Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential-determining step. Subsequently, novel cobalt oxide-based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water-oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non-ruthenium) complexes that efficiently mediate O2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox-potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.

The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis

We report the creation of a nanoscale electrochemical device inside a transmission electron microscope--consisting of a single tin dioxide (SnO(2)) nanowire anode, an ionic liquid electrolyte, and a bulk lithium cobalt dioxide (LiCoO(2)) cathode--and the in situ observation of the lithiation of the SnO(2) nanowire during electrochemical charging. Upon charging, a reaction front propagated progressively along the nanowire, causing the nanowire to swell, elongate, and spiral. The reaction front is a "Medusa zone" containing a high density of mobile dislocations, which are continuously nucleated and absorbed at the moving front. This dislocation cloud indicates large in-plane misfit stresses and is a structural precursor to electrochemically driven solid-state amorphization. Because lithiation-induced volume expansion, plasticity, and pulverization of electrode materials are the major mechanical effects that plague the performance and lifetime of high-capacity anodes in lithium-ion batteries, our observations provide important mechanistic insight for the design of advanced batteries.

https://science.sciencemag.org/content/sci/330/6010/1515.full.pdf

In Situ Observation of the Electrochemical Lithiation of a Single SnO2 Nanowire Electrode

Electrocatalysis is crucial for the development of clean and renewable energy technologies, which may reduce our reliance on fossil fuels. Multimetallic nanomaterials serve as state-of-the-art electrocatalysts as a consequence of their unique physico-chemical properties. One method of enhancing the electrocatalytic performance of multimetallic nanomaterials is to tune or control the surface strain of the nanomaterials, and tremendous progress has been made in this area in the past decade. In this Review, we summarize advances in the introduction, tuning and quantification of strain in multimetallic nanocrystals to achieve more efficient energy conversion by electrocatalysis. First, we introduce the concept of strain and its correlation with other key physico-chemical properties. Then, using the electrocatalytic reduction of oxygen as a model reaction, we discuss the underlying mechanisms behind the strain–adsorption–reactivity relationship based on combined classical theories and models. We describe how this knowledge can be harnessed to design multimetallic nanocrystals with optimized strain to increase the efficiency of oxygen reduction. In particular, we highlight the unexpectedly beneficial (and previously overlooked) role of tensile strain from multimetallic nanocrystals in improving electrocatalysis. We conclude by outlining the challenges and offering our perspectives on the research directions in this burgeoning field. Tuning the surface strain in multimetallic nanomaterials represents an effective strategy to improve their electrocatalytic properties. In this Review, using the oxygen reduction reaction as a model, the underlying relationship between surface strain and catalytic activity is discussed, along with the introduction, tuning and quantification of strain in nanocatalysts.

Strain-controlled electrocatalysis on multimetallic nanomaterials

Liqiang Mai,*,† Xiaocong Tian,† Xu Xu,† Liang Chang,‡ and Lin Xu†,§ †State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, WUT-Harvard Joint Nano Key Laboratory, Wuhan University of Technology, Wuhan 430070, China ‡Department of Materials Science and Engineering, Michigan Technological University, Houghton, Michigan 49931-1295, United States Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States

Nanowire electrodes for electrochemical energy storage devices.

Well-defined Li4Ti5O12 nanosheets terminated with rutile-TiO2 at the edges were synthesized by a facile solution-based method and revealed directly at atomic resolution by an advanced spherical aberration imaging technique. The rutile-TiO2 terminated Li4Ti5O12 nanosheets show much improved rate capability and specific capacity compared with pure Li4Ti5O12 nanosheets when used as anode materials for lithium ion batteries. The results here give clear evidence of the utility of rutile-TiO2 as a carbon-free coating layer to improve the kinetics of Li4Ti5O12 toward fast lithium insertion/extraction. The carbon-free nanocoating of rutile-TiO2 is highly effective in improving the electrochemical properties of Li4Ti5O12, promising advanced batteries with high volumetric energy density, high surface stability, and long cycle life compared with the commonly used carbon nanocoating in electrode materials.

Rutile-TiO2 nanocoating for a high-rate Li4Ti5O12 anode of a lithium-ion battery.

LiCoO2 is the most common lithium storage material for lithium rechargeable batteries, used widely to power portable electronic devices such as laptop computers. Lithium arrangements in the CoO2 framework have a profound effect on the structural stability and electrochemical properties of LixCoO2 (0 < x < 1), however, probing lithium ions has been difficult using traditional X-ray and neutron diffraction techniques. Here we have succeeded in simultaneously resolving columns of cobalt, oxygen, and lithium atoms in layered LiCoO2 battery material using experimental focal series of LiCoO2 images obtained at sub-Angstrom resolution in a mid-voltage transmission electron microscope. Lithium atoms are the smallest and lightest metal atoms, and scatter electrons only very weakly. We believe our observations of lithium to be the first by electron microscopy, and that they show promise to direct visualization of the ordering of lithium and vacancy in LixCoO2.

/pdf/atomic-resolution-of-lithium-ions-in-licoo2-ro5e13wff8.pdf

Atomic resolution of lithium ions in LiCoO2.

LiCoO{sub 2} is the most common lithium storage material used as positive electrode in lithium rechargeable batteries Ordering of lithium and vacancies has a profound effect on the physical properties of Li{sub x}CoO{sub 2} and the electrochemical performances of lithium batteries An exit surface wave (ESW) phase image reconstructed from experimental images obtained on the LBNL One-Angstrom Microscope (OAM) shows all three types of atoms in LiCoO{sub 2}

/pdf/atomic-resolution-of-lithium-ions-in-licoo-sub-2-1mpqs98fk6.pdf

Atomic resolution of lithium ions in LiCoO{sub 2}

The One-Angstrom Microscope (OAM) project was established at the NCEM to produce images at sub-Angstrom resolution (O'Keefe, 1993). The project was implemented using a Philips CM300FEG/UT with hardware modifications designed to correct objective lens three-fold astigmatism and extend information transfer to 0.8 Angstrom (O'Keefe et al., 2001a). A Gatan image filter (GIFTM) was used to bring the image magnification to more than three million times at the CCD camera to provide adequate real-space sampling. Phase-contrast imaging in the HRTEM produces images with peaks at atom positions by extracting the spatial distribution of the relative phase from the electron wave. Usually, the electron wave is imaged by direct interference of diffracted beams at optimum focus (Scherzer, 1949).

/pdf/sub-angstrom-resolution-with-a-mid-voltage-tem-56lv93mibd.pdf

Sub-Ångstrom Resolution with a Mid-Voltage TEM

Phase-contrast imaging in the high-resolution electron microscope produces images with peaks at atom positions by extracting the spatial distribution of the relative phase from the electron wave. Usually, the electron wave is imaged by direct interference of diffracted beams at optimum focus. Instead, the One-Angstrom Microscope uses focal-series reconstruction software to derive the relative electron phase from a series of images taken over a range of focus, with peaks that correspond to the atom positions at a resolution that extends to the microscope information limit. Tests using a silicon specimen tilted into [112] orientation show that the O Angstrom M has achieved a world-record resolution of 0.78 Angstrom.

/pdf/reaching-sub-angstrom-resolution-with-a-mid-voltage-tem-281fz8mtf7.pdf

Reaching sub-Angstrom resolution with a mid-voltage TEM

We have used the One-Angstrom Microscope (OAM) to image and apply focal-series reconstruction (FSR) of the exit-surface wave (ESW) to a 70Angstrom particle of gold supported on amorphous carbon. The phase of the complex ESW shows the positions of the atom columns in the specimen as white dots, and its diffractogram shows it contains information to 1.23Angstrom. The result demonstrates that through-focal reconstruction of the ESW does not need large crystal expanses to work properly. Although [110] Au structures may not need sub-Angstrom resolution to show all the useful structural details of the particle in this orientation, it is clear that focal reconstruction of the ESW can improve original data that is much more difficult to interpret directly. We expect this technique to prove even more useful when applied to nanoparticles containing finer spacings than the 2.35Angstrom separation of the 111 planes in the present gold nano-particle.

/pdf/focal-series-reconstruction-of-nanoparticle-exit-surface-2hghusovv7.pdf

E. Chris Nelson

Papers

Atomic resolution of lithium ions in LiCoO2.

Atomic resolution of lithium ions in LiCoO{sub 2}

Sub-Ångstrom Resolution with a Mid-Voltage TEM

Reaching sub-Angstrom resolution with a mid-voltage TEM

Focal-series reconstruction of nanoparticle exit-surface electron wave