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The introduction of inorganic compounds into the Sulfonated Poly Ether Ether Ketone, with Tungsto phosphoric acid matrix represents a possible solution to increase the efficiency of polymer electrolyte membrane water electrolyser. The electrochemical properties and morphology of these membranes were studied by Solartron test system and SEM. Furthermore comparative performance of the membranes and catalyst coating methods were reported. The influence of morphology on chemical-physical properties of membranes was also investigated. The composite inorganic SPEEK impregnated with Zirconium oxide and TPA performed at a high current density of 1.35 A/cm2 by impregnation reduction method, which closer to 1.4 A/cm2 is observed by conventional brush coating method.
Composite proton exchange membrane
Membrane electrode assembly
Impregnation reduction method
Brush coating method
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The development of solid polymer electrolyte (SPE) based water electrolyzer is appearing to be an efficient method of producing high purity hydrogen in large quantities with little or no environmental impact [1, 2]. Water Electrolysis is a very simple process that takes water and passes a supply of electricity through it using immersed electrodes to split into positive hydrogen (H+) and negative oxygen (O-) ions. These hydrogen and oxygen ions migrate through the water towards the cathode and anodes respectively, where electron transfers allow for the diatomic H2 and O2 molecules to form at high purity. The use of hydrogen as a power generation source is minimal at this stage due to its high cost of production.
Because of the challenges that conventional Nafion membranes faces significant research recently has been devoted to the development of more nonfluorinated polymers [3-7] Sulphoned Poly (Ether Ether Ketone) (SPEEK) [8-11], Poly (Benzimidazole) (PBI), Sulponated Poly sulfone (PSF), Poly (Ether Sulfone) (PES), poly(arylene ethers), polyimides, and polyphosphazene. [12-16] etc., have excellent chemical mechanical and thermo oxidative stability and are low cost. Among these membranes based on aromatic poly ether ether ketone (PEEK) were shown to be very promising for water electrolysis application since they possess good mechanical properties on degree of Sulfonation (DS).
In addition, the sulfonic acid group improves the chemical properties of the polymer such as water uptake and selectivity. Furthermore, the increased solubility in water and solvent limitations improves the application for proton conducting polymer. SPEEK was selected in this study. It was reported that the SPEEK membranes could have a life time longer than 3000 h [17-20] which can be used as a proton conducting membrane material. It was also found the degree of sulfonation has a strong influence on both conductivity and stability of SPEEK.
Recent years have witnessed the evolution of zero gap electrolysis cells [21, 22] As such, the use of membranes in zero gap electrolysis cells in the future will increases in direct proportion to the advancement on their synthesis and developmental applications. Heteropolyacids (HPA) [23-25], have dual role of being both hydrophilic and proton conducting. However, HPAs are generally water-soluble; the degrees of solubility are reduced in composite HPAs, while maintaining their high proton conductivity. Composite matrix reduces the leaching of HPA.
The objective of this research is to develop a new type of composite membrane by incorporating Heteropolyacids such as tungstophopsphoric acid (TPA) and Inorganic fillers such as titanium dioxide (TiO2), silicon dioxide (SiO2), zirconium dioxide (ZrO2) [26-28] into partially sulfonated polyether ether ketone (PEEK) polymer matrices.
In this work we have stretched the various catalytic formulations to evaluate if such a method could be applicable to zero gap water electrolyzers containing a wide range of current densities. Two types of catalyst coating were made on the membranes: Platinum- Platinum by non-equilibrium Impregnation-Reduction (I-R) method and Platinum - Iridium oxide by conventional brush coating method. Among all precious metals, platinum is the most active catalyst for the hydrogen evolution reaction (HER) at the cathode and can be applied at moderate loading. Iridium and iridium oxide is well known for its unique electro catalytic properties in respect to oxygen evolution processes. Since IrO2  catalyst exhibits high corrosion resistant properties but slightly lower electro catalytic activity than RuO2 . The conventional method coatings were made mechanically.
Poly(ether - ether ketone) (PEEK) was obtained from Victrex® US, Inc. (450 PF), 1-methyl-2-pyrrolidinone (NMP) was obtained from Merck, Titanium dioxide (TiO2), Silicon dioxide (SiO2) and Zirconium dioxide (ZrO2) and Phosphotungstic acid (PWA) were obtained from Sigma Aldrich and were used as received without further purification.
Synthesis of SPEEK membrane
PEEK was first dried at 100 oC in a vacuum oven for 8 h. Then 10 g of the polymer was dissolved in 500 ml of concentrated sulfuric acid (Merck) and vigorously stirred at room temperature for desired lengths of reaction time. Then, the polymer solution was gradually precipitated over ice-cold water under continuous mechanical agitation, and left to settle overnight. The degree of sulfonation of the polymers used is identical throughout the experiment. The precipitate was filtered and washed with distilled water. It was then dried under vacuum for 8-10 h at 100 oC. The final product is the sulfonic acid form of PEEK (SPEEK).
Preparation of SPEEK composite membranes
SPEEK membranes were prepared by dissolving the SPEEK sample in NMP solvent under constant stirring to form a homogenous solution at 70 oC to which 10 wt % phosphotungstic acid were added identical among the three and 10 wt% TiO2/SiO2/ZrO2 were added respectively and then refluxed at 80 oC for 6 h, to obtain a clear viscous gel. But higher phosphotungstic acid loading level has some retreats i.e, loss of the mechanical strength as well as leaching out of unbound acid. In order to solve these problems, the membranes were hot pressed between two Teflon films at 80 oC for 3 min is required. These approaches have certainly improved the mechanical properties. The film was cast on a clean glass plate with the desired thickness and dried at 80 oC for 12 h. The thickness of the wet composite polymer membrane was between 0.20 and 0.30 mm. Then, the composite membranes were detached from the glass tray by adding de-ionized water. Finally, the membranes were purified by heating at 70 oC in 3 % H2O2, 15% H2SO4 and in de-ionized water for 1 h, respectively.
Ionic conductivity and transport property measurements
Conductivity measurements were made for composite membrane by two-probe impedance technique, under potentiostatic conditions at a sweep rate of 5 mV.S-1 from 1-Hz to 1-MHz excitation signal. During the measurement each membrane was sandwiched between stainless steel plates and an ion blocking electrodes, the cross-sectional area of the surface was 0.502 cm2. The conductivity (σ) of the samples in the transverse direction was calculated from the impedance data, using the relationship σ = L/RA, where A is the Area of the electrodes, L and R are the thickness and bulk resistance of the films, respectively, The R was derived from the high frequency x-axis intercept of the complex impedance plot.
Water uptake (Wwd) after two hours of immersion was determined as the difference in weight (W) between the dried and the swollen membranes.
Where Wdry is the mass of the dry membrane.
Ion Exchange Capacity (IEC)
The ion exchange capacity of the membranes was determined using the titration procedure . The dry composite membrane was immersed in 100 ml of 0.1M hydro chloride aqueous solution for 48 hrs to change them into H+ form. The samples were then washed with distilled water to remove excess HCl, and then titrated with a standardized sodium hydroxide solution using phenolphthalein as an indicator.
The ion exchange capacity (IEC) was calculated using the following equation.
where IEC is the ion exchange capacity (mequiv. g-1), V the added titrant volume at the equivalent point (ml), M the molar concentration of the titrant and is the dry mass of the sample (g).
Water electrolysis to produce gaseous hydrogen and oxygen (Eq.1-3) is a long-established process.
Anode : → (1)
Cathode : → (2)
Cell : → (3)
In this study, we compare the performance capabilities of SPEEK based membranes with Nafion -117 by I-R method and brush coating methods at 80 oC. The resulting Current densities versus Voltage are also compared in Fig. 5 and 6. The experiments were repeated thrice to check for reproducibility.
2.7. Membrane Electrode Assemble (MEA)
In Impregnation Reduction (I-R) method, solutions of platinum anions, such as chloroplatinate ,0.01 M (PtCl6 2-) , and a reducing agent, typically sodium borohydrate ion 0.2M (BH4-), are revealed to opposite sides of a stationary solid Polymer Electrolyte membrane. (BH4- ) ions continuously penetrate the membrane and come into contact with (PtCl6 2-) ions on the opposite membrane face, at which point the platinum ions are reduced to platinum metal at the membrane surface in accordance with the impregnation reduction method . The other side of the membrane was impregnated and reduced by IrO2 under the conditions described in previous literature .
In Conventional Brush method, The Catalyst ink was prepared by first creating a stock solution of 5% Nafion solution, isopropyl alcohol, and occasionally water. The stock was sonicated for 30 minutes to thoroughly mix the components, and then added to the Platinum black and sonicated another 30 minutes until a homogenous suspension was obtained. Similarly the iridium ink is also made of the same procedure using iridium oxide salt. The catalyst coatings were made mechanically by brush. The loading of anode catalyst was about 2 mg cm-2 IrO2, while the cathode loading was 2 mg cm-2 of Pt.
After the reduction steps were completed, the MEA was soaked in 0.5M H2SO4 for 2 h. Finally the membranes were immersed in de-ionized water for 2 h before re-drying at 80 oC for 12 h. The membrane had an ion exchange capacity of 2.10 mequiv. g-1, a thickness of 0.210 mm, and an ionic conductivity of all the prepared composite membranes are in the order of 10-2 S.cm-1.
Results and discussion
3.1. Fourier Transform Infrared Spectroscopy Analysis
The FT-IR Spectra obtained for the membrane are shown as Fig. 1-3 shows a comparison between typical FTIR spectra of wet membranes of SPEEK/TPA/(Ti, Si, Zr O2) and of dried membranes of SPEEK/TPA/(Ti, Si, Zr O2). The membrane was first immersed in deionized water for 1 h. Then these wet membranes were used in FTIR spectra by quickly after removing the surface attached water to determine the wetted membrane spectra. The dry membrane spectra were determined after drying the membrane at 100oC for 1 h. There is also a clear difference between these spectra around the broadband in SPEEK samples appearing at 3480cm-1 was due to O-H vibration from sulfonic acid groups interacting with water molecules. The presence of absorption peaks in the range 1160 to 1100 cm-1 is observed. This indicates the presence of aromatic sulfonate group. The positions of vibration modes of all types of M-O bonds were strongly influenced by interaction of Phosphotungstic acid with the polymer and fillers. The rest of the peaks are assigned as follows: 550 cm-1: Ti-O asymmetric stretching. 1100 cm-1: Si-O stretching vibration. 530-560 cm-1: Zr-O vibration. 1230, 1075 cm-1: O-S-O symmetric stretching vibrations. 870 cm-1: Phosphotungstic acid. The bands around 610 cm-1 is due to the symmetric stretching of Ti-O dried membrane. The bands around 1460 cm-1 is due to the asymmetric stretching of Si-O dried membrane. In SPEEK/TPA/Zro2 spectra, the typical Zr-O stretching vibrations bands around 1680 cm-1 associated with the formation of a zirconium network were present. However, the presence of S=O bending vibrations around 1410 cm-1, confirms the incorporation of sulfonated groups. The strong vibrational bands at 1690-1750cm-1 were observed for the carbonyl group present in polymer backbone, which intensity was decreased with the dried membranes due to H-bonding. ( 33 )
IEC - Water uptake
The water uptake and ion exchange capacity (IEC) plays a vital role in membrane conductivity. At higher IEC values, the water uptake increased sharply at a proportional rate larger than that observed for lower IEC values, due to the hydration regions near the ions overlapping and the decreased volume fraction of the unfunctionalized phase. Fig. 4 shows the water uptake (%) and IEC for the membranes. It is observed that the SPEEK composite membranes exhibit lower water uptake when compared to pure SPEEK although the additives are hydrophilic and effects in the different IECs. Excessively high levels of water uptake can result in membrane dimensional change leading to failures in mechanical properties. Hence the relationship between IEC and water uptake (%) plays a crucial role in membrane morphology.
3.3. Scanning Electron Microscope
Surface morphology of the composite membrane as obtained by SEM are shown in Fig. 5. The distribution of inorganic fillers was relatively uniform in the organic matrix. Fig. 5 (a) and (b) shows the IrO2 coating and Platinum coating on the SPEEK membrane by conventional brush coating method. This shows the Agglomerated particles are observed in the interface as well as on the surface of the membranes. Fig. 5 (c) shows the Pt layer forms extensively on the membrane surface, as in non-equilibrium I-R method shows uniform distribution of the clusters of Pt particles. Fig. 5 (d) shows a highly porous composite membrane of (Pt/SPEEK-TPA-ZrO2/ IrO2) with pore diameters in the range 2-6 µm. The pore formation in the composite membranes could also be responsible for the increase in the conduction activation energy compared to pure SPEEK membranes. The addition of fillers resulted in a modification of the composite membranes; the surface homogeneity was reduced and an extended porosity was clearly revealed with no evidence of agglomeration.
3.4. Polymer electrolyte membrane electrolysis cell
The performance of the composite membranes were evaluated in a single cell configuration up to current densities cell up to current densities of 1.45 A/cm2 with a catalyst loading of 2 mg /cm2 in the cathode and anode. Fig. 6 and 7 shows the cell voltage vs. current density curves of MEA prepared using the I-R method and conventional brush method respectively. The water electrolysis cell was operated at 80 oC and under atmospheric pressure. Fig. 6 shows the effect of the voltage versus current density characteristics of the MEAs : (Pt/Nafion 117/ IrO2, Pt/SPEEK-TPA-TiO2/ IrO2, Pt/SPEEK-TPA-SiO2/ IrO2, Pt/SPEEK-TPA- ZrO2 / IrO2) prepared by impregnation reduction method.
Fig. 7 shows the effect of the voltage versus current density characteristics of the second MEAs : (Pt/Nafion 117/ IrO2, Pt/SPEEK-TPA-TiO2 / IrO2, Pt/SPEEK-TPA-SiO2 / IrO2, Pt/SPEEK-TPA-ZrO2 / IrO2) prepared by conventional brush coating method.
Note that the current consumed by the electrochemical cell increases with time and this increase is due to the increase in the cell temperature. It is also found that there is a higher increase in the cell current at increased inlet temperatures. Once the cell is stabilized, an appreciable amount of current density is observed and it is noted as a function of voltage at the applied temperature.
The data in Fig. 6 and 7. indicate that the increase in the current density is linear with the cell voltage. From the Table 1. It is also observed that the current density is higher in conventional brush method than I-R method. In I-R method, the maximum current density observed is 1.35 A/cm2 at a cell voltage of 2.0 volt at 80 oC for SPEEK-TPA-ZrO2 membrane. And the production of hydrogen is 1.9 L/hr . SPEEK-TPA-ZrO2 composite membrane showed near comparable performance to Nafion® 117, due to high proton conductivity as well as on IEC values. Furthermore, the addition of tungstophosphoric acid and zirconium oxide in the SPEEK polymer organic matrix enabled the preparation of composite membranes with a wide range of properties concerning proton conductivity, water uptake and IEC values. Therefore, these membranes can be used in the future to make a critical evaluation of the relationship between the proton electrolyte membrane properties and the water electrolysis performance.
The current density versus voltage curve of the SPEEK/TPA/ZrO2 membrane in the water electrolysis cell reveals the second best performance among the five membranes mentioned above, that is, Pt/Nafion117/IrO2, Pt/SPEEK/IrO2, Pt/SPEEK-TPA-TiO2/IrO2, Pt/SPEEK-TPA-SiO2/IrO2 and Pt/SPEEK-TPA-ZrO2/ IrO2.
The difference in the coating procedures depends on the catalyst activity, where the best compromise between density of active sites and electrical conductivity was found. High electrical conductivity of the catalysts is crucial in Solid Polymer Electrolyte Membrane electrolysis systems due to the porous design of the current collectors, where current transport must take place in squint direction of the catalytic layer besides in vertical direction. This leads to a longer current path and a lower cross section for electron transport compared to what is evidently found. Thus, electrical conductivity of the catalysts in solid polymer electrolyte membrane electrolysis of foremost splendor.
Since Pt-black is considerably less active towards oxygen evolution than IrO2. This can probably be explained by an increase in electric conductivity of the catalytic layer. The Pt particles quickly formed large agglomerates and segregated during the spraying stage. Metallic Pt possess different surface properties compared to oxide particles and interact differently with the ink solvent, It was believed that a pH gradient was produced through the membrane and that dissolved Iridium species precipitated when transcending its solubility limit at a certain PH. This was explained by the formation of a common d-band and a lowering of the heat of interaction between IrO2 and oxygen where the oxidation to IrO4 was suppressed.
Pt-black operated as a much better hydrogen catalyst at higher current densities, possibly caused by a more densely packed catalytic layer and a higher electrical conductivity.
Similarly IrO2 operated as a much better oxygen catalyst at higher current densities and a higher electrical conductivity.
The composite membranes were prepared by a straightforward sol-gel method. The electrochemical properties, tensile strength, swelling, and dimensional stability were found to improve with the addition of fillers and blended with TPA to avoid excessive water swelling and to reinforce their mechanical properties. Increasing the local concentration of sulfonic acid units as well as separating the hydrophilic moieties from the hydrophobic polymer main chain enabled the stabilization of the morphology of the water-swollen membranes and a promotion of the proton conduction.
SPEEK-TPA-ZrO2 composite membrane showed near comparable performance to Nafion® 117 due to high proton conductivity as well as on IEC values. There is also a clear difference between these spectra of dried membranes and wet membranes. Furthermore, the addition of tungstophosphoric acid and zirconium oxide in the SPEEK polymer organic matrix enabled the preparation of composite membranes with a wide range of properties concerning proton conductivity, water uptake and IEC values. Based on the above experiments, it can also be
observed that the coating method of conventional brush coating method showed better performance than the impregnation reduction method. Hence it is suggested that the brush coating is the most appropriate method for preparing the catalyst used in PEM electrolysis.