Impact of Textural Properties of Mesoporous Porphyrinic Carbon Electrocatalysts on Oxygen Reduction Reaction Activity

Graphene-Encapsulated Bifunctional Catalysts with High Activity and Durability for Zn-Air Battery

Carbon-based electrocatalysts with both high activity and high stability are desirable for use in Zn-air batteries. However, the carbon corrosion reaction (CCR) is a critical obstacle in rechargeable Zn-air batteries. In this study, a cost-effective carbon-based novel material is reported with a high catalytic effect and good durability for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), prepared via a simple graphitization process. In situ growth of graphene is utilized in a 3D-metal-coordinated hydrogel by introducing a catalytic lattice of transition metal alloys. Due to the direct growth of few-layer graphene on the metal alloy decorated 3d-carbon network, greatly reduced CCR is observed in a repetitive OER test. As a result, an efficient bifunctional electrocatalytic performance is achieved with a low ΔE value of 0.63 V and good electrochemical durability for 83 h at a current density of 10 mA cm^-2 in an alkaline media. Moreover, graphene-encapsulated transition metal alloys on the nitrogen-doped carbon supporter exhibit an excellent catalytic effect and good durability in a Zn-air battery system. This study suggests a straightforward way to overcome the CCR of carbon-based materials for an electrochemical catalyst with wide application in energy conversion and energy storage devices.

Steering Catalytic Selectivity with Atomically Dispersed Metal Electrocatalysts for Renewable Energy Conversion and Commodity Chemical Production

Electrocatalysis is a key driver in promoting the paradigm shift from the current fossil-fuel-based hydrocarbon economy to a renewable-energy-driven hydrogen economy. The success of electrocatalysis hinges primarily on achieving high catalytic selectivity along with maximum activity and sustained longevity. Many electrochemical reactions proceed through multiple pathways, requiring highly selective catalysts.Atomically dispersed metal catalysts have emerged as a new frontier in heterogeneous catalysis. In addition to the widely perceived advantages of maximized active site utilization and substantially reduced metal content, they have shown different catalytic selectivities in some electrocatalytic reactions compared to the traditional nanoparticle (NP)-based catalysts. Although there have been significant advances in their synthesis, the highly energetic nature of a single atomic site has made the preparation of atomically dispersed metal catalysts rely on empiricism rather than rational design. Consequently, the structural comprehension of a single atomic site and the understanding of its unusual electrocatalytic selectivity remain largely elusive.In this Account, we describe our endeavors toward developing general synthetic approaches for atomically dispersed metal catalysts for the discovery of new selective and active electrocatalysts and to understand their catalytic nature. We introduce synthetic approaches to produce a wide range of nonprecious- and precious-metal-based atomically dispersed catalysts and control their coordination environments. Metallomacrocyclic-compound-driven top-down and metal salt/heteroatom layer-based bottom-up strategies, coupled with a SiO₂-protective-layer-assisted method, have been developed that can effectively generate single atomic sites while mitigating the formation of metallic NPs. The low-temperature gas-phase ligand exchange method can reversibly tune the coordination structure of the atomically dispersed metal sites. We have used the prepared atomically dispersed metal catalysts as model systems to investigate their electrocatalytic reactivity for renewable energy conversion and commodity chemical production reactions, in which high selectivity is important. The reactions of our interest include the following: (i) the oxygen reduction reaction, where O₂ is reduced to either H₂O or H₂O₂ via the four-electron or two electron pathway, respectively; (ii) the CO₂ reduction reaction, which should suppress the hydrogen evolution reaction; and (iii) the chlorine evolution reaction, which competes with the oxygen evolution reaction. The type of metal center to which the reactant is directly bound is found to be the most important in determining the selectivity, which originates from the dramatic changes in the binding energy of each metal center with the reactants. The coordination structure surrounding the metal center also has a significant effect on the selectivity; its control can modulate the oxidation state of the metal center, thereby altering the binding strength with the reactants.We envisage that future advances in the synthesis of atomically dispersed metal catalysts, combined with the growing power of computational, spectroscopic, and microscopic methods, will bring their synthesis to the level of rational design. Elaborately designed catalysts can overcome the current limits of catalytic selectivity, which will help establish the field of atomically dispersed metal catalysts as an important branch of catalysis.

Coupling fine Pt nanoparticles and Co-Nx moiety as a synergistic bi-active site catalyst for oxygen reduction reaction in acid media

Fabricating high-efficiency catalysts of Pt nanoparticles coupled with single-atom sites (MNC) attracts intensive attention to accelerate the oxygen reduction reaction (ORR). Here we rationally design the low-Pt hybrid catalyst containing fine Pt nanoparticles coupled with Co-N_x moieties via a microwave-assisted heating process. The well-dispersed Pt nanoparticles are anchored by CoNC supports because of the metal-support interaction. Furthermore, the Co-N_x moiety acts as an electron donor to regulate the electronic structure of Pt through the electron synergistic effect, moderating the adsorption energy of oxygen intermediates on Pt sites, and then increasing the intrinsic activity of Pt. In addition, the overflow effect from CoNC to Pt facilitates a nearly four-electron process and enhances the kinetics of ORR. In acid media, the optimized 10% Pt/CoNC hybrid catalysts with Pt nanoparticles size (2.18 nm) exhibit improved ORR activity and robust durability, delivering a half-wave potential (E_1/2) of 0.886 V and negligible loss after accelerated durability test, exceeding the best-in-class commercial Pt/C. The finding of the synergy between CoNC supports and Pt nanoparticles offers a novel ideation to construct various low-loading Pt-based hybrid catalysts.

Pyrolysis-driven synthesis of nanoscale carambola-like carbon decorated with atomically dispersed Fe sites toward efficient oxygen reduction reaction

We report the pyrolysis-driven structural evolution of nanocapsule-shaped Fe^III porphyrin MOFs into nanocarambolas decorated with atomically dispersed Fe suitable for the ORR.

Iron-decorated nitrogen-rich carbons as efficient oxygen reduction electrocatalysts for Zn-air batteries

A low-cost and scalable method has been developed to synthesize Fe-decorated N-rich carbon electrocatalysts for the oxygen reduction reaction (ORR) based on pyrolysis of metal carbonyls containing metal-organic frameworks (MOFs). Such a method simultaneously optimizes the Fe-related active sites and the porous structure of the catalysts. Accordingly, the best-performing Fe-NC-900-M catalyst shows excellent ORR activity with a half-wave potential of 0.91 V vs. RHE, exceeding that of the 40% Pt/C catalyst in alkaline media. Furthermore, the zinc-air batteries constructed with Fe-NC-900-M as the cathode catalyst exhibit high open-circuit voltage (1.5 V) and peak power density (271 mW cm-2), and outperform most zinc-air batteries with noble-metal free ORR catalysts.