“…Although molecular catalysis by iron porphyrins has provided valuable mechanistic insights into the two‐electron/two‐proton vs. four‐electron/four‐proton reduction of O 2 , a large number of heterogeneous iron‐based materials were reported to act as much better catalysts for electrochemical reduction of O 2 than homogeneous iron complex catalysts . Even better ORR electrocatalytic performance has been reported for a two dimensional (2D) poly‐iron‐phthalocyanine PFe‐Pc, than for the state‐of‐the‐art 20 % platinum/carbon (Pt/C) catalyst in alkaline medium.…”
The two‐electron reduction of dioxygen with two protons produces hydrogen peroxide, which is directly used as a liquid fuel in hydrogen peroxide fuel cells, whereas the four‐electron reduction of dioxygen is combined with the two‐electron oxidation of hydrogen in hydrogen fuel cells. Platinum (Pt)‐based nanocomposites are the most efficient commercial electrocatalysts for the oxygen reduction reaction (ORR). However, the poor stability, scarcity and high cost of these Pt‐based oxygen electrocatalysts are major barriers for the large‐scale implementation of fuel cell technologies. Replacing noble metal‐based electrocatalysts with highly efficient and inexpensive earth‐abundant metal‐based oxygen electrocatalysts has been of critical importance for practical applications. To develop efficient catalysts for the two‐electron and four‐electron reduction of dioxygen, it is crucially important to clarify the catalytic mechanisms of two‐electron/two‐proton versus four‐electron/four‐proton reduction of dioxygen with earth‐abundant metal complexes. This review focused on the factors that control the two‐electron/two‐proton versus four‐electron/four‐proton reduction of dioxygen by electron donors (one electron reductants) such as ferrocene, catalyzed by earth‐abundant metal complexes such as iron, cobalt, copper, manganese and nickel complexes in the homogeneous phase, by detecting catalytic intermediates, which determine the catalytic pathways of the two‐electron versus four‐electron reduction of dioxygen. The electrocatalytic two‐electron or/and four‐electron reduction of dioxygen with earth‐abundant metal complexes and metal oxides has also been discussed in relation with the homogeneous catalysis.
“…Although molecular catalysis by iron porphyrins has provided valuable mechanistic insights into the two‐electron/two‐proton vs. four‐electron/four‐proton reduction of O 2 , a large number of heterogeneous iron‐based materials were reported to act as much better catalysts for electrochemical reduction of O 2 than homogeneous iron complex catalysts . Even better ORR electrocatalytic performance has been reported for a two dimensional (2D) poly‐iron‐phthalocyanine PFe‐Pc, than for the state‐of‐the‐art 20 % platinum/carbon (Pt/C) catalyst in alkaline medium.…”
The two‐electron reduction of dioxygen with two protons produces hydrogen peroxide, which is directly used as a liquid fuel in hydrogen peroxide fuel cells, whereas the four‐electron reduction of dioxygen is combined with the two‐electron oxidation of hydrogen in hydrogen fuel cells. Platinum (Pt)‐based nanocomposites are the most efficient commercial electrocatalysts for the oxygen reduction reaction (ORR). However, the poor stability, scarcity and high cost of these Pt‐based oxygen electrocatalysts are major barriers for the large‐scale implementation of fuel cell technologies. Replacing noble metal‐based electrocatalysts with highly efficient and inexpensive earth‐abundant metal‐based oxygen electrocatalysts has been of critical importance for practical applications. To develop efficient catalysts for the two‐electron and four‐electron reduction of dioxygen, it is crucially important to clarify the catalytic mechanisms of two‐electron/two‐proton versus four‐electron/four‐proton reduction of dioxygen with earth‐abundant metal complexes. This review focused on the factors that control the two‐electron/two‐proton versus four‐electron/four‐proton reduction of dioxygen by electron donors (one electron reductants) such as ferrocene, catalyzed by earth‐abundant metal complexes such as iron, cobalt, copper, manganese and nickel complexes in the homogeneous phase, by detecting catalytic intermediates, which determine the catalytic pathways of the two‐electron versus four‐electron reduction of dioxygen. The electrocatalytic two‐electron or/and four‐electron reduction of dioxygen with earth‐abundant metal complexes and metal oxides has also been discussed in relation with the homogeneous catalysis.
“…Benefiting from the small size of these abundant Fe 3 C and the hierarchically porous N-doped carbon structure, the as-prepared Fe 3 C@N-doped CNTs exhibited a remarkable ORR activity in alkaline media [54]. Covalent porphyrin polymers (CPP) synthesized from terephthalaldehyde and pyrrole incorporated with iron chlorides (Fe-CPP) were converted into porous Ndoped Fe 3 C@Cs as reported by Kong et al [55], and the derived Fe 3 C nanocrystals with 10-50 nm particle size were encapsulated in N-doped carbon shells.…”
Section: Iron Carbide Npsmentioning
confidence: 92%
“…The obtained catalyst showed excellent ORR activity that outperformed Pt/C in alkaline media. Organic iron sources such as Fe-based MOFs and covalent organic polymers (COPs) were utilized to prepare carbon encapsulated iron carbide catalysts [54,55]. It was reported that a Fe-based MIL-88B guest was introduced into a Zn-based ZIF-8 host to form a MOF-in-MOF structure.…”
It is crucial to develop low-cost and highly efficient catalysts for oxygen reduction reaction (ORR) which is the key process in electrochemical energy conversion and storage devices. Transition metal-based nanoparticles/carbon materials are an important class of non-noble metal catalysts that have attracted considerable research interest. The topic of this review is mainly focused on carbon encapsulated Fe/Cobased nanoparticles catalysts for ORR, and these catalysts are summarized in categories of metals, oxides, carbides, phosphides, sulfides and hybrid nanoparticles. The structures and morphologies of the carbon matrix as well as compositions of nanoparticles have great influence on the catalytic performance. Numerous catalysts display excellent ORR activity and stability in alkaline media but only a few are efficient in acidic media. In addition, challenges and further strategies on the development of this type of carbon encapsulated nanoparticles catalysts are also proposed.
“…Crystalline COFs and their derived porous materials have also been recently studied as precursors for efficient ORR catalysts. In a scalable one‐pot assembly, a covalent porphyrin polymer (CPP) incorporated with ferric chloride (Fe‐CPP) was synthesized, which upon pyrolysis at 900 °C for 4 h under N 2 yielded N‐doped Fe 3 C@C‐900 with unique core‐shell morphology (carbon shells encapsulating Fe 3 C nanoparticles) . The as‐prepared Fe‐CPP had a low BET surface area of 95.5 m 2 g −1 , which suggested the incorporation of FeCl 3 within the micropores of CPP.…”
“…e) Current–time CA responses of Fe 3 C@C‐900 and Pt/C recorded at 0.55 V in 0.1 m O 2 ‐ saturated KOH solution; f) CV curves run for 5000 cycles in 0.1 m O 2 ‐saturated KOH solution at a scan rate of 10 mV s −1 . Reproduced with permission . Copyright 2017, Elsevier.…”
The recent progress and challenges in developing highly active microporous materials as heterogeneous electrocatalysts for oxygen reduction reaction (ORR) are discussed. Due to the slow kinetics of ORR, the search for novel highly stable catalysts that can outperform Pt‐based ones is a growing demand. Functional microporous materials and exceptional chemical stability are of great interest due to their unique catalytic properties. The crucial rule of porosity and catalyst design strategies are thoroughly discussed with emphasis on the influence of the degree of microporosity on the material's intrinsic catalytic activity and the current best practice for catalyst testing and activity reporting. This Review focuses on microporous materials, which are classified into crystalline as metal–organic frameworks (MOFs), covalent–organic frameworks (COFs), or amorphous porous–organic polymers (POPs) as to their recent utilization in constructing efficient ORR electrocatalysts. The fundamental challenges in obtaining stable catalysts with appreciable kinetics from non‐noble metal ions, and ways to form them utilizing microporous solids as catalyst matrices, are reviewed. The fundamental criteria for future generation ORR catalysts based on microporous solids, including stability in acidic medium, high electrical conductivity, and accessibility of the active sites within the matrix are highlighted for guidance to future works.
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