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 a...
In spite of recent advances in the synthesis of hollow micro/nanostructures, the fabrication of three-dimensional electrodes on the basis of these structures remains a major challenge. Herein, we develop an electrochemical sacrificial-template strategy to fabricate hollow Co O microtube arrays with hierarchical porosity. The resultant unique structures and integrated electrode configurations impart enhanced mass transfer and electron mobility, ensuring high activity and stability in catalyzing oxygen and hydrogen evolution reactions. Impressively, the apparent performance can rival that of state-of-the-art noble-metal and transition-metal electrocatalysts. Furthermore, this bifunctional electrode can be used for highly efficient overall water splitting, even competing with the integrated performance of Pt/C and IrO /C.
has received utmost attention and are typically classified into two categories, namely, transition-metal-based (e.g., Fe, Co, Ni) and nonmetal candidates. [5][6][7][8][9] However, the insufficient electronic conductivity and inherent corrosion susceptibility largely inhibit the application potential of transition metals or their derivatives that generally underperform in comparison with the noble-metal benchmarks. On the other hand, carbon materials featuring adjustable molecular structures, fascinating physicochemical properties, and strong chemical stability are generally employed as supports for poorly conducting electrocatalysts to ensure instant electron transfer, thereby inducing synergistic effects to improve OER activity. [10][11][12] Therefore, it is of great significance to understand the intrinsic role of the carbonaceous substrates in these electrocatalytic systems, and insights thus received can be further applied to exploit novel carbon-based OER electrocatalysts. However, the development of highly efficient OER catalytic systems with little or even no metal for sustainable oxygen production is still a challenging task. Chemical modification of carbon materials via doping of foreign heteroatoms (such as N, P, B, and S) paves a way for improving their apparent electrocatalytic properties by modifying their electronic structures and chemical states. [13][14][15][16][17][18] Particularly, dual doping in carbon frameworks can result in synergistic coupling effects between the two types of heteroatoms, [19][20][21] and can consequently enhance the oxygen evolution performance to a large extent in terms of onset potential and current density. Nevertheless, current studies concerning chemically modified carbon for electrochemical OER are usually inconclusive, and deep investigation toward the corresponding electrocatalytic mechanism remains in its infancy.In addition to chemical doping, physical tailoring by the introduction of well-structured porosity into these carbon materials can expose abundant accessible active sites and offer interconnected channels for enhanced mass transfer, [22,23] both of which are beneficial in boosting electrocatalytic OER activity. Further, the electron transport capability, determined by the inherent electronic conductivity of the catalysts, represents another critical factor in governing the electrochemical activity. [24,25] Conventional electrocatalysts generally exist in the form of 1D or 2D particles (i.e., powdery materials), which should be further Developing earth-abundant and active electrocatalysts for the oxygen evolution reaction (OER) as replacements for conventional noble metal catalysts is of scientific and technological importance for achieving cost-effective and efficient conversion and storage of renewable energy. However, most of the promising candidates thus far are exclusively metal-based catalysts, which are disadvantaged by relatively restricted electron mobility, corrosion susceptibility, and detrimental environmental influences. Herein, hierarchically...
In spite of recent advances in the synthesis of hollow micro/nanostructures,t he fabrication of three-dimensional electrodes on the basis of these structures remains am ajor challenge.H erein, we develop an electrochemical sacrificialtemplate strategy to fabricate hollowC o 3 O 4 microtube arrays with hierarchicalporosity.The resultant unique structures and integrated electrode configurations impart enhanced mass transfer and electron mobility,e nsuring high activity and stability in catalyzing oxygen and hydrogen evolution reactions.Impressively,the apparent performance can rival that of state-of-the-art noble-metal and transition-metal electrocatalysts.F urthermore,t his bifunctional electrode can be used for highly efficient overall water splitting,even competing with the integrated performance of Pt/C and IrO 2 /C.
Abstract:Global climate change and increasing demands for clean energy have brought intensive interest in the search for proper electrocatalysts in order to reduce carbon dioxide (CO 2 ) to higher value carbon products such as hydrocarbons. Recently, transition-metal-centered molecules or organic frameworks have been reported to show outstanding electrocatalytic activity in the liquid phase. Their d-orbital electrons are believed to be one of the key factors to capture and convert CO 2 molecules to value-added low-carbon fuels. In this review, recent advances in electrocatalytic CO 2 reduction have been summarized based on the targeted products, ranging from homogeneous reactions to heterogeneous ones. Their advantages and fallbacks have been pointed out and the existing challenges, especially with respect to the practical and industrial application are addressed.
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