Yao Zheng

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

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.

Nanoporous Graphitic-C3N4@Carbon Metal-Free Electrocatalysts for Highly Efficient Oxygen Reduction

Based on theoretical prediction, a g-C(3)N(4)@carbon metal-free oxygen reduction reaction (ORR) electrocatalyst was designed and synthesized by uniform incorporation of g-C(3)N(4) into a mesoporous carbon to enhance the electron transfer efficiency of g-C(3)N(4). The resulting g-C(3)N(4)@carbon composite exhibited competitive catalytic activity (11.3 mA cm(-2) kinetic-limiting current density at -0.6 V) and superior methanol tolerance compared to a commercial Pt/C catalyst. Furthermore, it demonstrated significantly higher catalytic efficiency (nearly 100% of four-electron ORR process selectivity) than a Pt/C catalyst. The proposed synthesis route is facile and low-cost, providing a feasible method for the development of highly efficient electrocatalysts.

Advancing the Electrochemistry of the Hydrogen-Evolution Reaction through Combining Experiment and Theory

The electrocatalytic hydrogen-evolution reaction (HER), as the main step of water splitting and the cornerstone of exploring the mechanism of other multi-electron transfer electrochemical processes, is the subject of extensive studies. A large number of high-performance electrocatalysts have been developed for HER accompanied by recent significant advances in exploring its electrochemical nature. Herein we present a critical appraisal of both theoretical and experimental studies of HER electrocatalysts with special emphasis on the electronic structure, surface (electro)chemistry, and molecular design. It addresses the importance of correlating theoretical calculations and electrochemical measurements toward better understanding of HER electrocatalysis at the atomic level. Fundamental concepts in the computational quantum chemistry and its relation to experimental electrochemistry are also presented along with some featured examples.

Facile Oxygen Reduction on a Three‐Dimensionally Ordered Macroporous Graphitic C3N4/Carbon Composite Electrocatalyst

Honeycomb catalysis: a facile oxygen reduction reaction has been observed on a graphitic C(3)N(4)/carbon catalyst with three-dimensional interconnected macropores (see picture with SiO(2) template). This material not only shows catalytic activity that is comparable to that of commercial Pt/C, but also has much higher organic-fuel tolerance and long-term stability.

Origin of the Electrocatalytic Oxygen Reduction Activity of Graphene-Based Catalysts: A Roadmap to Achieve the Best Performance

The mutually corroborated electrochemical measurements and density functional theory (DFT) calculations were used to uncover the origin of electrocatalytic activity of graphene-based electrocatalysts for oxygen reduction reaction (ORR). A series of graphenes doped with nonmetal elements was designed and synthesized, and their ORR performance was evaluated in terms of four electrochemical descriptors: exchange current density, on-set potential, reaction pathway selectivity and kinetic current density. It is shown that these descriptors are in good agreement with DFT calculations, allowing derivation of a volcano plot between the ORR activity and the adsorption free energy of intermediates on metal-free materials, similarly as in the case of metallic catalysts. The molecular orbital concept was used to justify this volcano plot, and to theoretically predict the ORR performance of an ideal graphene-based catalyst, the ORR activity of which is comparable to the state-of-the-art Pt catalyst. Moreover, this study may stimulate the development of metal-free electrocatalysts for other key energy conversion processes including hydrogen evolution and oxygen evolution reactions and largely expand the spectrum of catalysts for energy-related electrocatalysis reactions.

Toward Design of Synergistically Active Carbon-Based Catalysts for Electrocatalytic Hydrogen Evolution

Replacement of precious Pt catalyst with cost-effective alternatives would be significantly beneficial for hydrogen production via electrocatalytic hydrogen evolution reaction (HER). All candidates thus far are exclusively metallic catalysts, which suffer inherent corrosion and oxidation susceptibility during acidic proton-exchange membrane electrolysis. Herein, based on theoretical predictions, we designed and synthesized nitrogen (N) and phosphorus (P) dual-doped graphene as a nonmetallic electrocatalyst for sustainable and efficient hydrogen production. The N and P heteroatoms could coactivate the adjacent C atom in the graphene matrix by affecting its valence orbital energy levels to induce a synergistically enhanced reactivity toward HER. As a result, the dual-doped graphene showed higher electrocatalytic HER activity than single-doped ones and comparable performance to some of the traditional metallic catalysts.

Nanostructured Metal‐Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction

Replacing precious and nondurable Pt catalysts with cheap and commercially available materials to facilitate sluggish cathodic oxygen reduction reaction (ORR) is a key issue in the development of fuel cell technology. The recently developed cost effective and highly stable metal-free catalysts reveal comparable catalytic activity and significantly better fuel tolerance than that of current Pt-based catalysts; therefore, they can serve as feasible Pt alternatives for the next generation of ORR electrocatalysts. Their promising electrocatalytic properties and acceptable costs greatly promote the R&D of fuel cell technology. This review provides an overview of recent advances in state-of-the-art nanostructured metal-free electrocatalysts including nitrogen-doped carbons, graphitic-carbon nitride (g-C(3) N(4) )-based hybrids, and 2D graphene-based materials. A special emphasis is placed on the molecular design of these electrocatalysts, origin of their electrochemical reactivity, and ORR pathways. Finally, some perspectives are highlighted on the development of more efficient ORR electrocatalysts featuring high stability, low cost, and enhanced performance, which are the key factors to accelerate the commercialization of fuel cell technology.

Molecule-level g-C3N4 coordinated transition metals as a new class of electrocatalysts for oxygen electrode reactions

Organometallic complexes with metal-nitrogen/carbon (M-N/C) coordination are the most important alternatives to precious metal catalysts for oxygen reduction and evolution reactions (ORR and OER) in energy conversion devices. Here, we designed and developed a range of molecule-level graphitic carbon nitride (g-C3N4) coordinated transition metals (M-C3N4) as a new generation of M-N/C catalysts for these oxygen electrode reactions. As a proof-of-concept example, we conducted theoretical evaluation and experimental validation on a cobalt-C3N4 catalyst with a desired molecular configuration, which possesses comparable electrocatalytic activity to that of precious metal benchmarks for the ORR and OER in alkaline media. The correlation of experimental and computational results confirms that this high activity originates from the precise M-N2 coordination in the g-C3N4 matrix. Moreover, the reversible ORR/OER activity trend for a wide variety of M-C3N4 complexes has been constructed to provide guidance for the molecular design of this promising class of catalysts.

Mesoporous hybrid material composed of Mn3O4 nanoparticles on nitrogen-doped graphene for highly efficient oxygen reduction reaction

The hybrid material composed of Mn3O4 nanoparticles on nitrogen-doped graphene was prepared via a solvothermal process and investigated for the first time as a catalyst for oxygen reduction reaction (ORR). Its high ORR activity, excellent durability and tolerance to methanol make this hybrid material a promising candidate for highly efficient ORR in fuel cells and metal-air batteries.

Surface and interface engineering of noble-metal-free electrocatalysts for efficient energy conversion processes

Developing cost-effective and high-performance electrocatalysts for renewable energy conversion and storage is motivated by increasing concerns regarding global energy security and creating sustainable technologies dependent on inexpensive and abundant resources. Recent achievements in the design and synthesis of efficient non-precious-metal and even non-metal electrocatalysts make the replacement of noble metal counterparts for the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) with earth-abundant elements, for example, C, N, Fe, Mn, and Co, a realistic possibility. It has been found that surface atomic engineering (e.g., heteroatom-doping) and interface atomic or molecular engineering (e.g., interfacial bonding) can induce novel physicochemical properties and strong synergistic effects for electrocatalysts, providing new and efficient strategies to greatly enhance the catalytic activities. In this Account, we discuss recent progress in the design and fabrication of efficient electrocatalysts based on carbon materials, graphitic carbon nitride, and transition metal oxides or hydroxides for efficient ORR, OER, and HER through surface and interfacial atomic and molecular engineering. Atomic and molecular engineering of carbon materials through heteroatom doping with one or more elements of noticeably different electronegativities can maximally tailor their electronic structures and induce a synergistic effect to increase electrochemical activity. Nonetheless, the electrocatalytic performance of chemically modified carbonaceous materials remains inferior to that of their metallic counterparts, which is mainly due to the relatively limited amount of electrocatalytic active sites induced by heteroatom doping. Accordingly, coupling carbon substrates with other active electrocatalysts to produce composite structures can impart novel physicochemical properties, thereby boosting the electroactivity even further. Although the majority of carbon-based materials remain uncompetitive with state-of-the-art metal-based catalysts for the aforementioned catalytic processes, non-metal carbon hybrids have already shown performance that typically only conventional noble metals or transition metal materials can achieve. The idea of hybridized carbon-based catalysts possessing unique active surfaces and macro- or nanostructures is addressed herein. For metal-carbon couples, the incorporation of carbon can effectively compensate for the intrinsic deficiency in conductivity of the metallic components. Chemical modification of carbon frameworks, such as nitrogen doping, not only can change the electron-donor character, but also can introduce anchoring sites for immobilizing active metallic centers to form metal-nitrogen-carbon (M-N-C) species, which are thought to facilitate the electrocatalytic process. With thoughtful material design, control over the porosity of composites, the molecular architecture of active metal moieties and macromorphologies of the whole catalysts can be achieved, leading to a better understanding structure-activity relationships. We hope that we can offer new insight into material design, particularly the role of chemical composition and structural properties in electrochemical performance and reaction mechanisms.