Investigation On Reaction Mechanism Of The Electrode In Advanced Power Sources And The Application Of In Situ FTIR Spectroscopy

Surface electrochemistry, which has a wide application field including energy, material science and life science, focuses on the research of interfacial change between two agglomerate phases. The electrochemical spectroscopy has been established and developed since 1970s in order to get the interfacial information at a molecular or atomic level. Among them in situ Fourier transform infrared spectroscopy (FTIR), in situ Raman spectroscopy and in situ UV-Vis spectroscopy have found many advantages. The development of spectro-electrochemistry prompts the research from macroscopic level to microscopic level and provides the useful information about the nature and chemical bond of the molecules at the interfaces. On the other hand, the battery industry, as an important section of energy, has been paid more attention world-widely. R&D of batteries, which have the advantages of high electrochemical energy, long life and environment friendly, etc., becomes one of the most attractive facets.In this work, the reaction mechanisms, especially the interfacial change on the electrode involving several kinds of high energy batteries, such as lithium or lithium ion battery, zinc-silver battery and direct alcohol fuel cells, were investigated by electrochemical techniques and in situ FTIR spectroscopy. In addition, the application of in situ FTIR spectroscopy on the electrochemical interfacial investigation has been discussed. The main results and conclusion are summarized as follows:1. A novel cathodic material, high-valence silver oxide (Ag₃O₄), was prepared by electrochemical methods. The electrochemical condition for the preparation, the growth process and electrochemical behaviors in alkaline solution of this material has been investigated for the first time.High-valence silver oxide (Ag₃O₄) with black color could be anodically electrodeposited on platinum electrode at the constant potential higher than 1.15V (vs. SCE) in 0.1mol/L AgF solution. Especially, the single crystal structure can be obtained at 1.38 V (vs. SCE). The growth process of Ag₃O₄ includes the typical the formation of crystal seed and the sequential development, which may undergo the following steps: Ag( I ) →Ag( II)→Ag(III).The electrochemical behavior of Ag₃O₄ in 1mol/L KOH was investigated by voltammetry and chronopotentiometry measurements. XRD analysis was used for the confirmation of the products at different reduction extent. It was found that thereduction process of Ag₃O₄ was more complicated. Ag₃O₄ could be recognized as the combination of Ag₂O₃ and AgO^*. During cathodic process Ag₂O₃ could be reduced through the reactions Ag₂O₃→AgO→Ag2O→Ag or Ag₂O₃→Ag₂O→Ag. And AgO* could be reduced to Ag directly. This mechanism has been confirmed by the results of chronopotentiometry experiment. Electrochemical impedance spectroscopic results suggest a possible reason for the above electrochemical behavior of Ag₃O₄.In addition, it should be noted that the theoretical capacity for Ag₃O₄ is 553.0mAh/g, which is 27.8% higher than that of common used AgO (theoretical capacity of 432. 8 mAh/g) in zinc-silver oxide battery. It can be calculated that the discharge rate is as high as 119C, and discharge capacity of Ag₃O₄ still kept 83% of the theoretical value. This material presents not only high capacity but also prominent electrochemical performance at very high discharge rate. As a new material it will benefit to the practical application in high power zinc-silver oxide battery.2. The interfacial changes of anodic materials during charging and discharging process in lithium and lithium ion battery, including MCMB, lithium metal and tin dioxide, were investigated by in situ spectroscopy and electrochemical techniques.The P (VdF-HFP)-based gel electrolyte was prepared using the organic electrolyte EC/DEC (1:1) with 1 mol/LLiPF₆ as the plasticizer. Electrochemical techniques and in situ Raman and FTIR spectroscopic methods were used to investigate the interfacial reaction on MCMB electrode in the prepared P (VdF-HFP)-based gel electrolyte. In situ Raman spectroscopy and charge-discharge profile prove that the structure changes of MCMB during lithium ions intercalation are through several stage transitions from dilute stage 1 to stage 4, then stage 3, stage 2 and finally to stage 1. An initial irreversible capacity about 50mAhg^-1 was observed in the initial charge-discharge cycle of MCMB electrode. It can be ascribed to the consumption for the SEI formation on the MCMB surface, which was further confirmed by in situ reflectance FTIR spectroscopy and EIS measurements. In situ FTIR spectra for MCMB electrode during initial charging process mainly indicates the depletion of EC and may suggest the formation of ROCO₂Li.The PMMA-based gel electrolyte was prepared using PC with 1 mol/L UClO₄ as the plasticizer. Electrochemical behavior and in situ FTIR spectroscopic results of MCMB electrode in PMMA-based gel electrolyte indicate that the electrolyte decomposed at about 1.3 V in the first cathodic process and almost no reversible capacity was produced by lithium intercalation and deintercalation.For lithium metal electrode in P (VdF-HFP)-based gel electrolyte, SEI film was composes by ROCO₂Li and Li₂CO₃, which was detected by in situ reflectance FTIR spectroscopy during the charge-discharge process. In addition, in order to avoid the contamination of impurities for lithium metal electrode in the cell-assembling process, lithium electrode was directly prepared by electro-deposition method within a sealed spectro-electrochemical cell with gel electrolyte. In situ FTIR spectroscopic results recorded on the electro-deposited lithium electrode are almost accorded with those on lithium metal electrode, which suggests that the electro-deposited lithium metal can be used as a feasible method, especially for the experiments where the impurities on Li surface have to be avoided.The interfacial change between tin dioxide electrode and liquid organic electrolyte or P (VdF-HFP)-based gel electrolyte during charge-discharge process has been investigated by in situ FTIR reflectance spectroscopy and electrochemical methods. The results suggest that interfacial character of tin dioxide in liquid electrolyte and gel electrolyte are very similar. The electrolyte was decomposed in the first cathodic process at the potential higher than 1.5 V and Li₂CO₃ was the dominant product. At the potential lower than 0.7 V, the products involved ROCO₂LiIt is very interesting here to note that the spectra recorded by in situ FTIR reflectance spectroscopy present a strange "reversible" change during the cathodic and anodic process for almost all the above experiments, which displayed the reversible change of the peak direction. This phenomenon may be popular for the electrodes in lithium or lithium ion battery, which is just related to the reversible change of solvation extent between lithium ion and the organic molecules and irrespective of electrode materials and the electrolytes.3. Solid state polymer electrolyte including PEO-based and PMSMA electrolyte were prepared and the interfacial character was discussed according to in situ FTIR reflectance spectroscopic investigation.PEO-based electrolyte was prepared with SBA-15 as the filler and LiN(CF₃SO₂)₂ serves as the lithium source. The ion conductivity of the composite electrolyte at room temperature reached to the order of 10^-5 S·cm^-1. FTIR results indicate that the addition of lithium salt can reduced the crystalline of PEO polymer and SBA-15 can decrease the gauche/trans conformer ration along PEO chain because of the grain boundary effect. Along with the increase of trans conformer, PEO molecular chains undergo the transition from order to disorder, which leads to a higher ion conductivity at roomtemperature. In situ FTIR spectra at different temperature suggest that pure PEO polymer melts between 60-70℃ and the temperature can be decreased to 40-50℃ and 30-40℃ for PEO- LiN(CF₃SO₂)₂ and the composite PEO electrolyte with 10% SBA-15, respectively.The interfacial change between lithium electrode and PMSMA-LiN(CF₃SO₂)₂ solid state electrolyte was investigated by in situ FTIR spectroscopy. The primary results indicate that chemical reaction occurs as the lithium electrode touches the solid electrolyte because of the activity of lithium. A passivate film may be formed between them, which can prevent the further reaction between lithium and the electrolyte. However, in situ FTIR spectroscopic results recorded during charge-discharge process suggest the depletion of the solid electrolyte, which may be produced by the reaction of lithium with the electrolyte.4. The electro-oxidation of ethanol on Au and Pt were investigated by in situ transmission FTIR spectroscopy and the mechanism was proposed.The electro-oxidation of ethanol on Au electrode was investigated in the different aqueous media including 1mol/L KOH, 1mol/L KClO₄ and 1mol/L HClO₄. Acetate (CH₃COO^-) is the only detectable product in alkaline solution and at the same time the products involve acetaldehyde (CH₃CHO), acetic acid (CH₃COOH) and carbon dioxide (CO₂) in both neutral and acidic solution. On the other hand, the electrochemical behaviors of non-aqueous ethanol on Au and Pt are very similar. The detectable products consist of acetaldehyde and acetal (CH₃CH(OC₂H₅)₂). The electro-catalytic of Pt electrode for the non-aqueous ethanol solution is higher than that of Au.The above results are summarized and the possible mechanism for electro-oxidation of ethanol on Au electrode may be displayed as follows: ethanol can dissociate and adsorb on Au surface and the production of acetaldehyde is a simple dehydrogenation process of the adsorbed ethanol. At the same time, the adsorbed ethanol can react with the gold when hydroxyl species are formed on the electrode surface. The nucleophilic attack of the activated hydroxyl leads to the O-adsorbed species and at the proper potential the Au electrode promotes the loss of one proton with the formation of acetic acid. In alkaline solution, activate hydroxyl can be formed easily on the electrode surface, which will facilitate the formation of acetic acid. Otherwise, the adsorbed hydroxyl is less in neutral and acidic solution and the adsorbed ethanol tends to be dehydrogenated leading to acetaldehyde. As the potentialis higher than that Au was oxidized with the O-adsorbed species formation, the formation of acetic acid will be promoted. In non-aqueous ethanol solution, few adsorbed hydroxyl can be formed and acetaldehyde is the dominant product, which may further react with ethanol and lead to acetal.5. The adsorption of surfactant sodium dodecyl sulfate (SDS) at the Au electrode-solution interface was firstly investigated by in situ transmission FTIR spectroscopy. Moreover, the kinetics of hydrolysis reaction of SDS in aqueous solution was investigated by measuring the variation of in situ transmission difference FTIR spectra with time.It has been substantiated that the charging of the solid electrode surface has a significant impact on the surface assembly of ionic surfactant (sodium dodecyl sulfate). Our results suggest that adsorption behavior on polycrystalline Au electrode may be different from that on single crystal electrode. With regard to 6mmol/L SDS solution, the adsorbed SDS molecules start to desorb at about -0.2 V and desorb entirely when the potential reaches to -0.8 V. However, more complicated phenomena have been inspected concerning 16mmol/L SDS solution. On the basis of the results, it may be inferred that the adsorbed SDS molecules initially transform from the hemimicells to an ordered adsorbed form (the condensed film with the CH terminal exposing to the surface) at the electrode surface at-0.3 V and then it will desorb from the surface with the potential further moving to -0.8 V. The reaction kinetics of SDS hydrolysis was also investigated and discussed in 6mmol/L and 16mmol/L SDS aqueous solutions. The rate constant of hydrolysis can be calculated based on the transmission FTIR spectroscopic results and the values are 6.00×10^-4 s^-1 and 4.99×10^-4 s^-1 for SDS solution with the concentration of 6mmol/L and 16mmol/L, respectively.Moreover, our results further illuminate that in situ transmission difference FTIR spectroscopy method is a very simple, convenient and efficient way to detect both the variations of adsorbed species at the interfaces and the species in the solution.