand Elements

A drastically efficient method for identifying electrocatalysts with desirable functionality is a pressing necessity for making a breakthrough in advanced water-electrolyzers toward large-scale green hydrogen production and addressing the significant challenge of carbon neutrality. Despite extensive investigations over the last several centuries, it remains a time-consuming task to identify even one promising affordable electrocatalyst without platinum-group-metal (PGM) for one electrochemical reaction due to its great complexities, particularly for the key anode reaction in the water-electrolyzer of the oxygen evolution reaction (OER). In this study, we demonstrate that a human-machine collaboration based on stepwise-evolving artificial intelligence (se-AI) can significantly shorten the development period of PGM-free multimetal OER electrocatalysts with performance beyond a PGM of RuO2. We were able to reach optimized materials only after 2% experimental trials of the entire candidate pool. The best PGM-free electrocatalyst discovered exhibited excellent activity comparable to RuO2 and, surprisingly, also demonstrated superior stability with a high current density of up to 1000 mA/cm2 at even pH 9.2, which condition is a thermodynamically challenging for typical PGM-free materials. This work illustrates that human's material discovery can be significantly accelerated through collaboration with AI.

Iridium-Cooperated, Symmetry-Broken Manganese Oxide Nanocatalyst for Water Oxidation

The water oxidation reaction, the most important reaction for hydrogen production and other sustainable chemistry, is efficiently catalyzed by the Mn4CaO5 cluster in biological photosystem II. However, synthetic Mn-based heterogeneous electrocatalysts exhibit inferior catalytic activity at neutral pH under mild conditions. Symmetry-broken Mn atoms and their cooperative mechanism through efficient oxidative charge accumulation in biological clusters are important lessons but synthesis strategies for heterogeneous electrocatalysts have not been successfully developed. Here, we report a crystallographically distorted Mn-oxide nanocatalyst, in which Ir atoms break the space group symmetry from I41/amd to P1. Tetrahedral Mn(II) in spinel is partially replaced by Ir, surprisingly resulting in an unprecedented crystal structure. We analyzed the distorted crystal structure of manganese oxide using TEM and investigated how the charge accumulation of Mn atoms is facilitated by the presence of a small amount of Ir.

Operando studies of Mn oxide based electrocatalysts for the oxygen evolution reaction

Inspired by photosystem II (PS II), Mn oxide based electrocatalysts have been repeatedly investigated as catalysts for the electrochemical oxygen evolution reaction (OER), the anodic reaction in water electrolysis. However, a comparison of the conditions in biological OER catalysed by the water splitting complex CaMn4Ox with the requirements for an electrocatalyst for industrially relevant applications reveals fundamental differences. Thus, a systematic development of artificial Mn-based OER catalysts requires both a fundamental understanding of the catalytic mechanisms as well as an evaluation of the practicality of the system for industrial scale applications. Experimentally, both aspects can be approached using in situ and operando methods including spectroscopy. This paper highlights some of the major challenges common to different operando investigation methods and recent insights gained with them. To this end, vibrational spectroscopy, especially Raman spectroscopy, absorption techniques in the bandgap region and operando X-ray spectroelectrochemistry (SEC), both in the hard and soft X-ray regime are particularly focused on here. Technical challenges specific to each method are discussed first, followed by challenges that are specific to Mn oxide based systems. Finally, recent in situ and operando studies are reviewed. This analysis shows that despite the technical and Mn specific challenges, three specific key features are common to most of the studied systems with significant OER activity: structural disorder, Mn oxidation states between III and IV, and the appearance of layered birnessite phases in the active regime.

A Reflection on Sustainable Anode Materials for Electrochemical Chloride Oxidation

Chloride oxidation is a key industrial electrochemical process in chlorine-based chemical production and water treatment. Over the past few decades, dimensionally stable anodes (DSAs) consisting of RuO2 - and IrO2 -based mixed-metal oxides have been successfully commercialized in the electrochemical chloride oxidation industry. For a sustainable supply of anode materials, considerable efforts both from the scientific and industrial aspects for developing earth-abundant-metal-based electrocatalysts have been made. This review first describes the history of commercial DSA fabrication and strategies to improve their efficiency and stability. Important features related to the electrocatalytic performance for chloride oxidation and reaction mechanism are then summarized. From the perspective of sustainability, recent progress in the design and fabrication of noble-metal-free anode materials, as well as methods for evaluating the industrialization of novel electrocatalysts, are highlighted. Finally, future directions for developing highly efficient and stable electrocatalysts for industrial chloride oxidation are proposed.

Light-Mediated Electrochemical Synthesis of Manganese Oxide Enhances Its Stability for Water Oxidation

New methods are needed to increase the activity and stability of earth-abundant catalysts for electrochemical water splitting to produce hydrogen fuel. Electrodeposition has been previously used to synthesize manganese oxide films with a high degree of disorder and a mixture of oxidation states for Mn, which has led to electrocatalysts with high activity but low stability for the oxygen evolution reaction (OER) at high current densities. In this study, we show that multipotential electrodeposition of manganese oxide under illumination produces nanostructured films with significantly higher stability for the OER compared to films grown under otherwise identical conditions in the dark. Manganese oxide films grown by multipotential deposition under illumination sustain a current density of 10 mA/cm2 at 2.2 V versus reversible hydrogen electrode for 18 h (pH 13). Illumination does not enhance the activity or stability of manganese oxide films grown using a constant potential, and films grown by multipotential deposition in the dark undergo a complete loss of activity within 1 h of electrolysis. Electrochemical and structural characterization indicate that photoexcitation of the films during growth reduces Mn ions and changes the content and structure of intercalated potassium ions and water molecules in between the disordered layers of birnessite-like sheets of MnOx, which stabilizes the nanostructured film during electrocatalysis. These results demonstrate that combining multiple external stimuli (i.e., light and an external potential) can induce structural changes not attainable by either stimulus alone to make earth-abundant catalysts more active and stable for important chemical transformations such as water oxidation.

Breaking the Activity and Stability Bottlenecks of Electrocatalysts for Oxygen Evolution Reactions in Acids

Oxygen evolution reaction (OER) is a cornerstone reaction for a variety of electrochemical energy conversion and storage systems such as water splitting, CO2 /N2 reduction, reversible fuel cells, and metal-air batteries. However, OER catalysis in acids suffers from extra sluggish kinetics due to the additional step of water dissociation along with its multiple electron transfer processes. Furthermore, OER catalysts often suffer from poor stability in harsh acidic electrolytes due to the severe dissolution/corrosion processes. The development of active and stable OER catalysts in acids is highly demanded. Here,  the recent advances in OER electrocatalysis in acids are reviewed and the key strategies are summarized to overcome the bottlenecks of activity and stability for both noble-metal-based and noble metal-free catalysts, including i) morphology engineering, ii) composition engineering, and iii) defect engineering. Recent achievements in operando characterization and theoretical calculations are summarized which provide an unprecedented understanding of the OER mechanisms regarding active site identification, surface reconstruction, and degradation/dissolution pathways. Finally, views are offered on the current challenges and opportunities to break the activity-stability relationships for acidic OER in mechanism understanding, catalyst design, as well as standardized stability and activity evaluation for industrial applications such as proton exchange membrane water electrolyzers and beyond.

Different metal cation-doped MnO2/carbon cloth for wide voltage energy storage

Aqueous gel supercapacitors, as an important component of flexible energy storage devices, have received widespread attention for their fast charging/discharging rates, long cycle life and high electrochemical stability under mechanical deformation condition. However, the low energy density of aqueous gel supercapacitors has greatly hindered their further development due to the narrow electrochemical window and limited energy storage capacity. Therefore, different metal cation-doped MnO₂/carbon cloth-based flexible electrodes herein are prepared by constant voltage deposition and electrochemical oxidation in various saturated sulphate solutions. The influence of different metal cations as K⁺, Na⁺ and Li⁺ doping and deposition conditions on the apparent morphology, lattice structure and electrochemical properties are explored. Furthermore, the pseudo-capacitance ratio of the doped MnO₂ and the voltage expansion mechanism of the composite electrode are investigated. The specific capacitance and pseudo-capacitance ratio of the optimized δ-Na_0.31MnO₂/carbon cloth as MNC-2 electrode could be reached 327.55 F/g at 10 mV/s and 35.56% of the pseudo-capacitance, respectively. The flexible symmetric supercapacitors (NSCs) with desirable electrochemical performances in the operating range of 0-1.4 V are further assembled with MNC-2 as the electrodes. The energy density is 26.8 Wh/kg at the power density of 300 W/kg, while the energy density can still reach 19.1 Wh/kg when the power density is up to 1150 W/kg. The energy storage devices with high-performance developed in this work can provide new ideas and strategic support for the application in portable and wearable electronic devices.

Mesoporous Manganese Oxides with High-Valent Mn Species and Disordered Local Structures for Efficient Oxygen Electrocatalysis

Active and nonprecious-metal bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are vital components of clean energy conversion devices such as regenerative fuel cells and rechargeable metal-air batteries. Porous manganese oxides (MnO_x) are promising electrocatalyst candidates because of their high surface area and the abundance of Mn. MnO_x catalysts exhibit various oxidation states and crystal structures, which critically affect their electrocatalytic activity. These effects remain elusive mainly because the synthesis of oxidation-state-controlled porous MnO_x with similar structural properties is challenging. In this work, four different mesoporous manganese oxides (m-MnO_x) were synthesized and used as model catalysts to investigate the effects of local structures and Mn valence states on the activity toward oxygen electrocatalysis. The following activity trends were observed: m-Mn₂O₃ > m-MnO₂ > m-MnO > m-Mn₃O₄ for the ORR and m-MnO₂ > m-Mn₂O₃ > m-MnO ≈ m-Mn₃O₄ for the OER. These activity trends suggest that high-valent Mn species (Mn(III) and Mn(IV)) with disordered atomic arrangements induced by nanostructuring significantly influence electrocatalysis. In situ X-ray absorption spectroscopy was used to analyze the changes in the oxidation states under the ORR and OER conditions, which showed the surface phase transformation and generation of active species during electrocatalysis.

Amorphous mixed-valent Mn-containing nanozyme with cocklebur-like morphology for specific colorimetric detection of cancer cells via Velcro effects

Designing nanozymes with excellent catalytic activity through valence state engineering and defect engineering is a widely applicable strategy. However, their development is hindered by the complexity of the design strategies. In this work, we employed a simple calcination method to regulate the valence of manganese and crystalline states in manganese oxide nanozymes. The oxidase-like activity of the nanozymes was found to benefit from a mixed valence state dominated by Mn (III). And the amorphous structure with more active defect sites significantly enhanced the catalytic efficiency. Moreover, we demonstrated that amorphous mixed-valent Mn-containing (amvMn) nanozymes with unique cocklebur-like biomimetic morphology achieved specific binding to cancer cells through the Velcro effects. Subsequently, the nanozymes mediated TMB coloration through their oxidase-like activity, enabling the colorimetric detection of cancer cells. This work not only provides guidance for optimizing nanozyme performance, but also inspire the development of equipment-free visual detection methods for cancer cells.

Fe(III) Docking-Activated Sites in Layered Birnessite for Efficient Water Oxidation

Non-noble metal catalysts for promoting the sluggish kinetics of oxygen evolution reaction (OER) are essential to efficient water splitting for sustainable hydrogen production. Birnessite has a local atomic structure similar to that of an oxygen-evolving complex in photosystem II, while the catalytic activity of birnessite is far from satisfactory. Herein, we report a novel Fe-Birnessite (Fe-Bir) catalyst obtained by controlled Fe(III) intercalation- and docking-induced layer reconstruction. The reconstruction dramatically lowers the OER overpotential to 240 mV at 10 mA/cm² and the Tafel slope to 33 mV/dec, making Fe-Bir the best of all the reported Bir-based catalysts, even on par with the best transition-metal-based OER catalysts. Experimental characterizations and molecular dynamics simulations elucidate that the catalyst features active Fe(III)-O-Mn(III) centers interfaced with ordered water molecules between neighboring layers, which lower reorganization energy and accelerate electron transfer. DFT calculations and kinetic measurements show non-concerted PCET steps conforming to a new OER mechanism, wherein the neighboring Fe(III) and Mn(III) synergistically co-adsorb OH* and O* intermediates with a substantially reduced O-O coupling activation energy. This work highlights the importance of elaborately engineering the confined interlayer environment of birnessite and more generally, layered materials, for efficient energy conversion catalysis.

Research on engineered electrocatalysts for efficient water splitting: a comprehensive review

Water electrolysis plays an interesting role toward hydrogen generation for overcoming global environmental crisis and solving the energy storage problem. However, there is still a deficiency of efficient electrocatalysts to overcome sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Great efforts have been employed to produce potential catalysts with low overpotential, rapid kinetics, and excellent stability for HER and OER. At present, hydrogen economy is driven by electrocatalysts with excellent characteristics; thus, systematic design strategy has become the driving force to exploit earth-abundant transition metal-based electrocatalysts toward H₂ economy. In this review, the recent progress on newer materials including metals, alloys, and transition metal oxides (manganese oxides, cobalt oxides, nickel oxides, PBA-derived metal oxides, and metal complexes) as photocatalysts/electrocatalysts has been overviewed together with some methodologies for efficient water splitting. Metal-organic framework (MOF)-based electrocatalysts have been highly exploited owing to their interesting functionalities. The photovoltaic-electrocatalytic (PV-EC) process focused on harvesting high solar-to-hydrogen efficiency (STH) among various solar energy conversion as well as storage systems. Electrocatalysts/photocatalysts with high efficiency have become an urgent need for overall water splitting. Also, cutting-edge achievements in the fabrication of electrocatalysts along with theoretical consideration have been discussed.

Ni-Doped MnO2 Nanosheet Arrays for Efficient Urea Oxidation

Urea oxidation reaction (UOR), with a low thermodynamic potential, offers great promise for replacing anodic oxygen evolution reaction of electrolysis systems such as water splitting, carbon dioxide reduction, etc., thus reducing the overall energy consumption. To promote the sluggish kinetics of UOR, highly efficient electrocatalysts are required, and Ni-based materials have been widely investigated. However, most of these reported Ni-based catalysts suffer from large overpotentials, as they generally undergo self-oxidation to form NiOOH species at high potentials, which act as catalytically active sites for UOR. Herein, Ni-doped MnO₂ (Ni-MnO₂) nanosheet arrays were successfully prepared on nickel foam. The as-fabricated Ni-MnO₂ shows distinct UOR behavior with most of the previously reported Ni-based catalysts, as urea oxidation on Ni-MnO₂ proceeds before the formation of NiOOH. Notably, a low potential of 1.388 V vs reversible hydrogen electrode was required to achieve a high current density of 100 mA cm^-2 on Ni-MnO₂. It is suggested that both Ni doping and nanosheet array configuration are responsible for the high UOR activities on Ni-MnO₂. The introduction of Ni modifies the electronic structure of Mn atoms, and more Mn³⁺ species are generated in Ni-MnO₂, contributing to its outstanding UOR performance.

Marigold Flower-Shaped Metal-Organic Framework Supported Manganese Vanadium Oxide Electrocatalyst for Efficient Oxygen Evolution Reactions in an Alkaline Medium

The electrochemical oxygen evolution reaction (OER) is a key process in many renewable energy systems. The development of low-cost, long-lasting alternatives to precious-metal catalysts, particularly functional electrocatalysts with high activity for OER processes, is crucial for reducing the operating expense and complexity of renewable energy generating systems. This work describes a concise method for generating marigold flower-like metal-organic frameworks (MOFs) aided manganese vanadium oxide via a hydrothermal procedure for increased OER activity. As synthesized MOF MnV oxide has a higher surface area due to the 3D flower-like structure, which is reinvented with enhanced electrocatalytic active sites. These distinctive structural features result in remarkable catalytic activity for MOF MnV oxide microflowers towards OER with a low overpotential of 310 mV at 50 mA cm^-2 and a Tafel slope with only 51.4 mV dec^-1 in alkaline conditions. This study provides a concise method for developing an optimized catalytic material with greater morphology and beneficial features for potential energy and environmental applications.

Alumina supported copper oxide nanoparticles (CuO/Al2O3) as high-performance electrocatalysts for hydrazine oxidation reaction

Direct hydrazine liquid fuel cell (DHFC) is perceived as effectual energy generating mean owing to high conversion efficiency and energy density. However, the development of well-designed, cost effective and high performance electrocatalysts is the paramount to establish DHFCs as efficient energy generating technology. Herein, gamma alumina supported copper oxide nanocatalysts (CuO/Al₂O₃) are synthesized via impregnation method and investigated for their electrocatalytic potential towards hydrazine oxidation reaction. CuO with different weight percentages i.e., 4%, 8%, 12%, 16% and 20% are impregnated on gamma alumina support. X-ray diffraction analysis revealed the cubic crystal structure and nanosized particles of the prepared metal oxides. Transmission electron microscopy also referred to the cubic morphology and nanoparticle formation. Electrochemical oxidation potential of the CuO/Al₂O₃ nanoparticles is explored via cyclic voltammetry as the analytical tool. Optimization of conditions and electrocatalytic studies shown that 16% CuO/Al₂O₃ presented the best electronic properties towards N₂H₂ oxidation reaction. BET analysis ascertained the high surface area (131.2546 m² g¹) and large pore diameter (0.279605 cm³ g^-1) for 16% CuO/Al₂O₃. Nanoparticle formation, high porosity and enlarged surface area of the proposed catalysts resulted in significant oxidation current output (600 μA), high current density (8.2 mA cm^-2) and low charge transfer resistance (3.7 kΩ). Electrooxidation of hydrazine on such an affordable and novel electrocatalyst opens a gateway to further explore the metal oxide impregnated alumina materials for different electrochemical applications.

Atomic Reconstruction and Oxygen Evolution Reaction of Mn3O4 Nanoparticles

Understanding the chemical states of individual surface atoms and their arrangements is essential for addressing several current issues such as catalysis, energy stroage/conversion, and environmental protection. Here, we exploit a profile imaging technique to understand the correlation between surface atomic structures and the oxygen evolution reaction (OER) in Mn₃O₄ nanoparticles. We image surface structures of Mn₃O₄ nanoparticles and observe surface reconstructions in the (110) and (101) planes. Mn³⁺ ions at the surface, which are commonly considered as the active sites in OER, disappear from the reconstructed planes, whereas Mn³⁺ ions are still exposed at the edges of nanoparticles. Our observations suggest that surface reconstructions can deactivate low-index surfaces of Mn oxides in OER. These structural and chemical observations are further validated by density functional theory calculations. This work shows why atomic-scale characterization of surface structures is crucial for a molecular-level understanding of a chemical reaction in oxide nanoparticles.

Stabilization of a Mn-Co Oxide During Oxygen Evolution in Alkaline Media

Improving the stability of electrocatalysts for the oxygen evolution reaction (OER) through materials design has received less attention than improving their catalytic activity. We explored the effects of Mn addition to a cobalt oxide for stabilizing the catalyst by comparing single phase CoOx and (Co0.7Mn0.3)Ox films electrodeposited in alkaline solution. The obtained disordered films were classified as layered oxides using X-ray absorption spectroscopy (XAS). The CoOx films showed a constant decrease in the catalytic activity during cycling, confirmed by oxygen detection, while that of (Co0.7Mn0.3)Ox remained constant within error as measured by electrochemical metrics. These trends were rationalized based on XAS analysis of the metal oxidation states, which were Co2.7+ and Mn3.7+ in the bulk and similar near the surface of (Co0.7Mn0.3)Ox, before and after cycling. Thus, Mn in (Co0.7Mn0.3)Ox successfully stabilized the bulk catalyst material and its surface activity during OER cycling. The development of stabilization approaches is essential to extend the durability of OER catalysts.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

How to Predict the pK a of Any Compound in Any Solvent

Acid-base properties of molecules in nonaqueous solvents are of critical importance for almost all areas of chemistry. Despite this very high relevance, our knowledge is still mostly limited to the pK a of rather few compounds in the most common solvents, and a simple yet truly general computational procedure to predict pK a's of any compound in any solvent is still missing. In this contribution, we describe such a procedure. Our method requires only the experimental pK a of a reference compound in water and a few standard quantum-chemical calculations. This method is tested through computing the proton solvation energy in 39 solvents and by comparing the pK a of 142 simple compounds in 12 solvents. Our computations indicate that the method to compute the proton solvation energy is robust with respect to the detailed computational setup and the construction of the solvation model. The unscaled pK a's computed using an implicit solvation model on the other hand differ significantly from the experimental data. These differences are partly associated with the poor quality of the experimental data and the well-known shortcomings of implicit solvation models. General linear scaling relationships to correct this error are suggested for protic and aprotic media. Using these relationships, the deviations between experiment and computations drop to a level comparable to that observed in water, which highlights the efficiency of our method.

Formation of the Metastable MnIII Water Oxidation Intermediate in Birnessite is Controlled by a Dissolution-Deposition Process Involving Labile MnII

Birnessite, the closest naturally occurring analog of the Mn₄ CaO₅ cluster of photosystem II, is an important model compound in the development of bio-inspired electrocatalysts for the water oxidation reaction. The present work reports the formation mechanism of the key Mn^III intermediate realized through the study of the effects of several electrolyte anions and cations on the catalytic efficiency of birnessite. In situ spectroelectrochemical measurements show that the activity is controlled by a dynamic dissolution-oxidation process, wherein Mn^III is formed through the oxidation of labile uncomplexed Mn^II that reversibly shuttles between the birnessite and the electrolyte in a manner similar to the photoactivation in photosystem II. The role of electrolyte cations of different ionic radii and hydration strengths is to control the interlayer spacing, whereas electrolyte anions control the extent of deprotonation of complexed Mn^II in the lattice. Both in turn govern the shuttling efficiency of uncomplexed Mn^II and its subsequent electro-oxidation to Mn^III .

Inter-relationships between Oxygen Evolution and Iridium Dissolution Mechanisms

The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir-based catalysts is, therefore, crucial for a scale-up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state-of-the-art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.

Bridging electrocatalyst and cocatalyst studies for solar hydrogen production via water splitting

Solar-driven water-splitting has been considered as a promising technology for large-scale generation of sustainable energy for succeeding generations. Recent intensive efforts have led to the discovery of advanced multi-element-compound water-splitting electrocatalysts with very small overpotentials in anticipation of their application to solar cell-assisted water electrolysis. Although photocatalytic and photoelectrochemical water-splitting systems are more attractive approaches for scaling up without much technical complexity and high investment costs, improving their efficiencies remains a huge challenge. Hybridizing photocatalysts or photoelectrodes with cocatalysts has been an effective scheme to enhance their overall solar energy conversion efficiencies. However, direct integration of highly-active electrocatalysts as cocatalysts introduces critical factors that require careful consideration. These additional requirements limit the design principle for cocatalysts compared with electrocatalysts, decelerating development of cocatalyst materials. This perspective first summarizes the recent advances in electrocatalyst materials and the effective strategies to assemble cocatalyst/photoactive semiconductor composites, and further discusses the core principles and tools that hold the key in designing advanced cocatalysts and generating a deeper understanding on how to further push the limits of water-splitting efficiency.

Oxygen evolving reactions catalyzed by different manganese oxides: the role of oxidation state and specific surface area

A set of the four manganese oxide powders α-MnO2 (hollandite), δ-MnO2 (birnessite), Mn2O3 (bixbyite), and Mn3O4 (hausmannite) have been synthesized in a phase-pure form and tested as catalysts in three different oxygen evolution reactions (OER): electrochemical OER in KOH (1 mol L−1), chemical OER using aqueous cerium ammonium nitrate, and H2O2 decomposition. The trends in electrochemical (hollandite >> bixbyite > birnessite > hausmannite) and chemical OER (hollandite > birnessite > bixbyite > hausmannite) are different, which can be explained by differences in electric conductivity. H2O2 decomposition and chemical OER, on the other hand, showed the same trend and even a linear correlation of their initial OER rates. A linear correlation between the catalytic performance and the manganese oxidation state of the catalysts was observed. Another trend was observed related to the specific surface area, highlighting the importance of these properties for the OER. Altogether, hollandite was found to be the best performing catalyst in this study due to a combination of the high manganese oxidation state and a large specific surface area. Likely, due to a sufficient electrical conductivity, this intrinsically high OER performance is also found to some extent in electrocatalysis for this specific example.

The effects of pH on U(VI)/Th(IV) and Ra(II)/Ba(II) adsorption by polystyrene-nano manganese dioxide composites: Fourier Transform Infra-Red spectroscopic analysis

Fourier Transform Infra-Red (FTIR) spectroscopy provides structural information of prime importance to understand ions coordination to adsorbents. This consequently aids in the design of improved ion exchange materials and help in deriving the optimum adsorption conditions. In the present work, the adsorption mechanism of both U(VI)/Th(IV) and Ra(II)/Ba(II) radionuclides couples onto polystyrene-nano manganese dioxide (PS-NMO) composite is reported in relation to the effect of working solution pH. The separation of each radionuclide couple; i.e. U(VI)/Th(IV) and Ra(II)/Ba(II); could be effectively achieved at pH = 3 and pH = 1 respectively. The pH values not only determine the species of the respected elements that are mainly present in aqueous solution before applying the adsorbent, but it also alters the structure of the composite adsorbent. FTIR spectroscopy showed that Th(IV) formed inner sphere complexes and occupied the A site in the dioxide layer, while U(VI) formed outer sphere complexes on the surface of the composite. Spectra subtraction showed that some aromatic bands and vinyl C-H bands were split or shifted to lower wavenumbers with the loading of Ba(II). This was attributed to changes in the composite stereochemistry to accommodate Ba(II). The working solution pH could be the key in the separation process of both U(VI)/Th(IV) and Ra(II)/Ba(II) from their mixture, and FTIR spectroscopy stands as a useful technique to explain the difference between metal ions responses to adsorbants.

Surprisingly Low Reactivity of Layered Manganese Oxide toward Water Oxidation in Fe/Ni-Free Electrolyte under Alkaline Conditions

So far, many studies on the oxygen-evolution reaction (OER) by Mn oxides have been focused on activity; however, the identification of the best performing active site and corresponding catalytic cycles is also of critical importance. Herein, the real intrinsic activity of layered Mn oxide toward OER in Fe/Ni-free KOH is studied for the first time. At pH ≈ 14, the onset of OER for layered Mn oxide in the presence of Fe/Ni-free KOH happens at 1.72 V (vs reversible hydrogen electrode (RHE)). In the presence of Fe ions, a 190 mV decrease in the overpotential of OER was recorded for layered Mn oxide as well as a significant decrease (from 172.8 to 49 mV/decade) in the Tafel slope. Furthermore, we find that both Ni and Fe ions increase OER remarkably in the presence of layered Mn oxide, but that pure layered Mn oxide is not an efficient catalyst for OER without Ni and Fe under alkaline conditions. Thus, pure layered Mn oxide and electrolytes are critical factors in finding the real intrinsic activity of layered Mn oxide for OER. Our results call into question the high efficiency of layered Mn oxides toward OER under alkaline conditions and also elucidate the significant role of Ni and Fe impurities in the electrolyte in the presence of layered Mn oxide toward OER under alkaline conditions. Overall, a computational model supports the conclusions from the experimental structural and electrochemical characterizations. In particular, substitutional doping with Fe decreases the thermodynamic OER overpotential up to 310 mV. Besides, the thermodynamic OER onset potential calculated for the Fe-free structures is higher than 1.7 V (vs RHE) and, thus, not in the stability range of Mn oxides.

Electrode surface embedded manganese(iii)–pincer complexes: efficient electrocatalysts for the oxygen evolution reaction

Highly stable and robust gold electrode surface anchored Mn(iii)–pincer complex exhibits an excellent electrocatalytic activity towards the oxygen evolution reaction at a low overpotential with a medium Tafel slope under neutral pH condition.

Chloride ion inhibition of electrochemical oscillations on the anode of an electrolytic manganese metal cell

In industrial electrolytic manganese metal process, the energy consumption closely related to the electrolysis of cathode and anode. The effect of Cl^- concentration on electrochemical oscillation at the anode of the electrolytic manganese metal cell was investigated. The results showed that the electrochemical oscillation at the anode was inhibited by Cl^-, and the amplitude and frequency of the electrochemical oscillation decreased as the increase of Cl^- concentration. When the concentration of Cl^- was 2.68 g/L, the cathode and anode electrodes could be effectively activated, and the manganese current efficiency reached its minimum, correspondingly, the power consumption reached its maximum. In addition, the presence of the chloride reduced the production of MnO₂ at the anode surface. ClO₄^- and free ions formed insoluble amorphous structures on the surface of the anode with the increase in reaction time and chloride ion concentration, and the insoluble amorphous structures prevented further generation of MnO₂. Thus, electrolytic manganese metal energy consumption decreased.

Redox nanomedicine ameliorates chronic kidney disease (CKD) by mitochondrial reconditioning in mice

Targeting reactive oxygen species (ROS) while maintaining cellular redox signaling is crucial in the development of redox medicine as the origin of several prevailing diseases including chronic kidney disease (CKD) is linked to ROS imbalance and associated mitochondrial dysfunction. Here, we have shown that a potential nanomedicine comprising of Mn3O4 nanoparticles duly functionalized with biocompatible ligand citrate (C-Mn3O4 NPs) can maintain cellular redox balance in an animal model of oxidative injury. We developed a cisplatin-induced CKD model in C57BL/6j mice with severe mitochondrial dysfunction and oxidative distress leading to the pathogenesis. Four weeks of treatment with C-Mn3O4 NPs restored renal function, preserved normal kidney architecture, ameliorated overexpression of pro-inflammatory cytokines, and arrested glomerulosclerosis and interstitial fibrosis. A detailed study involving human embryonic kidney (HEK 293) cells and isolated mitochondria from experimental animals revealed that the molecular mechanism behind the pharmacological action of the nanomedicine involves protection of structural and functional integrity of mitochondria from oxidative damage, subsequent reduction in intracellular ROS, and maintenance of cellular redox homeostasis. To the best of our knowledge, such studies that efficiently treated a multifaceted disease like CKD using a biocompatible redox nanomedicine are sparse in the literature. Successful clinical translation of this nanomedicine may open a new avenue in redox-mediated therapeutics of several other diseases (e.g., diabetic nephropathy, neurodegeneration, and cardiovascular disease) where oxidative distress plays a central role in pathogenesis.

Nafion-Induced Reduction of Manganese and its Impact on the Electrocatalytic Properties of a Highly Active MnFeNi Oxide for Bifunctional Oxygen Conversion

Electrocatalysts for bifunctional oxygen reduction (ORR) and oxygen evolution reactions (OER) are commonly studied under hydrodynamic conditions, rendering the use of binders necessary to ensure the mechanical stability of the electrode films. The presence of a binder, however, may influence the properties of the materials under examination to an unknown extent. Herein, we investigate the impact of Nafion on a highly active ORR/OER catalyst consisting of MnFeNi oxide nanoparticles supported on multi-walled carbon nanotubes. Electrochemical studies revealed that, in addition to enhancing the mechanical stability and particle connectivity, Nafion poses a major impact on the ORR selectivity, which correlates with a decrease in the valence state of Mn according to X-ray absorption spectroscopy. These findings call for awareness regarding the use of electrode additives, since in some cases the extent of their impact on the properties of electrode films cannot be regarded as negligible.

Progress of Nonprecious-Metal-Based Electrocatalysts for Oxygen Evolution in Acidic Media

Water oxidation, or the oxygen evolution reaction (OER), which combines two oxygen atoms from two water molecules and releases one oxygen molecule, plays the key role by providing protons and electrons needed for the hydrogen generation, electrochemical carbon dioxide reduction, and nitrogen fixation. The multielectron transfer OER process involves multiple reaction intermediates, and a high overpotential is needed to overcome the sluggish kinetics. Among the different water splitting devices, proton exchange membrane (PEM) water electrolyzer offers greater advantages. However, current anode OER electrocatalysts in PEM electrolyzers are limited to precious iridium and ruthenium oxides. Developing highly active, stable, and precious-metal-free electrocatalysts for water oxidation in acidic media is attractive for the large-scale application of PEM electrolyzers. In recent years, various types of precious-metal-free catalysts such as carbon-based materials, earth-abundant transition metal oxides, and multiple metal oxide mixtures have been investigated and some of them show promising activity and stability for acidic OER. In this review, the thermodynamics of water oxidation, Pourbaix diagram of metal elements in aqueous solution, and theoretical screening and prediction of precious-metal-free electrocatalysts for acidic OER are first elaborated. The catalytic performance, reaction kinetics, and mechanisms together with future research directions regarding acidic OER are summarized and discussed.