1 Introduction

Fuel cells are an electrochemical cell that converts chemical energy into electrical energy through electrochemical reactions. Fuel cells are a unique array for a potential application, and similar to batteries but do not need to recharge. The basic component of a fuel cell is anode, cathode, and electrolytes. The core source of a fuel cell is hydrogen which helps to transfer ions from cathode to anode. Solid electrolytes are exhibit high ionic conductivity, low activation energy, good mechanical properties, and are found to be promising materials for solid-state batteries, fuel cells, sensors, memory cells, solid-state batteries, etc. [1, 2]. There are two reasons for the high ionic conductivity of these superionic materials, one is the presence of a large number of defects and vacancies in the lattice at room temperature and the second is the low coordination number of the mobile ions [3,4,5]. There are several new materials for efficient proton conductivity, among them cesium dihydrogen phosphate (CDP), showing a superprotonic phase transition at 230 °C [6, 7]. The high-temperature conductivity is increased several orders of the magnitude at the transition temperature from 223 °C to 233 °C, the ionic conductivity increases from 8.5 × 10–6 S cm−1 to 1.8 × 10–2 S cm−1 [8] and CDP with 80 wt% represent a high ionic conductivity of 1.1 × 10–2 S cm−1at 259 °C [9]. At the transition temperature CDP changed from the monoclinic phase to the tetragonal (cubic) phase, this changed increase ionic conductivity followed by the Grotthuss mechanism [10]. Above 230 °C CDP chemical transformation decomposed as

$${\text{2CsH}}_{{2}} {\text{PO}}_{{4}} \to {\text{Cs}}_{{2}} {\text{H}}_{{2}} {\text{P}}_{{2}} {\text{O}}_{{7}} + {\text{H}}_{{2}} {\text{O}}$$

So, the superprotonic phase of CDP is unstable because of its dehydration under normal conditions [11]. It is thermally stable in an atmosphere containing more than 30 mol% H2O, which is not favorable for fuel cell electrolytes [12]. We need some modification by using heterogeneous doping of highly inert oxide ZrO2, SiO2, TiO2, Al2O3 and rare-earth phosphate with CDP produces a super-cooling effect on the conductivity by ball milling method. Ball milling is a method for enhancing the material reactivity and uniformity of distribution of elements which is economically and environmentally sustainable. The ability of thermal and mechanical stability improves by such type of doping above the transition temperature [13,14,15,16]. An enhancement of the ionic conductivity was detected for CsH2PO4–SrZrO3 [17] and CsH2PO4-SiO2 [18, 19] composite electrolytes. Ponomareva et al. [20] observed that the conductivity of CDP at low temperatures was improving several orders by small amount doping of SiO2. Kikuchi et al. [21] observed CsH5(PO4)2/SiP2O7 exhibit higher conductivity 500 mS cm−1 than pure CsH5(PO4)2 160 mS cm−1.

In the present work, we will observe the transport, thermal properties and ionic conductivity of the composite electrolytes 0.9CDP-0.1ZrO2, 0.8CDP-0.2ZrO2, 0.7CDP-0.3ZrO2, and 0.6CDP-0.4ZrO2 at different temperature.

2 Experimental

Cesium dihydrogen phosphate (CDP), was synthesized by slow evaporation under ambient conditions from an aqueous solution of Cs2CO3 (99%, Alfa Aesar) and phosphoric acid H3PO4 (≥ 85 wt. % in H2O, Alfa Aesar) according to the following reaction [22]:

$${\text{Cs}}_{{2}} {\text{CO}}_{{3}} + {\text{2H}}_{{3}} {\text{PO}}_{{4}} \to {\text{ 2CsH}}_{{2}} {\text{PO}}_{{4}} + {\text{ CO}}_{{2}} + {\text{ H}}_{{2}} {\text{O}}$$

Dissolve 25 gm Cs2CO3 in 200 ml distilled water, 15 ml 98% H3PO4 previously dissolve in 200 ml distilled water was added to the solution prepared above dropwise, using a burette. The total volume of the solution becomes 400 ml. The solution was stirred for 6 h. Thereafter, the obtained solution was heated in an oven at 150 °C until it turns in a hard-solid amorphous bottom layer. The solid layer was washed with methanol and vacuum-filtered then dried at 120 °C for 2 h to remove the residual water and grinded to produce the powder of CDP [7]. X-ray diffraction (XRD) pattern of initial powders and mixtures were recorded with Proto Axrd Benchtop (Kα1,2 or Kβ wavelength, the energy resolution of 200 eV FWHM, 640 channel high-speed detector). Finally, the (1−x) CsH2PO4/x ZrO2 where x = (0.0–0.4) composites synthesized. ZrO2 particles that were purchased from Alfa Aesar and added into dry crystalline/amorphous precipitate produced in the above method according to their weight percent. The constituents were mixed in an agate mortar for 4 h and a homogeneous powder mixture was prepared. The powder was pressed in the form of a pallet by hydraulic pellet pressing at 7 tons for 20 min. The diameter and thickness of the pellet were 10.0 mm and 3 mm, respectively. The pellets were calcined at 80 °C for 2 h. Silver was used for making electrodes on the surface of pellets by advanced vacuum coating unit (Hind High Vacuum-12A4D). The conductivity was measured by LC Impedance Meter (Hikoki 3532–50) in the frequency range 1 Hz to 400 kHz at the temperature range from room temperature to 300 °C. The ionic conductivity value was calculated from the resistance value of R. Temperature dependences of the conductivity plotted in Arrhenius plots form by the following equation,

$$\sigma \left( {{\text{S}}/{\text{cm}}} \right) \, = {\text{L}}\left( {{\text{cm}}} \right)/{\text{A}}\left( {{\text{cm}}^{{2}} } \right){\text{ R}}\left( \Omega \right)$$

where R is the resistance, L is the thickness of the electrolyte pellet and A is the area of the electrolyte pellet.

Differential scanning calorimetry (DSC) data were obtained by the (DSC-60) Unit (Shimadzu, Japan) system in the temperature range of 20 °C to 400 °C at a heating rate of 5 °C/min under 50 ml/min nitrogen flow. Phase transitions of the materials were measured by the DSC. Thermogravimetric analysis (TGA) and Differential thermal analysis (DTA) data were obtained with a Perkins Elemer 4000 system. Fourier Transform Infrared Spectroscopy (FTIR) was used to identify the number of components and the presence of any functional group in the material. FTIR data was collected by FTIR- 8400S Unit (Schimadzu, Japan). RAMAN data was collected by micro –Raman system (Horiba-Jobin T64000) equipped with an Ar laser in the range 100–1000 cm−1.

3 Results and discussion

3.1 X-ray diffraction (XRD)

The X-ray diffraction (XRD) patterns of the CDP and combination of CDP with ZrO2 in the range 2θ = 20°–60° are plotted in Fig. 1.

The XRD patterns of the CDP and CDP/ZrO2 electrolyte composites were measured in the powdered form under the atmospheric condition, after heat treatment of 100 °C for 2 h. The sharp and well resolved diffraction peaks obtained at 2θ values of 23.6°, 29.0°, 36.8°, 38.1° and 48.2° of CDP correspond to (011), (111), (220), (\(\stackrel{-}{2}\) 21), and (311) planes respectively. The experimentally measured diffraction pattern for CDP is the same as standard data from JCPDS of CDP and ZrO2 card No. 84–0122 and 37–1484 respectively [23, 24]. CDP powder showed a monoclinic phase with space group P21/m and ZrO2 powder also monoclinic phase at room temperature. All peaks of the composites are the same as observed for CDP and ZrO2.

3.2 Conductivity

Temperature dependence conductivity of composites (1−x) CDP-xZrO2 where x ≤ 0.4 (CDP/ZrO2 ratio of 0.9/0.1, 0.8/0.2, 0.7/0.3, and 0.6/0.4) were studied. The arrehenius plots at various temperatures are shown in Fig. 2.

Fig. 1
figure 1

XRD patterns of powdered CsH2PO4/ZrO2 composites. CsH2PO4 and monoclinic ZrO2 are indexed according to JCPDS card No. 84–0122 and 37–1484 respectively

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Fig. 2
figure 2

Arrhenius plots of Temperature dependence conductivity for CDP and its Composites 0.9CDP/0.1ZrO2,0.8CDP/0.2ZrO2, 0.7CDP/0.3ZrO2 and 0.6CDP/0.4ZrO2

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The conductivity of CDP increases by three orders of magnitude at the transition temperature. The ionic conductivity of CDP was found out 10−6S cm−1 at 150 °C which is increased 10−2S cm−1at 230 °C with an activation energy ≈ 0.4 eV. The Arrhenius plot of conductivity jumped with an activation energy of 0.88 eV at a higher temperature which was 1.28 eV below the 180 °C [25, 26]. At cold sintering process, ionic conductivity of CDP reaches 2.30 × 10–4 S cm−1 at 200 °C because it is highly dissolving material with water [2]. The presence of ZrO2 increases the stability of solid acid composites due to the imperfect interconnections between the solid acid and ZrO2 [27]. Above 250 °C CDP dehydrates so conductivity starts to decreases for removing this problem introduced ZrO2. In this work, the conductivity of CDP reached a maximum of 2.0 × 10–2 S cm−1 at the 240 °C and 6.4 × 10–3 S cm−1 at the 260 °C. Only the temperature between 230 °C (transition temperature) to 250 °C (decomposition temperature) pure CDP exhibited high conductivity, after introducing ZrO2 in CDP this range increase from 230 to 300 °C. The conductivity of 0.7CDP/0.3ZrO2 and 0.6CDP/0.4ZrO2 was observed 1.14 × 10–2 S cm−1 at the temperature of 290 °C and 4.76 × 10–3 S cm−1 at the temperature of 300 °C, respectively which shows the stability increases and conductivity slightly decreases with increasing of x. Introducing of ZrO2 was a good increment to stopping decomposition. After 280 °C the conductivity of CDP/ZrO2 mixture starts decreases slowly as temperature crossed 300 °C, it decreases fast. Conductivity measured for pure CDP and CDP/ZrO2 at the temperature 25 °C, 50 °C, 75 °C, 100 °C, 125 °C, 150 °C, 175 °C, 200 °C, 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C and 300 °C. Leal et al. [28], Anfimova et al. [29], Ponomerva et al. [30] reported a result for enhancing the conductivity and stability of other composites electrolytes.

If the container of pellet confined in a high pressure, the protonic conductivity of 0.9CDP/0.1ZrO2 and 0.8CDP/0.2ZrO2 was 1.8 × 10–2 S cm−1 for 2000 min and 1.3 × 10–2 S cm−1 for 2200 min, respectively as soon in Fig. 3.

Fig. 3
figure 3

Time dependence of the conductivity for the CDP/ZrO2 in open atmosphere and hermetically sealed in a container

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3.3 Differential scanning calorimetry (DSC)

Typical thermal analyses for CDP and CDP with ZrO2 at different composition ratios with the constant heating rate (5 °C/min) are presented in Fig. 4.

Fig. 4
figure 4

Differential Scanning Calorimetry (DSC) curves for CDP and its composites heating rate 5 °C/min

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The DSC data for the CDP shows endothermic peaks at 150 °C, 230 °C, 270 °C, and 295 °C which are most likely related to the protonic phase transition. The transition at 150 °C is quasi-irreversible because hydrogen-bonded in CDP are ferroelectrics, so at the 150 °C peak is due to the presence of a small amount of CsH5(PO4)2 and ferroelectric transition take place which is quasi-irreversible [31]. Transition at 230 °C occurs due to the formation of monoclinic transition to cubic (superprotonic) transition and 270 °C due to dehydration of composites electrolytes [32] as shown in Fig. 4. 295 °C transition occurs due to the formation of polymers dehydration of the phosphate ions. The endothermic peaks were observed in composites 0.9CDP-0.1ZrO2, 0.8CDP-0.2ZrO2, and 0.7CDP-0.3ZrO2 at the temperature of 264 °C and 335 °C, 270 °C and 347 °C, 288 °C and 359 °C respectively which is more stable then pure CDP. The enthalpy of phase transition decreases more rapidly on increasing the value of x in (1−x) CDP/xZrO2 (where x = 0.1 to 0.4), which is indicating that the thermal stability of the CDP enhanced [18]. The CDP with different compositions of ZrO2 shows distinctive peaks between 150 °C and 400 °C due to the dehydration of present additives.

3.4 Thermogravimetric analysis (TGA) and differential thermal analysis (DTA)

A plot of the TGA and DTA investigated at a different temperature from a powder of CDP and CDP/ZrO2 composites is shown in Fig. 5A and Fig. 5B. The weight loss for CDP/ZrO2 is less as compared with the pure CDP, So the weight loss decreases as the value of x increases. The important decrease of the weight was obtained after 200 °C, which indicated dehydration of CDP as in Fig. 5A. Thus, the conductivity is directly influenced by dehydration of CDP and its composites.

Fig. 5
figure 5

TGA (a) and DTA (b) of powder CDP and composite electrolytes CDP/ZrO2 with dry air flow 60 ml/minute with a heating rate 5 °C/minute in the temperature range 30 °C —500 °C

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As well as the value of x increases, the peak of DTA is shifted towards the higher temperature which indicates the mixing of ZrO2 increases the stability of CDP as in TGA data. The DTA curve of pure CDP and its composites in Fig. 5B showed two endothermic peaks at the temperature near about 230 °C and 320 °C. The endothermic peak arises due to phase transition from the monoclinic phase to the cubic phase and the second one endothermic peak at the temperature of 320 °C is due to completely dehydration of the liquid phase as a sharp mass loss.

3.5 Fourier transform infrared spectroscopy (FTIR)

The FTIR spectra of the CDP, binary mixtures of CDP with ZrO2, and the intense band of the group Zr-OH, Zr-O-Zr, and Zr-O are shown in the Fig. 6.

Fig. 6
figure 6

O–H modes, Stretching P–O, bending P–OH, O–P–O modes and Cs–O modes in FT-IR Spectra of CDP and comparison its composites 0.9CDP/0.1ZrO2, 0.8CDP/0.2ZrO2, 0.7CDP/0.3ZrO2 and 0.6CDP/0.4ZrO2 in the range of 400 to 4000 cm−1

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To determine the presence of hydrogen bonds and PO43− anions in the present composite electrolytes, infrared transmittance spectra of the material was taken in the range of 400 cm−1 to 4000 cm−1. There are many well-separated regions in the infrared spectrum in high frequency (OH) modes 3600 cm−1 to 1300 cm−1, Stretching P–O, and bending P–OH modes 1300 cm−1 to 800 cm−1 and O–P–O modes and Cs-O belongs to 800 cm−1 to 400 cm−1[33]. The material monoclinic ZrO2 sharp band is appearing at 746 cm−1. The proton-conducting behavior of the electrolytes exists with the type of hydrogen bonds [34]. O–H band separated into ν (OH), δ (OH), γ (OH) due to Fermi resonance overtones of the deformation modes appear as the ABC structure [35, 36]. In our study, the peaks were observed at 2760 cm−1, 2332 cm−1, and 1700 cm−1 which are indicated to the O–H stretching bands. The strong bands δ(PO–H) measured at 1216 cm−1, ν3PO4 asymmetric and symmetric stretching observed at 1085 cm−1 and 954 cm−1 respectively. The weak bands δ(PO–H) in-plane and γ(PO–H) out of the plane are also observed at 1104 cm−1 and 1214 cm−1 respectively. The Cs–OH band is arising at 531 cm−1 regions. Similar symmetry was also measured for other composite electrolytes. The well-separated peaks of CDP powder are shown in Table 1.

Table 1 Functional groups which are present in CDP
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3.6 Raman spectra

Raman spectra of CDP versus temperature between the range of room temperature to − 191 °C and frequency range 100 cm−1 to 1000 cm−1 are given in Fig. 7.

Fig. 7
figure 7

Raman spectra of Cesium dihydrogen phosphate at different temperature. Phosphate ions giving rise to bands at 620 cm−1 (P–O–P bridge stretching)

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Similar spectra at low temperatures up to room temperature have been discussed [11, 35], which shows the same result as our work. In the CDP, the bands are less visible due to OH– stretching. The spectrum of P–O–P bridges appears at 620 cm−1. There is no large effect by temperature in the range of room temperature to − 191 °C on Raman Spectra. All the bands well defined in the spectrum of the superionic phase in CDP. The bands arising due to internal vibration of H2PO4− ions, is likewise appeared by the low-frequency spectra.

4 Conclusions

The structural, thermal, and proton conductivity behavior of (1−x) CsH2PO4/x ZrO2 (0 ≤ x ≤ 0.4) composites electrolytes for the fuel cell. Based on an evaluation of the literature and our recent conductivity measurements, CDP undergoes a superprotonic transition at 230 °C. A three order of magnitude ionic conductivity increases in the unmixed sample at the transition temperature. The CDP was dehydrated up to 250 °C. We were observed that CDP with low additives of ZrO2 (contents x = 0.1 and 0.2) was good agreement with ionic conductivity and stability. CDP and ZrO2 perform between the temperatures range from 230 °C to 280 °C which is a good increment. The highest conductivity of confined electrolytes 0.9CDP/0.1ZrO2 and 0.8CDP/0.2ZrO2 were observed 1.8 × 10–2 S cm−1 for 2000 min and 1.3 × 10–2 S cm−1 for 2200 min, respectively. The intensity was decreasing in Raman spectroscopy as well as temperature decreases. The high ionic conductivity and thermal stability of these composites make them attractive for use in different chemical devices and fuel cells.