Elucidating the Reactivity and Mechanism of CO₂ Electroreduction at Highly Dispersed Cobalt Phthalocyanine

Transforming carbon dioxide to carbon monoxide with electrochemical methods allows for small-scale, modular conversion of point sources of carbon dioxide. In this work, through the preparation of a well-dispersed cobalt phthalocyanine model catalyst immobilized on carbon paper, we revealed high turnover frequencies for reducing carbon dioxide at low catalyst loadings, which are obscured at higher loadings due to aggregation. The low catalyst loadings have also enabled mechanistic studies that provide a detailed understanding of the molecular-level picture of how cobalt phthalocyanine facilitates proton and electron transfers in the rate-limiting step. We are able to tune the rate-limiting step from electron transfer to concerted proton–electron transfer, enabling higher rates of carbon dioxide reduction. Our results highlight the significance of dispersion for understanding the intrinsic catalytic performance of metal phthalocyanines for electroreduction of CO₂

Revisiting Whitlockite, the Second Most Abundant Biomineral in Bone: Nanocrystal Synthesis in Physiologically Relevant Conditions and Biocompatibility Evaluation

The synthesis of pure whitlockite (WH: Ca₁₈Mg₂(HPO₄)₂(PO₄)₁₂) has remained a challenge even though it is the second most abundant inorganic in living bone. Although a few reports about the precipitation of WH in heterogeneous phases have been published, to date, synthesizing WH without utilizing any effects of a buffer or various other ions remains difficult. Thus, the related research fields have encountered difficulties and have not been fully developed. Here, we developed a large-scale synthesis method for pure WH nanoparticles in a ternary Ca(OH)₂–Mg(OH)₂–H₃PO₄ system based on a systematic approach. We used excess Mg²⁺ to impede the growth of hydroxyapatite (HAP: Ca₁₀(PO₄)₆(OH)₂) and the formation of other kinetically favored calcium phosphate intermediate phases. In addition, we designed and investigated the synthesis conditions of WH under the acidic pH conditions required to dissolve HAP, which is the most thermodynamically stable phase above pH 4.2, and to incorporate the HPO₄^2– group into the chemical structure of WH. We demonstrated that pure WH nanoparticles can be precipitated under Mg²⁺-rich and acidic pH conditions without any intermediate phases. Interestingly, this synthesized nano-WH showed comparable biocompatibility with HAP. Our methodology for determining the synthesis conditions of WH could provide a new platform for investigating other important precipitants in aqueous systems

A New Water Oxidation Catalyst: Lithium Manganese Pyrophosphate with Tunable Mn Valency

The development of a water oxidation catalyst has been a demanding challenge for the realization of overall water-splitting systems. Although intensive studies have explored the role of Mn element in water oxidation catalysis, it has been difficult to understand whether the catalytic capability originates mainly from either the Mn arrangement or the Mn valency. In this study, to decouple these two factors and to investigate the role of Mn valency on catalysis, we selected a new pyrophosphate-based Mn compound (Li₂MnP₂O₇), which has not been utilized for water oxidation catalysis to date, as a model system. Due to the monophasic behavior of Li₂MnP₂O₇ with delithiation, the Mn valency of Li_2‑<i>x</i>MnP₂O₇ (<i>x</i> = 0.3, 0.5, 1) can be controlled with negligible change in the crystal framework (e.g., volume change ∼1%). Moreover, inductively coupled plasma mass spectrometry, X-ray photoelectron spectroscopy, ex-situ X-ray absorption near-edge structure, galvanostatic charging–discharging, and cyclic voltammetry analysis indicate that Li_2‑<i>x</i>MnP₂O₇ (<i>x</i> = 0.3, 0.5, 1) exhibits high catalytic stability without additional delithiation or phase transformation. Notably, we observed that, as the averaged oxidation state of Mn in Li_2‑<i>x</i>MnP₂O₇ increases from 2 to 3, the catalytic performance is enhanced in the series Li₂MnP₂O₇ < Li_1.7MnP₂O₇ < Li_1.5MnP₂O₇ < LiMnP₂O₇. Moreover, Li₂MnP₂O₇ itself exhibits superior catalytic performance compared with MnO or MnO₂. Our study provides valuable guidelines for developing an efficient Mn-based catalyst under neutral conditions with controlled Mn valency and atomic arrangement

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

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

Hydrated Manganese(II) Phosphate (Mn₃(PO₄)₂·3H₂O) as a Water Oxidation Catalyst

The development of a water oxidation catalyst has been a demanding challenge in realizing water splitting systems. The asymmetric geometry and flexible ligation of the biological Mn₄CaO₅ cluster are important properties for the function of photosystem II, and these properties can be applied to the design of new inorganic water oxidation catalysts. We identified a new crystal structure, Mn₃(PO₄)₂·3H₂O, that precipitates spontaneously in aqueous solution at room temperature and demonstrated its high catalytic performance under neutral conditions. The bulky phosphate polyhedron induces a less-ordered Mn geometry in Mn₃(PO₄)₂·3H₂O. Computational analysis indicated that the structural flexibility in Mn₃(PO₄)₂·3H₂O could stabilize the Jahn–Teller-distorted Mn­(III) and thus facilitate Mn­(II) oxidation. This study provides valuable insights into the interplay between atomic structure and catalytic activity

Biofunctionalized Ceramic with Self-Assembled Networks of Nanochannels

Nature designs circulatory systems with hierarchically organized networks of gradually tapered channels ranging from micrometer to nanometer in diameter. In most hard tissues in biological systems, fluid, gases, nutrients and wastes are constantly exchanged through such networks. Here, we developed a biologically inspired, hierarchically organized structure in ceramic to achieve effective permeation with minimum void region, using fabrication methods that create a long-range, highly interconnected nanochannel system in a ceramic biomaterial. This design of a synthetic model-material was implemented through a novel pressurized sintering process formulated to induce a gradual tapering in channel diameter based on pressure-dependent polymer agglomeration. The resulting system allows long-range, efficient transport of fluid and nutrients into sites and interfaces that conventional fluid conduction cannot reach without external force. We demonstrate the ability of mammalian bone-forming cells placed at the distal transport termination of the nanochannel system to proliferate in a manner dependent solely upon the supply of media by the self-powering nanochannels. This approach mimics the significant contribution that nanochannel transport plays in maintaining living hard tissues by providing nutrient supply that facilitates cell growth and differentiation, and thereby makes the ceramic composite “alive”