Surface Activation And Stability Of Water-Splitting Electrocatalysts

Electrocatalysis,a critical branch of heterogeous catalysis,play a central role in energy conversion,green syntheses,environment and chemical engineer.Electrocatalytic water-splitting reactions have attracted intensive attention for it's a sustainable pathway for the production of clean energy–hydrogen.Electrocatalytic water-splitting reaction can be devided into two half-reactions:the hydrogen evolution reaction?HER?and oxygen evolution reaction?OER?.Both of them are multi-electron processes,thus suffering form sluggish thermodynamic and kinetic behaviors.Hence,in order to reduce overpotential,highly efficient and stable electrocatalysts are needed to expedite these processes.As a kind of heterogeneous catalyst,the composition and structure of the surface play a determined role in the electrocatalytic performances.Many researches concentrated on following two aspects have been conducted:?i?surface activation:tune the electronic structure of them by strain engineering,doping and heterointeface engineering of the electrocatalysts to enhance their activity;?ii?structure-activity relationship:the electrocatalysts usually operate in strong acid/alkaline and oxidation/reduction conditions,hence surface reconstruction is observed in many electrocatalysts,which leading to the different surface structure and composition from initial bulk structure.To establish reasonable structure-activity relationship,the real structure responsible for the activity according to the post-OER electrode should be identified.In this thesis,we aim to study the surface activation and reconstruction of water-splitting electrocatalysts.We obtain high-performance HER electrocatalysts by the introduction of heteroatom in alkaline medium and highlight the surface reconstruction of some typical OER catalyst during long-time stability measurement,thus generating deeper understandings of the relationship between surface structure and electrocatalytic performance.The content of the thesis are as follows:1.HER is used in energy-intensive water-alkali electrolyzers,and thus highly active and stable electrocatalysts are required in alkaline media.Titanates,a family of typical materials possessed two-dimensional structure,are chemically and structurally stable under the basic conditions.Different form the widely studied MoS₂ and layer double hydroxide?LDH?,the main layers of titanate are negatively charged and they have never been reported to exbibit activity for electrocatalysis.In this chapter,we discover 3d metal cations(Fe³⁺,Co²⁺,Ni²⁺and Cu²⁺ions)intercalated titanates can serve as efficient and robust HER electrocatalysts in alkaline.By rapid cation-exchange reaction between Na⁺-containing titanates and 3d metal cations at room temperature,the 3d metal cations can intercalate into the interlayer space of titanate.Among these 3d metal-intercalated titanates,the Co²⁺exchanged titanate is found to exhibit the best electrocatalytic activity.Theoretical and experimental results reveal that there is strong electronic interaction between transition metal cations and main[TiO₆]_?layers,which acitivate the outermost oxygen atoms in the[TiO₆]_?layer and endow the efficient activity of 3d metal-intercalated titanate for the HER in alkaline medium.2.Despite metal selenide exhibit superior electrocatalytic activity for OER in alkaline,the origin of their activity remains to be explored.In this part,we conduct a thorough study on the NiSe oxygen evolution nanoelectrocatalyst?a typical metal selenide that has been demonstrated to be efficient for OER.Our results clearly show that NiSe experiences deselenization under the corrosive electrocatalytic condition and finally is transformed into NiSe/NiO_x core/shell nanostructure.The core/shell nanostructure exhibits an excellent activity towards OER,affording a current density of 10 mA/cm² at an overpotential of 243 mV and outperforming most of the previously reported nickel?hydro?oxide electrocatalysts.These results further indicate that the OER activity of NiSe should derive from the amorphous NiO_x shell in situ formed during electrocatalytic measurment,while NiSe facilitate the charge-transfer process.Additionally,the NiSe/NiO_x core-shell structure is also found to possess a higher electrocatalytic activity for HER than pristine NiSe in alkaline media.3.Development of low-cost and active OER catalysts performing well in acid is a major challenge in proton exchange membrane water electrolyzer?PEMWE?.In this chapter,we investigate the electrocatalytic activity and surface reconstruction behavior of a family of iridium-based 12L-perovskites(Ba₄MIr₃O₁₂;M=Pr,Bi,Nb)for OER in acid medium.These 12L-perovskites is composed of trinuclear face-shared IrO₆ octahedral strings,a unique subunits have never been found in recently reported Ir-based electrocatalysts.Accoring to the experimental and theoretical results,we found the catalytic activities of them are correlated with the location of O 2p-band center,which is modulated by the B-site nonprecious element?i.e.,Pr,Bi or Nb?.Among the three12L-perovskitess,Ba₄PrIr₃O₁₂ demonstrate highest activity and contains 55 wt%less iridium than the state-of-art catalyst IrO₂.During electrocatalytic measurement,transient leaching behavior of Ba and Pr in Ba₄PrIr₃O₁₂ leads to the surface reconstruction of the initial catalysts.Further experimental results reveal that the good activity of Ba₄PrIr₃O₁₂can be ascribed to the formation of under-coordinated IrO_x-rich surface and more efficient generation of Ir^V active intermediate.