Immobilization of planar CoII‐2,3‐naphthalocyanine (NapCo) complexes onto doped graphene resulted in a heterogeneous molecular Co electrocatalyst that was active and selective to reduce CO2 into CO ...in aqueous solution. A systematic study revealed that graphitic sulfoxide and carboxyl dopants of graphene were the efficient binding sites for the immobilization of NapCo through axial coordination and resulted in active Co sites for CO2 reduction. Compared to carboxyl dopants, the sulfoxide dopants further improved the electron communication between NapCo and graphene, which led to the increase of turnover frequency of the Co sites by about 3 times for CO production with a Faradic efficiency up to 97 %. Pristine NapCo in the absence of a graphene support did not display efficient electron communication with the electrode and thus failed to serve as the electrochemical active site for CO2 reduction under the identical conditions.
No plain, no gain: Immobilization of planar CoII‐2,3‐naphthalocyanine (NapCo) complexes onto sulfoxide or carboxyl doped graphene resulted in a heterogeneous molecular Co electrocatalyst that was active and selective to reduce CO2 into CO in aqueous solution.
The oxygen evolution reaction (OER) is the bottleneck in the efficient production of hydrogen gas fuel via the electrochemical splitting of water. In this work, we present and elucidate the workings ...of an OER catalytic system which consists of cobalt oxide (CoO x ) with adsorbed Fe3+ ions. The CoO x was electrodeposited onto glassy-carbon-disk electrodes, while Fe3+ was added to the 1 M KOH electrolyte. Linear sweep voltammetry and chronopotentiometry were used to assess the system’s OER activity. The addition of Fe3+ significantly lowered the average overpotential (η) required by the cobalt oxide catalyst to produce 10 mA/cm2 O2 current from 378 to 309 mV. The Tafel slope of the CoO x + Fe3+ catalyst also decreased from 59.5 (pure CoO x ) to 27.6 mV/dec, and its stability lasted ∼20 h for 10 mA/cm2 O2 evolution. Cyclic voltammetry showed that oxidation of the deposited CoO x , from Co2+ to Co3+ occurred at a more positive potential when Fe3+ was added to the electrolyte. This could be attributed to interactions between the Co and Fe atoms. Comprehensive X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopy were conducted. The in situ XANES spectra of Co sites in the CoO x , CoO x + Fe3+, and control Fe48Co52O x catalysts were similar during the OER, which indicates that the improved OER performance of the CoO x + Fe3+ catalyst could not be directly correlated to changes in the Co sites. The XANES spectra of Fe indicated that Fe3+ adsorbed on CoO x did not further oxidize under OER conditions. However, Fe’s coordination number was notably reduced from 6 in pure FeO x to 3.7 when it was adsorbed on CoO x . No change in the Fe–O bond lengths/strengths was found. The nature and mechanistic role of Fe adsorbed on CoO x are discussed. We propose that Fe sites with oxygen vacancies are responsible for the improved OER activity of CoO x + Fe3+ catalyst.
Unveiling the impact of a single parameter on the catalytic descriptor is fundamental to guide rational design principles for high‐activity catalysts. Facets with distinct surface coordination that ...exhibit a central role in the kinetics modulation (reactivity) of surface electrochemistry, have remained elusive in oxygen evolution reactions (OERs). Here, the relationship between the predominant facets and catalytic reactivity is revealed, and it is recognized that facets decisively govern the oxygen evolution activity descriptor in hematite nanocrystals. Specifically, the hematite shows facet‐dependent activity that follows the computed binding energy of surface‐oxygenated intermediates. Moreover, a lower kinetics energy barrier is observed on a highly coordinated surface, both experimentally and computationally, in the light of molecular orbital principles. Consequently, a record‐low overpotential and Tafel slope in iron oxides toward OER are manifested, competing against the benchmark binary transition metal oxide electrocatalysts and expelling the stereotype of the passive oxygen evolution activity of iron oxides. Significantly, the identification of facet‐governing reactivity, construction of favorable facets, and strategic regulation of surface covalency enlighten design strategies for highly active catalysts.
Facet‐governing reactivity toward oxygen evolution is identified in hematites, where three facet‐enclosed nanocrystals show distinct activity that follows computed trends. The favorable high‐index (012) facet hematite nanocrystal exhibits high activity, expelling the stereotype passiveness of iron oxides toward oxygen evolution.
Abstract
The ability to precisely engineer the doping of sub-nanometer bimetallic clusters offers exciting opportunities for tailoring their catalytic performance with atomic accuracy. However, the ...fabrication of singly dispersed bimetallic cluster catalysts with atomic-level control of dopants has been a long-standing challenge. Herein, we report a strategy for the controllable synthesis of a precisely doped single cluster catalyst consisting of partially ligand-enveloped Au
4
Pt
2
clusters supported on defective graphene. This creates a bimetal single cluster catalyst (Au
4
Pt
2
/G) with exceptional activity for electrochemical nitrogen (N
2
) reduction. Our mechanistic study reveals that each N
2
molecule is activated in the confined region between cluster and graphene. The heteroatom dopant plays an indispensable role in the activation of N
2
via an enhanced back donation of electrons to the N
2
LUMO. Moreover, besides the heteroatom Pt, the catalytic performance of single cluster catalyst can be further tuned by using Pd in place of Pt as the dopant.
Cobalt‐containing spinel oxides are promising electrocatalysts for the oxygen evolution reaction (OER) owing to their remarkable activity and durability. However, the activity still needs further ...improvement and related fundamentals remain untouched. The fact that spinel oxides tend to form cation deficiencies can differentiate their electrocatalysis from other oxide materials, for example, the most studied oxygen‐deficient perovskites. Here, a systematic study of spinel ZnFexCo2−xO4 oxides (x = 0–2.0) toward the OER is presented and a highly active catalyst superior to benchmark IrO2 is developed. The distinctive OER activity is found to be dominated by the metal–oxygen covalency and an enlarged CoO covalency by 10–30 at% Fe substitution is responsible for the activity enhancement. While the pH‐dependent OER activity of ZnFe0.4Co1.6O4 (the optimal one) indicates decoupled proton–electron transfers during the OER, the involvement of lattice oxygen is not considered as a favorable route because of the downshifted O p‐band center relative to Fermi level governed by the spinel's cation deficient nature.
Metal–oxygen covalency has great implications for efficient spinel oxides toward the oxygen evolution reaction (OER). An enlarged CoO covalency by optimum Fe substitution in ZnFexCo2−xO4 gives a promoted OER superior to IrO2. Although the pH‐dependent OER suggests non‐concerted proton–electron transfer, the lattice oxygen involvement is not considered favorable because of the downshifted O p‐band governed by the spinel's cation deficient nature.
Cobalt spinel oxides are a class of promising transition metal (TM) oxides for catalyzing oxygen evolution reaction (OER). Their catalytic activity depends on the electronic structure. In a spinel ...oxide lattice, each oxygen anion is shared amongst its four nearest transition metal cations, of which one is located within the tetrahedral interstices and the remaining three cations are in the octahedral interstices. This work uncovered the influence of oxygen anion charge distribution on the electronic structure of the redox‐active building block Co−O. The charge of oxygen anion tends to shift toward the octahedral‐occupied Co instead of tetrahedral‐occupied Co, which hence produces strong orbital interaction between octahedral Co and O. Thus, the OER activity can be promoted by pushing more Co into the octahedral site or shifting the oxygen charge towards the redox‐active metal center in CoO6 octahedra.
The oxygen evolution activity of Co‐based spinel oxides is dominated by the catalytically critical TMO6 octahedra. Pushing more active Co into octahedral sites and shifting the oxygen charge to octahedral Co significantly enhance the activity.
Mn–Co containing spinel oxides are promising, low‐cost electrocatalysts for the oxygen reduction reaction (ORR). Most studies are devoted to the design of porous Mn–Co spinels or to strongly coupled ...hybrids (e.g., MnCo2O4/N‐doped‐rmGO) to maximize the mass efficiency. The lack of analyses by metal oxide intrinsic activity (activity normalized to catalysts' surface area) hinders the development of fundamental understanding of the physicochemical principles behind the catalytic activities. A systematic study on the composition dependence of ORR in ZnCoxMn2−xO4 (x = 0.0–2.0) spinel is presented here with special attention to the role of edge sharing CoxMn1−xO6 octahedra in the spinel structure. The ORR specific activity of ZnCoxMn2−xO4 spans across a potential window of 200 mV, indicating an activity difference of ≈3 orders of magnitude. The curve of composition‐dependent ORR specific activity as a function of Co substitution exhibits a volcano shape with an optimum Mn/Co ratio of 0.43. It is revealed that the modulated eg occupancy of active Mn cations (0.3–0.9), as a consequence of the superexchange effect between edge sharing CoO6 and MnO6, reflects the ORR activity of edge sharing CoxMn1−xO6 octahedra in the ZnCoxMn2−xO4 spinel oxide. These findings offer crucial insights in designing spinel oxide catalysts with fine‐tuned eg occupancy for efficient catalysis.
The superexchange interaction between CoO6 and MnO6 in spinel structured ZnCoxMn2−xO4 has a significant influence on its activity toward the oxygen reduction reaction. With Co substitution, the evolution of the Mn antibonding orbital state in the edge‐sharing CoxMn1−xO6 octahedron is rearranged by the superexchange interaction.
High‐energy Li‐rich layered cathode materials (≈900 Wh kg−1) suffer from severe capacity and voltage decay during cycling, which is associated with layered‐to‐spinel phase transition and oxygen redox ...reaction. Current efforts mainly focus on surface modification to suppress this unwanted structural transformation. However, the true challenge probably originates from the continuous oxygen release upon charging. Here, the usage of dielectric polarization in surface coating to suppress the oxygen evolution of Li‐rich material is reported, using Mg2TiO4 as a proof‐of‐concept material. The creation of a reverse electric field in surface layers effectively restrains the outward migration of bulk oxygen anions. Meanwhile, high oxygen‐affinity elements of Mg and Ti well stabilize the surface oxygen of Li‐rich material via enhancing the energy barrier for oxygen release reaction, verified by density functional theory simulation. Benefited from these, the modified Li‐rich electrode exhibits an impressive cyclability with a high capacity retention of ≈81% even after 700 cycles at 2 C (≈0.5 A g−1), far superior to ≈44% of the unmodified counterpart. In addition, Mg2TiO4 coating greatly mitigates the voltage decay of Li‐rich material with the degradation rate reduced by ≈65%. This work proposes new insights into manipulating surface chemistry of electrode materials to control oxygen activity for high‐energy‐density rechargeable batteries.
A dielectric inverse spinel‐structured Mg2TiO4 coating on Li‐rich cathode material significantly suppresses the continuous oxygen release, endowing batteries with remarkable cyclability and well‐inhibited voltage decay, e.g., showing a capacity retention of ≈81% and voltage degradation of only 151 mV after 700 cycles, far superior to 44% and 432 mV of the unmodified counterpart.
Electrochemical CO2 reduction relies on the availability of highly efficient and selective catalysts. Herein, we report a general strategy to boost the activity of metal–organic frameworks (MOFs) ...towards CO2 reduction via ligand doping. A strong electron‐donating molecule of 1,10‐phenanthroline was doped into Zn‐based MOFs of zeolitic imidazolate framework‐8 (ZIF‐8) as CO2 reduction electrocatalyst. Experimental and theoretical evidences reveal that the electron‐donating nature of phenanthroline enables a charge transfer, which induces adjacent active sites at the sp2 C atoms in the imidazole ligand possessing more electrons, and facilitates the generation of *COOH, hence leading to improved activity and Faradaic efficiency towards CO production.
Organic doping: Electron‐donating 1,10‐phenanthroline molecules are doped into ZIF‐8 to form a CO2 reduction electrocatalyst. The dopant enables charge transfer to the sp2 C atom active site in the imidazole ligand and thus facilitates the generation of *COOH, hence leading to improved activity and Faradaic efficiency towards CO production.
Abstract
Metal/oxide interface is of fundamental significance to heterogeneous catalysis because the seemingly “inert” oxide support can modulate the morphology, atomic and electronic structures of ...the metal catalyst through the interface. The interfacial effects are well studied over a bulk oxide support but remain elusive for nanometer-sized systems like clusters, arising from the challenges associated with chemical synthesis and structural elucidation of such hybrid clusters. We hereby demonstrate the essential catalytic roles of a nanometer metal/oxide interface constructed by a hybrid Pd/Bi
2
O
3
cluster ensemble, which is fabricated by a facile stepwise photochemical method. The Pd/Bi
2
O
3
cluster, of which the hybrid structure is elucidated by combined electron microscopy and microanalysis, features a small Pd-Pd coordination number and more importantly a Pd-Bi spatial correlation ascribed to the heterografting between Pd and Bi terminated Bi
2
O
3
clusters. The intra-cluster electron transfer towards Pd across the as-formed nanometer metal/oxide interface significantly weakens the ethylene adsorption without compromising the hydrogen activation. As a result, a 91% selectivity of ethylene and 90% conversion of acetylene can be achieved in a front-end hydrogenation process with a temperature as low as 44 °C.
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