Achievements, challenges and perspectives on cathode catalysts in proton exchange membrane fuel cells for transportation

197

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: Discussionmentioning

confidence: 99%

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

Gong

Zhang

Lin

et al. 2019

197

122

“…[ 122,124 ] In addition, the long‐term activity loss of the SACs was ascribed to the constant electrochemical carbon oxidation above 0.207 V versus standard hydrogen electrode (SHE). [ 125 ] In turn, such carbon corrosion could accelerate the demetallation of the active sites, which would lead to a chain reaction to further accelerate the performance decrease of the fuel cells. [ 125 ]…”

Section: Characterization Of Sacs: From Rde To Pemfcsmentioning

confidence: 99%

Molecular Design of Single‐Atom Catalysts for Oxygen Reduction Reaction

Wan

Duan

Huang

2020

339

210

His thesis mainly focuses on developing graphene-based single-atom catalysts and the surface engineering of Pt-based nanostructures for energy conversion and storage. Xiangfeng Duan received his Ph.D. in physical chemistry from Harvard University and his B.S. degree in chemistry from the University of Science and Technology of China. His research group at UCLA focuses on developing new material systems for highly efficient energy harvesting, conversion, and storage via the rational design and nanoscale integration of distinct materials. Taking advantage of unique 2D materials, he conducts research on the facile and controllable synthesis of novel single atoms catalysts for energy demanding applications and investigates their structure-activity relationships at atomic level to improve their catalytic activity for future application in real-life devices. Yu Huang received her Ph.D. in physical chemistry from Harvard University and her B.S. degree in chemistry from the University of Science and Technology of China.Her research group at UCLA explores the unique technological opportunities that result from the structure and assembly of nanoscale building blocks. Focusing at phenomena that occur at molecular level, she aims to unravel the fundamental principles that govern the synthesis and assembly of nanoscale material. Moreover, she uses such principles to design nanostructures and nanodevices with unique functions and properties to address critical challenges in electronics, energy science, and biomedicine.

“…In an energy cycle driven by renewable sources, electricity is first converted into chemical energy in the form of H 2 and O 2 through water splitting, utilizing electrochemical reactions such as the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) . Then, the generated H 2 and O 2 are converted back to water in fuel cells, through the oxygen reduction reaction (ORR), regenerating the electricity . The discharging and charging processes in rechargeable metal–air batteries are driven by the and OER, respectively .…”

Section: Introductionmentioning

confidence: 99%

Engineering Local Coordination Environments of Atomically Dispersed and Heteroatom‐Coordinated Single Metal Site Electrocatalysts for Clean Energy‐Conversion

Zhu

Sokolowski

Song

et al. 2019

Self Cite

293

240

Carbon‐based heteroatom‐coordinated single‐atom catalysts (SACs) are promising candidates for energy‐related electrocatalysts because of their low‐cost, tunable catalytic activity/selectivity, and relatively homogeneous morphologies. Unique interactions between single metal sites and their surrounding coordination environments play a significant role in modulating the electronic structure of the metal centers, leading to unusual scaling relationships, new reaction mechanisms, and improved catalytic performance. This review summarizes recent advancements in engineering of the local coordination environment of SACs for improved electrocatalytic performance for several crucial energy‐convention electrochemical reactions: oxygen reduction reaction, hydrogen evolution reaction, oxygen evolution reaction, CO2 reduction reaction, and nitrogen reduction reaction. Various engineering strategies including heteroatom‐doping, changing the location of SACs on their support, introducing external ligands, and constructing dual metal sites are comprehensively discussed. The controllable synthetic methods and the activity enhancement mechanism of state‐of‐the‐art SACs are also highlighted. Recent achievements in the electronic modification of SACs will provide an understanding of the structure–activity relationship for the rational design of advanced electrocatalysts.