Preparation, Characterization, and Stability of the (1-x) CsH2PO4 + xTiO2 Composite Electrolytes for Fuel Cells

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Introduction
Today, the consumption of oil in the world is increasing day by day, which is limited.We need to focus on technologies that reduce oil consumption.Fuel cell vehicles are considered a good way to reduce oil consumption.The purpose of the fuel cell is to eliminate air pollution and further greenhouse gases.A fuel cell converts chemical potential energy directly into electrical energy without creating any noise.The fundamental parts of a fuel cell are electrolytes, gas transport processes, anode, and cathode [1,2].We will discuss the intermediate temperature fuel cell electrolyte, which is an important part of the fuel cell.
CDP is considered as one of the highly conductive materials [3][4][5] in the family of acid salts M α H β (NO 4 ) γ (M = Li, Na, K, Rb, Cs, NH 4 ): N = P, S, As, Se, and α, β, γ, are integers for fuel cell electrolytes [6][7][8][9].CsH 2 PO 4 has proton conductivity 1.8×10 − 6 at a temperature of 150°C [10] and showed high conductivity 2.2× 10 − 2 S cm -1 at the temperature of 240°C [11].Botez et al. [3] reported 1.5×10 − 2 S cm -1 high conductivity of CDP at the temperature of 260°C.CDP achieved high proton conductivity of 2.0× 10 -2 S cm -1 up to 260°C for 50 hours under high atmospheric pressure and high humidity when the container is con ned in a small container [12].It is a good conductive electrolyte but shows some disadvantages such as poor mechanical properties, water solubility, and availability of high conductivity in a short range of temperature 230 to 250°C.CDP decomposes into Cs 2 H 2 P 2 O 7 over 250°C, which reduces conductivity [1,13].To overcome this problem, we introduce heterogeneous doping of highly inert oxides such as (SiO 2 , ZrO 2 , TiO 2 , Al 2 O 3 , rare earth phosphates, and solid acid) for long-term durability as well as thermal and mechanical stability [14][15][16][17].Hermetically con nement of a pellet in a container is one of the methods to improve the stability and increase the conductivity of superprotonic CDP and its composites.

Sample Preparation
Cesium dihydrogen phosphate (CDP) was prepared from a precursor solution used 25 gm Cs 2 CO 3 (Alfa Aesar, 99%) and 15 ml of aqueous phosphoric acid (H 3 PO 4 -Alfa Aesar, 85%) were dissolved in deionized water (DI).The solution was stirred for 24 hours.After that, the obtained solution was heated in an oven until it turns into a hard-solid amorphous bottom layer.The solid layer was introduced with methanol and the resultant material was then ltered from the mother solution.The precipitate of CDP was dried at 150 o C for 6 hours to get it in solid form, remove residual water, and grinded to produce the powder of CDP [13,22,23].TiO 2 powder was purchased (Alfa Aesar, 99.99%), and added to dry CDP powder, which was produced by the above method.The constituents of composites (1-x) CDP-xTiO 2 where x= (0.0-0.4) were mixed by an agate mortar for 4 hours and a homogeneous powder mixture was prepared.The powder was pressed at a pressure of 7 tonnes (metric)/cm 2 (0.68646 GPa) for 10 minutes to make the pallets by a hydraulic pellet pressing machine.The diameter and thickness of the pellet were 12 mm and 3 mm, respectively.The pellets were calcined at 100 o C for 2 hours.Silver wires were used for making the electrodes on the surface of pellets by vacuum coating unit (Hind High Vacuum-12A4D).The prepared pellets were used for measuring ionic conductivity.

Characterization
The XRD was recorded in the range of 2θ from 20 o to 60 o with Proto AXRD Benchtop (Kα 1,2 or K β wavelength, the energy resolution of 200 eV FWHM, 640 channel high-speed detector).EDX and FESEM data were recorded by FESEM-Quanta 200 FEG.The conductivity was measured by LC Impedance Meter (Hikoki 3532-50) in the frequency range of 50 Hz to 4 MHz at the temperature range from room temperature to 320 o C. The ionic conductivity value was calculated by the resistance value (R).Temperature and time-dependent conductivity plotted by the Arrhenius plots by the following equation where R is the resistance, L is the thickness of the pellet and A is the area of the pellet.
Fourier Transform Infrared Spectroscopy (FTIR) was used to identify the number of components and the presence of functional groups present in the material.FTIR data was collected by FTIR-8400S Unit (Shimadzu, Japan).The decomposition and the phase change of solid acid composite are identi ed in the form of Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Derivative Thermogravimetric (DTG).DSC data were obtained by the (DSC-60) Unit (Shimadzu, Japan) system in the temperature range of 20 to 400 o C at a heating rate of 05 o C /min under Nitrogen ow.TGA, DTA, and DTG were analyzed by SII 6300 EXSTAR at a rate of 10 o C/minutes under air ow 200 ml/minutes.
The sharp and well resolved diffraction peaks of CDP obtained at the 2θ values of 23 [24,25].By increasing the concentrations of TiO 2 , the intensity of the peaks is reduced which suggests the rise in the degree of the amorphousity in the composites.The interaction between Cs and Ti in the composites is being displaced from their lattice sites.The displacement caused the dislocation and imperfection; hence, the intensity of the diffracted X-ray peaks is identi ed.This phenomenon leads to ion diffusivity with ionic conductivity and stability because of a rise in the amorphous nature of the composites [26,27].Thus, XRD patterns of the composite electrolytes show the appearance of two independent phases (CsH 2 PO 4 and TiO 2 ) in disordered and dispersed states.

FESEM and EDX Analysis
EDX and FESEM images of CDP and TiO 2 are shown in Figs. 2 and 3.The surface morphology of pure CDP and TiO 2 showed by FSEM in Fig. 2(a, c and d) and Fig. 3(a, c, and d) respectively.FESEM compose almost spherical, hexagonal, and clustered to form larger aggregates on the surface with different sizes, and it is observed CDP and TiO2 homogeneous decorated.The chemical composition of pure CDP and TiO 2 was investigated by EDX analysis as shown in Fig. 2(b) and Fig. 2(c).
The obtained results clearly illustrate that the CDP and TiO 2 were well prepared, which also supported the XRD analysis.The EDX spectrum indicated the presence of the Cesium, phosphorus, oxygen in CDP, and titanium, oxygen in TiO 2, and have the value of weight percent and atomic percent indicated in Table 1 (a   and b).
Table 1.The value of weight percentage and atomic percentage of (a) CDP (b) TiO 2 in the EDX spectrum.

Fourier Transform Infrared Spectroscopy (FTIR)
Infrared transmittance spectra of the pure CDP and composites electrolytes CDP/TiO 2 were taken in the range of 400 cm − 1 to 4000 cm − 1 to con rm the presence of anions PO 4 − 3 hydrogen and other bonds as shown in Fig. 4.
On the breaking of the hydrogen bonds, CDP showed high conductivity at the temperature of 230 o C. CDP was stable at room temperature due to the paraelectric phase.The IR spectra consist of high-frequency well-separated H modes between 3600 and 1300 cm − 1 .Stretching P-OH and Bending P-O modes belong to 1300 cm − 1 to 800 cm − 1 .Cs-O and O-P-O belong to 800 cm − 1 to 400 cm − 1 [28].The ABC structure of the stretching vibrations appears due to the Fermi resonance and observed for the ν(OH)-2791 cm − 1 , δ(OH)-2332 cm − 1 , and γ(OH)-1700 cm − 1 .The strong absorption of the samples in the region 900-1200 cm − 1 of the IR spectra can undoubtedly be assigned to the components of the PO 4 while the four bands at 1216 cm − 1 , 1110 cm − 1 , 1080 cm − 1, and 930 cm − 1 are attributed to the P-O stretching modes in the H 2 PO 4  − anion that is compatible to reported data.The generated bond due to Cs…O-H is observed at 549 cm − 1 .FTIR spectrum of TiO 2 observed at 3516 cm − 1 , 1691 cm − 1, and 1260 cm − 1 corresponding to the stretching vibration of the O-H group, bending modes of the water Ti-OH group, a Ti-O group, respectively, in TiO 2 .Ti-O-Ti bonds in TiO 2 lattice shown in the range of 700 − 500 cm − 1 [29,30].We observed after adding TiO 2 the intensity of composite electrolytes decreases with the increase of doped TiO 2 concentration.This indicates that the amorphous region in the composite material was increased, which was in agreement with the XRD results and also be observed the small shift in some bonds.

Conductivity
Temperature dependence Arrhenius plots of pure CDP and its four composites are shown in g .5.
Figure 5 demonstrates that a phase transition of CDP occurs at the temperature of 230 o C, and the conductivity increasing by ~ 3 orders of magnitude from the monoclinic phase (10 − 6 S cm − 1 ) to the cubic phase (2.0 × 10 − 2 S cm − 1 ).CDP dehydrated above the temperature of 250 o C matched with [18], so conductivity starts to decrease.If the CDP pellet is con ned hermetically in a tide and sealed container, the dehydration is greatly reduced.The proton conductivity slightly increased concerning temperature range between 235 to 260 o C [3,30].We observed that the conductivity of CDP at a lower temperature is more than its composites, while at a higher temperature the conductivity of its composites is more than CDP.As the value of x is increasing the arc of the curve is being linear at a higher temperature, which showed the stable value of the conductivity and agrees with XRD, FTIR, and thermal analysis discussion.
The addition of TiO 2 increases the stable conductivity of phosphoric acid composites due to the grain boundaries and imperfect interconnections between the acid and TiO 2 [8].Composite electrolyte 0.9 CDP/0.1TiO 2 exhibited high conductivity 1.7×10 − 2 S cm − 1 in the temperature range of 240-280 o C for 45 hours of a pellet which was bounded hermetically in a small container as shown in Fig. 6.
Time dependence of the proton conductivity measured on a pellet which was sealed in a small container.Mohammad et.al reported that the conductivity of CDP is 1.9×10 − 2 S cm − 1 [15].CDP ionic conductivity was found 2.30× 10 − 4 S cm − 1 at 200 o C at the cold sintering process [1].A proton-conductive electrolyte based on a CsH 2 PO 4 /SiP 2 O 7 the maximum conductivity achieved was 44 mS cm − 1 at 266°C [19].
CsH 2 PO 4 -SiO 2 composites with modi ed silica exhibited high proton conductivity 10 − 3 -10 − 2 S cm − 1 at 130-250°C and have higher thermal stability at lower water vapor partial pressures compared with the pure salt [6].Conductivity was measured for pure CDP and composites CDP/TiO 2 at different temperatures.
3.5 Thermal Analysis (DSC, TGA, DTA, DTG) DSC thermal analyses of CDP and its composites electrolytes with a constant heating rate (5 o C /minute) at different composition ratios are shown in Fig. 8.
Dehydration reaction occurs [32] in the following way Figure 8 con rmed that CDP phase transition monoclinic to cubic occurred at the temperature of 230 o C and another phase transition, which is corresponding to dehydration at 250 o C [15].CDP shows endothermic peaks at 150, 230, 250 o C. The sharp peak at 150 o C is quasi-irreversible because hydrogenbonded in CDP are ferroelectrics, it occurs due to the presence of a small amount of CsH 5 (PO 4 ) 2 [33].In pure CDP dehydration start after 250 o C while four composites 0.9CDP-0.1TiO 2 , 0.8CDP-0.2TiO 2 , 0.7CDP-0.3TiO 2 , 0.6CDP-0.4TiO 2 showed dehydration behavior disappears as the value of x increased.
The results of the TGA were performed under an ambient atmosphere, as shown in Fig. 9 (a).
Weight loss of 7.2, 3. previous results [7,34,35].In the case of four composites, dehydration is slowed, which is indicating that the chemical interaction between CDP and TiO 2 .Overall weight loss is decreasing with increasing x value and found that the minimum weight loss for the composite 0.6CDP/0.4TiO 2 .Our result shows that the thermal behavior and stability increase with increases the value of x.
According to DTA plots, CDP showed main peaks at 150, 230, 332, and 354 o C due to the formation phase of CsH5(PO4), phase transition, formation of dimmers, and polymers, respectively.After the transition temperature the dehydration process slowdown in Fig. 9 (b) and matched with other results [1,36,37].
In the DTG curve Fig. 9 (c) also showed the variation of amorphization and phase composition.The dehydration peaks also disappeared as shown in DSC, DTA curves, and supported each other.DSC, TGA, DTA, DTG is favorable with conductivity, XRD, and FTIR.

Conclusions
We have observed the structural, thermal, and proton conductivity behavior of (1-x) CsH 2 PO 4 /xTiO 2 (0 ≤ x ≤ 0.4) composites electrolytes for the fuel cell.Based on the evaluation of the literature and our recent conductivity measurements, CDP undergoes a superprotonic transition at 230 o C. Three orders of magnitude ionic conductivity increases in the unmixed sample at the transition temperature.It was also observed that CDP with low additives of TiO 2 (contents x = 0.1 and 0.2) appeared in good agreement with ionic conductivity and stability.The highest ionic conductivity 1.7×10 − 2 S cm − 1 has measured the composites 0.9CDP/0.1TiO 2 at 270 o C, which is more stable for long-term durability than pure CDP.It was achieved the protonic conductivity of 0.9CDP/0.1TiO 2 and 0.8CDP/0.2TiO 2 over a time span t = 45 and t = 50 hours, respectively, which is con ned hermetically in a small container.A mixture of CDP and TiO 2 was useful for protonic conductivity between the temperature range from 240 to 310 o C, this result favors to establish fuel cell technology.Thermal analysis (DSC, TGA, DTA, and DTG) revealed enhancement of the thermal stability of CDP as TiO 2 was introduced.The high ionic conductivity and thermal stability of these composites make them attractive for use in different chemical devices and fuel cells.

Declarations
Figures Time dependence of the ionic conductivity for the sealed pellet of 0.9CDP/0.1TiO2composite electrolytes.
Time dependence of the proton conductivity for 0.8CDP/0.2TiO2 in a small con ned region.

5 .
0.8CDP/0.2TiO 2 exhibited high conductivity1.2×10− 2 S cm − 1 in the temperature range of 240-290 o C for 50 hours as shown in Fig. 7. 0.7CDP/0.3TiO 2 exhibited 7.0 × 10 − 3 S cm − 1 in the temperature range 240-300 o C and 0.6CDP/0.4TiO2exhibited 1.0 × 10 − 3 S cm − 1 in the temperature range of 240-310 o C in open atmosphere as shown in Fig. Similar results were presented for other types of materials at different molar and weight ratio.N.

Figure 4 O
Figure 4