Determination of the Electron Transfer Number for the Oxygen Reduction Reaction: From Theory to Experiment

177

122

energy such as wind and solar energy has attracted enormous interest for its significant roles in mitigating CO 2 emissions and reducing dependence on petrochemicals. [1,2] At the heart of the CO 2 conversion technology, electrocatalysts are needed to promote a critical reaction, CO 2 reduction reaction (CO 2 RR) that determines the efficiency and selectivity. To date, the electrocatalysts have confronted severe bottlenecks issue: poor selectivity about various accessory products in CO 2 conversion process, and loss of efficiency toward competing hydrogen evolution. [3] The former involves associated multielectron transfer process and is difficult to accurately control the reaction process by external conditions, [4] while the latter is mainly due to the fact that the equilibrium potentials for most of the CO 2 RR are very close to hydrogen evolution reaction (HER) toward undesirable side-products in aqueous electrolytes, which degrades the electrocatalytic performance during the CO 2 RR process. [5,6] Therefore, CO 2 RR is much more complex than other energy-related electrochemical reactions such as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), and it is still a great challenge to design and synthesize electrocatalytic materials with higher product selectivity and catalytic activity for CO 2 RR.Among the electrocatalysts including metals oxides and metal-doped carbon materials, single-atom catalysts (SACs) represent an exciting class of catalysts with monodispersed metal catalytic centers and have emerged as the frontier science in both homogeneous and heterogeneous catalysis, including CO 2 conversion. [7][8][9][10] This type of catalysts contains M-N-C moiety with single atoms and is common in building metalorganic frameworks (MOFs), [11] covalent organic frameworks (COFs) [12] with transition metal macrocyclic clusters, such as porphyrin, phthalocyanine, and tetraazannulene, as well as metal-doped carbon materials [13] (e.g., graphene, carbon nanotubes, fullerene). In nature, the biomolecules, like chlorophyll (Mg-porphyrin) in leaves, and iron porphyrins in cytochrome c oxidase in blood cells, have similar structures as SACs, with special ability in photosynthesis and transforming CO 2 from cells with high efficiency and selectivity. [14] Through the selection of appropriate motifs, the construction principles of Direct conversion of CO 2 into carbon-neutral fuels or industrial chemicals holds a great promise for renewable energy storage and mitigation of greenhouse gas emission. However, experimentally finding an electrocatalyst for specific final products with high efficiency and high selectivity poses serious challenges due to multiple electron transfer, complicated intermediates, and numerous reaction pathways in electrocatalytic CO 2 reduction. Here, an intrinsic descriptor that correlates the catalytic activity with the topological, bonding, and electronic structures of catalytic centers on M-N-C based single-atom catalysts is discovered. The "volcano"-shaped relationships betwee...

Section: Reaction Pathways and Scaling Relationshipmentioning

confidence: 99%

Catalytic Mechanisms and Design Principles for Single‐Atom Catalysts in Highly Efficient CO₂ Conversion

Gong

Zhang

Lin

et al. 2019

177

122

“…[64] The calculated electron transfer number was 4, signifying the ideal four-electron dominated ORR pathway, similar to the ORR catalyzed by the commercial Pt/C catalyst. The electron transfer number per oxygen molecule was calculated using the below relation [66] 4 d d r n I The electron transfer number per oxygen molecule was calculated using the below relation [66] 4 d d r n I…”

Section: Oxygen Reduction Reactionmentioning

confidence: 99%

“…[66] Finally, the durability evaluation was carried out for 25 h, and the NiCoP/CNF900 has exhibited remarkable electrocatalytic stability throughout the length of analysis as shown in Figure 8e. Moreover, both the RDE and RRDE results of NiCoP/CNF900 suggest an efficient four-electron pathway: O 2 + 2H 2 O + 4e − → 4OH − .…”

Section: Oxygen Reduction Reactionmentioning

confidence: 99%

Electrospun Carbon Nanofibers Encapsulated with NiCoP: A Multifunctional Electrode for Supercapattery and Oxygen Reduction, Oxygen Evolution, and Hydrogen Evolution Reactions

Surendran

Shanmugapriya

Sivanantham

et al. 2018

267

148

Functionalizing nanostructured carbon nanofibers (CNFs) with bimetallic phosphides enables the material to become an active electrode for multifunctional applications. A facile electrospinning technique is utilized for the first time to develop NiCoP nanoparticles encapsulated CNFs that are used as an energy storage system of supercapattery, and as an electrocatalyst for oxygen reduction, oxygen evolution, and hydrogen evolution reaction in KOH electrolyte. Evolving from the inclusion of bimetallic phosphide nanoparticles, the NiCoP/CNF electrode unveils superior‐specific capacitance (333 Fg−1 at 2 Ag−1) and rate capability (87%). The fabricated supercapattery device offers a voltage of 1.6 V that supplies a remarkable energy density (36 Wh kg−1) along with an improved power density (4000 W kg−1) and unwavering cyclic stability (25 000 cycles). Meanwhile, the NiCoP/CNF electrode has simultaneously performed well as a multifunctional electrocatalyst for oxygen reduction reaction at a half‐wave potential of 0.82 V versus reversible hydrogen electrode and can attain a current density of 10 mA cm−2 at a very low overpotential of 268 and 130 mV for the oxygen evolution reaction and hydrogen evolution reaction, respectively. Thus, the NiCoP/CNF with all its inimitable electrode properties has profoundly proved its proficiency at handling multifunctional challenges in terms of both storage and conversion.

“…[1][2][3][4][5][6][7] However, the sluggish ORR kinetics at the cathode greatly limits the practical application of these technologies. [1][2][3][4][5][6][7] However, the sluggish ORR kinetics at the cathode greatly limits the practical application of these technologies.…”

mentioning

confidence: 99%

Graphene Layers‐Wrapped Fe/Fe₅C₂ Nanoparticles Supported on N‐doped Graphene Nanosheets for Highly Efficient Oxygen Reduction

Chen

et al. 2017

209

Synthesis of highly efficient nonprecious metal electrocatalysts for the oxygen reduction reaction (ORR) superior to platinum (Pt) is still a big challenge. Herein, a new highly active ORR electrocatalyst is reported based on graphene layers‐wrapped Fe/Fe5C2 nanoparticles supported on N‐doped graphene nanosheets (GL‐Fe/Fe5C2/NG) through simply annealing a mixture of bulk graphitic carbon nitride (g‐C3N4) and ferrocene. An interesting exfoliation–denitrogen mechanism underlying the conversion of bulk g‐C3N4 into N‐doped graphene nanosheets is revealed. Owing to the high graphitic degree, optimum N‐doping level and sufficient active sites from the graphene layers‐wrapped Fe/Fe5C2 nanoparticles, the as‐prepared GL‐Fe/Fe5C2/NG electrocatalyst obtained at 800 °C exhibits outstanding ORR activity with a 20 mV more positive half‐wave potential than the commercial Pt/C catalyst in 0.1 m KOH solution and a comparable onset potential of 0.98 V. This makes GL‐Fe/Fe5C2/NG an outstanding electrocatalyst for ORR in alkaline solution.