7 research outputs found
Elucidating the Reactivity and Mechanism of CO<sub>2</sub> 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 protonâelectron 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 CO<sub>2</sub>
Revisiting Whitlockite, the Second Most Abundant Biomineral in Bone: Nanocrystal Synthesis in Physiologically Relevant Conditions and Biocompatibility Evaluation
The synthesis of pure whitlockite (WH: Ca<sub>18</sub>Mg<sub>2</sub>(HPO<sub>4</sub>)<sub>2</sub>(PO<sub>4</sub>)<sub>12</sub>) 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)<sub>2</sub>âMg(OH)<sub>2</sub>âH<sub>3</sub>PO<sub>4</sub> system based on a systematic approach. We used excess Mg<sup>2+</sup> to impede the growth of hydroxyapatite (HAP: Ca<sub>10</sub>(PO<sub>4</sub>)<sub>6</sub>(OH)<sub>2</sub>) 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 HPO<sub>4</sub><sup>2â</sup> group into the chemical structure of WH. We demonstrated that pure WH nanoparticles can be precipitated under Mg<sup>2+</sup>-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 (Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub>), which has not been utilized for water oxidation catalysis
to date, as a model system. Due to the monophasic behavior of Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> with delithiation, the Mn valency
of Li<sub>2â<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> (<i>x</i> = 0.3, 0.5, 1) can be controlled with negligible
change in the crystal framework (e.g., volume change âŒ1%).
Moreover, inductively coupled plasma mass spectrometry, X-ray photoelectron
spectroscopy, ex-situ X-ray absorption near-edge structure, galvanostatic
chargingâdischarging, and cyclic voltammetry analysis indicate
that Li<sub>2â<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> (<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<sub>2â<i>x</i></sub>MnP<sub>2</sub>O<sub>7</sub> increases from 2 to 3, the catalytic performance is enhanced in
the series Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> < Li<sub>1.7</sub>MnP<sub>2</sub>O<sub>7</sub> < Li<sub>1.5</sub>MnP<sub>2</sub>O<sub>7</sub> < LiMnP<sub>2</sub>O<sub>7</sub>. Moreover,
Li<sub>2</sub>MnP<sub>2</sub>O<sub>7</sub> itself exhibits superior
catalytic performance compared with MnO or MnO<sub>2</sub>. 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 PhotovoltaicâElectrolysis 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 photovoltaicâwater 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, MnÂ(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 MnÂ(II) to MnÂ(III) is no
longer the rate-determining step for water oxidation and that MnÂ(IV)î»O
species are generated as reaction intermediates during catalysis
Hydrated Manganese(II) Phosphate (Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O) 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 Mn<sub>4</sub>CaO<sub>5</sub> 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,
Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O, 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 Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O.
Computational analysis indicated that the structural flexibility in
Mn<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>·3H<sub>2</sub>O could
stabilize the JahnâTeller-distorted MnÂ(III) and thus facilitate
MnÂ(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 âaliveâ