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.

Capturing Manganese Oxide Intermediates in Electrochemical Water Oxidation at Neutral pH by In Situ Raman Spectroscopy.

Electrochemical water splitting is a promising means to produce eco-friendly hydrogen fuels. Inspired by the Mn4 CaO5 cluster in nature, substantial works have been performed to develop efficient manganese (Mn)-based heterogeneous catalysts. Despite improvements in catalytic activity, the underlying mechanism of the oxygen evolution reaction (OER) is not completely elucidated owing to the lack of direct spectroscopic evidence for the active Mn-oxo moieties. We identify water oxidation intermediates on the surface of Mn3 O4 nanoparticles (NPs) in the OER at neutral pH by in situ Raman spectroscopy. A potential-dependent Raman peak was detected at 760 cm-1 and assigned to the active MnIV =O species generated during water oxidation. Isotope-labeling experiments combined with scavenger experiments confirmed the generation of surface terminal MnIV =O intermediates in the Mn-oxide NPs. This study provides an insight into the design of systems for the observation of reaction intermediates.

Electrocatalytic Reduction of CO2 to Ethylene by Molecular Cu-Complex Immobilized on Graphitized Mesoporous Carbon.

The electrochemical reduction of carbon dioxide (CO2 ) to hydrocarbons is a challenging task because of the issues in controlling the efficiency and selectivity of the products. Among the various transition metals, copper has attracted attention as it yields more reduced and C2 products even while using mononuclear copper center as catalysts. In addition, it is found that reversible formation of copper nanoparticle acts as the real catalytically active site for the conversion of CO2 to reduced products. Here, it is demonstrated that the dinuclear molecular copper complex immobilized over graphitized mesoporous carbon can act as catalysts for the conversion of CO2 to hydrocarbons (methane and ethylene) up to 60%. Interestingly, high selectivity toward C2 product (40% faradaic efficiency) is achieved by a molecular complex based hybrid material from CO2 in 0.1 m KCl. In addition, the role of local pH, porous structure, and carbon support in limiting the mass transport to achieve the highly reduced products is demonstrated. Although the spectroscopic analysis of the catalysts exhibits molecular nature of the complex after 2 h bulk electrolysis, morphological study reveals that the newly generated copper cluster is the real active site during the catalytic reactions.

Anion Extraction-Induced Polymorph Control of Transition Metal Dichalcogenides.

Controlled phase conversion in polymorphic transition metal dichalcogenides (TMDs) provides a new synthetic route for realizing tunable nanomaterials. Most conversion methods from the stable 2H to metastable 1T phase are limited to kinetically slow cation insertion into atomically thin layered TMDs for charge transfer from intercalated ions. Here, we report that anion extraction by the selective reaction between carbon monoxide (CO) and chalcogen atoms enables predictive and scalable TMD polymorph control. Sulfur vacancy, induced by anion extraction, is a key factor in molybdenum disulfide (MoS2) polymorph conversion without cation insertion. Thermodynamic MoS2-CO-CO2 ternary phase diagram offers a processing window for efficient sulfur vacancy formation with precisely controlled MoS2 structures from single layer to multilayer. To utilize our efficient phase conversion, we synthesize vertically stacked 1T-MoS2 layers in carbon nanofibers, which exhibit highly efficient hydrogen evolution reaction catalytic activity. Anion extraction induces the polymorph conversion of tungsten disulfide (WS2) from 2H to 1T. This reveals that our method can be utilized as a general polymorph control platform. The versatility of the gas-solid reaction-based polymorphic control will enable the engineering of metastable phases in 2D TMDs for further applications.

Mechanistic Investigation of Water Oxidation Catalyzed by Uniform, Assembled MnO Nanoparticles.

The development of active water oxidation catalysts is critical to achieve high efficiency in overall water splitting. Recently, sub-10 nm-sized monodispersed partially oxidized manganese oxide nanoparticles were shown to exhibit not only superior catalytic performance for oxygen evolution, but also unique electrokinetics, as compared to their bulk counterparts. In the present work, the water-oxidizing mechanism of partially oxidized MnO nanoparticles was investigated using integrated in situ spectroscopic and electrokinetic analyses. We successfully demonstrated that, in contrast to previously reported manganese (Mn)-based catalysts, Mn(III) species are stably generated on the surface of MnO nanoparticles via a proton-coupled electron transfer pathway. Furthermore, we confirmed as to MnO nanoparticles that the one-electron oxidation step from Mn(II) to Mn(III) is no longer the rate-determining step for water oxidation and that Mn(IV)═O species are generated as reaction intermediates during catalysis.

Ru-Doped Co3O4 Nanoparticles as Efficient and Stable Electrocatalysts for the Chlorine Evolution Reaction.

The electrochemical chlorine evolution reaction (CER) is one of the most important electrochemical reactions. Typically, iridium (Ir)- or ruthenium (Ru)-based mixed metal oxides have been used as electrocatalysts for the CER due to their high activities and durabilities. However, the scarcity of Ir and Ru has indicated the need to develop alternative earth-abundant transition-metal-based CER catalysts. In this study, we report a Co3O4 nanoparticle (NP) catalyst synthesized by a hydrothermal method. Furthermore, Ru was successfully incorporated into the Co3O4 NPs (RuxCo3-xO4 NPs) for further improvement of catalytic performance in chlorine generation. Electrokinetic analyses combined with in situ X-ray absorption near-edge structure (XANES) results suggested an identical CER mechanism for the Co3O4 NPs and RuxCo3-xO4 NPs. Various characterization techniques demonstrated that the homogeneous substitution of Ru4+ ions into the Co3+ octahedral sites enhanced the structural disorder and changed the electronic state of Co3O4, resulting in additional exposed active sites. Remarkably, the Ru0.09Co2.91O4 NP electrode exhibited outstanding stability for more than 150 h even at a high current density of 500 mA/cm2, which shows its commercial viability for active chlorine generation.

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.

Complex Impedance Analysis on Charge Accumulation Step of Mn3O4 Nanoparticles during Water Oxidation.

The development of efficient water-oxidizing electrocatalysts is a key issue for achieving high performance in the overall water electrolysis technique. However, the complexity of multiple electron transfer processes and large activation energies have been regarded as major bottlenecks for efficient water electrolysis. Thus, complete electrochemical processes, including electron transport, charge accumulation, and chemical bond formation/dissociation, need to be analyzed for establishing a design rule for film-type electrocatalysts. In light of this, complex capacitance analysis is an effective tool for investigating the charge accumulation and dissipation processes of film-type electrocatalysts. Here, we conduct complex capacitance analysis for the Mn3O4 nanocatalyst, which exhibits superb catalytic activity for water oxidation under neutral conditions. Charge was accumulated on the catalyst surface by the change in Mn valence between Mn(II) and Mn(IV) prior to the rate-determining O-O bond forming step. Furthermore, we newly propose the dissipation ratio (D) for understanding the energy balance between charge accumulation and charge consumption for chemical O-O bond formation. From this analysis, we reveal the potential- and thickness-dependent contribution of the charge accumulation process on the overall catalytic efficiency. We think that an understanding of complex capacitance analysis could be an effective methodology for investigating the charge accumulation process on the surface of general film-type electrocatalysts.

Mechanistic Investigation of Biomass Oxidation Using Nickel Oxide Nanoparticles in a CO2-Saturated Electrolyte for Paired Electrolysis.

A highly efficient CO2 electrolysis system could be created by introducing biomass oxidation as an alternative anodic reaction to the sluggish oxygen evolution reaction in a CO2-saturated and near-neutral electrolyte. Here, we successfully demonstrate anodic biomass oxidation by synthesizing 5 nm nickel oxide nanoparticles (NiO NPs). NiO NPs show a unique electrocatalytic activity for 5-hydroxymethylfurfural (HMF) oxidation under near-neutral conditions, exhibiting an anodic current onset (1 mA cm-2) at 1.524 V versus the reversible hydrogen electrode and a total Faradaic efficiency of ≤70%. Electrokinetic and in situ ultraviolet-visible spectroscopic analyses suggest that a redox active nickel hydroxide species is formed on the surface of NiO electrocatalysts during HMF oxidation, and this oxidation of Ni(II) hydroxide to Ni(III) oxyhydroxide could be the rate-determining step. This mechanistic study of biomass oxidation in a CO2-saturated electrolyte provides insight into constructing a highly efficient system for the paired electrolysis of CO2 reduction and biomass oxidation.

Proton-enabled activation of peptide materials for biological bimodal memory.

The process of memory and learning in biological systems is multimodal, as several kinds of input signals cooperatively determine the weight of information transfer and storage. This study describes a peptide-based platform of materials and devices that can control the coupled conduction of protons and electrons and thus create distinct regions of synapse-like performance depending on the proton activity. We utilized tyrosine-rich peptide-based films and generalized our principles by demonstrating both memristor and synaptic devices. Interestingly, even memristive behavior can be controlled by both voltage and humidity inputs, learning and forgetting process in the device can be initiated and terminated by protons alone in peptide films. We believe that this work can help to understand the mechanism of biological memory and lay a foundation to realize a brain-like device based on ions and electrons.

Spectroscopic capture of a low-spin Mn(IV)-oxo species in Ni-Mn3O4 nanoparticles during water oxidation catalysis.

High-valent metal-oxo moieties have been implicated as key intermediates preceding various oxidation processes. The critical O-O bond formation step in the Kok cycle that is presumed to generate molecular oxygen occurs through the high-valent Mn-oxo species of the water oxidation complex, i.e., the Mn4Ca cluster in photosystem II. Here, we report the spectroscopic characterization of new intermediates during the water oxidation reaction of manganese-based heterogeneous catalysts and assign them as low-spin Mn(IV)-oxo species. Recently, the effects of the spin state in transition metal catalysts on catalytic reactivity have been intensely studied; however, no detailed characterization of a low-spin Mn(IV)-oxo intermediate species currently exists. We demonstrate that a low-spin configuration of Mn(IV), S = 1/2, is stably present in a heterogeneous electrocatalyst of Ni-doped monodisperse 10-nm Mn3O4 nanoparticles via oxo-ligand field engineering. An unprecedented signal (g = 1.83) is found to evolve in the electron paramagnetic resonance spectrum during the stepwise transition from the Jahn-Teller-distorted Mn(III). In-situ Raman analysis directly provides the evidence for Mn(IV)-oxo species as the active intermediate species. Computational analysis confirmed that the substituted nickel species induces the formation of a z-axis-compressed octahedral C4v crystal field that stabilizes the low-spin Mn(IV)-oxo intermediates.

Highly Selective Active Chlorine Generation Electrocatalyzed by Co3O4 Nanoparticles: Mechanistic Investigation through in Situ Electrokinetic and Spectroscopic Analyses.

The reaction mechanism of electrochemical chloride oxidation at neutral pH is different from that at acidic pH, in which a commercial chlor-alkali process has been developed. Different proton concentrations and accelerated hydrolysis of the generated chlorine into hypochlorous acid at high pH can change the electrokinetics and stability of reaction intermediates. We have investigated a unique reaction mechanism of Co3O4 nanoparticles for chloride oxidation at neutral pH. In contrast with water oxidation, the valency of cobalt was not changed during chloride oxidation. Interestingly, a new intermediate of Co-Cl was captured spectroscopically, distinct from the reaction intermediate at acidic pH. In addition, Co3O4 nanoparticles exhibited high selectivity for active chlorine generation at neutral pH, comparable to commercially available RuO2-based catalysts. We believe that this study provides insight into designing efficient electrocatalysts for active chlorine generation at neutral pH, which can be practically applied to electrochemical water treatment coupled to hydrogen production.

Efficient Water Splitting Cascade Photoanodes with Ligand-Engineered MnO Cocatalysts.

The band edge positions of semiconductors determine functionality in solar water splitting. While ligand exchange is known to enable modification of the band structure, its crucial role in water splitting efficiency is not yet fully understood. Here, ligand-engineered manganese oxide cocatalyst nanoparticles (MnO NPs) on bismuth vanadate (BiVO4) anodes are first demonstrated, and a remarkably enhanced photocurrent density of 6.25 mA cm-2 is achieved. It is close to 85% of the theoretical photocurrent density (≈7.5 mA cm-2) of BiVO4. Improved photoactivity is closely related to the substantial shifts in band edge energies that originate from both the induced dipole at the ligand/MnO interface and the intrinsic dipole of the ligand. Combined spectroscopic analysis and electrochemical study reveal the clear relationship between the surface modification and the band edge positions for water oxidation. The proposed concept has considerable potential to explore new, efficient solar water splitting systems.

Design Principle and Loss Engineering for Photovoltaic-Electrolysis Cell System.

The effects of exchange current density, Tafel slope, system resistance, electrode area, light intensity, and solar cell efficiency were systematically decoupled at the converter-assisted photovoltaic-water electrolysis system. This allows key determinants of overall efficiency to be identified. On the basis of this model, 26.5% single-junction GaAs solar cell was combined with a membrane-electrode-assembled electrolysis cell (EC) using the dc/dc converting technology. As a result, we have achieved a solar-to-hydrogen conversion efficiency of 20.6% on a prototype scale and demonstrated light intensity tracking optimization to maintain high efficiency. We believe that this study will provide design principles for combining solar cells, ECs, and new catalysts and can be generalized to other solar conversion chemical devices while minimizing their power loss during the conversion of electrical energy into fuel.