Ruthenium-based oxides as electrocatalysts for acidic oxygen evolution
The problems of energy scarcity,cost volatility and climate change are accelerating the move away from fossil fuels.Diversifying energy sources,investing in renewable energy sources,such as solar and wind power,can combat both the energy and climate crises.Hydrogen is a clean energy carrier that could be produced by water electrolysis.Electricity can be generated from off-grid power generators,such as solar and wind power.The oxygen evolution reaction(OER)is a key half reaction for hydrogen production by water electrolysis.The sluggish kinetics of the OER,caused by a complex multistep electron transfer process result in a high overpotential,inevitably limiting the energy conversion effiiciency of water electrolysis.Thus,the development of highly active and stable OER electrocatalysts is an urgent task.Proton exchange membrane(PEM)electrolysis,a promising technology for green hydrogen production,has more advantages(e.g.,higher current density,operating pressure,gas purity,and system compactness)compared to alkaline water electrolysis using an alkaline solution as electrolyte.However,the acidic operating environment imposes more stringent requirements on the stability of the catalyst.To date,the commercially available oxygen evolution reaction(OER)catalysts for PEM electrolysis are mainly iridium(Ir)-based materials.To achieve large-scale application of hydrogen production from water,relatively low-cost electrocatalysts are pursued to replace Ir in acidic media.In comparison with Ir-based materials,the ruthenium(Ru)-based oxide catalysts have the advantage of much lower cost but competitive OER activity.Thus,the Ru-based oxide catalysts are one of the most promising candidates for acidic OER in PEM water electrolysis.However,Ru-based oxide catalysts usually suffer from poor electrochemical stability in acidic media.The development of efficient and stable Ru-based oxide catalysts for acidic OER has received increasing attention.This review covers the theoretical stability,OER mechanism,and theoretical activity of Ru-based oxide catalysts.We start with the price analysis of metallic Ru and Ir within a five-year range.Theoretical stability,including chemical stability and electrochemical stability,is then discussed based on Pourbaix diagrams.The OER mechanisms commonly used in acidic media are reviewed,including adsorbate evolution mechanism(AEM),lattice oxygen-mediated mechanism(LOM),and oxide path mechanism(OPM).Furthermore,the theoretical OER activity of RuO2 compared with IrO2 and other oxides is also discussed.To unravel the origin of the poor electrochemical stability for Ru-based oxide catalysts in acidic media,this review further delves into the fundamental understanding of degradation mechanisms,including anodic dissolution(or anodic oxidation with subsequent chemical dissolution),transient dissolution,LOM-derived dissolution,and poor electrical contact between catalysts and substrate.Based on the understanding of the degradation mechanisms,we further summarize and discuss the design strategies used to improve the activity and stability of Ru-based oxide catalysts,such as doping/substitution,phase design,heterostructure engineering,electrochemical lithiation,morphology control,surface coating,support design,and defect engineering.The different stabilization ways(e.g.,inhibition of over-oxidation and stabilization of lattice oxygen or inhibition of lattice oxygen participation during OER)to overcome the stability issue are also addressed.Finally,we propose the critical challenges and perspectives regarding the use of Ru-based oxide catalysts for acidic OER.These challenges include finding guidelines for the rational design of Ru-based oxide compositions as promising OER electrocatalysts and managing the activity/stability trade-off of Ru-based oxide electrocatalysts.We believe that by addressing these challenges,a new generation of commercial Ru-based oxide catalysts for acidic OER can be developed.
water electrolysisruthenium-based oxidesdegradation mechanismcatalyst design strategy
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