Engineering Carbon Materials for Electrochemical Oxygen Reduction Reactions

Abstract

exchange membrane fuel cells (PEMFCs) and metal-air batteries, the ORR starts from the adsorption of O 2 molecules at the cathode, electrochemical activation of the OO bond, and then the formation of O-containing groups such as OOH*, O*, and OH*. In this way, O 2 is reduced by the electrons (Figure 1a). [4] The efficiency of these energy devices is usually determined by that of the ORR process. However, the strong bond energy of OO (498 kJ mol −1 ) means that the ORR at the electrode is not easy, especially compared to the hydrogen oxidation reaction (HOR: H 2 →2H + + 2e − ) at the anode of hydrogen-oxygen fuel cells (Figure 1a). Improvement of OO bond activation and cleavage with catalysts is therefore highly important in developing efficient energy storage and conversion systems as well as fast and efficient chemical production techniques.Nevertheless, in industry, the most common ORR catalysts are still Pt-based materials, which are prohibitively expensive for wider application (Figure 1b). [5] There is a pressing need to explore alternative electrocatalysts that are cheap and stable. Significant efforts have been made to develop alternative materials (e.g., transition metal oxides, alloys, metal-organic frameworks/MOFs, single atom catalyst) and investigate their catalytic mechanisms, [6][7][8][9][10][11][12] but challenges remain. In 2009, Dai's group reported using doped carbon nanotubes (CNTs) to catalyze ORR that exhibited better catalytic efficiency than commercial Pt/C electrodes in alkaline media. [13] It should be noted that, in alkaline media, the hydroxide conducting polymeric membranes (e.g., in anion exchange membrane fuel cells) are unstable and have poor resistance to CO 2 , while the overpotentials for hydrogen oxidation are also high. [14][15][16] Compared to metal-based catalysts, carbon materials have significant merits of outstanding anti-corrosion performance and electrochemical durability, as well as the possibility of low-cost manufacture. This combination means that carbon materials are seen as highly promising candidates to replace precious metals as ORR. In the years following Dai's pioneering work, various other doped carbon materials were further developed and their catalytic mechanisms investigated. [17][18][19][20][21][22] Some more recent studies explored this further and suggested that active intrinsic carbon structural defects are associated with efficient ORR catalysis (Figure 2b). [23][24][25][26] Although great progress has been achieved on this topic, the origin and mechanism of the ORR activity are still not fully clear. Beyond the catalytic activity, no pattern has been established in the selectivityThe electrochemical oxygen reduction reaction (ORR) is the key energy conversion reaction involved in fuel cells, metal-air batteries, and hydrogen peroxide production. Proliferation and improvement of the ORR requires wider use of new and existing high performance catalysts; unfortunately, most of these are still based on precious metals and become uneconomical in massuse ...

Bioderived Ionic Liquids and Salts with Various Cyano Anions as Precursors for Doped Carbon Materials

“…The onset potential for the ORR varied between materials and generally increased with an increase in the carbonization temperature (Table 3). This enhanced activity can be ascribed to the increased amount of intrinsic carbon structural defects, which was confirmed by Raman spectroscopy (Figure 3c) [37]. Moreover, the highest onset potential, −0.12 V, was observed for the carbon material derived at 800 °C from the [Carb][SCN] precursor, also indicating the remarkable effect of N and S dual-doping [38].…”

Plasma modified and tailored defective electrocatalysts for water electrolysis and hydrogen fuel cells

“…The edges on carbon materials exhibit better catalytic activity than the basal planes in ORR and HOR 76,160,161 and therefore, exposure of carbon edges is a popular strategy in the design of fuel cell electrocatalysts. Plasma-assisted etching is suitable for creating more edges and local or atomic defects on carbon materials and thus plasma etching of non-noble metal electrocatalysts has aroused interests.…”

Recent Advances in the Application and Mechanism of Carbon Dots/Metal‐Organic Frameworks Hybrids in Photocatalysis and the Detection of Environmental Pollutants

“…), whose high price and corrosion-prone defects prompt researchers to search for cheaper and more stable alternatives. [54] The CDs@MOFs composite catalysts provide a new idea for the development of new electrocatalysts. Rehman and coworkers [55] reported a novel CDs@MOFs (Ni) electrocatalyst containing two ligands, 1, 3, 5-benzenetricarboxylic acid (BTC) and 4, 4-bipyridine (bpy) (Figure 5b).…”

“…At present, the most common electrochemical catalysts are noble metal‐based electrocatalysts (e. g. Pt, Pd, Au, etc. ), whose high price and corrosion‐prone defects prompt researchers to search for cheaper and more stable alternatives [54] . The CDs@MOFs composite catalysts provide a new idea for the development of new electrocatalysts.…”