Low-Spin-Induced Optimization of Intermediate Adsorption in Selenium-Incorporated Layered Double Hydroxides for Enhanced Electrochemical Water Splitting.

Various spin states of Co3+ were tested in Co-doped BiFeO3 (BFO) through first-principle method, aimed to reveal the role of spin state transition of Co3+ in developing ferrimagnetism. It is found that low-spin (LS) Co3+ contributed much more to resultant magnetic moment than high-spin (HS) Co3+ (~ 3.914 µB). To explore the electronic origins of the enhanced magnetism, a more detailed analysis is carried out including electronic configuration of HS and LS Co3+ as well as Co–O hybridization. It is demonstrated that the LS Co3+ which is nonmagnetic could destroy the cycloid–dal spin structure of BFO. Moreover, LS Co3+ has vacant 3d orbitals so that the p–d covalency is enhanced and the local ferrimagnetism is improved. The structural origin of the Co3+ spin-state transition is ascribed to the distortion of CoO6 octahedra. To explore the influence of Co doping experimentally, the 8% Co-doped and un-doped BFO powders were prepared. The Hysteresis loops indicated that the substitution of Co led to a slight increase of magnetization, i.e. HS Co3+ made limited contribution to the magnetism of BFO.

An approach of the exchange interaction phenomenon in polynuclear complexes grounded on the concepts of magnetic orbitals and of overlap density between these magnetic orbitals is proposed. The aim of this work is to show how these simple concepts can lead to a prevision of the variations of the JAFntiferromagnetic and the Jf ferromagnetic contributions to the J exchange parameter (J = Jaf + Jf) versus small structural changes in closely related complexes. In section 2, it is demonstrated that in binuclear complexes without direct metal-metal interaction, the exchange interaction can be studied by focusing on the overlap density between the magnetic orbitals around the bridging atoms. The section 3 deals with copper (II) dimers with the Open image in new window network; the influences of both the bridging angle θ defined as θ=2 CuXX and the dihedral angle D between the two CuXX planes are studied. The section 4 is devoted to Cu(II) — VO(II) and Cu(II)-low spin CO(II) heterobinuclear complexes in which a strict orthogonality of the magnetic orbitals is realized. The strong ferromagnetic coupling experimentally observed in CuV0(fsa)2en, CH3OH [H4(fsa)2en = N, N’-(2-hydroxy, 3-carboxy benzilidene) 1, 2-diaminoethane] is explained from the overlap density map for the appropriate model complex. A prospective study of the influence of an axial ligand coming near the Co(II) ion on the magnetic properties of Cu(II)- low spin Co(II) complexes is presented.

Catalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are key steps involved in artificial photosynthesis for solar-energy conversion for the production of clean, carbon-free, and renewable energy sources, and thus they are considered to be appealing ways to eliminate the energy and environmental problems caused by burning fossil fuels. Extensive efforts have been made to develop efficient and robust catalysts for these two processes. Research on mononuclear metal complexes has attracted increasing attention recently because not only they are capable of catalyzing HER and OER as efficiently as multinuclear metal complexes, but also there are several advantages in synthesis and mechanism studies by using single site catalysts. In this talk, we will introduce hydrogen and oxygen evolution reactions catalyzed by single site metal porphyrins and corroles.1-6 References (1) Zhang, W.; Lai, W. Z.; Cao, R. Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems. Chem. Rev. 2017, 117, 3717-3797. (2) Han, Y. Z.; Fang, H. Y.; Jing, H. Z.; Sun, H. L.; Lei, H. T.; Lai, W. Z.; Cao, R. Singly Versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen-Evolution Reaction. Angew. Chem. Int. Ed. 2016, 55, 5457-5462. (3) Lei, H. T.; Fang, H. Y.; Han, Y. Z.; Lai, W. Z.; Fu, X. F.; Cao, R. Reactivity and Mechanism Studies of Hydrogen Evolution Catalyzed by Copper Corroles. ACS Catal. 2015, 5, 5145-5153. (4) Lei, H. T.; Liu, C. Y.; Wang, Z. J.; Zhang, Z. Y.; Zhang, M. N.; Chang, X. M.; Zhang, W.; Cao, R. Noncovalent Immobilization of a Pyrene-Modified Cobalt Corrole on Carbon Supports for Enhanced Electrocatalytic Oxygen Reduction and Oxygen Evolution in Aqueous Solutions. ACS Catal. 2016, 6, 6429-6437. (5) Han, Y. Z.; Wu, Y. Z.; Lai, W. Z.; Cao, R. Electrocatalytic Water Oxidation by a Water-Soluble Nickel Porphyrin Complex at Neutral pH with Low Overpotential. Inorg. Chem. 2015, 54, 5604-5613. (6) Sun, H. L.; Han, Y. Z.; Lei, H. T.; Chen, M. X.; Cao, R. Cobalt Corroles with Phosphonic Acid Pendants as Catalysts for Oxygen and Hydrogen Evolution from Neutral Aqueous Solution. Chem. Commun. 2017, 53, 6195-6198. Figure 1

Advances demonstrate that the incorporation of phosphorous into the network of nitrogen, sulfur, or fluorine‐doped carbon materials can remarkably enhance their oxygen and hydrogen evolution activities. However, the electrocatalytic behaviors of pristine phosphorous single‐doped carbon catalysts toward the oxygen and hydrogen evolution reactions (OER and HER) are rarely investigated and their corresponding active species are not yet explored. To clearly ascertain the effects of phosphorous doping on the OER and HER and identify the active sites, herein, phosphorous unitary‐doped graphite layers with different phosphorous species distributions are prepared and the correlations between the oxygen or hydrogen evolution activity and different phosphorous species are investigated, respectively. Results indicate that phosphorous single‐doped graphite layers show a superior oxygen evolution activity to most of the reported OER catalysts and the commercial IrO2 in alkaline medium, and comparable hydrogen evolution activity to most reported carbon catalysts in acidic medium. Moreover, the relevancies unveil that the COP species are the main OER active species, and the defects derived from the decomposition of C3P = O species are the main active sites for HER, as evidenced by density functional theory calculations, showing a new perspective for the design of more effective phosphorous‐containing water‐splitting catalysts.

Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Advanced Hierarchically 2D and 3D Nanostructured Materials for Electrochemical Clean Energy Conversion

For many decades, fossil fuels, including gas, oil, and coal, have acted as the primary contributors to electricity generation [1]. Nevertheless, the combustion of carbon-based fuels causes substantial emissions of greenhouse gases, notably carbon dioxide (CO2), thereby precipitating climate change and adversely affecting human well-being and the environment [2]. Consequently, there is an endeavour to innovate and develop efficient and environmentally acceptable energy sources. The oxygen evolution reaction (OER) constitutes a pivotal half-reaction within diverse renewable energy technologies. Despite its significance, the OER is intrinsically sluggish and energy-intensive, necessitating the advancement of electrocatalysts that are both efficient and stable to facilitate this reaction [3]. In recent years, layered double hydroxides (LDH) have emerged as potential candidates for OERs due to their cost-effectiveness, versatile composition, and favourable electrocatalytic properties [4]. Ni-based LDHs, particularly NiFe-LDH, have been extensively investigated and proven to be effective OER electrocatalysts in alkaline environments [5]. However, its limited electrical conductivity hinders further enhancement of its OER catalytic activity. Meanwhile, Co-based LDHs, such as NiCo-LDH and CoFe-LDH have been proven to have excellent electrocatalytic activity [6]. In contrast to binary LDHs, the ternary LDHs, incorporating diverse transition elements can exhibit higher capacitance and contain more abundant active sites [7]. However, the utilization of LDH electrode materials is limited by their low conductivity and tendency for agglomeration. Graphene serves as an excellent substrate for catalyst immobilisation in electrocatalysis, owing to its superior electrical conductivity, high surface areas, and impressive stability [8]. Therefore, the combination of ternary LDHs, characterised by reversible redox activity, and conductive graphene is expected to represent an efficient approach for fabricating hybrid materials with enhanced oxygen evolution reaction (OER) activity, facilitated by the advantageous interplay between LDHs and graphene.Accordingly, in this study, trimetallic CoNiFe-LDHs were designed and grown on graphene (G) through a one-step hydrothermal approach to obtain a structure that promotes efficient charge transfer, optimizing the OER kinetics. A 2-level full-factorial model was used to determine the effect of Co (1.5, 3 and 4.5 mmol) and graphene (10, 30 and 50 mg) concentrations on the OER onset potential, which was the chosen response parameter. The independent and dependent variables were fitted to the linear model equation, using ANOVA analysis. The F-values, the ratio of noise to response, confirmed that the model is significant (p<0.05). The p-values less than 0.05 indicate the model terms, such as cobalt and graphene concentrations and their interaction, are significant, implying that the OER onset potential is strongly correlated to these parameters. The polarization curves of CoNiFe-LDH/G composites for OER are shown in Figure 1. The OER was run in triplicate using the Co3Ni3Fe3-LDH/G30 (central point) to estimate the variability (0.58%). The comparison study showed that a low onset potential (1.54 V) and overpotential at 10 mA cm-2 (1.58 V) was achieved for Co1.5Ni3Fe3-LDH/G10, demonstrating that a low concentration of cobalt and graphene could make for an ideal electrocatalyst in practical applications.[1] R.F. Hirsh, J.G. Koomey, Electricity Consumption and Economic Growth: A New Relationship with Significant Consequences?, Electricity Journal 28 (2015) 72–84.[2] X.H. Chen, K. Tee, M. Elnahass, R. Ahmed, Assessing the environmental impacts of renewable energy sources: A case study on air pollution and carbon emissions in China, J Environ Manage 345 (2023) 118525.[3] F. Zeng, C. Mebrahtu, L. Liao, A.K. Beine, R. Palkovits, Stability and deactivation of OER electrocatalysts: A review, Journal of Energy Chemistry 69 (2022) 301–329.[4] J. Qian, Y. Zhang, Z. Chen, Y. Du, B.J. Ni, NiCo layered double hydroxides/NiFe layered double hydroxides composite (NiCo-LDH/NiFe-LDH) towards efficient oxygen evolution in different water matrices, Chemosphere 345 (2023) 140472.[5] S. He, R. Yue, W. Liu, J. Ding, X. Zhu, N. Liu, R. Guo, Z. Mo, Nano-NiFe LDH assembled on CNTs by electrostatic action as an efficient and durable electrocatalyst for oxygen evolution, Journal of Electroanalytical Chemistry 946 (2023) 117718.[6] Y. Li, G. Zhou, J. Yin, F. Li, Q. Zou, W. Chen, W. Yan, Q. Li, C. Liu, A. Khataee, L. Zhang, Aboundent oxygen defects in CoFe-LDH derivatives for enhanced photo-thermal synergistic catalytic hydrogen production from NaBH4, Int J Hydrogen Energy 48 (2023) 16745–16755.[7] A. Raja, N. Son, Y. Il Kim, M. Kang, Hybrid ternary NiCoCu layered double hydroxide electrocatalyst for alkaline hydrogen and oxygen evolution reaction, J Colloid Interface Sci 647 (2023) 104–114.[8] W. Gao, D. Havas, S. Gupta, Q. Pan, N. He, H. Zhang, H.L. Wang, G. Wu, Is reduced graphene oxide favorable for nonprecious metal oxygen-reduction catalysts?, Carbon N Y 102 (2016) 346–356. Figure 1

Improved hydrogen evolution activity of layered double hydroxide by optimizing the electronic structure

Hybrid ternary NiCoCu layered double hydroxide electrocatalyst for alkaline hydrogen and oxygen evolution reaction

Electrochemical water splitting is a promising method for the renewable production of high-purity hydrogen via the hydrogen evolution reaction (HER). Ni-Fe layered double hydroxides (Ni-Fe LDHs) are highly efficient materials for mediating the oxygen evolution reaction (OER), a half-reaction for water splitting at the anode, but LDHs typically display poor HER performance. Here, we report the preparation of self-organized Ag@NiFe layered double hydroxide core-shell electrodes on Ni foam (Ag@NiFe/NF) prepared by galvanic etching for mediating both the HER and OER (bifunctional water-splitting electrocatalysis). This synthetic strategy allowed for the preparation of organized hierarchical architectures which displayed improved the electrochemical performance by tuning the electronic structure of the catalyst and increasing the surface area utilization. X-ray photoelectron spectroscopy (XPS) and theoretical calculations revealed that electron transfer from the Ni-Fe LDH to Ag influenced the adsorption of the reaction intermediates leading to enhanced catalytic activity. The Ag@NiFe/NF electrode displayed overpotentials as low as 180 and 80 mV for oxygen and hydrogen evolution, respectively, at a current density of 10 mA cm-2, and improvements in the specific activity by ∼5× and ∼1.5× for the oxygen and hydrogen evolution reaction, respectively, compared to benchmark NiFe hydroxide materials. Additionally, an integrated water-splitting electrolyzer electrode can be driven by an AA battery.

The bifunctional mechanism that involves adsorbed hydroxide in the alkaline hydrogen oxidation and evolution reactions, important in hydrogen fuel cells and water electrolysers, is hotly debated. Hydroxide binding has been suggested to impact activity, but the exact role of adsorbed hydroxide in the reaction mechanism is unknown. Here, by selectively decorating steps on a Pt single crystal with other metal atoms, we show that the rate of alkaline hydrogen evolution exhibits a volcano-type relationship with the hydroxide binding strength. We find that Pt decorated with Ru at the step edge is 65 times more active for the hydrogen evolution reaction (HER) than is the bare Pt step. Simulations of electrochemical water dissociation show that the activation energy correlates with the OH* adsorption strength, even when the adsorbed hydroxide is not a product, which leads to a simulated volcano curve that matches the experimental curve. This work not only illustrates the alkaline HER mechanism but also provides a goal for catalyst design in targeting an optimum hydroxide binding strength to yield the highest rate for the alkaline HER. A three-dimensional (H and OH adsorbed species) HER activity volcano and the implications for hydrogen oxidation are discussed. The appropriate descriptors for a catalyst’s hydrogen evolution activity in alkaline electrolyte are debated. Combining simulations and single-crystal studies of metal-decorated Pt surfaces, McCrum and Koper show that activity exhibits a volcano-type relationship with the hydroxide binding strength of the catalyst, providing a target for catalyst design.

Tetrafunctional electrocatalyst for oxygen reduction, oxygen evolution, hydrogen evolution, and carbon dioxide reduction reactions

In the context of the gradual depletion of global fossil fuel resources, it is increasingly necessary to explore new alternative energy. Hydrogen energy has attracted great interest from researchers because of its green and pollution-free characteristics. Moreover, the methanol oxidation reaction (MOR) can combine the hydrogen evolution reaction (HER), replacing the anode reaction (oxygen evolution reaction-OER) in overall water splitting and efficiently producing hydrogen. In this study, platinum-palladium nanoparticles on reduced graphene oxide (PtPd/rGO) were successfully synthesized as HER and MOR bifunctional electrocatalysts under alkaline conditions by the stepwise loading of Pt and Pd bimetallic nanoparticles on rGO using a simple liquid-phase reduction method. PtPd/rGO-2 with 0.99 wt% Pt and 2.86 wt% Pd in the HER has the lowest overpotential (87.16 mV at 100 mA cm-2), with the smallest Tafel slope (18.9 mV dec-1). The exceptional mass activity of PtPd/rGO-2 in the MOR reaches 10.75 A mg-1PtPd, which is 18.22 and 53.75 times greater than that of commercial Pt/C (Pt/C) and commercial Pd/C (Pd/C), respectively. PtPd/rGO-2 is 0.935 V lower in the coupling reaction of HER and MOR (MOR ∥ HER) compared to the overall water splitting (OER ∥ HER) without methanol (10 mA cm-2). This is probably because appropriate Pt and Pd loading exposes many more catalytic sites, and the synergistic interaction between Pt, Pd, and Pt-Pd enhances the catalytic performance. This strategy can be used for the synthesis of novel bifunctional electrocatalysts.

To achieve Net-Zero Emissions by 2050, hydrogen and hydrogen fuel cells will play a significant role in powering vehicles. In this presentation, I will show, through first-principles-based modeling, how to achieve rigorous modeling of solid-solid interface for the hydrogen and oxygen evolution reactions in electrolysis and the oxygen reduction reaction in fuel cells; and how the fundamental understanding paves the way toward developing both model electrocatalysts and industrial electrocatalysts with significantly improved performance. Specifically, I will introduce the following three topics.[1] metal-metal interfaces: strain evolution and strain tuning of epitaxial, stepped and free-standing platinum group metals for the oxygen reduction reaction in fuel cells.(1)[2] metal-oxide interfaces: stability and activity of monolayer oxide/Pt interface toward improving the hydrogen evolution reaction in alkaline conditions.(2)[3] oxide-oxide interfaces: self-consistent modeling of active phases, reaction centers, and catalytic mechanisms of Ni- and Co-based layered double hydroxides for the oxygen evolution reaction.(3, 4) L. Wang, Z. Zeng, W. Gao, T. Maxson, D. Raciti, M. Giroux, X. Pan, C. Wang, J. Greeley, Tunable intrinsic strain in two-dimensional transition metal electrocatalysts. Science 363, 870-874 (2019).Z. Zeng, K.-C. Chang, J. Kubal, N. M. Markovic, J. Greeley, Stabilization of ultrathin (hydroxy)oxide films on transition metal substrates for electrochemical energy conversion. Nature Energy 2, 17070 (2017).F. Dionigi, Z. Zeng, I. Sinev, T. Merzdorf, S. Deshpande, M. B. Lopez, S. Kunze, I. Zegkinoglou, H. Sarodnik, D. Fan, A. Bergmann, J. Drnec, J. F. d. Araujo, M. Gliech, D. Teschner, J. Zhu, W.-X. Li, J. Greeley, B. R. Cuenya, P. Strasser, In-situ structure and catalytic mechanism of NiFe and CoFe layered double hydroxides during oxygen evolution. Nature Communications 11, 2522 (2020).F. Dionigi, J. Zhu, Z. Zeng, T. Merzdorf, H. Sarodnik, M. Gliech, L. Pan, W.-X. Li, J. Greeley, P. Strasser, Intrinsic Electrocatalytic Activity for Oxygen Evolution of Crystalline 3d-Transition Metal Layered Double Hydroxides. Angew. Chem., Int. Ed. 60, 14446-14457 (2021). Figure 1

Designing cost-effective and highly efficient electrocatalysts for water splitting is a significant challenge. We have systematically investigated a series of quasi-2D oxides, LaSrMn0.5M0.5O4 (M = Co, Ni, Cu, Zn), to enhance the electrocatalytic properties of the two half-reactions of water-splitting, namely oxygen and hydrogen evolution reactions (OER and HER). The four materials are isostructural, as confirmed by Rietveld refinements with X-ray diffraction. The oxygen contents and metal valence states were determined by iodometric titrations and X-ray photoelectron spectroscopy. Electrical conductivity measurements in a wide range of temperatures revealed semiconducting behavior for all four materials. Electrocatalytic properties were studied for both half-reactions of water-splitting, namely, oxygen-evolution and hydrogen-evolution reactions (OER and HER). For the four materials, the trends in both OER and HER were the same, which also matched the trend in electrical conductivities. Among them, LaSrMn0.5Co0.5O4 showed the best bifunctional electrocatalytic activity for both OER and HER, which may be attributed to its higher electrical conductivity and favorable electron configuration.