MOF/PCP-based Electrocatalysts for the Oxygen Reduction Reaction

Rational Design of Organic Electrocatalysts for Hydrogen and Oxygen Electrocatalytic Applications

Efficient electrocatalysts are pivotal for advancing green energy conversion technologies. Organic electrocatalysts, as cost-effective alternatives to noble-metal benchmarks, have garnered attention. However, the understanding of the relationships between their properties and electrocatalytic activities remains ambiguous. Plenty of research articles regarding low-cost organic electrocatalysts started to gain momentum in 2010 and have been flourishing recently though, a review article for both entry-level and experienced researchers in this field is still lacking. This review underscores the urgent need to elucidate the structure-activity relationship and design suitable electrode structures, leveraging the unique features of organic electrocatalysts like controllability and compatibility for real-world applications. Organic electrocatalysts are classified into four groups: small molecules, oligomers, polymers, and frameworks, with specific structural and physicochemical properties serving as activity indicators. To unlock the full potential of organic electrocatalysts, five strategies are discussed: integrated structures, surface property modulation, membrane technologies, electrolyte affinity regulation, and addition of anticorrosion species, all aimed at enhancing charge efficiency, mass transfer, and long-term stability during electrocatalytic reactions. The review offers a comprehensive overview of the current state of organic electrocatalysts and their practical applications, bridging the understanding gap and paving the way for future developments of more efficient green energy conversion technologies.

Bifunctional electrocatalytic reduction performance of nitrogen containing biomass based nanoreactors loaded with Ni nanoparticles for oxygen and carbon dioxide

In the face of increasing energy demand, the approach of transformation that combines energy restructuring and environmental governance has become a popular research direction. As an important part of electrocatalytic reactions for gas molecules, reduction reactions of oxygen (ORR) and carbon dioxide (CO2RR) are very indispensable in the field of energy conversion and storage. However, the non-interchangeability and irreversibility of electrode materials have always been a challenge in electrocatalysis. Hereon, nickel and nitrogen decorated biomass carbon-based materials (Ni/N-BC) has been prepared by high temperature pyrolysis using agricultural waste straw as raw material. Surprisingly, it possesses abundant active sites and specific surface area as a bifunctional electrocatalyst for ORR and CO2RR. The three-dimensional porous cavity structure for the framework of biomass could not only provide a strong anchoring foundation for the active site, but also facilitate the transport and enrichment of reactants around the site. In addition, temperature modulation during the preparation process also optimizes the composition and structure of biomass carbon and nitrogen. Benefit from above structure and morphology advantages, Ni/N-BC-800 exhibits the superior electrocatalytic activity for both ORR and CO2RR simultaneously. More specifically, Ni/N-BC-800 exhibits satisfactory ORR activity in terms of initial potential and half wave potential, while also enables the production of CO under high selective. The research results provide ideas for the development and design of electrode materials and green electrocatalysts, and also expand new applications of agricultural waste in fields such as energy conversion, environmental protection, and resource utilization.

First-Row Transition Metals for Catalyzing Oxygen Redox

Rechargeable zinc-air batteries are widely recognized as a highly promising technology for energy conversion and storage, offering a cost-effective and viable alternative to commercial lithium-ion batteries due to their unique advantages. However, the practical application and commercialization of zinc-air batteries are hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, extensive research has focused on the potential of first-row transition metals (Mn, Fe, Co, Ni, and Cu) as promising alternatives to noble metals in bifunctional ORR/OER electrocatalysts, leveraging their high-efficiency electrocatalytic activity and excellent durability. This review provides a comprehensive summary of the recent advancements in the mechanisms of ORR/OER, the performance of bifunctional electrocatalysts, and the preparation strategies employed for electrocatalysts based on first-row transition metals in alkaline media for zinc-air batteries. The paper concludes by proposing several challenges and highlighting emerging research trends for the future development of bifunctional electrocatalysts based on first-row transition metals.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Advanced BIFs with Co, B, N, and S for Electrocatalytic Oxygen Reduction and Oxygen Evolution Reactions

In this work, a new alkaline-stable boron imidazolate framework (BIF-90) was rationally designed and successfully synthesized by solvothermal reaction. Due to its potential electrocatalytic active sites (Co, B, N, and S) and chemical stabilities, BIF-90 was explored as a bifunctional electrocatalyst toward electrochemical oxygen reactions, namely, oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). This work will open new avenues toward the design of stable, cheap, and more active BIFs as bifunctional catalysts.

Engineering carbon semi-tubes supported platinum catalyst for efficient oxygen reduction electrocatalysis

Innovation of catalyst structure is extremely important to develop the high-performance electrocatalysts for oxygen-reduction reaction (ORR). Herein, nitrogen-doped carbon semi-tube (N-CST) is used as a functional support for stabilizing the microwave-reduced Pt nanoparticles with an average size of ∼2.8 nm to synthesize the semi-tubular Pt/N-CST catalyst. The contribution of interfacial Pt-N bond between N-CST support and Pt nanoparticles with electrons transfer from N-CST support to Pt nanoparticles is found by electron paramagnetic resonance (EPR) and X-ray absorption fine structure (XAFS) spectroscopy. This bridged Pt-N coordination can simultaneously help ORR electrocatalysis and promote electrochemical stability. As a result, the innovative Pt/N-CST catalyst exhibits excellent catalytic performance, realizing ORR activity and electrochemical stability superior to the commercial Pt/C catalyst. Furthermore, density functional theoretical (DFT) calculations suggest that the interfacial Pt-N-C site with unique affinity of O∗ + OH∗ can provide new active routes for the enhanced electrocatalytic ORR capacity.

Metal Oxide-Supported Metal Catalysts for Electrocatalytic Oxygen Reduction Reaction: Characterization Methods, Modulation Strategies, and Recent Progress

The sluggish kinetics of the oxygen reduction reaction (ORR) with complex multielectron transfer steps significantly limits the large-scale application of electrochemical energy devices, including metal-air batteries and fuel cells. Recent years witnessed the development of metal oxide-supported metal catalysts (MOSMCs), covering single atoms, clusters, and nanoparticles. As alternatives to conventional carbon-dispersed metal catalysts, MOSMCs are gaining increasing interest due to their unique electronic configuration and potentially high corrosion resistance. By engineering the metal oxide substrate, supported metal, and their interactions, MOSMCs can be facilely modulated. Significant progress has been made in advancing MOSMCs for ORR, and their further development warrants advanced characterization methods to better understand MOSMCs and precise modulation strategies to boost their functionalities. In this regard, a comprehensive review of MOSMCs for ORR is still lacking despite this fast-developing field. To eliminate this gap, advanced characterization methods are introduced for clarifying MOSMCs experimentally and theoretically, discuss critical methods of boosting their intrinsic activities and number of active sites, and systematically overview the status of MOSMCs based on different metal oxide substrates for ORR. By conveying methods, research status, critical challenges, and perspectives, this review will rationally promote the design of MOSMCs for electrochemical energy devices.

High-Efficiency Oxygen Reduction Reaction Revived from Walnut Shell

The development of inexpensive and efficient electrocatalysts for oxygen reduction reactions (ORR) remains a challenge with respect to renewable energy technologies. In this research, a porous, nitrogen-doped ORR catalyst is prepared using the hydrothermal method and pyrolysis with walnut shell as a biomass precursor and urea as a nitrogen source. Unlike past research, in this study, urea is not directly doped; instead, a new type of doping is carried out after annealing at 550 °C. In addition, the sample's morphology and structure are analyzed and characterized by scanning electron microscopy (SEM) and X-ray powder diffraction (XRD). A CHI 760E electrochemical workstation is used to test NSCL-900's performance in terms of oxygen reduction electrocatalysis (ORR). It has been found that the catalytic performance of NSCL-900 is significantly improved compared with that of NS-900 without urea doping. In a 0.1 mol/L KOH electrolyte, the half-wave potential can reach 0.86 V (vs. RHE) and the initial potential is 1.00 V (vs. RHE). The catalytic process is close to four-electron transfer and there are large quantities of pyridine nitrogen and pyrrole nitrogen.

Generalized Encapsulations of ZIF-Based Fe-N-C Catalysts with Controllable Nitrogen-Doped Carbon for Significantly-Improved Stability Toward Oxygen Reduction Reaction

The vigorous development of efficient platinum group metal-free catalysts is considerably important to facilitate the universal application of proton exchange membrane fuel cells. Although nitrogen-coordinated atomic iron intercalated in carbon matrix (Fe-N-C) catalysts exhibit promising catalytic activity, the performance in fuel cells, especially the short lifetime, remains an obstacle. Herein, a highly-active Fe-N-C catalyst with a power density of >1 w cm^-2 and prolonged discharge stability with a current density of 357 mA cm^-2 after 40 h of constant voltage discharge at 0.7 V in H₂ -O₂ fuel cells using a controllable and efficient N-C coating strategy is developed. It is clarified that a thicker N-C coating may be more favorable to enhance the stability of Fe-N-C catalysts at the expense of their catalytic activity. The stability enhancement mechanism of the N-C coating strategy is proven to be the synergistic effect of reduced carbon corrosion and iron loss. It is believed that these findings can contribute to the development of Fe-N-C catalysts with high activity and long lifetimes.

Mesoscale Mass Transport Enhancement on Well-Defined Porous Carbon Platform for Electrochemical H2O2 Synthesis

Two-electron oxygen reduction toward hydrogen peroxide (H₂O₂) offers a promising alternative for H₂O₂ production, but its commercial utilization is still hindered by the difficulty of transferring lab-observed catalyst performance to the practical reactor. Here we report the investigation of the porosity engineering effect on catalytic performance inconsistency through a material platform consisting of a series of hollow mesoporous carbon sphere (HMCS) samples. The performance comparison of HMCS samples in rotating ring-disk electrode and Zn-air battery together with the simulation of diffusion behavior reveals that, in low current density conditions, large surface area is preferred, but the mass transport governs the performance in high current density regions. On account of the favorable porous structure, HMCS-8 nm delivers the most excellent practical performance (166 mW cm^-2) and performs well in the bifunctional Zn-air battery for the wastewater purification (70% RhB degraded after 2 min and 99% after 32 min).

Thermal Migration Promotes the Formation of Manganese and Nitrogen Doped Polyhedral Surface for Boosted Oxygen Reduction Electrocatalysis

Increasing the oxygen reduction reaction (ORR) catalytic activity of carbon-based electrocatalysts with robust stability is of great significance for their application. Herein, a feasible thermal migration strategy was proposed to construct manganese- and nitrogen-doped carbonaceous polyhedron frameworks coupled with manganese monoxide microrods (MnO-NC). Mn species were migrated to the surface of polyhedron frameworks, the shape of which was maintained at the high-temperature treatment. The Mn thermal migration not only created highly dispersed Mn-N_x active sites but also promoted graphitization, which benefited ORR electrocatalysis. Moreover, the MnO microrod-supported polyhedron frameworks provide beneficial mass transfer channels for electrocatalysis. Therefore, MnO-NC exhibited impressive ORR catalytic activity and stability in both alkaline and neutral electrolytes compared to commercial Pt/C catalysts. A magnesium-air battery (MAB) driven by MnO-NC delivered a high open circuit voltage and peak power density comparable to that driven by Pt/C. Notably, MnO-NC-driven MAB possessed a longer discharge time than the Pt/C-driven one, indicative of the superior catalytic performance of Mn-NC. This work provides a simple but effective strategy to construct carbonaceous framework electrocatalysts for boosted ORR, promoting the widespread application of metal-air batteries and fuel cells.

Phosphorus-Doped Graphene Electrocatalysts for Oxygen Reduction Reaction

Developing cheap and earth-abundant electrocatalysts with high activity and stability for oxygen reduction reactions (ORRs) is highly desired for the commercial implementation of fuel cells and metal-air batteries. Tremendous efforts have been made on doped-graphene catalysts. However, the progress of phosphorus-doped graphene (P-graphene) for ORRs has rarely been summarized until now. This review focuses on the recent development of P-graphene-based materials, including the various synthesis methods, ORR performance, and ORR mechanism. The applications of single phosphorus atom-doped graphene, phosphorus, nitrogen-codoped graphene (P, N-graphene), as well as phosphorus, multi-atoms codoped graphene (P, X-graphene) as catalysts, supporting materials, and coating materials for ORR are discussed thoroughly. Additionally, the current issues and perspectives for the development of P-graphene materials are proposed.