Review of Mechanisms of Transition Metal-Based Electrocatalysts in Oxygen Evolution Reaction
Abstract and Introduction
The oxygen evolution reaction (OER) is a key step in the solar energy conversion process, playing a crucial role in the renewable energy field. Developing efficient, stable, and cost-effective OER electrocatalysts is an important prerequisite for achieving sustainable energy conversion. This paper systematically reviews the progress in mechanistic studies on transition metal-based heterogeneous electrocatalysts for OER, focusing on catalytic systems based on manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu).
The OER is a complex electrochemical process involving four-electron transfer with slow kinetics and high overpotential that severely limits overall energy conversion efficiency. In nature, the oxygen-evolving center (OEC) of photosystem II (PSII) achieves efficient water oxidation through Mn4CaO5 clusters, providing significant insights for designing artificial catalysts. Understanding the mechanisms of transition metal-based catalysts for OER not only aids in developing new highly efficient catalysts but also offers new perspectives on understanding natural and artificial energy conversion processes.
Reaction Mechanism of Transition Metal-Based Catalysts
Manganese-Based Catalyst Systems
As a core component of PSII's OEC in nature, studying manganese's catalytic mechanism holds special significance. Under alkaline conditions, manganese oxide catalysts typically follow an adsorption evolution mechanism (AEM), where Mn(III)/Mn(IV) redox pairs play critical roles. The formation of Mn(IV)=O intermediates can be observed via in situ Raman spectroscopy as important active species during the OER process. Under neutral conditions, Mn3O4 nanoparticles exhibit different catalytic behaviors potentially involving lattice oxygen participation mechanisms (LOM).
Notably, the valence state stability of manganese directly affects its catalytic performance. Studies show that suppressing charge delocalization phenomena from Mn(III) can significantly enhance water oxidation efficiency by MnO2. Additionally, single-atom Mn embedded within nitrogen-doped graphene (Mn-NG) exhibits unique electronic structures with coordination environments similar to homogeneous catalysts.
Iron-Nickel Based Catalyst Systems
Bimetallic iron-nickel catalysts demonstrate excellent OER activity under alkaline conditions. Research indicates that NiFe layered double hydroxides (LDH) undergo surface reconstruction during reactions to form NiOOH/FeOOH heterostructures. Particularly noteworthy is that high-valent iron Fe(VI) species may form during reactions; this unusual oxidation state could play a pivotal role in OER.
For FeCoNi ternary catalyst systems, their catalytic mechanisms are more complex. Techniques such as synchrotron radiation X-ray absorption spectroscopy (XAS) allow tracking structural evolution throughout reactions; experimental evidence suggests preferential OH- adsorption at Fe sites while Ni sites tend toward forming OOH* intermediates—this synergistic effect significantly lowers reaction barriers.
Characterization Techniques and Mechanistic Research Methods
In-depth research into OER mechanisms relies heavily on advanced characterization techniques like extended X-ray absorption fine structure spectroscopy (EXAFS), which provides information about local coordination environments at active sites while in situ Raman spectroscopy allows direct observation of intermediate formations and transformations during reactions.Electrochemical impedance spectroscopy(EIS )and differential electrochemical mass spectrometry(DEMS )are essential tools for understanding charge transfer processes along with gas evolution dynamics . n nIt’s particularly emphasized that most transition metal-based catalysts undergo surface reconstruction under actual working conditions . Thus , distinguishing between “pre-catalyst”and“real catalyst”is vital to mechanistic studies.Developing suitable operando methods while establishing reliable structure-function relationship models remains one major challenge within this field . n n### Challenges & Future Perspectives nDespite notable advancements made regarding transition-metal based-O ER catalysis , numerous challenges persist.Firstly , breaking scaling relations between HO *and HOO *adsorption energies represents key steps towards enhancing catalysis efficiencies.Secondly , deeper investigations into competitive relationships among lattice oxygen participation mechanisms versus adsorbate evolution require further exploration.Additionally applying knowledge gained from homogeneous catalyst mechanics onto heterogeneous systems presents promising avenues worth pursuing moving forward .Future efforts should prioritize several aspects:developing novel characterization technologies capable capturing transient intermediates ;designing stable model systems exhibiting desirable surfaces ;combining theoretical calculations alongside experiments elucidating true essence behind active site characteristics ;exploring stabilization strategies non-precious metals operating acidic environments.These inquiries will propel development surrounding higher-efficiency lower-costs associated specifically targeting future generations pertaining towards advancing our understanding concerning effective utilization across various platforms relating back ultimately driving progress around sustainable solutions.