ION TRANSPORT STUDIES IN SOME DOPED HYDROGEL AND ALCOGEL SYSTEMS

 

 

ABSTRACT

 

THESIS SUBMITTED FOR THE DEGREE OF

 

DOCTOR OF PHILOSOPHY

IN

PHYSICS

 

BY

RITU SRIVASTAVA

 

(SUPERVISOR: PROF. S. CHANDRA)

 

 

 

 

 

DEPARTMENT OF PHYSICS

BANARAS HINDU UNIVERSITY

VARANASI- 221005,INDIA

                

Submitted on : 17^th APRIL 2001                                                                                          Awarded on  08.03.2002

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olid state materials with high ionic conductivity (commonly termed as Superionic Conductor, Solid Electrolytes or Fast Ion Conductors; s~10^-1-10^-4 S. cm^-1) have attracted great attention recently due to their potential applications in the solid state electrochemical devices like batteries, fuel cells, sensors, electrochemical display devices etc. These are broadly classified as:

(i)                  framework crystalline solids

(ii)                ion conducting glasses

(iii)               composite or dispersed phase electrolytes

(iv)              polymer electrolytes

(v)                ion conducting gels.

Out of these, ion conducting gels are the most recent sought after materials in the field of solid state ionics particularly because of the possibilities of easily moulding these into desire shapes and configurations. The present thesis primarily focuses on the development and characterization of proton conducting gels.

                   Gels, in general, are macroscopically solid-like but exhibit liquid-like characteristics microscopically due to the presence of a large no of liquid field micropores. The latter offers the possibility of synthesizing good ion conducting materials. Gels can be synthesized by “sol-gel process” which gives a solid-like polymerized network of precursor materials like silicate, vanadate, titanate etc as a result of hydrolysis leading to the formation of many…O-H-O…  bridging bonds. Two distinct hydrolysis routes in the “sol-gel” process are followed depending upon the precursor salt used as given below:

(i)                  inorganic precursor(metal salt like Na₂SiO₃)

(ii)                organic precursor(metal alkoxide like tetraethyl orthosilicate, TEOS).

The hydrolysis steps in these precursors involve respectively water or alcohol and hence, the respective derived gels are sometimes termed as “hydrogel and “alcogels”. The protonic species (H⁺, OH^-, H₃O⁺ etc.) arising out of the …O-H-O…  bridging links or from the dissociation of some suitable dopant salts/acids may migrate in the ion migrating channels provided by the ultraporous structure of the gels. This strategy has been adopted in developing the proton conducting gels reported in this thesis.

                   Our effort is directed towards developing some “doped hydrogels” starting from a metal salt and some “doped alcogels” starting from a metal alkoxide. Specific systems studied are:

(a)               Doped Hydrogels

                                                    Inorganic Precursor –         Na₂SiO₃ (SiO₂ gel matrix)

                                                    Dopant                   -        NH₄BF₄

(b)               Doped Alcogels

Organic Precursor   -        TEOS (tetraethyl orthosilicate)

                                      (SiO₂-gel matrix)

Dopant                   -        NH₄BF₄, NH₄H₂PO₄, KH₂PO₄

& H₃PO₄

 

                   A detailed structural and ion transport studies of all the above gel systems have been carried out. As an off-shoot, it has been noted that “whiskers” of dopant grow as a result of the slow evaporation of the dopant electrolyte entrapped in the “pores” of the gel matrix. A systematic study for such a phenomenon is also reported in this thesis.

                   Chapter-1 contains a general introduction of the Superionic solids with special emphasis on the gel-electrolytes. This sets the background and scope of the research work reported in the thesis.

                    Chapter-2 describes the different structural and electrical characterization techniques used in the present work. The techniques include:

Ø      X-ray Diffraction

Ø      Differential Thermal Analysis/ Thermogravimetric Analysis and Differential Scanning Calorimerty

Ø      Infrared Spectroscopy

Ø      Wagner’s polarisation method for total ionic transference number

Ø      Cationic (or anionic) transference number from coulometry and/or combined a.c./d.c. techniques.

Ø      Complex impedance spectroscopy for studying the electrical conductivity.

Another specific study reported in this thesis is growth of whisker from SiO₂ doped gel matrix for which Scanning Electron Microscopy and Optical Microscopy have also been employed as additional experimental techniques.

Chapter – 3 reports our results on sodium metasilicate based “Hydrogel” doped with the salt NH₄BF₄. To prepare this hydrogel xNa₂SiO₃+(100-x)NH₄BF₄, separately prepared aqueous solutions of desired concentrations of precursor material Na₂SiO₃ and dopant NH₄BF₄ were mixed together and left at ~40⁰C for 3-4days to hydrolyze and condense in order to gellify. The gellification was complete in nearly 7 days after which the gel was dried at ~200⁰C to obtain xerogel. The structural studies (XRD, IR, DTA) showed that the dopant salt complexes with SiO₂-gel network and results in the formation of a new material. Gellification is limited to x= 15to 50 mol%. The electrical conductivity of the complexed gel as a function of the composition, temperature, and relative humidity has been studied and discussed in this chapter. The Wagner’s polarization studies showed that the gel system is predominately ionically conducting (t_ion ~1) upto 300⁰C. Coulometric studies suggested that H⁺ and OH^- ions are the dominant mobile species in the gel system at room temperature, but at higher temperature (T>100⁰C) some Na⁺ ion mobility was also suspected. Therefore, in order to get pure protonic conduction only, we switched over to “alcogel” systems prepared by organic precursor so that the presence of Na⁺ could be avoided in the gel-matrix. These results are discussed in Chapter 4 and 5.

Chapter – 4 contains the ion transport and structural studies tetraethyl orthosilicate(TEOS) precursor derived silica alcogel doped with proton conducting salts NH₄BF₄, NH₄H₂PO₄ & KH₂PO₄ to obtain different gel compositions xSiO₂+(100-x) dopant salt. These gel system are prepared by the usual “sol-gel” process. Separately prepared ethanolic solution of TEOS and aqueous solution of dopant salts were mixed together to which 2-3 drops of HCl was added as catalyst. This “sol” was left for 3-4 days at ~40⁰C to “gel” through the process of hydrolysis and condensation. After the gellification was complete, the “gel” ingot was dried at ~200⁰C. In all our gel systems, the gellification was only possible in a limited compositional range (viz. x=95 to 40 mol% for NH₄BF₄, x= 95 to 65 mol% for NH₄H₂PO₄ and x= 95 to 70 mol% for KH₂PO₄). All these gels are transparent and the measured porosity is ~80%. The surface morphologies of fresh and stored gels are different. On cutting a freshly prepared gel ingot, some liquid was seen oozing out which suggested the entrapment of liquid electrolyte in the porous SiO₂ gel matrix. The XRD and DSC results also confirm this conjecture. In IR studies also, no new peaks other than the peaks for the doped salts NH₄BF₄ or NH₄H₂PO₄ or KH₂PO₄ and SiO₂-gel matrix are observed which also led us to conclude the same as above that the dopant salts are entrapped as solution in the pores of the gel matrix. The gels are predominately ionically conducting ( t_ion ~1). The electrical conductivity has been found to be composition, temperature and humidity dependent. The electrical conductivity Vs composition studies on each gel system showed that the conductivity is maximum for some optimum composition like

s_max.~1.76x10^-4S.cm^-1 for 70 mol%SiO₂+30mol%NH₄BF₄;

s_max.~3.28x10^-4S.cm^-1 for 70 mol%SiO₂+30 mol%NH₄H₂PO₄;

s_max.~1.76x10^-4S.cm^-1 for 70 mol%SiO₂+30 mol%KH₂PO₄;

gel system at room temperature. These values are very high in comparison to that of pure SiO₂ gel (~4.0x10^-8Scm^-1). The electrical conductivity as a function of temperature for all the three gel systems has also been discussed in this chapter. The whole temperature range of study is divided in to three regions:- Region1 (RT to 100⁰C) in which the conductivity decreases due to the desorption of water; Region II (100-160⁰C) in which the conductivity variation is slow or negligible which is typical of liquid- like conductivity and Region III (>160⁰C) in which the electrolyte evaporates rapidly and the conductivity drops sharply. Based on the above general structural and electrical characterization, we have modeled that the gel system is essentially a solid-liquid electrolyte composite in which the liquid electrolyte filled pores are embedded in an apparently solidified gel matrix.

           Chapter – 5 contains the ion transport and structural studies on proton conducting acid H3PO4 doped tetraethyl orthosilicate (TEOS) precursor derived silica alcogels of different compositions i.e. xSiO₂+(100-x) H₃PO₄. These are also prepared by sol-gel process as described in chapter4. Gelling took place only in the compositional range of x=40 to 90 mol%. The XRD and DTA/TGA results indicate that H₃PO₄solution is entrapped in the porous SiO₂ gel matrix and no new compound is formed as a result of H₃PO₄ doping in the silica gel i.e. SiO₂ gel matrix and H₃PO₄ remain as separate phase in the doped gel system. The infrared spectral studies showed that a two phase composite has been formed and no chemical modification and interface reaction has occurred. All gels were ionically conducting with tion~1. For understanding the transport mechanism, the variation of electrical conductivity as a function of composition, temperature and relative humidity has been carried out. The highest room temperature electrical conductivity has been found to be ~ 1.6x10^-2 S.cm^-1 for the composition x=40 mol% which is higher than that of pure SiO₂ gel (~4.0x10^-8 S.cm^-1) by about 6 orders of magnitude. In the temperature dependence of conductivity, the s Vs 1/T plot can be divided into three regions:- Region I (RT to 100°C) in which the conductivity decreases due to the desorption of physiosorbed water; Region II (100-160⁰C) in which the conductivity is almost independent of temperature typical of liquid-like conductivity behaviour.  Region III  (>160⁰C) in which a sudden decrease in the conductivity is observed due to the loss of H₃PO₄. The liquid-like protonic transport has been interpreted in terms of self-dissociation of entrapped H₃PO_4­ to yield mobile H₂PO₄^- / HPO₄^2- acid-base pairs.

               Chapter – 6 gives results of a significant “off-shoot” of the studies on TEOS-derived alcogels discussed in Chapter-4. The gels xSiO₂+(100-x) NH₄BF₄, xSiO₂+(100-x) NH₄H₂PO₄ and xSiO₂+(100-x) KH₂PO₄ have been interpreted by us as solid-liquid electrolyte composites formed by the entrapment of respective liquid electrolytes solution in the pores silica alcogel matrix. When these gels were allowed to stay (or stored) for long time in high humidity ambient, then long whiskers appeared from the gel surface. The growth of whiskers can be interpreted in terms of the slow movement of the entrapped liquid electrolyte towards the surface and subsequently evaporating in the ambient at the gel-air interface. Given the proper conditions of nucleation, whisker type crystallization occurred. The growth was monitored with the help of optical and scanning electron microscopy. Identification of the whiskers has been done with the help of IR, XRD, and DSC, and these were found to be the same as the respective dopant salt. Thus, the SiO₂-gel porous matrix has been demonstrated to provide a simple and good platform for the growth of whiskers.

               Chapter – 7, finally, summarizes all the results reported in this thesis.