Activity Trends of Binary Silver Alloy Nanocatalysts for Oxygen Reduction Reaction in Alkaline Media

The electrocatalytic activity of Pt‐based alloys exhibits a strong dependence on their electronic structures, but a relationship between electronic structure and oxygen reduction reaction (ORR) activity in Ag‐based alloys is still not clear. Here, a vapor deposition based approach is reported for the preparation of Ag₇₅M₂₅ (M = Cu, Co, Fe, and In) and Ag_x Cu_100?x (x = 0, 25, 45, 50, 55, 75, 90, and 100) nanocatalysts and their electronic structures are determined by valence band spectra. The relationship of the d‐band center and ORR activity exhibits volcano‐shape behaviors, where the maximum catalytic activity is obtained for Ag₇₅Cu₂₅ alloys. The ORR enhancement of Ag₇₅Cu₂₅ alloys originates from the 0.12 eV upshift in d‐band center relative to pure Ag, which is different from the downshift in the d‐band center in Pt‐based alloys. The activity trend for these Ag₇₅M₂₅ alloys is in the order of Ag₇₅Cu₂₅ > Ag₇₅Fe₂₅ > Ag₇₅Co₂₅. These results provide an insight to understand the activity and stability enhancement of Ag₇₅Cu₂₅ and Ag₅₀Cu₅₀ catalysts by alloying.     

Nanostructured Metal‐Free Electrochemical Catalysts for Highly Efficient Oxygen Reduction

Replacing precious and nondurable Pt catalysts with cheap and commercially available materials to facilitate sluggish cathodic oxygen reduction reaction (ORR) is a key issue in the development of fuel cell technology. The recently developed cost effective and highly stable metal‐free catalysts reveal comparable catalytic activity and significantly better fuel tolerance than that of current Pt‐based catalysts; therefore, they can serve as feasible Pt alternatives for the next generation of ORR electrocatalysts. Their promising electrocatalytic properties and acceptable costs greatly promote the R&D of fuel cell technology. This review provides an overview of recent advances in state‐of‐the‐art nanostructured metal‐free electrocatalysts including nitrogen‐doped carbons, graphitic‐carbon nitride (g‐C₃N₄)‐based hybrids, and 2D graphene‐based materials. A special emphasis is placed on the molecular design of these electrocatalysts, origin of their electrochemical reactivity, and ORR pathways. Finally, some perspectives are highlighted on the development of more efficient ORR electrocatalysts featuring high stability, low cost, and enhanced performance, which are the key factors to accelerate the commercialization of fuel cell technology.     

Co‐N‐Doped Mesoporous Carbon Hollow Spheres as Highly Efficient Electrocatalysts for Oxygen Reduction Reaction

Rational design of cost‐effective, nonprecious metal‐based catalysts with desirable oxygen reduction reaction (ORR) performance is extremely important for future fuel cell commercialization, etc. Herein, a new type of ORR catalyst of Co‐N‐doped mesoporous carbon hollow sphere (Co‐N‐mC) was developed by pyrolysis from elaborately fabricated polystyrene@polydopamine‐Co precursors. The obtained catalysts with active Co sites distributed in highly graphitized mesoporous N‐doped carbon hollow spheres exhibited outstanding ORR activity with an onset potential of 0.940 V, a half‐wave potential of 0.851 V, and a small Tafel slope of 45 mV decade^?1 in 0.1 m KOH solution, which was comparable to that of the Pt/C catalyst (20%, Alfa). More importantly, they showed superior durability with little current decline (less than 4%) in the chronoamperometric evaluation over 60 000 s. These features make the Co‐N‐mC one of the best nonprecious‐metal catalysts to date for ORR in alkaline condition.     

Pt‐Based Nanocrystal for Electrocatalytic Oxygen Reduction

Currently, Pt‐based electrocatalysts are adopted in the practical proton exchange membrane fuel cell (PEMFC), which converts the energy stored in hydrogen and oxygen into electrical power. However, the broad implementation of the PEMFC, like replacing the internal combustion engine in the present automobile fleet, sets a requirement for less Pt loading compared to current devices. In principle, the requirement needs the Pt‐based catalyst to be more active and stable. Two main strategies, engineering of the electronic (d‐band) structure (including controlling surface facet, tuning surface composition, and engineering surface strain) and optimizing the reactant adsorption sites are discussed and categorized based on the fundamental working principle. In addition, general routes for improving the electrochemical surface area, which improves activity normalized by the unit mass of precious group metal/platinum group metal, and stability of the electrocatalyst are also discussed. Furthermore, the recent progress of full fuel cell tests of novel electrocatalysts is summarized. It is suggested that a better understanding of the reactant/intermediate adsorption, electron transfer, and desorption occurring at the electrolyte–electrode interface is necessary to fully comprehend these electrified surface reactions, and standardized membrane electrode assembly (MEA) testing protocols should be practiced, and data with full parameters detailed, for reliable evaluation of catalyst functions in devices.     

2D Nanoporous Fe−N/C Nanosheets as Highly Efficient Non‐Platinum Electrocatalysts for Oxygen Reduction Reaction in Zn‐Air Battery

It is an ongoing challenge to fabricate nonprecious oxygen reduction reaction (ORR) catalysts that can be comparable to or exceed the efficiency of platinum. A highly active non‐platinum self‐supporting Fe?N/C catalyst has been developed through the pyrolysis of a new type of precursor of iron coordination complex, in which 1,4‐bis(1H‐1,3,7,8–tetraazacyclopenta(1)phenanthren‐2‐yl)benzene (btcpb) functions as a ligand complexing Fe(II) ions. The optimal catalyst pyrolyzed at 700 °C (Fe?N/C?700) shows the best ORR activity with a half‐wave potential (E_1/2) of 840 mV versus reversible hydrogen electrode (RHE) in 0.1 m KOH, which is more positive than that of commercial Pt/C (E_1/2: 835 mV vs RHE). Additionally, the Fe?N/C?700 catalyst also exhibits high ORR activity in 0.1 m HClO₄ with the onset potential and E_1/2 comparable to those of the Pt/C catalyst. Notably, the Fe?N/C?700 catalyst displays superior durability (9.8 mV loss in 0.1 m KOH and 23.6 mV loss in 0.1 m HClO₄ for E_1/2 after 8000 cycles) and better tolerance to methanol than Pt/C. Furthermore, the Fe?N/C?700 catalyst can be used for fabricating the air electrode in Zn–air battery with a specific capacity of 727 mA hg^?1 at 5 mA cm^?2 and a negligible voltage loss after continuous operation for 110 h.     

Active Sites Engineering toward Superior Carbon‐Based Oxygen Reduction Catalysts via Confinement Pyrolysis

Developing efficient and low‐cost defective carbon‐based catalysts for the oxygen reduction reaction (ORR) is essential to metal–air batteries and fuel cells. Active sites engineering toward these catalysts is highly desirable but challenging to realize boosted catalytic performance. Herein, a sandwich‐like confinement route to achieve the controllable regulation of active sites for carbon‐based catalysts is reported. In particular, three distinct catalysts including metal‐free N‐doped carbon (NC), single Co atoms dispersed NC (Co–N–C), and Co nanoparticles‐contained Co–N–C (Co/Co–N–C) are controllably realized and clearly identified by synchrotron radiation‐based X‐ray spectroscopy. Electrochemical measurements suggest that the Co/Co–N–C catalyst delivers optimized ORR performance due to the rich Co–N_x active sites and their synergistic effect with metallic Co nanoparticles. This work provides deep insight for rationally designing efficient ORR catalyst based on active sites engineering.     

Progress in the Development of Fe‐Based PGM‐Free Electrocatalysts for the Oxygen Reduction Reaction

Development of alternative energy sources is crucial to tackle challenges encountered by the growing global energy demand. Hydrogen fuel, a promising way to store energy produced from renewable power sources, can be converted into electrical energy at high efficiency via direct electrochemical conversion in fuel cells, releasing water as the sole byproduct. One important drawback to current fuel‐cell technology is the high content of platinum‐group‐metal (PGM) electrocatalysts required to perform the sluggish oxygen reduction reaction (ORR). Addressing this challenge, remarkable progress has been made in the development of low‐cost PGM‐free electrocatalysts synthesized from inexpensive, earth‐abundant, and easily sourced materials such as iron, nitrogen, and carbon (Fe–N–C). PGM‐free Fe–N–C electrocatalysts now exhibit ORR activities approaching that of PGM electrocatalysts but at a fraction of the cost, promising to significantly reduce overall fuel‐cell technology costs. Herein, recent developments in PGM‐free electrocatalysis, demonstrating increased fuel‐cell performance, as well as efforts aimed at understanding the key limiting factor, i.e., the nature of the PGM‐free active site, are summarized. Further improvements will be accomplished through the controlled and/or rationally designed synthesis of materials with higher active‐site densities, while at the same time establishing methods to mitigate catalyst degradation.