BACKGROUND With a rising global population, increasing energy demands, and impending climate change, major concerns have been raised over the security of our energy future. Developing sustainable, fossil-free pathways to produce fuels and chemicals of global importance could play a major role in reducing carbon dioxide emissions while providing the feedstocks needed to make the products we use on a daily basis. One prospective goal is to develop electrochemical conversion processes that can convert molecules in the atmosphere (e.g., water, carbon dioxide, and nitrogen) into higher-value products (e.g., hydrogen, hydrocarbons, oxygenates, and ammonia) by coupling to renewable energy. Electrocatalysts play a key role in these energy conversion technologies because they increase the rate, efficiency, and selectivity of the chemical transformations involved. Today’s electrocatalysts, however, are inadequate. The grand challenge is to develop advanced electrocatalysts with the enhanced performance needed to enable widespread penetration of clean energy technologies. ADVANCES Over the past decade, substantial progress has been made in understanding several key electrochemical transformations, particularly those that involve water, hydrogen, and oxygen. The combination of theoretical and experimental studies working in concert has proven to be a successful strategy in this respect, yielding a framework to understand catalytic trends that can ultimately provide rational guidance toward the development of improved catalysts. Catalyst design strategies that aim to increase the number of active sites and/or increase the intrinsic activity of each active site have been successfully developed. The field of hydrogen evolution, for example, has seen important breakthroughs over the years in the development of highly active non–precious metal catalysts in acid. Notable advancements have also been made in the design of oxygen reduction and evolution catalysts, although there remains substantial room for improvement. The combination of theory and experiment elucidates the remaining challenges in developing further improved catalysts, often involving scaling relations among reactive intermediates. This understanding serves as an initial platform to design strategies to circumvent technical obstacles, opening up opportunities and approaches to develop higher-performance electrocatalysts for a wide range of reactions. OUTLOOK A systematic framework of combining theory and experiment in electrocatalysis helps to uncover broader governing principles that can be used to understand a wide variety of electrochemical transformations. These principles can be applied to other emerging and promising clean energy reactions, including hydrogen peroxide production, carbon dioxide reduction, and nitrogen reduction, among others. Although current paradigms for catalyst development have been helpful to date, a number of challenges need to be successfully addressed in order to achieve major breakthroughs. One important frontier, for example, is the development of both experimental and computational methods that can rapidly elucidate reaction mechanisms on broad classes of materials and in a wide range of operating conditions (e.g., pH, solvent, electrolyte). Such efforts would build on current frameworks for understanding catalysis to provide the deeper insights needed to fine-tune catalyst properties in an optimal manner. The long-term goal is to continue improving the activity and selectivity of these catalysts in order to realize the prospects of using renewable energy to provide the fuels and chemicals that we need for a sustainable energy future.

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Combining theory and experiment in electrocatalysis: Insights into materials design

BACKGROUND Materials by design is an appealing idea that is very hard to realize in practice. Combining the best of different ingredients in one ultimate material is a task for which we currently have no general solution. However, we do have some successful examples to draw upon: Composite materials and III-V heterostructures have revolutionized many aspects of our lives. Still, we need a general strategy to solve the problem of mixing and matching crystals with different properties, creating combinations with predetermined attributes and functionalities. ADVANCES Two-dimensional (2D) materials offer a platform that allows creation of heterostructures with a variety of properties. One-atom-thick crystals now comprise a large family of these materials, collectively covering a very broad range of properties. The first material to be included was graphene, a zero-overlap semimetal. The family of 2D crystals has grown to includes metals (e.g., NbSe 2 ), semiconductors (e.g., MoS 2 ), and insulators [e.g., hexagonal boron nitride (hBN)]. Many of these materials are stable at ambient conditions, and we have come up with strategies for handling those that are not. Surprisingly, the properties of such 2D materials are often very different from those of their 3D counterparts. Furthermore, even the study of familiar phenomena (like superconductivity or ferromagnetism) in the 2D case, where there is no long-range order, raises many thought-provoking questions. A plethora of opportunities appear when we start to combine several 2D crystals in one vertical stack. Held together by van der Waals forces (the same forces that hold layered materials together), such heterostructures allow a far greater number of combinations than any traditional growth method. As the family of 2D crystals is expanding day by day, so too is the complexity of the heterostructures that could be created with atomic precision. When stacking different crystals together, the synergetic effects become very important. In the first-order approximation, charge redistribution might occur between the neighboring (and even more distant) crystals in the stack. Neighboring crystals can also induce structural changes in each other. Furthermore, such changes can be controlled by adjusting the relative orientation between the individual elements. Such heterostructures have already led to the observation of numerous exciting physical phenomena. Thus, spectrum reconstruction in graphene interacting with hBN allowed several groups to study the Hofstadter butterfly effect and topological currents in such a system. The possibility of positioning crystals in very close (but controlled) proximity to one another allows for the study of tunneling and drag effects. The use of semiconducting monolayers leads to the creation of optically active heterostructures. The extended range of functionalities of such heterostructures yields a range of possible applications. Now the highest-mobility graphene transistors are achieved by encapsulating graphene with hBN. Photovoltaic and light-emitting devices have been demonstrated by combining optically active semiconducting layers and graphene as transparent electrodes. OUTLOOK Currently, most 2D heterostructures are composed by direct stacking of individual monolayer flakes of different materials. Although this method allows ultimate flexibility, it is slow and cumbersome. Thus, techniques involving transfer of large-area crystals grown by chemical vapor deposition (CVD), direct growth of heterostructures by CVD or physical epitaxy, or one-step growth in solution are being developed. Currently, we are at the same level as we were with graphene 10 years ago: plenty of interesting science and unclear prospects for mass production. Given the fast progress of graphene technology over the past few years, we can expect similar advances in the production of the heterostructures, making the science and applications more achievable.

2D materials and van der Waals heterostructures

Fuel cells powered by hydrogen from secure and renewable sources are the ideal solution for non-polluting vehicles, and extensive research and development on all aspects of this technology over the past fifteen years has delivered prototype cars with impressive performances. But taking the step towards successful commercialization requires oxygen reduction electrocatalysts--crucial components at the heart of fuel cells--that meet exacting performance targets. In addition, these catalyst systems will need to be highly durable, fault-tolerant and amenable to high-volume production with high yields and exceptional quality. Not all the catalyst approaches currently being pursued will meet those demands.

Electrocatalyst approaches and challenges for automotive fuel cells

A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

Controlling the structure of thermoelectric materials on all length scales (atomic, nanoscale and mesoscale) relevant for phonon scattering makes it possible to increase the dimensionless figure of merit to more than two, which could allow for the recovery of a significant fraction of waste heat with which to produce electricity.

High-performance bulk thermoelectrics with all-scale hierarchical architectures

The operation of batteries, fuel cells, and other energy storage and conversion devices is underpinned by the intricate synergy of ion transport, phase transformations, and electrochemical reactions. While electrochemical processes in well-defined molecular systems are amenable to classical chemical techniques including in situ optical spectroscopies and nuclear magnetic resonance (NMR), phase transformations and reactivity in solids and at solid interfaces remain largely an enigma. The presence of multiple types of structural defects and imperfections, competing electron and ion transport pathways, and multiple possible surface reconstruction types, combined with the directionality of ion flows and state of charge transformation fronts render these phenomena inaccessible to macroscopic interrogation techniques. At the same time, knowledge of the individual mechanisms involved in electrochemical reactivity of solids at the level of individual, atomic size structural elements is required for understanding the fundamental mechanisms of energy storage, factors controlling charge/discharge rates, degradation and failure processes, and ultimately knowledge-driven prediction and optimization of energy materials. 1-3

Functional Electron Microscopy for Electrochemistry Research: From the Atomic to the Micro Scale

https://onlinelibrary.wiley.com/doi/pdf/10.1002/adma.201370249

Phase Transitions, Phase Coexistence, and Piezoelectric Switching Behavior in Highly Strained BiFeO3 Films (Adv. Mater. 39/2013)

We report a solvothermal method for the facile and reproducible synthesis of BaTiO3 nanocubes by employing anatase TiO2 nanorods as a precursor to titanium hydroxide. The TiO2 nanorods were, in turn, synthesized in advance through another solvothermal process. We could obtain high-quality BaTiO3 nanocubes by reacting the TiO2 nanorods with Ba(NO3)2 under solvothermal conditions in the presence of oleic acid and acetic acid as dual capping agents, followed by a purification process involving chemical washing and physical filtration. The addition of acetic acid alongside oleic acid was critical to the formation of well-defined {100} facets on the nanocrystals and also increased batch-to-batch reproducibility without the use of a glove box or similar air-free experimental design. Using this method, BaTiO3 nanocubes crystallized in the cubic phase could be readily obtained with an average edge length around 8 nm.

Facile Synthesis of BaTiO3 Nanocubes with the Use of Anatase TiO2 Nanorods as a Precursor to Titanium Hydroxide

Li0.625Al0.125H0.25Cl0.75O0.25 Superionic Conductor with Disordered Rock-Salt Structure

Recent development of electrochemical technologies for renewable energy conversion and storage has enlightened the importance of tailoring the structures of catalytic materials at the nanoscale. In particular, the design and synthesis of nanocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have attracted great attention and been extensively studied. Remarkable progress has been made in improving the catalytic activity and stability of ORR catalysts by controlling and tuning the particle size, shape, composition and surface structures, as well as building up sophisticated composite nanostructures in core/shell and nanoporous configurations. Here a brief review is provided for the recent development of Pt-based nanocatalysts for the ORR. Instead of providing a complete list of the great amount of work reported in this topic, our focus is placed on fundamental understanding of the structure-property relationships of platinum-based nanomaterials, in particular alloy nanoparticles, in the electrochemical environment. The discussion is guided by correlations between well-defined extended surfaces and practical high-surface-area catalysts. Conclusions are made on challenges that remain and potential implications on other catalytic systems.

Miaofang Chi

Papers

Functional Electron Microscopy for Electrochemistry Research: From the Atomic to the Micro Scale

Phase Transitions, Phase Coexistence, and Piezoelectric Switching Behavior in Highly Strained BiFeO3 Films (Adv. Mater. 39/2013)

Facile Synthesis of BaTiO3 Nanocubes with the Use of Anatase TiO2 Nanorods as a Precursor to Titanium Hydroxide

Li0.625Al0.125H0.25Cl0.75O0.25 Superionic Conductor with Disordered Rock-Salt Structure

Recent Development of Platinum-Based Nanocatalysts for Oxygen Reduction Electrocatalysis