Metastable Structure Design Of Nano-Ruthenium Oxide Electrocatalysts And Their Mechanisms For Stable Overall Seawater Splitting

Hydrogen production through seawater electrolysis technology can combine the abundant wind and solar power in marine areas,which can convert renewable energy into clean energy of hydrogen energy with high gravimetric specific energy,and realize energy conversion and storage.The chloride ions in seawater can easily corrode the structure of catalyst,resulting in performance degradation.Ruthenium oxide（Ru O₂）,as one of the benchmark catalysts for the oxygen evolution reaction（OER）,has good electrochemical activity and chemical corrosion resistance.However,the structure of Ru O₂ is unstable at high potential and Ru O₂ can be oxidized to soluble high-valence oxides,which leads to the structural destruction and performance degradation of the catalyst.The chloride ion in seawater will further aggravate the structural destruction due to the coordination.Therefore,catalytic overpotential need to be greatly reduced for Ru O₂.Among the strategies of reducing overpotential and enhancing stability of Ru O₂,regulation of crystallinity is one of the most basic method.The metastable amorphous structure could promote the catalytic activity,while the crystalline structure is beneficial for enhancing the stability.The realization of the enhanced stability of the highly active metastable structure of Ru O₂ is an important way for overall seawater splitting.The metastable-stable of amorphous-crystalline structure is expected to simultaneously improve the electrocatalytic activity and stability of Ru O₂.Therefore,this dissertation mainly focused on the reasonable design of metastable nanostructred Ru O₂ to obtain a metastable-stable composite structure,which could enhance the catalytic activity and stability for electrolytic seawater splitting.The specific work is as follows:Design of metastable-structured composites.The Ru was uniformly deposited on cobalt hydroxide（Co（OH）₂）nanosheets through cation exchange strategy.The metastable-structured Ru Co O_x-Co₃O₄ composite nanosheets was obtained by a low-temperature thermal treatment.The Ru Co O_x catalyst exhibits excellent performance for alkaline water electrolysis,with overpotentials of only 32 m V and 211 m V for HER and OER at a current density of 10 m A cm^-2,respectively,as well as good catalytic stability.The amorphous structure has been proved to be mainly Ru Co O_x,and the highly dispersed nanoclusters are Co₃O₄.The amorphous structure of Ru Co O_x is supposed to be the main reason for high activity,which not only provides a large number of electrochemically active sites,but also acts as high active center for the electrolysis of water.The highly dispersed Co₃O₄ nanoclusters could enhance the stability of structure to a certain degree.However,due to the overall amorphous structure of the material,the anode performance in the alkaline seawater is not stable enough.Therefore,the next chapter further enhanced the catalytic durability in alkaline seawater by regulating the crystallinity of catalyst.Design of metastable heterogeneous interface of Ru O₂.The Ru OS with an amorphous structure was prepared by a co-precipitation method and the nano-Ru O₂-Ru heterostructure was formed in situ during the high-temperature thermal treatment of Ru OS in inert atmosphere.The crystal fusion region appeared at the heterointerface has a metastable structure with low crystallinity.The catalyst exhibits excellent HER and OER performance in alkaline environment,with overpotentials of 19 m V and 220m V at 10 m A cm^-2 current density is,respectively.It only needs potential of 1.483 V to drive the overall water splitting,and can run for more than 100 hours.What’s more,it could run for 60 hours with potential of～1.560 V to drive the alkaline overall seawater splitting.Both experiment and calculation reveal that the Ru O₂-Ru heterointerface with a metastable structure is supposed to play a key role in the efficient electrolysis of water,which could promote the dissociation process of water molecules,then accelerate the reaction kinetics.The crystalline Ru O₂ and metal Ru in the nanostructure also play a role in stabilizing the metastable heterointerface,which is the main reason for increasing of stability.The high valence anion of SO₄^2-in the structure could repel Cl^-in seawater,thus also protects the surface Ru atoms.Regeneration-design of metastable hydroxylated structure of Ru O₂.Hydroxylated Ru O₂ was obtained by a facile alkali-ethylene glycol treatment of commercial Ru O₂.It has been proved that the insertion of H atoms into the lattice of Ru O₂ leads to the metastable of structure,and the surface hydroxyl（OH）groups are also metastable.Surprisingly,the Ru O₂ hydroxylation is reversible and the high performance is retained even after 10 cycles of regeneration by calcination and alkali/EG treatment.The hydroxylated Ru O₂ exhibits excellent performances for H₂（HER,26 m V）and O₂evolution（OER,254 m V）at 10 m A cm^-2 current density,which are much higher than the performance of untreated commercial Ru O₂（HER:135 m V;OER:351 m V）.It can run for 240 hours in the overall water splitting test,and the Faradaic efficiency can reach 98%of the photovoltaic power driven overall seawater splitting,and it can run for 60 hours.The central role of surface hydroxyl（OH）to HER and OER has been proven via Experiment-Calculation-Prediction-Verification,that surface OH can enhance hydrophilicity of water and regulate the Ru dz²-band center（near-0.6 e V）for appropriate adsorption of intermediates in both HER and OER.The lower valence of Ru caused by hydroxylation can alleviate its excessive oxidation at high potential,and the surface OH can stabilize the surface Ru atoms and prevent Cl^-corrosion especially in alkaline seawater,thereby enhance its electrolytic seawater stability.