Despite the dedicated search for novel catalysts for fuel cell applications, the intrinsic oxygen reduction reaction (ORR) activity of materials has not improved significantly over the past decade. Here, we review the role of theory in understanding the ORR mechanism and highlight the descriptor-based approaches that have been used to identify catalysts with increased activity. Specifically, by showing that the performance of the commonly studied materials (e.g., metals, alloys, carbons, etc.) is limited by unfavorable scaling relationships (for binding energies of reaction intermediates), we present a number of alternative strategies that may lead to the design and discovery of more promising materials for ORR.

Understanding Catalytic Activity Trends in the Oxygen Reduction Reaction.

Metal–organic frameworks (MOFs) are porous crystalline materials constructed from metal ions or clusters and multidentate organic ligands. Recently, the use of MOFs or MOF composites as catalysts for synergistic catalysis and tandem reactions has attracted increasing attention due to their tunable open metal centres, functional organic linkers, and active guest species in their pores. In this review, the applications of MOFs with multiple active sites in synergistic organic catalysis, photocatalysis and tandem reactions are discussed. These multifunctional MOFs can be categorized by the type of active centre as follows: (i) open metal centres and functional organic linkers in the MOF structure, (ii) active guest sites in the pores and active sites in the MOF structure, and (iii) bimetallic nanoparticles (NPs) on MOF supports. The types of synergistic catalysis and tandem reactions promoted by multifunctional MOFs and their proposed mechanisms are presented in detail. Here, catalytic MOFs with a single type of active site and MOFs that only serve as supports to enhance substrate adsorption are not discussed.

Multifunctional metal–organic framework catalysts: synergistic catalysis and tandem reactions

Photoreduction of CO2 into sustainable and green solar fuels is generally believed to be an appealing solution to simultaneously overcome both environmental problems and energy crisis. The low selectivity of challenging multi-electron CO2 photoreduction reactions makes it one of the holy grails in heterogeneous photocatalysis. This Review highlights the important roles of cocatalysts in selective photocatalytic CO2 reduction into solar fuels using semiconductor catalysts. A special emphasis in this review is placed on the key role, design considerations and modification strategies of cocatalysts for CO2 photoreduction. Various cocatalysts, such as the biomimetic, metal-based, metal-free, and multifunctional ones, and their selectivity for CO2 photoreduction are summarized and discussed, along with the recent advances in this area. This Review provides useful information for the design of highly selective cocatalysts for photo(electro)reduction and electroreduction of CO2 and complements the existing reviews on various semiconductor photocatalysts.

Cocatalysts for Selective Photoreduction of CO2 into Solar Fuels.

Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, ...

Emerging Two-Dimensional Nanomaterials for Electrocatalysis

Advances in electrocatalysis at solid-liquid interfaces are vital for driving the technological innovations that are needed to deliver reliable, affordable and environmentally friendly energy. Here, we highlight the key achievements in the development of new materials for efficient hydrogen and oxygen production in electrolysers and, in reverse, their use in fuel cells. A key issue addressed here is the degree to which the fundamental understanding of the synergy between covalent and non-covalent interactions can form the basis for any predictive ability in tailor-making real-world catalysts. Common descriptors such as the substrate-hydroxide binding energy and the interactions in the double layer between hydroxide-oxides and H---OH are found to control individual parts of the hydrogen and oxygen electrochemistry that govern the efficiency of water-based energy conversion and storage systems. Links between aqueous- and organic-based environments are also established, encouraging the 'fuel cell' and 'battery' communities to move forward together.

Energy and fuels from electrochemical interfaces

Pt-Ni bimetallic alloys with various nanostructures have shown excellent activity toward oxygen reduction reaction (ORR). The ORR activity is highly dependent on the structure of the catalyst. In this paper, Pt- Ni nanourchins were synthesized with an average size of 50 nm consisting of 10–20 nanorods and nanooctahedra by adjusting the synthesis condition. The formation of Pt-Ni nanourchins is mainly dependent on the adding order of solvents (benzyl ether, oleylamine and oleic acid). Pt-Ni nanourchins present a reasonable high ORR activity (0.81 A/mg at 0.9 V).

Pt-Ni nanourchins as electrocatalysts for oxygen reduction reaction

Selective electrochemical oxygen reduction (ORR) toward a two-electron (2e−) pathway is an eco-friendly alternative method for H2O2 synthesis to replace the energy-intensive anthraquinone oxidation process. Carbon-based electrocatalysts (CBEs) show great potential for practical H2O2 synthesis. However, their complex structures make it challenging to determine the nature of active sites and to precisely control them. Herein, we show that precise modulation of the chemistry and structures of holey graphene with edge sites enriched by oxygen-containing functional groups can facilitate 2e− ORR. These combined functionalities could improve ORR performance under various pH conditions, for example, resulting in an average of 95% H2O2 selectivity, ~97% Faraday efficiency, high productivity of 2360 mol kgcat−1 h−1 in alkaline media. Density functional theory calculations on the oxygen functional groups at the edge sites revealed the most active site for 2e− ORR is a synergy between ether (COC) and carbonyl (CO) functional groups with nearly zero overpotential. 

Elucidation and modulation of active sites in holey graphene electrocatalysts for
 
 H
 2
 O
 2
 
 production

Doping the PtNi octahedra with trace amount of transition metals has recently been shown to significantly improve the activity and durability of the oxygen reduction reaction (ORR). However, the role of excessive amounts of doped elements on ORR performance is unclear. In this work, PtNi octahedra were synthesized with and without excess Mo dopant. Also, spherical PtNi nanoparticles containing excess Co dopant were synthesized. The ORR performance revealed that PtNiMo octahedra and PtNiCo nanoparticles showed inferior ORR activities compared to bimetallic PtNi octahedra. Based on the electrochemical analysis, we found that unfavorable oxygen-binding strengths of PtNiMo and PtNiCo catalysts decrease the ORR performances. 

Excess dopant effect in
 platinum‐based
 alloys toward the oxygen electroreduction reaction

To effectively utilize glycerol as a fuel for electrochemical fuel cells, it is necessary to optimize catalysts for effective C-C bond cleavage and complete oxidation of reaction intermediates to achieve maximum efficiency. The current work showed that the synergistic interactions of platinum (Pt) with transition metal carbide (TMC) substrates, such as tungsten carbide (WC) and tantalum carbide (TaC), fulfilled these criteria. The TMC-supported Pt catalysts showed higher activity and selectivity for complete glycerol oxidation than commercial 10 wt% Pt/C. In-situ FTIR analysis revealed that 5 wt% Pt/WC was the most effective catalyst among those tested for complete glycerol oxidation at 0.9 V vs RHE. In-situ X-ray absorption fine structure characterization and density functional theory calculations provided additional insight into the synergistic interactions for glycerol oxidation over Pt/TMC catalysts. 

Enhancing Glycerol Electrooxidation from Synergistic Interactions of Platinum and Transition Metal Carbides

Oxygen evolution reaction (OER) is the anodic half‐reaction for crucial energy devices, such as water electrolysis, metal–air battery, and electrochemical CO2 reduction. Fe‐based materials are recognized as one of the most promising electrocatalysts for OER because of its extremely low price and high activity. In particular, iron oxyhydroxide (FeOOH) is not only highly active toward OER, but also widely accepted as the true active species of Fe‐based OER electrocatalysts for plenty of Fe‐based materials are converted into FeOOH during OER test. Herein, the recent advances of FeOOH‐based nano‐structure and its application in OER are reviewed. The relationship between FeOOH structure and its catalytic performance, followed by the introduction of current strategies for enhancing the OER activity (i.e., crystalline phase engineering, element doping, and construction of hybrid materials) is mainly focused. Finally, a summary and perspective about the remaining challenges and future opportunities in this area and further the design of Fe‐based OER electrocatalysts are provided.

Qiaowan Chang

Papers

Pt-Ni nanourchins as electrocatalysts for oxygen reduction reaction

Elucidation and modulation of active sites in holey graphene electrocatalysts for H 2 O 2 production

Excess dopant effect in platinum‐based alloys toward the oxygen electroreduction reaction

Enhancing Glycerol Electrooxidation from Synergistic Interactions of Platinum and Transition Metal Carbides

Iron Oxyhydroxide: Structure and Applications in Electrocatalytic Oxygen Evolution Reaction