Conduction Behaviors Of SnP₂O₇-based Solid Electrolytes At Intermediate Temperature

Solid electrolyte materials are a kind of important functional materials and haveattracted much attention because of their potential application values and wideapplication prospects in solid oxide fuel cells （SOFCs）, gas sensors, steam electrolyzes,separation and purification of hydrogen, hydrogenation and dehydrogenation of someorganic compounds and ammonia synthesis at atmospheric pressure, etc.The traditional Y2O3stabilized ZrO₂（YSZ） has been extensively used aselectrolyte material for many electrochemical devices due to its high oxide-ionicconductivity and good chemical stability in both oxidizing and reducing atmospheres.However, relatively high operating temperature may result in a series of seriousproblems including high fabrication cost for electrochemical devices, diffusion ofcomponents by interface between electrolyte and electrode, difficult in selectivity ofmaterials, easy material aging, etc.Compared to low-or high-temperature electrochemical devices, intermediatetemperature electrochemical devices have many advantages:（1） fast electrode reactionrate;（2） high tolerance performance for CO and other impurities;（3） easy electivity forsealing materials and connection materials;（4） simple structure of electrochemicaldevice;（5） easy management in water and thermal cycles. Therefore, considerable efforthas been devoted toward the intermediate-temperature solid electrolyte materials（100―600℃）. Recently, a new type of intermediate temperature protonic conductor,AP₂O₇（A=Sn, Ti, Ge, Si, Zr）, has attracted considerable attention. However, up to thepresent, the reports on the conduction in AP₂O₇only focous on the proton conduction.There are little reports on the oxide-ionic conduction, mixed protonic and oxide-ionicconduction in SnP₂O₇,and the conduction mechanism.Therefore, in the thesis，we focused on the conduction behaviors at intermediatetemperature of Sn1-xRxP2O7(R=M³⁺, M²⁺) and their applications. Main works and results in this paper are as follows:（1） Chapter1—Introduction. We introduced briefly some typical solid electrolytematerials, concerned defect chemistry, ionic transference mechanism, preparationmethods and their applications. In addition, we summarized the internal and externalresearch background on SnP₂O₇-based intermediate temperature electrolyte materials,proposed the present theme and the research meaning.（2） Chapter2—Preparation and intermediate temperature conducting performancesof SnP₂O₇. The influence of H₃PO₄concentration, initial molar ratio of P vs Sn （Pini/Sn）and heat treating temperature on phase purity and conducting performances of SnP₂O₇was investigated. It was found that1) the samples with a single cubic phase of SnP₂O₇could be obtained by the reaction between SnO₂and85%H₃PO₄with Pini/Sn≥2.4at500℃;2) The heating temperature remarkably influenced the conductivity in theorder: σ（800℃）<σ（600℃）<σ（500℃）;3) The experimental atmosphereremarkably influenced the conductivity in the order: σ（dry air）<σ（wet air）<σ（wetH₂）;4) The Pini/Sn also remarkably influenced the conductivity in the order: σ（Pini/Sn=2.2）<σ （Pini/Sn=2.4）<σ（Pini/Sn=2.8）<σ（Pini/Sn=3.0）. When Pini/Sn=2.4, theresultant molar ratio of P vs Sn, Pfin/Sn, was equal to2.The intermediate temperature （100–250℃） conducting performances of thesample of Pfin/Sn=2were investigated. The ionic, protonic, oxide-ionic, and electronictransport numbers were ti=0.96–0.98，tH=0.76–0.79，tO=0.17–0.21and te=0.02–0.04, respectively, under wet hydrogen atmosphere. The results indicate thatSnP₂O₇is almost a pure ionic conductor, has dominant protonic conduction,oxide-ionic conduction to a certain extent, and a little electronic conduction. Therelationship of logσ–log（pO₂） between conductivity （σ） and oxygen partial pressure（pO₂） was measured. The plots was horizontal at high oxygen partial pressure range(about10^-10–1atm), indicating that the total conductivity is independent of pO₂andthe sample is almost a pure ion conductor. Whereas at low oxygen partial pressurerange (about10^-10–10^-20atm), the total conductivity increases with decreasing oxygenpartial pressure, indicating that it is a mixed conductor of proton and electron. This isbecause Sn4+is reduced to Sn2+at low oxygen partial pressure range. （3） Chapter3—Preparation and intermediate temperature conducting performancesof Sn_1-xGa_xP₂O₇. A novel series of samples Sn_1-xGa_xP₂O₇（x=0.00,0.01,0.03,0.06,0.09,0.12,0.15） were synthesized. The doping limit of Ga3+in SnP₂O₇is9mol%. Thedopant content x in the samples remarkably influenced the conduction properties.Conductivities increased in the order: σ （SnP₂O₇）<σ （x=0.01）<σ （x=0.03）<σ （x=0.06）<σ （x=0.12）<σ （x=0.09）. The highest conductivities were observed for thesample of x=0.09to be4.6×10^-2S·cm^-1in wet H₂and2.9×10^-2S·cm^-1in dry air at175℃, respectively. The result of oxygen concentration cell showed a mixedconduction of oxygen ion and electron hole in dry oxygen-containing atmosphere. Theresult of the hydrogen concentration cell suggested that protons were the major chargecarriers in wet hydrogen atmosphere. The H₂/air fuel cell using x=0.09as electrolyte（thickness:1.45mm） generated a maximum power density of19.2mW·cm^-2at150℃and22.1mW·cm^-2at175℃, respectively. The result indicated that Sn0.91Ga0.09P2O7may be a potential electrolyte candidate for intermediate temperature fuel cell.（4） Chapter4—Preparation and intermediate temperature conducting performancesof Sn_1-xSc_xP₂O₇. The chapter4synthesized a novel series of samples Sn_1-xSc_xP₂O₇（x=0.03,0.06,0.09,0.12）. The doping limit of Sc3+in SnP₂O₇is9mol%. The dopantcontent x in the samples remarkably influenced the conduction properties.Conductivities increased in the order: σ （x=0.12）<σ （x=0.03）<σ （x=0.09）<σ （x=0.06）. The experimental atmosphere also remarkably influenced the conductivity inthe order: σ （dry O₂）<σ （dry H₂）<σ （wet O₂）<σ （wet H₂）. The highest conductivitywas observed to be2.76×10^-2S·cm^-1for the sample of x=0.06under wet H₂atmosphere at200℃. The ionic conduction was contributed mainly to proton andpartially to oxide ion and electron in wet hydrogen atmosphere. The H₂/air fuel cellsusing Sn_1-xSc_xP₂O₇（x=0.03,0.06,0.09） as electrolytes （1.7mm in thickness）generated the maximum power densities of11.2mW·cm^-2for x=0.03,25.0mW·cm^-2for x=0.06and14.3mW·cm^-2for x=0.09at150℃, respectively. The resultindicated that Sn0.94Sc0.06P2O7may be a potential electrolyte candidate for intermediatetemperature fuel cell.（5） Chapter5—Preparation and intermediate temperature conducting performances of Sn1-xMxP2O7（M=In3+, Mg2+）. Three samples, Sn_0.97In_0.03P₂O₇, Sn_0.95Mg_0.05P₂O₇and Sn_0.9Mg_0.1P₂O₇were synthesized. All the samples are in agreement with the cubicphase structure of SnP₂O₇. The highest conductivities were observed to be2.9×10^-2S·cm^-1for Sn_0.97In_0.03P₂O₇at175℃,5.9×10^-2S·cm^-1for Sn0.95Mg0.05P2O7at200℃,and5.0×10^-2S·cm^-1for Sn_0.9Mg_0.1P₂O₇, respectively, at150℃under wet H₂atmosphere. The H₂/air fuel cells generated the maximum power output densities of30.7mW·cm^-2(Sn_0.97In_0.03P₂O₇),19.1mW·cm^-2(Sn_0.95Mg_0.05P₂O₇) and33.9mW·cm^-2(Sn_0.9Mg_0.1P₂O₇). The results indicated that the samples may be potential electrolytecandidates for intermediate temperature fuel cells.（6） Chapter6—Preparation of Mg-doped SnP₂O₇-SnO₂composite ceramic and itsintermediate temperature conducting performance. A dense Mg-doped SnP₂O₇-SnO₂composite ceramic (5mol%Mg²⁺) was synthesized by reaction of concentratedphosphoric acid with porous Mg-doped SnO₂and heat-treated at lower temperature（600℃）. The heat-treated temperature is much lower than the conventional sinteringtemperature （about1200℃）. The highest conductivity was observed in dry H₂at275℃to be3.8×10^-2S·cm^-1which increased by two orders of magnitude ascompared with the highest conductivity (2.4×10-4S·cm^-1) of Sn_0.91Zn_0.09P₂O₇under H₂atmosphere. This may be related to the vaporization of phosphorus species during thesintering process at higher temperature. The H₂/air fuel cell based on the densecomposite ceramic electrolyte prepared by us exhibited good cell performances with amaximum power output densities of39.7mW·cm^-2at150℃and66.9mW·cm^-2at200℃and93.7mW·cm^-2at250℃, respectively.The methode is not only easy and effective for preparing the dense SnP₂O₇-SnO₂composite ceramic electrolyte fuel cell and increasing cell performances, but also maybe used to prepare dense ZrP2O7-ZrO₂and TiP2O7-TiO₂etc. composite ceramicelectrolyte fuel cells.