Synchrotron Radiation Study On Structure And Properties Of Transition Metal Electrocatalysts

Electrochemical decomposition of water is an attractive hydrogen production technology,which has the advantages of zero carbon emissions,high purity.As an electrochemical reaction,the operating voltage of water electrolytes is usually much greater than the theoretical limit of 1.23 V,ranging from 1.8 V to 2.0 V.Therefore,the use of efficient hydrogen evolution/oxygen evolution reaction(HER/OER)electrocatalyst can effectively improve the slow kinetic process of hydrogen evolution/oxygen evolution reaction,thereby reducing the overpotential,to achieve the effect of twice the result with half the effort.At present,the most advanced HER and OER electrocatalysts are Pt based materials and Ru or Ir oxides,respectively.While their scarcity and high cost make it difficult for them to be widely used.Therefore,people focus on inexpensive and highly active transition metal catalysts and non-noble metal electrocatalysts(including transition metal compounds such as sulfide),and have carried out extensive research.In this paper,two-dimensional transition metal materials and the nonmetallic carbon and nitrogen two-dimensional material catalysts were prepared by mild hydrothermal method,and the coordination relationship between atoms inside the material and electrons outside the nucleus can be accurately observed by using XAFS technology,and then the structure-activity relationship between the material structure and its properties can be defined.SR-FTIR is to use infrared spectroscopy to observe the material reaction process from time to time,so as to identify the new functional groups and new intermediates in the reaction process.The materials were studied precisely through the trinity of preparation,property characterization and theoretical calculation.The research content of this paper is divided into the following parts:1.Kinetic study of electrocatalytic performance of Ni-Fe MOF materials by operando FTIR.We demonstrate that robust bifunctional oxygen reduction reaction(ORR)and oxygen evolution reaction(OER)activity can be achieved by inducing lattice strain in noble-metal-free metal-organic frameworks(MOFs).Lattice-strained NiFe MOFs exhibit mass activities of 500 A gmetal-1 at a half-wave potential of 0.83 V for the ORR and 2,000 A gmetal-1 at an overpotential of 0.30 V for the OER,which are 50-100 times that of pristine NiFe metal-organic frameworks.The catalyst maintains～97%of its initial activity after 200 h of continuous ORR/OER reaction at a high current density of 100-200 mA Cm-2.Using operandp synchrotron spectroscopies,We observed a key superoxide*OOH intermediate emerging on Ni4+ sites during both the ORR and OER processes,which suggests a four-electron mechanistic pathway.2.Kinetic study of oxygen evolution performance of metal-free carbon-based materials in acidic solution by operando FTIR.We enable a ubiquitous carbon material as an efficient acidic oxygen evolution reaction(OER)electrocatalyst,synthesized via a facile and controllable "amino-assisted polymerization and carbonization" strategy.This as-developed metal-free amino-rich hierarchical-network carbon(amino-HNC)framework directly supported on carbon paper can catalyze OER at a quite low overpotential of 281 mV and a small Tafel slope of 96 mV dec-1 in an acid solution,and maintain～98%of its initial catalytic activity after 100 h oxygen evolution operation.By using the operando synchrotron infrared spectroscopy,a crucial structurally evolved H2N-(*O-C)-C,formed by adsorbing the*O intermediate on the active H2N-C=C moiety,is observed on amino-HNC electrocatalysts during the OER process in the acid medium.Furthermore,theoretical calculations reveal that the optimization of the sp2 electronic structure of C=C by amino radicals could effectively lower the kinetic formation barrier of the*O intermediate on the H2N-C=C moiety,contributing to a prominent acidic oxygen-involved catalysis.3.Study on oxygen evolution performance of Co based compounds in alkaline environment by XAFS.Via an efficient electrophilic extraction of local electrons in cobalt oxyhydroxide(CoOOH)nanosheets realized by confining high-valence transition-metal ions(Mn4+)in cation sites of the basal plane,we significantly facilitate the proton-electron transfer kinetics and reduce the charge transfer resistance by more than 50%for high-efficiency water oxidation.The assynthesized Mn-doped CoOOH nanosheets exhibit an excellent OER performance with a quite low overpotential of 255 mV at 10 mA cm-2 and a small Tafel slope of-38 mV dec-1.X-ray absorption spectroscopy and first-principles calculations demonstrate that the high-valence Mn4+ ion with an unpaired 3d3 configuration extracts local electrons from Co active sites and reduces the adsorption free energy of OH by 0.7 eV for efficient oxygen evolution.