Lowering the Water Oxidation Overpotential by Spin-Crossover in Cobalt Hexacyanoferrate

Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates

A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

Interplay between Defects and Short-Range Disorder Manipulating the Oxygen Evolution Reaction on a Layered Double Hydroxide Electrocatalyst

Improving the efficiency of the oxygen evolution reaction (OER) is crucial for advancing sustainable and environmentally friendly hydrogen energy. Layered double hydroxides (LDHs) have emerged as promising electrocatalysts for the OER. However, a thorough understanding of the impact of structural disorder and defects on the catalytic activity of LDHs remains limited. In this work, a series of NiAl-LDH models are systematically constructed, and their OER performance is rigorously screened through theoretical density functional theory. The acquired results unequivocally reveal that the energy increase induced by structural disorder is effectively counteracted at the defect surface, indicating the coexistence of defects and disorder. Notably, it is ascertained that the simultaneous presence of defects and disorder synergistically augments the catalytic activity of LDHs in the context of the OER. These theoretical findings offer valuable insights into the design of highly efficient OER catalysts while also shedding light on the efficacy of LDH electrocatalysts.

Harnessing the electronic structure of active metals to lower the overpotential of the electrocatalytic oxygen evolution reaction

Despite substantial advancements in the field of the electrocatalytic oxygen evolution reaction (OER), the efficiency of earth-abundant electrocatalysts remains far from ideal. The difficulty stems from the complex nature of the catalytic system, which limits our fundamental understanding of the process and thus the possibility of a rational improvement of performance. Herein, we shed light on the role played by the tunable 3d configuration of the metal centers in determining the OER catalytic activity by combining electrochemical and spectroscopic measurements with an experimentally validated computational protocol. One-dimensional coordination polymers based on Fe, Co and Ni held together by an oxonato linker were selected as a case study because of their well-defined electronic and geometric structure in the active site, which can be straightforwardly correlated with their catalytic activity. Novel heterobimetallic coordination polymers were also considered, in order to shed light on the cooperativity effects of different metals. Our results demonstrate the fundamental importance of electronic structure effects such as metal spin and oxidation state evolutions along the reaction profile to modulate ligand binding energies and increase catalyst efficiency. We demonstrated that these effects could in principle be exploited to reduce the overpotential of the electrocatalytic OER below its theoretical limit, and we provide basic principles for the development of coordination polymers with a tailored electronic structure and activity.

Electrochemical Potential-Driven Shift of Frontier Orbitals in M-N-C Single-Atom Catalysts Leading to Inverted Adsorption Energies

Electronic structure is essential to understanding the catalytic mechanism of metal single-atom catalysts (SACs), especially under electrochemical conditions. This study delves into the nuanced modulation of "frontier orbitals" in SACs on nitrogen-doped graphene (N-C) substrates by electrochemical potentials. We observe shifts in Fermi level and changes of d-orbital occupation with alterations in electrochemical potentials, emphasizing a synergy between the discretized atomic orbitals of metals and the continuous bands of the N-C based environment. Using O2 and CO2 as model adsorbates, we highlight the direct consequences of these shifts on adsorption energies, unveiling an intriguing inversion of adsorption energies on Co/N-C SAC under negative electrochemical potentials. Such insights are attributed to the role of the dxz and dz2 orbitals, pivotal for stabilizing the π* orbitals of O2. Through this exploration, our work offers insights on the interplay between electronic structures and adsorption behaviors in SACs, paving the way for enhanced catalyst design strategies in electrochemical processes.

Targeted Spin-State Regulation to Boost Oxygen Reduction Reaction

Catalytic reactions are known to be significantly affected by spin states and their variations during reaction processes, yet the mechanisms behind them remain not fully understood, thus preventing the rational optimization of catalysis. Here, we explore the relationship between the spin states of active sites and their catalytic performance, taking the oxygen reduction reaction as an example. We demonstrate that the catalytic performance is spin-state-dependent and can be improved by adjusting spin states during the catalytic process. To this end, we further investigate the possibility of altering the spin states of transition metals through the application of external fields, such as adsorbed species. By studying the influence of the strength of adsorbed ligands on spin states and its impact on catalytic performance, our results show that optimal catalytic performance is achieved when the strength of the external field is neither too strong nor too weak, forming a volcano-like relationship between the catalytic performance and the external field strength. Our findings can have far-reaching implications for the rational design of high-performance catalysis.

Manipulating Intermetallic Charge Transfer for Switchable External Stimulus-Enhanced Water Oxidation Electrocatalysis

Electrocatalytic processes involving the oxygen evolution reaction (OER) present a kinetic bottleneck due to the existence of linear-scaling relationships, which bind the energies of the different intermediates in the mechanism limiting optimization. Here, we offer a way to break these scaling relationships and enhance the electrocatalytic activity of a Co-Fe Prussian blue modified electrode in OER by applying external stimuli. Improvements of ≈11 % and ≈57 % were achieved under magnetic field (0.2 T) and light irradiation (100 mW cm-2 ), respectively, when working at fixed overpotential, η=0.6 V at pH 7. The observed enhancements strongly tie in with the intermetallic charge transfer (IMCT) intensity between Fe and Co sites. Density Functional Theory simulations suggest that tuning the IMCT can lead to a change of the OER mechanism to an external stimuli-sensitive spin crossover-based pathway, which opens the way for switchable electrocatalytic devices.

Chemically Etched Prussian Blue Analog-WS2 Composite as a Precatalyst for Enhanced Electrocatalytic Water Oxidation in Alkaline Media

The electrochemical water-splitting reaction is a promising source of ecofriendly hydrogen fuel. However, the oxygen evolution reaction (OER) at the anode impedes the overall process due to its four-electron oxidation steps. To address this issue, we developed a highly efficient and cost-effective electrocatalyst by transforming Co-Fe Prussian blue analog nanocubes into hollow nanocages using dimethylformamide as a mild etchant and then anchoring tungsten disulfide (WS2) nanoflowers onto the cages to boost OER efficiency. The resulting hybrid catalyst-derived oxide demonstrated a low overpotential of 290 mV at a current density of 10 mA cm-2 with a Tafel slope of 75 mV dec-1 in 1.0 M KOH and a high faradaic efficiency of 89.4%. These results were achieved through the abundant electrocatalytically active sites, enhanced surface permeability, and high electronic conductivity provided by WS2 nanoflowers and the porous three-dimensional (3D) architecture of the nanocages. Our research work uniquely combines surface etching of Co-Fe PBA with WS2 growth to create a promising OER electrocatalyst. This study provides a potential solution to the challenge of the OER in electrochemical water-splitting, contributing to UN SDG 7: Affordable and clean energy.

Sodium Tartrate-Assisted Synthesis of High-Purity NiFe2O4 Nano-Microrods Supported by Porous Ketjenblack Carbon for Efficient Alkaline Oxygen Evolution

Herein, a simple two-step synthetic method was developed for the synthesis of NiFe₂O₄ nano-microrods supported on Ketjenblack carbon (NiFe₂O₄/KB). A sodium tartrate-assisted hydrothermal method was employed for the synthesis of a NiFe-MOF/KB precursor, which was then pyrolyzed under N₂ at 500 °C to yield NiFe₂O₄/KB. Benefiting from the presence of high-valence Ni³⁺ and Fe³⁺, high conductivity, and a large electrochemically active surface area, NiFe₂O₄/KB delivered outstanding OER electrocatalytic performance under alkaline conditions, including a very low overpotential of 258 mV (vs RHE) at 10 mA cm^-2, a small Tafel slope of 43.01 mV dec^-1, and excellent durability in 1.0 M KOH. Density functional theory calculations verified the superior alkaline OER electrocatalytic activity of NiFe₂O₄ to IrO₂. While both catalysts possessed a similar metallic ground state, NiFe₂O₄ offered a lower energy barrier in the rate-determining OER step (*OOH → O₂) compared to IrO₂, resulting in faster OER kinetics.

The Journey of 1-Keto-1,2,3,4-Tetrahydrocarbazole Based Fluorophores: From Inception to Implementation

Carbazole is a unique template associated with several biological activities. It is due to the diverse and versatile biological properties of carbazole derivatives that they are of immense interest to the research community. 1-keto-1,2,3,4-tetrahydrocarbazoles are important synthetic intermediates to obtain carbazole derivatives. Several members of this family emit fluorescence on photoexcitation. In the context of biochemical and biophysical research, designing and characterising small molecule environment sensitive fluorophores is extremely significant. This article aims to be a state of the art review with synthetic and photophysical details of a variety of fluorophores based on 1-keto-1,2,3,4-tetrahydrocarbazole skeleton.