Oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are three key reactions for the development of green and sustainable energy systems. Efficient electrocatalysts for these reactions are highly desired to lower their overpotentials and promote practical applications of related energy devices. Metal–organic frameworks (MOFs) have recently emerged as precursors to fabricate carbon-based electrocatalysts with high electrical conductivity and uniformly distributed active sites. In this review, the current progress of MOF-derived carbon-based materials for ORR/OER/HER electrocatalysis is presented. Materials design strategies of MOF-derived carbon-based materials are firstly summarized to show the rich possibilities of the morphology and composition of MOF-derived carbon-based materials. A wide range of applications based on these materials for ORR, OER, HER and multifunctional electrocatalysis are discussed, with an emphasis on the required features of MOF-derived carbon-based materials for the electrocatalysis of corresponding reactions. Finally, perspectives on the development of MOF-derived carbon-based materials for ORR, OER and HER electrocatalysis are provided.

MOF-derived electrocatalysts for oxygen reduction, oxygen evolution and hydrogen evolution reactions

We determined the size-dependent specific and mass activities of the oxygen reduction in HClO(4) solutions on the Pt particles in the range of 1-5 nm. The maximal mass activity at 2.2 nm is well explained based on density functional theory calculations performed on fully relaxed nanoparticles. The presence of the edge sites is the main reason for the low specific activity in nanoparticles due to very strong oxygen binding energies at these sites. Our results clearly demonstrate that the catalytic activity highly depends on the shape and size of the nanoparticles.

http://ma.ecsdl.org/content/MA2011-01/42/1931.full.pdf

Electrocatalysis on Platinum Nanoparticles: Particle Size Effect on Oxygen Reduction Reaction Activity

Electrocatalysis plays a central role in clean energy conversion, enabling a number of processes for future sustainable technologies. Atomic site electrocatalysts (ASCs), including single-atomic site catalysts (SASCs) and diatomic site catalysis (DASCs), are being pursued as economical alternatives to noble-metal-based catalysts for these reactions by virtue of their exceptionally high atom utilization efficiencies, well-defined active sites and high selectivities. In this review, we start from a systematic review on the fabrication routes of ASCs followed by an overview of some new and effective characterization methods to precisely probe the atomic structure. Then we give a comprehensive summary on the current advances in some typical clean energy reactions: water splitting, including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER); oxygen reduction reaction (ORR), including selective 4e- - ORR toward H2O/OH- and 2e- - ORR toward H2O2/HO2-; selective electrooxidation of formic acid, methanol and ethanol (FAOR, MOR and EOR). At the end of this paper, we present a brief conclusion, and discuss the challenges and opportunities on the further development of more selective, active, stable and less expensive ASCs.

Atomic site electrocatalysts for water splitting, oxygen reduction and selective oxidation.

Advanced Electrocatalysts for the Oxygen Reduction Reaction in Energy Conversion Technologies

Transition metal nitrides (TMNs), by virtue of their unique electronic structure, high electrical conductivity, superior chemical stability, and excellent mechanical robustness, have triggered tremendous research interest over the past decade, and showed great potential for electrochemical energy conversion and storage. However, bulk TMNs usually suffer from limited numbers of active sites and sluggish ionic kinetics, and eventually ordinary electrochemical performance. Designing nanostructured TMNs with tailored morphology and good dispersity has proved an effective strategy to address these issues, which provides a larger specific surface area, more abundant active sites, and shorter ion and mass transport distances over the bulk counterparts. Herein, the most up-to-date progress on TMN-based nanomaterials is comprehensively reviewed, focusing on geometric-structure design, electronic-structure engineering, and applications in electrochemical energy conversion and storage, including electrocatalysis, supercapacitors, and rechargeable batteries. Finally, we outline the future challenges of TMN-based nanomaterials and their possible research directions beyond electrochemical energy applications.

Transition metal nitrides for electrochemical energy applications.

DOI: 10.1002/aenm.201802856 For the quick development and initial commercialization of proton exchange membrane fuel cells (PEMFCs), one of the serious challenges is the use of scarce, costly platinum as a catalyst to overcome the sluggish kinetics and high overpotential of the oxygen reduction reaction (ORR) at the cathode[1–3] In the last decade, tremendous efforts have been made to explore highly efficient non-precious-metal catalysts, such as doped carbon, to replace Pt/C.[4,5] Singleatom Fe-embedded nitrogen-doped carbon (Fe–Nx/C) has attracted particular attention recently, showing superior ORR performance that makes it a promising material for further investigation[6–11] Though great progress has already been achieved, the rational design and preparation of a single-atom Fe-doped catalyst (Fe–Nx/C) with a large surface area, beneficial porous structure, high active-site density, and high performance in an acidic PEMFC remains a considerable hurdle.[12–15] Zeolitic imidazole frameworks (ZIFs) are self-assembling, ordered, porous materials with bridging metal atoms and N-containing ligands. ZIFs provide a good platform for the preparation of high-performance carbon catalysts doped with single metal atoms[16,17] Several groups have reported preparing carbon catalysts doped with single Fe (or other metal) atoms using ZIFs or other metal–organic frameworks (MOFs) as precursors[18] For instance, Jiang and coworkers[19] used a porphyrinic MOF as the precursor to prepare single-atom Fe-implanted N-doped porous carbon via a mixed-ligand strategy; their catalyst exhibited excellent oxygen reduction activity in an acidic medium, with a half-wave potential of 0.776 V. Gao and co-workers[20] reported creating a ZIF-derived catalyst doped with N-coordinated single-atom Fe, using a ferrocene encapsulated in ZIF-8 strategy whereby ferrocene was introduced during the assembly of Zn2+ ions with 2-methylimidazole; the catalyst exhibited good oxygen reduction activity in an alkaline medium. However, it is still a great challenge for obtaining satisfactory performance in a single acidic PEMFC. To date, few intensive investigations of single Fe-doped carbon as the cathode catalyst in single PEMFCs have been reported[14,19] Generally, the inadequate performance of doped-carbon A metalorganic gaseous doping approach for constructing nitrogen-doped carbon polyhedron catalysts embedded with single Fe atoms is reported. The resulting catalysts are characterized using scanning transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy; for the optimal sample, calculated densities of Fe–Nx sites and active N sites reach 1.75812 × 1013 and 1.93693 × 1014 sites cm-2, respectively. Its oxygen reduction reaction half-wave potential (0.864 V) is 50 mV higher than that of 20 wt% Pt/C catalyst in an alkaline medium and comparable to the latter (0.78 V vs 0.84 V) in an acidic medium, along with outstanding durability. More importantly, when used as a hydrogen–oxygen polymer electrolyte membrane fuel cell (PEMFC) cathode catalyst with a catalyst loading as low as 1 mg cm-2 (compared with a conventional loading of 4 mg cm-2), it exhibits a current density of 1100 mA cm-2 at 0.6 V and 637 mA cm-2 at 0.7 V, with a power density of 775 mW cm-2, or 0.775 kW g–1 of catalyst. In a hydrogen–air PEMFC, current density reaches 650 mA cm-2 at 0.6 V and 350 mA cm-2 at 0.7 V, and the maximum power density is 463 mW cm-2, which makes it a promising candidate for cathode catalyst toward high-performance PEMFCs.

Atomic Fe‐Doped MOF‐Derived Carbon Polyhedrons with High Active‐Center Density and Ultra‐High Performance toward PEM Fuel Cells

The investigation of highly effective, durable, and cost-effective electrocatalysts for the hydrogen evolution reaction (HER) is a prerequisite for the upcoming hydrogen energy society. To establish a new hydrogen energy system and gradually replace the traditional fossil-based energy, electrochemical water-splitting is considered the most promising, environmentally friendly, and efficient way to produce pure hydrogen. Compared with the commonly used platinum (Pt)-based catalysts, ruthenium (Ru) is expected to be a good alternative because of its similar hydrogen bonding energy, lower water decomposition barrier, and considerably lower price. Analyzing and revealing the HER mechanisms, as well as identifying a rational design of Ru-based HER catalysts with desirable activity and stability is indispensable. In this review, the research progress on HER electrocatalysts and the relevant describing parameters for HER performance are briefly introduced. Moreover, four major strategies to improve the performance of Ru-based electrocatalysts, including electronic effect modulation, support engineering, structure design, and maximum utilization (single atom) are discussed. Finally, the challenges, solutions and prospects are highlighted to prompt the practical applications of Ru-based electrocatalysts for HER.

/pdf/engineering-ruthenium-based-electrocatalysts-for-effective-49238140ub.pdf

Jing Li

Papers

Atomic Fe‐Doped MOF‐Derived Carbon Polyhedrons with High Active‐Center Density and Ultra‐High Performance toward PEM Fuel Cells

Engineering Ruthenium-Based Electrocatalysts for Effective Hydrogen Evolution Reaction

Advanced Atomically Dispersed Metal–Nitrogen–Carbon Catalysts Toward Cathodic Oxygen Reduction in PEM Fuel Cells

Recent advances in two-dimensional Pt based electrocatalysts for methanol oxidation reaction

Single atomic cobalt catalyst for efficient oxygen reduction reaction