Bipyridine Modified Conjugated Carbon Aerogels as a Platform for the Electrocatalysis of Oxygen Reduction Reaction

Aerogels offer a great platform for heterogeneous electrocatalysis owing to their high surface area and porosity. Atomically dispersed transition metal ions can be imbedded in these platforms at ultra-high site density to make them catalytically active for various reactions. Herein, the synthesis of a new class of conjugated microporous organic aerogels that are used as covalent 3D frameworks for the electrocatalysis of oxygen reduction reaction (ORR) is reported. Modified aerogels functionalized with bipyridine ligands enable copper ion complexation in a single-step synthesis. The aerogels’ structures are fully characterized using a wide array of spectroscopic and microscopic methods, and heat-treated in order to make them electronically conductive. After heat treatment at 600 °C, the aerogels maintained their macrostructure and became active ORR catalysts in alkaline environment, showing high mass activity and ultra-high site density.     

Hybrid of Iron Nitride and Nitrogen‐Doped Graphene Aerogel as Synergistic Catalyst for Oxygen Reduction Reaction

It is extremely desirable but challenging to create highly active, stable, and low‐cost catalysts towards oxygen reduction reaction to replace Pt‐based catalysts in order to perform the commercialization of fuel cells. Here, a novel iron nitride/nitrogen doped‐graphene aerogel hybrid, synthesized by a facile two‐step hydrothermal process, in which iron phthalocyanine is uniformly dispersed and anchored on graphene surface with the assist of π–π stacking and oxygen‐containing functional groups, is reported. As a result, there exist strong interactions between Fe _x N nanoparticles and graphene substrates, leading to a synergistic effect towards oxygen reduction reaction. It is worth noting that the onset potential and current density of the hybrid are significantly better and the charge transfer resistance is much lower than that of pure nitrogen‐doped graphene aerogel, free Fe _x N and their physical mixtures. The hybrid also exhibits comparable catalytic activity as commercial Pt/C at the same catalyst loading, while its stability and resistance to methanol crossover are superior. Interestingly, it is found that, apart from the active nature of the hybrid, the large surface area and porosity are responsible for its excellent onset potential and the high density of Fe–N–C sties and small size of Fe _x N particles boost charge transfer rate.     

Identification and Understanding of Active Sites of Non-Noble Iron-Nitrogen-Carbon Catalysts for Oxygen Reduction Electrocatalysis

Non-noble iron-nitrogen-carbon (Fe-N-C) catalysts have been explored as one type of the most promising alternatives of precious platinum (Pt) in catalyzing the oxygen reduction reaction (ORR). However, their catalytic ORR activity and stability still cannot meet the requirement of practical applications. Active sites in such catalysts are the key factors determining the catalytic performance. This review gives a critical overview on identification and understanding of active sties of non-pyrolytic and pyrolytic Fe-N-C catalysts in terms of design strategies, synthesis, characterization, functional mechanisms and performance validation. The diversity and complexity of active sites that greatly dominate the progress of Fe-N-C catalysts include Fe-containing sites (Fe-based nanoparticles and single-atom Fe-species) and metal-free sites (heteroatoms doping and defects). Meanwhile, synergistic effects are also discussed in this review with emphasis on the interaction among multiple active sites. Although substantial endeavors have been devoted to develop the efficient Fe-N-C catalysts, some challenges still remain. To facilitate further research on Fe-N-C catalysts toward practical applications, some research perspectives are prospected in the aspects of innovative synthesis methods, active-sites modulation strategies, high-resolution ex situ/in situ/operando characterization techniques, theoretical calculations, and so on. This review may provide a guideline for identifying and understanding active-sites for developing high-performance Fe-N-C catalysts.     

Single-Atom Catalysts for H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Reaction

Oxygen reduction reaction via the two-electron route (2e⁻ ORR) provides a green method for the direct production of hydrogen peroxide (H₂O₂) along with in situ utilization. The effective catalysts with high ORR activity, 2e⁻ selectivity, and stability are essential for the application of this technology. Single-atom catalysts (SACs) have attracted intensively attention for H₂O₂ electrosynthesis owing to the unique geometric and electronic configurations. In this review, the mechanism and theoretical predictions for 2e⁻ ORR over SACs are first introduced. Then, the recent advances of various SACs for the electrosynthesis of H₂O₂ are documented. And the correlation between the central atom, coordination atoms, and coordination environment of SACs and the corresponding electrocatalytic ORR performance including activity, selectivity, and stability are emphatically analyzed and summarized. Finally, the major challenges and opportunities regarding the future design of SACs for the H₂O₂ production are pointed out.     

Bimetallic-Based Electrocatalysts for Oxygen Evolution Reaction

The electrochemical oxygen evolution reaction (OER) is a core electrode reaction for the renewable production of high-purity hydrogen, carbon-based fuel, synthetic ammonia, etc. However, the sluggish kinetics of the OER result in a high overpotential and limit the widespread application of OER-based technologies. Recent studies have shown that bimetallic-based materials with the synergism of different metal components to regulate the adsorption and dissociation energy of intermediates are promising OER electrocatalyst candidates with a lower cost and energy consumption. In the past two decades, tremendous efforts have been devoted to developing OER applications of bimetallic-based materials with a focus on compositions, phase, structure, etc., to highlight the synergism of different metal components. However, there is a lack of critical thinking and organized analysis of OER applications with bimetallic-based materials. This review critically discusses the challenges of developing bimetallic-based OER materials, summarizes the current optimization strategies to enhance both activity and stability, and highlights the state-of-the-art electrocatalysts for OER. The relationship between the componential/structural features of bimetallic-based materials and their electrocatalytic properties is presented to form comprehensive electronic and geometric modifications based on thorough analysis of the reported works and discuss future efforts to realize sustainable bimetallic-based OER applications.     

Polyformamidine‐Derived Non‐Noble Metal Electrocatalysts for Efficient Oxygen Reduction Reaction

A facile approach for the template‐free synthesis of highly active non‐noble metal based oxygen reduction reaction (ORR) electrocatalysts is presented. Porous Fe?N?C/Fe/Fe₃C composite materials are obtained by pyrolysis of defined precursor mixtures of polyformamidine (PFA) and FeCl₃ as nitrogen‐rich carbon and iron sources, respectively. Selection of pyrolysis temperature (700–1100 °C) and FeCl₃ loading (5–30 wt%) yields materials with differing surface areas, porosity, graphitization degree, nitrogen and iron content, as well as ORR activity. While the ORR activity of Fe‐free materials is limited (i.e., synthesized from pure PFA), a huge increase in activity is observed for catalysts containing Fe, revealing the participation of the metal dopant in the construction of active electrocatalytic sites. Further activity improvement is achieved via acid‐leaching and repeated pyrolysis, a result which is attributed to the creation of new active sites located at the surface of the porous nitrogen‐doped carbon by dissolution of the Fe and Fe₃C nanophases. The best performing catalyst, which was synthesized with a low Fe loading (i.e., 5 wt%) and at a pyrolysis temperature of 900 °C, exhibits high activity, excellent H₂O selectivity, extended stability, in both basic and acidic media as well as a remarkable tolerance toward methanol.     

Dual 3D Ceramic Textile Electrodes: Fast Kinetics for Carbon Oxidation Reaction and Oxygen Reduction Reaction in Direct Carbon Fuel Cells at Reduced Temperatures

Direct carbon fuel cells (DCFCs) are an efficient energy‐conversion technology capable of generating electricity with carbon‐dioxide‐capture chemistry with solid carbon as fuels. The efficiency and performance of DCFCs depend on the kinetics of the carbon oxidation reactions (COR) and the oxygen reduction reactions (ORR), each occurring at anode and cathode, respectively. The limited active sites paired with reduced temperatures greatly decrease the efficiency of the electrochemical reactions. Ultraporous dual‐3D ceramic textiles (dual‐3DCT) are integrated into electrolyte‐supported DCFCs to enhance charge and mass transfer at the electrodes. Improved COR at the anode is achieved by the synergy between the 3DCT NiO–Ce_0.8Gd_0.2O_1.95 (GDC) structure and optimal carbon fuel choice. In a comparative study, DCFCs using graphitic carbon (GC) as fuel show the best COR performance when compared to DCFCs utilizing alternative fuels such as carbon black (CB) and activated carbon (AC). The 3DCT Sm_0.5Sr_0.5CoO_3‐δ–GDC (SSC–GDC) composite cathode shows electrochemical performance superior to that of the conventional screen‐printed SSC–GDC. A peak power density of 392 mW cm^?2 at 600 °C is obtained in a DCFC using the 3DCT‐anode/electrolyte/3DCT‐cathode configuration, an unprecedented value for any reported DCFC as of yet. This points toward promising applications of dual‐3DCT electrodes for reduced‐temperature DCFCs.     

Biological Redox Mediation in Electron Transport Chain of Bacteria for Oxygen Reduction Reaction Catalysts in Lithium–Oxygen Batteries

Governing the fundamental reaction in lithium–oxygen batteries is vital to realizing their potentially high energy density. Here, novel oxygen reduction reaction (ORR) catalysts capable of mediating the lithium and oxygen reaction within a solution‐driven discharge, which promotes the solution‐phase formation of lithium peroxide (Li₂O₂), are reported, thus enhancing the discharge capacity. The new catalysts are derived from mimicking the biological redox mediation in the electron transport chain in Escherichia coli, where vitamin K2 mediates the oxidation of flavin mononucleotide and the reduction of cytochrome b in the cell membrane. The redox potential of vitamin K2 is demonstrated to coincide with the suitable ORR potential range of lithium–oxygen batteries in aprotic solvent, thereby enabling its successful functioning as a redox mediator (RM) triggering the solution‐based discharge. The use of vitamin K2 prevents the growth of film‐like Li₂O₂ even in an ether‐based electrolyte, which has been reported to induce surface‐driven discharge and early passivation of the electrode, thus boosting the discharge capacity by ≈30 times. The similarity of the redox mediation in the biological cell and lithium–oxygen “cell” inspires the exploration of redox active bio‐organic compounds for potential high‐performance RMs toward achieving high specific energies for lithium–oxygen batteries.     

Coordination Shell Dependent Activity of CuCo Diatomic Catalysts for Oxygen Reduction,Oxygen Evolution,and Hydrogen Evolution Reaction

Challenges in rational designing dual-atom catalysts (DACs) give a strong motivation to construct coordination-activity correlations. Here, thorough coordination-activity correlations of DACs based on how the changes in coordination shells (CSs) of dual-atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is constructed. First, Cu,Co DACs with different CSs modifications are fabricated by using a controlled “precursors-preselection” approach. Three DACs with unique coordination environments are characterized as secondary S atoms that directly bond to Cu,Co-N₆ in lower CSs, indirectly bond in neighboring CSs, and are doped in higher CSs, respectively. Then, experimentally and theoretically, a coordination correlation resembling a planet-satellite system, where satellite coordinated atoms (heteroatom N, S) surround Cu-Co dual-atom entity in various orbitals CSs. By evaluating electrocatalytic activity indicators, differences are identified in electronic structure and electrocatalytic performance of Cu and Co centers in ORR, OER, and HER. Interestingly, initial CSs modifications for DACs may not always be advantageous for electrocatalysis. This work offers valuable insight for designing DACs for diverse applications.     

Ammonia Tolerance of Atomically Dispersed Single Metal Site Catalysts: Mechanistic Understanding and High-Performance Oxygen Reduction Electrocatalysis

The anion-exchange membrane direct ammonia fuel cell, as a carbon-free fuel cell type, has recently received increasing attention albeit suffering from high cost of using the platinum-group metal oxygen reduction reaction (ORR) catalysts. To pave the development of this promising power source, the atomically dispersed transition metal-nitrogen-carbon (M-N-C) materials with low cost and high ORR performance have allured to investigate their ammonia tolerance during the ORR. Herein, it is initially deconvoluted how compositional and structural elements of FeN₄ sites modulate catalyst's performance. Furthermore, ORR catalytic activities of the M-N-C (M = Fe, Co or Mn) and Pt/C catalysts are investigated in ammonia-containing electrolytes, showing that M-N-C catalysts have better ammonia tolerance than Pt/C. Among others, the Fe-N-C exhibits the best ammonia tolerance with only 4 mV negative shifts of half-wave potential, 2.7% decrease of current, and negligibly irreversible activity loss. The superior ammonia tolerance of MN₄ sites to Pt (111) surface is further confirmed by density functional theory calculations. The adsorption capacity of MN₄ for O₂ is higher than NH₃ and the bonding force between MN₄ and O₂ is stronger than NH₃, whereas opposite adsorption capacity and bonding force trends are observed on Pt (111) surface.     

Catalyst Design for Electrochemical Oxygen Reduction toward Hydrogen Peroxide

Precise electrochemical synthesis under ambient conditions has provided emerging opportunities for renewable energy utilization. Among many promising systems, the production of hydrogen peroxide (H₂O₂) from the cathodic oxygen reduction reaction (ORR) has attracted considerable interest in past decades due to the increasing market demands and the vital role of ORR in the electrocatalysis field. This work describes recent advances in cathodic materials for H₂O₂ synthesis from 2e^- ORR. By using Pt as a stereotype, the tuning knobs are overviewed, including the intrinsic binding strength of oxygenated species, the intermediate diffusion path and the isolation of Pt–Pt ensembles that enable 2e^- ORR pathway from 4e^- total reduction. This knowledge is successfully applied to other transition metal systems and leads to the discovery of more efficient alloy catalysts with balanced improvement on both activity and selectivity. In addition, mesostructure engineering and heteroatoms doping strategies on carbon‐based materials, which significantly boost the H₂O₂ production efficiency as compared to intact carbon sites, are also reviewed. Finally, future directions and challenges of transferring developed catalysts from lab scale tests to pilot plant operations are briefly outlooked.     

Recent Progress in MOF‐Derived,Heteroatom‐Doped Porous Carbons as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction in Fuel Cells

Currently, developing nonprecious‐metal catalysts to replace Pt‐based electrocatalysts in fuel cells has become a hot topic because the oxygen reduction reaction (ORR) in fuel cells often requires platinum, a precious metal, as a catalyst, which is one of the major hurdles for commercialization of the fuel cells. Recently, the newly emerging metal‐organic frameworks (MOFs) have been widely used as self‐sacrificed precursors/templates to fabricate heteroatom‐doped porous carbons. Here, the recent progress of MOF‐derived, heteroatom‐doped porous carbon catalysts for ORR in fuel cells is systematically reviewed, and the synthesis strategies for using different MOF precursors to prepare heteroatom‐doped porous carbon catalysts, including the direct carbonization of MOFs, MOF and heteroatom source mixture carbonization, and MOF‐based composite carbonization are summarized. The emphasis is placed on the precursor design of MOF‐derived metal‐free catalysts and transition‐metal‐doped carbon catalysts because the MOF precursors often determine the microstructures of the derived porous carbon catalysts. The discussion provides a useful strategy for in situ synthesis of heteroatom‐doped carbon ORR electrocatalysts by rationally designing MOF precursors. Due to the versatility of MOF structures, MOF‐derived porous carbons not only provide chances to develop highly efficient ORR electrocatalysts, but also broaden the family of nanoporous carbons for applications in supercapacitors and batteries.     

Nanoporous Graphene Enriched with Fe/Co‐N Active Sites as a Promising Oxygen Reduction Electrocatalyst for Anion Exchange Membrane Fuel Cells

Here, a simple but efficient way is demonstrated for the preparation of nanoporous graphene enriched with Fe/Co–nitrogen‐doped active sites (Fe/Co‐NpGr) as a potential electrocatalyst for the electrochemical oxygen reduction reaction (ORR) applications. Once graphene is converted into porous graphene (pGr) by a controlled oxidative etching process, pGr can be converted into a potential electrocatalyst for ORR by utilizing the created edge sites of pGr for doping nitrogen and subsequently to utilize the doped nitrogens to build Fe/Co coordinated centers (Fe/Co‐NpGr). The structural information elucidated using both XPS and TOF‐SIMS study indicates the presence of coordination of the M–N (M = Fe and Co)‐doped carbon active sites. Creation of this bimetallic coordination assisted by the nitrogen locked at the pore openings is found to be helping the system to substantially reduce the overpotential for ORR. A 30 mV difference in the overpotential (η) with respect to the standard Pt/C catalyst and high retention in half wave potential after 10 000 cycles in ORR can be attained. A single cell of an anion exchange membrane fuel cell (AEMFC) by using Fe/Co‐NpGr as the cathode delivers a maximum power density of ≈35 mWcm^?2 compared to 60 mWcm^?2 displayed by the Pt‐based system.     

Recent Advances in Atomic‐Level Engineering of Nanostructured Catalysts for Electrochemical CO2 Reduction

Electrochemical reduction of CO₂ into value‐added chemicals provides a promising approach to mitigate climate change caused by CO₂ from excess consumption of fossil fuels. As the CO₂ molecule is chemically inert and the reaction kinetics is sluggish, efficient electrocatalysts are thus highly required for promoting the conversion of CO₂. With great efforts devoted to improving the catalytic performance, the development of electrocatalysts for CO₂ reduction has gone from bulk metals with poor control to nanostructures with atomic precision. Nanostructured electrocatalysts with atomic precision are believed to be capable of combining the advantages of heterogeneous and homogenous catalysts. In this review, the recent advances in designing nanostructured electrocatalysts at the atomic level for boosting the catalytic performance toward CO₂ reduction and revealing the structure–property relationship are summarized. The challenges and opportunities in the near future are also proposed for paving the development of electrocatalytic CO₂ reduction.     

Zn Single Atom Catalyst for Highly Efficient Oxygen Reduction Reaction

The authors report first a new type of nitrogen‐triggered Zn single atom catalyst, demonstrating high catalytic activity and remarkable durability for the oxygen reduction reaction process. Both X‐ray absorption fine structure spectra and theoretical calculations suggest that the atomically dispersed Zn‐N₄ site is the main, as well as the most active, component with O adsorption as the rate‐limiting step at a low overpotential of 1.70 V. This work opens a new field for the exploration of high‐performance Pt‐free electrochemical oxygen reduction catalysts for fuel cells.     

Platinum Porous Nanosheets with High Surface Distortion and Pt Utilization for Enhanced Oxygen Reduction Catalysis

Unlike the well‐established shape/composition control, surface distortion is a newly emerged yet largely unexplored nanosurface engineering for boosting electrocatalysis. Tapping into the novel electrocatalysts for taking full use of the distortion effect is therefore of importance but remains a formidable challenge. Here, an approach to designing highly distorted porous Pt nanosheets (NSs) by electrochemical erosion of ultrathin PtTe₂ NSs is reported. The inherent ultrathin feature and massive leaching of Te have conspired to produce a highly distorted structure. As a result, the generated Pt NSs exhibit a much‐enhanced oxygen reduction reaction (ORR) mass and specific activity of 2.07 A mg_Pt^?1 and 3.1 mA cm^?2 at 0.90 V versus reversible hydrogen electrode, 9.8 and 10.7 times higher than those of commercial Pt/C. The highly distorted Pt NSs can endure 30 000 cycles with negligible activity decay and structure variation. Density functional theory calculations reveal that the electrochemical corrosion induced nanopores, boundaries, and vacancies consist of Pt sites with substantially low coordination numbers deviating from the one of pristine Pt (111) surface. These Pt sites actively act as electron‐depleting centers for highly efficient electron transfer toward the adsorbing O‐species. This study opens a new design for fully using the distortion effect to promote ORR performance and beyond.     

High‐Performance Metal‐Free Nanosheets Array Electrocatalyst for Oxygen Evolution Reaction in Acid

Development of low cost electrocatalysts with outstanding catalytic activity and stability for oxygen evolution reaction (OER) in acid is a major challenge to produce hydrogen energy from water splitting. Herein, a novel metal‐free electrocatalyst consisting of a oxygen‐functionalized electrochemically exfoliated graphene (OEEG) nanosheets array is reported. Benefitting from a vertically aligned arrays structure and introducing oxygen functional groups, the metal‐free OEEG nanosheets array exhibits superior electrocatalytic activity and stability toward OER with a low overpotential of 334 mV at 10 mA cm^?2 in acidic electrolyte. Such a high OER performance is thus far the best among all previously reported metal‐free carbon‐based materials, and even superior to commercial Ir/C catalysts (420 mV at 10 mA cm^?2) in acid. Characterization results and electrochemical measurements identify the COOH species in the OEEG acting as active sites for acidic OER, which is further supported by atomic‐scale scanning transmission electron microscopy imaging and electron energy‐loss spectroscopy. Density functional theory calculations reveal that the reaction pathway of dual sites that is mixed by zigzag and armchair edges (COOH‐zig‐corner) is better than the pathway of single site.