In-situ Self-reconstruction Of Fe/Co/Ni-based Catalysts In Electrochemical Oxygen And Hydrogen Evolution Reactions

With the excessive consumption of traditional fossil energy and the increasingly serious environmental pollution caused by it,the development of renewable green energy is extremely urgent.Hydrogen energy has attracted much attention in new energy systems due to the advantages of high energy density,no pollution,and recyclable utilization.Among many hydrogen production methods,water electrolysis for hydrogen production is considered an ideal method for its simplicity,reliability,and high efficiency.At present,the commercial hydrogen production catalysts are Pt group precious metal materials（eg.Pt/C,Ir O₂/Ru O₂）.However,they are expensive,scarce resources,and degrade or inactivate in strong acid and alkaline media.Therefore,inexpensive transition metal catalysts have become a research hotspot.Among them,iron（Fe）,cobalt（Co）,and nickel（Ni）-based catalysts have high electrocatalytic activity.More importantly,the in-situ self-reconstruction occurs in oxygen evolution reaction（OER）because of their diversity of valence states,thus further improving the catalytic process.However,the mechanism of in-situ self-reconstruction is still lacking.In addition,it remains to be explored whether or not such catalysts also have the ability of in-situ self-reconstruction in hydrogen evolution reaction（HER）.Hence,a series of Fe/Co/Ni-based catalysts were prepared with Fe,Co,and Ni as metal cations and P,S,Se,and F as non-metallic ligands.In-situ self-reconstruction behavior and mechanism of the catalysts in the process of OER were deeply investigated.Meanwhile,the feasibility and mechanism of in-situ self-reconstruction of this type of catalyst in the process of HER were explored for the first time.The specific research contents are shown as follows:（1）In-situ self-reconstruction of P-doped carbon encapsulated bimetallic diphosphide pair nanocage catalyst during the OER process.Fe₃Ni₂ alloy was pre-treated with H₂O₂ to form nanocages and mixed with glucose and carbon,and then red P was selected as a phosphorus source for direct phosphating to obtain P-doped carbon encapsulated bimetallic diphosphide pair（Fe P₂-Ni P₂@PC）nanocage catalyst.Conventional electrochemical tests were carried out to explore the external factors of in-situ self-reconstruction.As a result,OER catalytic activity of Fe P₂-Ni P₂@PC in the alkaline electrolyte was significantly improved during long-time electrochemical cyclic voltammetry（CV）and chronoamperometry（i-t）acceleration.Therefore,the structure and morphology of catalyst after CV acceleration were studied to preliminarily explore the mechanism of in-situ self-reconstruction.Specifically,in alkaline electrolytes,the electrochemical acceleration boosts in-situ phase transformation of phosphides to hydroxides and further to oxyhydroxides as the vital intermediates of OER in Fe P₂-Ni P₂@PC catalyst during OER,enhancing the catalytic activity.（2）In-situ self-reconstruction of cobalt phosphide（Co P）catalyst during the OER process.Flower-like cobalt phosphide（Co P）catalyst supported on carbon cloth was synthesized by simple two steps of hydrothermal and phosphating.The electrochemical CV cycle was used to accelerate Co P in the alkaline OER potential range to produce reconstruction with different depths.The results showed that the catalytic activity increases with the degree of reconstruction.Then,the structure and morphology of catalysts with different degrees of reconstruction were systematically recorded by ex-situ characterization to deeply explore the mechanism of in-situ self-reconstruction of catalysts under electrochemical acceleration conditions.The results directly confirmed that Co P first is oxidized to Co（OH）₂,then further oxidized to Co OOH,and even transformed to Co^IV species.The mainly existed in the form of stable trivalent Co OOH.Such produced oxidized species are responsible for the excellent OER activity and stability.In addition,it is preliminarily found that Co P can also undergo an in-situ self-reconstruction during the HER process,and the reconstruction conditions found in OER are also applicable to HER.（3）In-situ self-reconstruction of Fe/Co/Ni-based fluoride catalysts during the HER and OER process.A kind of carbon cloth supported fluoride catalysts were constructed by anion regulation strategy(Co F₂,Ni F₂,and Fe F₃·（H₂O）_0.33).The reasons for the in-situ self-reconstruction and self-stability of the catalyst were mainly observed by chemical and electrochemical tests.As a result,fluorides are capable of achieving ultra-fast complete reconstruction only soaked in alkaline electrolytes,and the catalytic activity is continuously improved during the HER electrochemical test.Herein,take Co F₂ as an example,in-situ and ex-situ structure characterization,and DFT calculations are carried out.It was found that the internal origin of fast in-situ self-reconstruction is the unique surface structure of the fluorides,the external origin is the alkaline electrolyte and the bias voltage.Specifically,fluorides possessing a surface F-atom enrichment structure with more hydrophilic characters endows the feasibility of continuous self-reconstruction.When contacting the alkaline electrolyte,the F^-is rapidly leached from the fluorides due to high iconicity,then the OH^-derived from alkaline electrolyte immediately coordinates with Co sites to form amorphous Co（OH）₂.As the reaction proceeds,bias voltage drives the amorphous crystallization and boosts the process of reconstruction.The completely reconstructed catalyst is featured by a nanoscale catalytic unit with abundant lattice defects and porous structure,which accelerate the mass transfer and kinetics processes of alkaline HER.Meantime,this kind of catalyst also has the ability of rapid in-situ self-reconstruction during the OER process,and the product is Co（OH）₂,which is consistent with HER,proving that the fluoride catalysts have a similar reconstruction process in both HER and OER.（4）In-situ self-reconstruction of Co/Ni-based catalysts regulated by S,P,and Se anions during the HER process.Co/Ni-based catalysts（Co S₂,Co P₂,Co Se₂,Ni S₂,and Ni S₂）supported on carbon cloth were synthesized by simple hydrothermal and S/P/Se/F reaction steps.The catalysts were tested under i-t acceleration at the high potential in the range of HER.It was found that the catalytic activities of catalysts with different anions and cations were significantly improved with i-t acceleration.Take Co S₂ as an example,the in-situ self-reconstruction mechanism of the catalyst during HER acceleration was deeply studied.In the electrochemical test,the S element in the catalyst gradually migrates to its surface and leaches into the electrolyte,and produces unsaturated Co sites.The alkaline electrolyte provides OH^-to coordinate with unsaturated Co sites to form amorphous Co（OH）₂.The applied bias voltage drives the crystallization of amorphous Co（OH）₂.In addition,in-situ self-reconstruction also brings significant morphology changes.The catalyst first changes from sub-micron particles of Co S₂ to nanoparticles crosslinking porous structure.With the electrochemical reaction,the nanoparticles were powdered and transformed into smaller nanoparticles.Finally,partial nanoparticles were reassembled into a large-sized fusiform structure.This unique large specific surface area structure contributes to the penetration of the electrolyte,exposing more active sites,shortening the path of charge transfer,thus accelerating the HER process.Moreover,Co P₂,Co Se₂,Ni S₂,and Ni S₂ are completely reconstructed,the reconstruction-derived components are hydroxides of corresponding cation（Co（OH）₂ and Ni（OH）₂）with a similar morphology evolution process.Therefore,this paper systematically investigates the external（bias voltage,electrolyte）and internal（self-stability）origins of in-situ self-reconstruction during OER and HER for a series of Fe/Co/Ni-based catalysts,which not only builds the in-depth comprehension for the research of in-situ self-reconstruction of catalysts but also provides an available reference for the development and synthesis of new materials.