Spin-Selective Coupling in Mott-Schottky Er2 O3 -Co Boosts Electrocatalytic Oxygen Reduction

Highly dispersed ultrafine Ru nanoparticles on a honeycomb-like N-doped carbon matrix with modified rectifying contact for enhanced electrochemical hydrogen evolution

The manipulation of rectifying contact between metal and semiconductor represents a powerful strategy to modify the electronic configuration of active sites for improved electrocatalytic performance. Herein, we present an NaCl template-assisted approach to rationally construct a Schottky electrocatalyst consisting of a honeycomb-like N-doped carbon matrix decorated with uniformly ultrasmall Ru nanoparticles with an average diameter of 2.5 nm (hereafter abbreviated as Ru NPs@HNC). It is found that the Fermi level difference between Ru and HNC can cause self-driven migration of electrons from Ru NPs to the HNC substrate, which leads to the generation of a built-in electric field and directional flow of electrons, thereby enhancing the intrinsic activity. In addition, the immobilization of ultrafine Ru NPs on the honeycomb-like carbon skeleton can effectively inhibit the undesired migration, agglomeration and detachment of the active sites, thus ensuring remarkable structural stability. As a result, the Ru NPs@HNC with optimal rectifying contact delivers superior electrochemical activity with a small overpotential of 28 mV at 10 mA cm-2 and outstanding long-term stability in an alkaline solution. The design philosophy of grain-size modulation and Schottky contact may widen up insight into the preparation of high-performance electrocatalysts in sustainable energy conversion systems.

Manipulating the Spin State of Spinel Octahedral Sites via a π-π Type Orbital Coupling to Boost Water Oxidation

Spin state is often regarded as the crucial valve to release the reactivity of energy-related catalysts, yet it is also challenging to precisely manipulate, especially for the active center ions occupied at the specific geometric sites. Herein, a π-π type orbital coupling of 3d (Co)-2p (O)-4f (Ce) was employed to regulate the spin state of octahedral cobalt sites (CoOh) in the composite of Co3O4/CeO2. More specifically, the equivalent high-spin ratio of CoOh can reach to 54.7 % via tuning the CeO2 content, thereby triggering the average eg filling (1.094) close to the theoretical optimum value. The corresponding catalyst exhibits a superior water oxidation performance with an overpotential of 251 mV at 10 mA cm-2, rivaling most cobalt-based oxides state-of-the-art. The π-π type coupling corroborated by the matched energy levels between Ce t1u/t2u-O and CoOh t2g-O π type bond in the calculated crystal orbital Hamilton population and partial density of states profiles, stimulates a π-donation between O 2p and π-symmetric Ce 4fyz 2 orbital, consequently facilitating the electrons hopping from t2g to eg orbital of CoOh. This work offers an in-depth insight into understanding the 4f and 3d orbital coupling for spin state optimization in composite oxides.

Electron Transfer Induced by the Change of Spin States as a Catalytic Descriptor on C2N-TM Single-Atom Catalysts

The catalytic activity and selectivity of metal single-atom catalysts strongly depend upon their spin states. However, their intrinsic connections are not yet clear. In this work, we evaluate the catalytic activity and selectivity of oxygen reduction reactions (ORRs) on C2N-supporting 3d transition metal (TM = Mn/Co/Ni/Cu) single-atom catalysts (SACs) using the density functional theory calculations. It is found that all of the SACs with different spin states tend to follow the 2e- H2O2 pathway, except for C2N-Mn (S = 1/2), which takes the 4e- OOH pathway. Interestingly, we found that the sum of the changes in the electron spin moments of the metal active centers and the reaction intermediate OOH affects the OOH electron transfer, and the electron transfer promotes the catalytic activity of the 2e- H2O2 pathway on C2N-TM SACs. Moreover, there is a strong linear relationship between the OOH electron transfer and the catalytic activity of the 2e- H2O2 pathway on C2N-TM SACs. These findings indicate that electron transfer induced by the change of spin states serves as a descriptor of the catalytic activity of the 2e- H2O2 pathway on C2N-TM SACs, which is very helpful for designing more powerful SACs.

Spin-dependent electrocatalysis

The shift towards sustainable energy requires efficient electrochemical conversion technologies, emphasizing the crucial need for robust electrocatalyst design. Recent findings reveal that the efficiency of some electrocatalytic reactions is spin-dependent, with spin configuration dictating performance. Consequently, understanding the spin's role and controlling it in electrocatalysts is important. This review succinctly outlines recent investigations into spin-dependent electrocatalysis, stressing its importance in energy conversion. It begins with an introduction to spin-related features, discusses characterization techniques for identifying spin configurations, and explores strategies for fine-tuning them. At the end, the article provides insights into future research directions, aiming to reveal more unknown fundamentals of spin-dependent electrocatalysis and encourage further exploration in spin-related research and applications.

Facile synthesis of MnO/NC nanohybrids toward high-efficiency ORR for zinc-air battery

The development of inexpensive non-precious metal materials as high-efficiency stable oxygen reduction reaction (ORR) catalysts holds significant promise for application in metal-air batteries. Here, we synthesized a series of nanohybrids formed from MnO nanoparticles anchored on N-doped Ketjenblack carbon (MnO/NC) via a facile hydrothermal reaction and pyrolysis strategy. We systematically investigated the influence of pyrolysis temperature (600 to 900 °C) on the ORR activities of the MnO/NC samples. At the optimized pyrolysis temperature of 900 °C, the resulting MnO/NC (referred to as MnO/NC-900) exhibited superior ORR activity (onset potential = 0.85 V; half-wave potential = 0.74 V), surpassing other MnO/NC samples and nitrogen-doped Ketjenblack carbon (NC). Additionally, MnO/NC-900 demonstrated better stability than the Pt/C catalyst. The enhanced ORR activity of MnO/NC-900 was attributed to the synergy effect between MnO and NC, abundant surface carbon defects and surface-active components (N species and oxygen vacancies). Notably, the Zinc-air battery (ZAB) equipped MnO/NC-900 as the cathode catalyst delivered promising performance metrics, including a high peak power density of 146.5 mW cm-2, a large specific capacity of 795 mA h gZn -1, and an excellent cyclability up to 360 cycles. These results underscore the potential of this nanohybrid for applications in energy storage devices.

Ultrafine Nanoclusters Unlocked 3d-4f Electronic Ladders for Efficient Electrocatalytic Water Oxidation

The vast extensional planes of two-dimensional (2D) nanomaterials are recognized as desirable ground for electrocatalytic reactions. However, they tend to exhibit catalytic inertia due to their surface-ordered coordination configurations. Herein, an in situ autoxidation strategy enables high-density grafting of ultrafine CeO2 nanoclusters on 2D Co(OH)2. Affluent active units were activated at the inert interface of Co(OH)2 via the formation of Co-O-Ce units. The optimized catalyst exhibits oxygen evolution reaction activity with an overpotential of 83 mV lower than that of Co(OH)2 at 10 mA cm-2. The cascade orbital coupling between Co (3d) and Ce (4f) in Co-O-Ce units drives electron transfer by unlocking a "d-f electron ladder". Meanwhile, the bond-order theorem analyses and the d-band center show that the occupancy of Co-3d-eg is optimized to balance the adsorption-desorption process of active sites to the key reaction intermediate *OOH, thereby making it easier to release oxygen. This work will drive the development of wider area electron modulation methods and provide guidance for the surface engineering of 2D nanomaterials.

Lanthanide-regulating Ru-O covalency optimizes acidic oxygen evolution electrocatalysis

Precisely modulating the Ru-O covalency in RuOx for enhanced stability in proton exchange membrane water electrolysis is highly desired. However, transition metals with d-valence electrons, which were doped into or alloyed with RuOx, are inherently susceptible to the influence of coordination environment, making it challenging to modulate the Ru-O covalency in a precise and continuous manner. Here, we first deduce that the introduction of lanthanide with gradually changing electronic configurations can continuously modulate the Ru-O covalency owing to the shielding effect of 5s/5p orbitals. Theoretical calculations confirm that the durability of Ln-RuOx following a volcanic trend as a function of Ru-O covalency. Among various Ln-RuOx, Er-RuOx is identified as the optimal catalyst and possesses a stability 35.5 times higher than that of RuO2. Particularly, the Er-RuOx-based device requires only 1.837 V to reach 3 A cm-2 and shows a long-term stability at 500 mA cm-2 for 100 h with a degradation rate of mere 37 μV h-1.

Controlled establishment of advanced local high-entropy NiCoMnFe-based layered double hydroxide for zinc batteries and low-temperature supercapacitors

The development of high-performance electrodes is essential for improving the charge storage performance of rechargeable devices. In this study, local high-entropy C, N co-doped NiCoMnFe-based layered double hydroxide (C/N-NiCoMnFe-LDH, C/N-NCMF) were designed using a novel method. Multi-component synergistic effects can dramatically modulate the surface electron density, crystalline structure, and band-gap of the electrode. Thus, the electrical conductivity, electron transfer, and affinity for the electrolyte can be optimized. Additionally, the C/N-NCMF yielded a high specific capacitance (1454F·g-1) at 1 A·g-1. The electrode also exhibited excellent cycling stability, with 62 % capacitance retention after 5000 cycles. Moreover, the assembled Zn||C/N-NCMF battery and the C/N-NCMF//AC hybrid supercapacitor yielded excellent energy densities of 63.1 and 35.4 Wh·kg-1 at power densities of 1000 and 825 W·kg-1, and superior cycling performance with 69 % and 88.7 % capacitance retention after 1000 and 30,000 cycles, respectively. Furthermore, the electrode maintained high electrochemical activity and stability and ensured high energy density, power density, and cycling stability of the rechargeable devices even at a low temperature (-20 °C). This study paves a new pathway for regulating the electrochemical performance of LDH-based electrodes.

Engineering High-Spin State Cobalt Cations in Sulfide Spinel for Enhancing Water Oxidation

The spin states of active sites have a significant impact on the adsorption/desorption ability of the reaction intermediates during the oxygen evolution reaction (OER). Sulfide spinel is not generally considered a highly efficient OER catalyst owing to the low spin state of Co3+ and the lack of unpaired electrons available for adsorption of reaction intermediates. Herein, it is proposed a novel Nd-evoked valence electronic adjustment strategy to engineer the spin state of Co ions. The unique f-p-d orbital electronic coupling effect stimulates the rearrangement of Co d orbital electrons and increases the eg electron filling to achieve high-spin state Co ions, which promotes charge transport by propagating a spin channel and generates a high number of active sites for intermediate adsorption. The optimized CuCo1.75 Nd0.25 S4 catalyst exhibits outstanding electrocatalytic properties with a low overpotential of 320 mV at 500 mA cm-2 and a 48 h stability at 300 mA cm-2 . In situ synchrotron radiation infrared spectra confirm the quick accumulation of key *OOH and *O intermediates. This work deepens the comprehensive understanding of the relationship between OER activity and spin configurations of Co ions and offers a new design strategy for spinel compound catalysts.

Importing Antibonding-Orbital Occupancy through Pd-O-Gd Bridge Promotes Electrocatalytic Oxygen Reduction

The active-site density, intrinsic activity, and durability of Pd-based materials for oxygen reduction reaction (ORR) are critical to their application in industrial energy devices. This work constructs a series of carbon-based rare-earth (RE) oxides (Gd2 O3 , Sm2 O3 , Eu2 O3 , and CeO2 ) by using RE metal-organic frameworks to tune the ORR performance of the Pd sites through the Pd-REx Oy interface interaction. Taking Pd-Gd2 O3 /C as a representative, it is identified that the strong coupling between Pd and Gd2 O3 induces the formation of the Pd-O-Gd bridge, which triggers charge redistribution of Pd and Gd2 O3 . The screened Pd-Gd2 O3 /C exhibits impressive ORR performance with high onset potential (0.986 VRHE ), half-wave potential (0.877 VRHE ), and excellent stability. Similar ORR results are also found for Pd-Sm2 O3 /C, Pd-Eu2 O3 /C, and Pd-CeO2 /C catalysts. Theoretical analyses reveal that the coupling between Pd and Gd2 O3 promotes electron transfer through the Pd-O-Gd bridge, which induces the antibonding-orbital occupancy of Pd-*OH for the optimization of *OH adsorption in the rate-determining step of ORR. The pH-dependent microkinetic modeling shows that Pd-Gd2 O3 is close to the theoretical optimal activity for ORR, outperforming Pt under the same conditions. By its ascendancy in ORR, the Pd-Gd2 O3 /C exhibits superior performance in Zn-air battery as an air cathode, implying its excellent practicability.

High-Valence Mo Doping and Oxygen Vacancy Engineering to Promote Morphological Evolution and Oxygen Evolution Reaction Activity

The rational design of high-efficiency, low-cost electrocatalysts for electrochemical water oxidation in alkaline media remains a huge challenge. Herein, combined strategies of metal doping and vacancy engineering are employed to develop unique Mo-doped cobalt oxide nanosheet arrays. The Mo dopants exist in the form of high-valence Mo6+, and the doping amount has a significant effect on the structure morphology, which transforms from 1D nanowires/nanobelts to 2D nanosheets and finally 3D nanoflowers. In addition, the introduction of vast oxygen vacancies helps to modulate the electronic states and increase the electronic conductivity. The optimal catalyst MoCoO-3 exhibits greatly increased active sites and enhanced reaction kinetics. It gives a dramatically lower overpotential at 50 mA cm-2 (288 mV), much smaller than that of the undoped counterpart (418 mV) and comparable to those of the recently reported electrocatalysts. Density functional theory results further verify that the increased electronic conductivity and optimized adsorption energy toward oxygen evolution reaction intermediates are mainly responsible for the enhanced catalytic activity. Moreover, the assembled two-electrode electrolyzer (MoCoO-3||Pt/C) exhibits superior performance with the cell potential decreased by 233 mV to reach a current density of 50 mA cm-2 with respect to the benchmark counterpart catalysts (RuO2||Pt/C). This work might contribute to the rational design of effective, low-cost electrocatalyst materials by combining multiple strategies.

Axial Oxygen Ligands Regulating Electronic and Geometric Structure of Zn-N-C Sites to Boost Oxygen Reduction Reaction

Zn-N-C possesses the intrinsic inertia for Fenton-like reaction and can retain robust durability in harsh circumstance, but it is often neglected in oxygen reduction reaction (ORR) because of its poor catalytic activity. Zn is of fully filled 3d10 4s2 configuration and is prone to evaporation, making it difficult to regulate the electronic and geometric structure of Zn center. Here, guided by theoretical calculations, five-fold coordinated single-atom Zn sites with four in-plane N ligands is constructed and one axial O ligand (Zn-N4 -O) by ionic liquid-assisted molten salt template method. Additional axial O not only triggers a geometry transformation from the planar structure of Zn-N4 to the non-planar structure of Zn-N4 -O, but also induces the electron transfer from Zn center to neighboring atoms and lower the d-band center of Zn atom, which weakens the adsorption strength of *OH and decreases the energy barrier of rate determining step of ORR. Consequently, the Zn-N4 -O sites exhibit improved ORR activity and excellent methanol tolerance with long-term durability. The Zn-air battery assembled by Zn-N4 -O presents a maximum power density of 182 mW cm-2 and can operate continuously for over 160 h. This work provides new insights into the design of Zn-based single atom catalysts through axial coordination engineering.