A High-Entropy Single-Atom Catalyst Toward Oxygen Reduction Reaction in Acidic and Alkaline Conditions

The design of high-entropy single-atom catalysts (HESAC) with 5.2 times higher entropy compared to single-atom catalysts (SAC) is proposed, by using four different metals (FeCoNiRu-HESAC) for oxygen reduction reaction (ORR). Fe active sites with intermetallic distances of 6.1 Å exhibit a low ORR overpotential of 0.44 V, which originates from weakening the adsorption of OH intermediates. Based on density functional theory (DFT) findings, the FeCoNiRu-HESAC with a nitrogen-doped sample were synthesized. The atomic structures are confirmed with X-ray photoelectron spectroscopy (XPS), X-ray absorption (XAS), and scanning transmission electron microscopy (STEM). The predicted high catalytic activity is experimentally verified, finding that FeCoNiRu-HESAC has overpotentials of 0.41 and 0.37 V with Tafel slopes of 101 and 210 mVdec-1 at the current density of 1 mA cm-2 and the kinetic current densities of 8.2 and 5.3 mA cm-2, respectively, in acidic and alkaline electrolytes. These results are comparable with Pt/C. The FeCoNiRu-HESAC is used for Zinc-air battery applications with an open circuit potential of 1.39 V and power density of 0.16 W cm-2. Therefore, a strategy guided by DFT is provided for the rational design of HESAC which can be replaced with high-cost Pt catalysts toward ORR and beyond.

High-performance atomic Co/N co-doped porous carbon catalysts derived from Co-doped metal-organic frameworks for oxygen reduction

Improving the activity and durability of carbon-based catalysts is a key challenge for their application in fuel cells. Herein, we report a highly active and durable Co/N co-doped carbon (CoNC) catalyst prepared via pyrolysis of Co-doped zeolitic-imidazolate framework-8 (ZIF-8), which was synthesized by controlling the feeding sequence to enable Co to replace Zn in the metal-organic framework (MOF). The catalyst exhibited excellent oxygen reduction reaction (ORR) performance, while the half-wave potential decreased by only 8 mV after 5,000 accelerated stress test (AST) cycles in an acidic solution. Furthermore, the catalyst exhibited satisfactory cathodic catalytic performance when utilized in a hydrogen/oxygen single proton exchange membrane (PEM) fuel cell and a Zn-air battery, yielding maximum power densities of 530 and 164 mW cm^-2, respectively. X-ray absorption spectroscopy (XAS) and high-angle annular dark field-scanning transmission electron microscopy (HAAD-STEM) analyses revealed that Co was present in the catalyst as single atoms coordinated with N to form Co-N moieties, which results in the high catalytic performance. These results show that the reported catalyst is a promising material for inclusion into future fuel cell designs.

Tailoring the microenvironment in Fe-N-C electrocatalysts for optimal oxygen reduction reaction performance

Fe-N-C electrocatalysts, comprising FeN4 single atom sites immobilized on N-doped carbon supports, offer excellent activity in the oxygen reduction reaction (ORR), especially in alkaline solution. Herein, we report a simple synthetic strategy for improving the accessibility of FeN4 sites during ORR and simultaneously fine-tuning the microenvironment of FeN4 sites, thus enhancing the ORR activity. Our approach involved a simple one-step pyrolysis of a Fe-containing zeolitic imidazolate framework in the presence of NaCl, yielding a hierarchically porous Fe-N-C electrocatalyst containing tailored FeN4 sites with slightly elongated Fe-N bond distances and reduced Fe charge. The porous carbon structure improved mass transport during ORR, whilst the microenvironment optimized FeN4 sites benefitted the adsorption/desorption of ORR intermediates. Accordingly, the developed electrocatalyst, possessing a high FeN4 site density (9.9 × 1019 sites g-1) and turnover frequency (2.26 s-1), delivered remarkable ORR performance with a low overpotential (a half-wave potential of 0.90 V vs. reversible hydrogen electrode) in 0.1 mol L-1 KOH.

In Situ Electrochemical Mn(III)/Mn(IV) Generation of Mn(II)O Electrocatalysts for High-Performance Oxygen Reduction

Highlights

MnO rich in oxygen vacancies has been synthesized. The synthesized MnO demonstrates excellent oxygen reduction reaction performance and high output power in Zn–air battery. The high catalytic activity is attributed to the synergetic catalytic effect between oxygen vacancies and in situ generated Mn3+/Mn4+.

Abstract

Among various earth-abundant and noble metal-free catalysts for oxygen reduction reaction (ORR), manganese-based oxides are promising candidates owing to the rich variety of manganese valence. Herein, an extremely facile method for the synthesis of cubic and orthorhombic phase coexisting Mn(II)O electrocatalyst as an efficient ORR catalyst was explored. The obtained MnO electrocatalyst with oxygen vacancies shows a significantly elevated ORR catalytic activity with a half-wave potential (E 1/2) of as high as 0.895 V, in comparison with that of commercial Pt/C (E 1/2 = 0.877 V). More impressively, the MnO electrocatalyst exhibits a marked activity enhancement after test under a constant applied potential for 1000 s thanks to the in situ generation and stable presence of high-valence manganese species (Mn3+ and Mn4+) during the electrochemical process, initiating a synergetic catalytic effect with oxygen vacancies, which is proved to largely accelerate the adsorption and reduction of O2 molecules favoring the ORR activity elevation. Such an excellent ORR catalytic performance of this MnO electrocatalyst is applied in Zn–air battery, which shows an extra-high peak power density of 63.2 mW cm−2 in comparison with that (47.4 mW cm−2) of commercial Pt/C under identical test conditions.

Electronic supplementary material

The online version of this article (10.1007/s40820-020-00500-7) contains supplementary material, which is available to authorized users.

Hierarchical Carbon Microtube@Nanotube Core-Shell Structure for High-Performance Oxygen Electrocatalysis and Zn-Air Battery

Zinc-air batteries (ZABs) hold tremendous promise for clean and efficient energy storage with the merits of high theoretical energy density and environmental friendliness. However, the performance of practical ZABs is still unsatisfactory because of the inevitably decreased activity of electrocatalysts when assembly into a thick electrode with high mass loading. Herein, we report a hierarchical electrocatalyst based on carbon microtube@nanotube core-shell nanostructure (CMT@CNT), which demonstrates superior electrocatalytic activity for oxygen reduction reaction and oxygen evolution reaction with a small potential gap of 0.678 V. Remarkably, when being employed as air-cathode in ZAB, the CMT@CNT presents an excellent performance with a high power density (160.6 mW cm-2), specific capacity (781.7 mAhg Zn -1 ) as well as long cycle stability (117 h, 351 cycles). Moreover, the ZAB performance of CMT@CNT is maintained well even under high mass loading (3 mg cm-2, three times as much as traditional usage), which could afford high power density and energy density for advanced electronic equipment. We believe that this work is promising for the rational design of hierarchical structured electrocatalysts for advanced metal-air batteries.