CALCIUM SILICATE HYDRATE ANION EXCHANGE MEMBRANE USEFUL FOR WATER ELECTROLYSIS AND FUEL CELLS AND A PROCESS FOR THE PREPARATION THEREOF

Abstract

The present invention relates to a process for the preparation of Calcium Silicate Hydrate anion exchange membrane (cement paste) with an ionic conductivity of the order of 10.sup.−3 S/cm. The membrane can be formulated by mixing Ordinary Portland Cement (OPC) and water with the cement to water ratio of 1:0.45. After initial setting time, the membrane undergoes curing in 7% calcium chloride solution and the Cl.sup.− ions in the membrane is converted to OH.sup.− form by immersing into saturated Ca(OH).sub.2 solution with pH 14 and it has been washed to remove the excess alkali. This membrane has high mechanical strength (Ultimate Tensile Strength: 6.3 MPa) and does not deteriorate even at high temperature (up to 450° C.) and alkaline atmosphere (pH 11.5-14). Also disclosed is a method of producing in-situ formation of membrane electrode assembly. This invention encompasses a process for producing and using the membrane in water electrolysis and fuel cell.

Claims

1. A Calcium Silicate Hydrate (C—S—H) anion exchange membrane useful for water electrolysis and fuel cells having ionic conductivity in the range of 6.4 to 80 mS/cm with Ion Exchange Capacity (IEC) in the range of 0.22 to 0.45 mequiv/g.

2. The Calcium Silicate Hydrate anion exchange membrane as claimed in claim 1, wherein the membrane is stable up to 450° C. and in alkaline medium (pH 11.5-14) and non-resistant to hydroxyl ions, having high mechanical strength and tensile strength in the range of 6-7 MPa and 25-30% of elongation at break.

3. The Calcium Silicate Hydrate anion exchange membrane as claimed in claim 1 wherein, the membrane is useful for electrochemical water splitting to generate hydrogen and in a solid alkaline fuel cell.

4. A process for the preparation of a Calcium Silicate Hydrate (C—S—H) anion exchange membrane comprising the steps of: a) mixing portland cement with water to obtain a mixture; b) curing the mixture as obtained in step (a) with a calcium chloride solution to obtain a cured mixture; c) treating the cured mixture as obtained from step (b) with a calcium hydroxide solution with pH 11-14 followed by washing to get the Calcium Silicate Hydrate (C—S—H) anion exchange membrane.

5. The process as claimed in claim 4 wherein the ratio of cement to water is in the range of 1:0.30 to 1:0.70.

6. The process as claimed in claim 5 wherein the ratio of cement to water is in the range of 1:0.45.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0009] FIG. 1 depicts the Nyquist plot of the invented anion exchange membrane.

[0010] FIG. 2 depicts the chemical composition of the invented anion exchange membrane (prepared at pH 14) using Energy Dispersive X-ray Analysis.

[0011] FIG. 3 depicts the surface charge of the invented anion exchange membrane (prepared at pH 14) using zeta potential analyzer.

[0012] FIG. 4 depicts the thermal stability of the invented anion exchange membrane (prepared at pH 14) is given by Thermogravimetric Analysis(TGA)/Differential thermal analysis(DTA) curve.

[0013] FIG. 5 depicts the mechanical stability of the invented anion exchange membrane (prepared at pH 14) using stress-strain graph.

[0014] FIG. 6 depicts a cross sectional view of in-situ membrane electrode assembly of the embodiment of the present invention.

[0015] FIG. 7 (A) & (B) depicts the schematic cross sectional view of the water electrolysis and fuel cell system respectively.

[0016] FIG. 8 (i) & (ii) depict the polarization curve and durability test of water electrolysis cell comprising the invented membrane.

[0017] FIG. 9 depicts the H.sub.2/O.sub.2 fuel cell performance curve obtained with the invented membrane.

SUMMARY OF INVENTION

[0018] Accordingly the present invention describes a Calcium Silicate Hydrate (C—S—H) anion exchange membrane useful for water electrolysis and fuel cells having ionic conductivity in the range of 6.4 to 80 mS/cm with Ion Exchange Capacity (IEC) in the range of 0.22 to 0.45 mequiv/g.

[0019] Yet in an embodiment of the present invention the Calcium Silicate Hydrate anion exchange membrane is stable up to 450° C. and in alkaline medium (pH 11.5-14) and non-resistant to hydroxyl ions, having high mechanical strength and tensile strength in the range of 6-7 MPa and 25-30% of elongation at break.

[0020] Yet in another embodiment of the present invention the membrane is useful for electrochemical water splitting to generate hydrogen and in a solid alkaline fuel cell

[0021] Yet in another embodiment of the present invention, a process for the preparation of Calcium Silicate Hydrate (C—S—H) anion exchange membrane comprising the steps of: [0022] a) mixing portland cement with water to obtain a mixture; [0023] b) curing the mixture as obtained in step (a) with calcium chloride solution to obtain cured mixture; [0024] c) treating the cured mixture as obtained from step (b) with calcium hydroxide solution with pH 11-14 followed by washing to get Calcium Silicate Hydrate (C—S—H) anion exchange membrane.

[0025] Yet in another embodiment of the present invention, the process for the preparation of Calcium Silicate Hydrate (C—S—H) anion exchange membrane wherein the ratio of cement to water is in the range of 1:0.30 to 1:0.70 preferably 1:0.45.

[0026] Yet in another embodiment of the present invention, the ionic conductivity of the membrane is analyzed using Nyquist plot (Impedance spectra). The Ion Exchange Capacity (IEC), static and dynamic transport number of the membrane is studied by Mohr's method, Henderson and Hittorf's method respectively. The thermal and mechanical stability of the membrane is investigated by TGA and stress-strain graph respectively. Elemental analysis of the membrane is carried out by EDX and surface charge analysis using a zeta potential analyzer.

[0027] Yet in another embodiment of the present invention, this invention furnishes a Membrane Electrode Assembly (MEA) constituting the in-situ formation of catalyst layers and invented anion exchange membrane, where the membrane is in between anode catalyst and cathode catalyst. This new design (in-situ MEA) avoids the use of electrode binders, catalyst ink preparation, membrane/electrode attachment schemes (hot pressing, decal methods) and minimizes membrane-electrode contact resistance losses and condensed-phase transport losses.

[0028] Yet in another embodiment of the present invention, the water splitting behavior of the membrane is studied in a MEA where the electronic conductive metal electrodes (Titanium) are separated from one another by an in-situ MEA. Three electrodes system is used to evaluate the performance with Ti metal as an electrode (anode and cathode) and Hg/HgO as the reference electrode in 0.1 M KOH under a potential of 2.0 V.

[0029] Yet in another embodiment of the present invention, the fuel cell performance of the membrane is investigated in a MEA where H.sub.2 and O.sub.2 gases are passed at anode and cathode respectively.

DETAILED DESCRIPTION OF INVENTION

[0030] The present invention is the development of calcium silicate hydrate anion exchange membrane for water electrolysis and fuel cells. Ordinary Portland Cement with the cement to water ratio of 1:0.45 is mixed; After initial setting time, the membrane undergoes curing in 7% calcium chloride solution, then the Cl.sup.− ions in the membrane is converted to OH.sup.− form by immersing into saturated Ca(OH).sub.2 solution with pH 14 (pH is adjusted using 1 M KOH) and it is washed thoroughly to remove the excess alkali.

[0031] In an embodiment of the present invention, solid inorganic anion exchange membrane is provided, having the following structure:

[0032] In the present invention, Ordinary Portland Cement (OPC) is used to synthesize the membrane and it is brought from aditya birla group-ultratech cement ltd.

[0033] The basic components of OPC are:

Lime (CaO):61-63%

Silica (SiO.SUB.2.):17-25%

[0034] Alumina (Al.sub.2O.sub.3):4-8%
Sulphur tri oxide (SO.sub.3):1.3-3%
Iron oxide (Fe.sub.2O.sub.3):0.5-0.6%

Magnesia (MgO):0.1-4%

[0035] Soda and/or potash (Na.sub.2O+K.sub.2O):0.4-1.3%

Chloride ion (Cl.SUP.−):.0.01-0.1%

Insoluble Residue (IR):0.6-1.75%

[0036] The four major compounds in OPC are tricalcium silicate (Ca.sub.3SiO.sub.5 in short form C.sub.3S), dicalcium silicate (Ca.sub.2SiO.sub.4 in short form C.sub.2S), tricalcium aluminate (Ca.sub.3Al.sub.2O.sub.6 in short form C.sub.3A), tetracalcium aluminoferrite (Ca.sub.4Al.sub.2Fe.sub.2O.sub.10 in short form C.sub.4AF) and the extent of its composition are given as follows:

TABLE-US-00001 Compound Composition as % C.sub.3S 48-52 C.sub.2S 22-26 C.sub.3A  6-10 C.sub.4AF 13-16 Free lime 1-2

[0037] Since C.sub.3S, C.sub.2S are the major compositions of OPC, they are playing a crucial role in determining its properties.

TABLE-US-00002 Element Weigh percent Atom percent O 57.11 76.63 Ca 41.15 22.04 Si 1.73 1.32 Total 100.00 100.00

[0038] When OPC is mixed with water, C.sub.3S, C.sub.2S are hydrated and produce Calcium Silicate Hydrate (3CaO.Math.2SiO.sub.2.Math.3H.sub.2O in short form C—S—H) by the following exothermic reaction:

2Ca.sub.3SiO.sub.5+6H.sub.2O3 CaO.Math.2SiO.sub.2.Math.3H.sub.2O+3Ca (OH).sub.2

2Ca.sub.2SiO.sub.4+4H.sub.2O3 CaO.Math.2SiO.sub.2.Math.3H.sub.2O+Ca (OH).sub.2

[0039] So the major composition of the membrane is Calcium Silicate Hydrate (3CaO.Math.2SiO.sub.2.Math.3H.sub.2O in short form C—S—H). When C—S—H undergoes curing in calcium chloride solution followed by immersion in saturated Ca(OH).sub.2 solution with pH 14, its surface is overcompensated by Ca.sup.2+ ions.

[0040] The novelty of the present invention resides in providing highly conductive and alkaline stable inorganic anion exchange membrane for use in electrolyzer and fuel cell.

[0041] The present invention relates to a water electrolyzer and solid alkaline fuel cell comprising solid inorganic anion exchange membrane, Calcium Silicate Hydrate (C—S—H). Ordinary Portland Cement with the cement to water ratio of 1:0.45 is mixed. Lower water/cement ratio has been used to avoid the porosity. After initial setting time, the membrane undergoes curing in 7% calcium chloride solution. Then the Cl.sup.− ions in the membrane is converted to OH.sup.− form by immersing into saturated Ca(OH).sub.2 solution with pH 14 (pH is adjusted using 1 M KOH). Repeated washing has been done to remove the excess alkali.

[0042] The Ion Exchange Capacity (IEC) and percentage of conversion (Cl.sup.− into OH.sup.− ions) of the membrane is studied by Mohr's method. The ionic conductivity, static and dynamic transport of the invented membrane is analyzed using Nyquist plot (Impedance spectra), Henderson's and Hittorf's method respectively. The thermal and mechanical stability of the invented membrane is investigated by TGA and stress-strain graph respectively. Elemental analysis is carried out by EDX and surface charge analysis using zeta potential analyzer.

[0043] When Ordinary Portland Cement is mixed with water, the cement grains react to form a continuous inorganic phase of Calcium Silicate Hydrate (C—S—H). Thus the “hydrated form of cement paste” at the micrometer level will be in the form of “nanoporous Calcium Silicate Hydrate gel.” It has a “sandwich”-like layered structure with a layer of calcium oxide clipped in two layers of silicon-oxygen tetrahedron.

[0044] As soon as Ordinary Portland Cement is mixed with water, the dissolved Ca.sup.2+ ion from the clinker phase is adsorbed on the silica surface and immediately replenished. Thus the surface charge of a Calcium Silicate Hydrate system depends on the molar ratio of CaO to SiO.sub.2 (C/S ratio). When Ca/Si ratio is high, the reactive Ca.sup.2+ ion is adsorbed on the vicinity of surface and makes the surface charge positive. At high surface charge densities, the surface charge of C—S—H is overcompensated by Ca.sup.2+ ions. According to B. Jonsson, H. Wennerstrom, A. Nonat, and B. Cabane, Langmuir (2004), 20, 6702-6709, this distribution (ionic clouds) of Ca.sup.2+ will form bond (bridges) only near the isoelectric point of the surfaces (pH 11.6). Thus when the pH is beyond 12, the silanol reacts with OH.sup.− ions of the solution to give SiO.sup.− groups. From the evolution of stoichiometry of C—S—H dispersions in calcium hydroxide solutions, labile Ca.sup.2+ ion becomes supersaturated at a high value with respect to the precipitation of a Calcium-Silicate-Hydrate (“C—S—H”) and the chemical binding of Ca.sup.2+ ions occur, making the surface charge extremely positive.

[0045] Thus the surface site hopping of hydroxyl anions occurs by the fixed Ca.sup.2+ sites present on the membrane. The small size of the fixed cations Ca.sup.2+, enhances the exchange sites which improves the ionic conductivity compared to the state of prior art.

[0046] In addition to that, mobility of OH.sup.− ion will be increased due to the pore structure and the chemistry of pore solution. In well hydrated phases, capillary pore network (<10 nm) within the interlayer of C—S—H will adsorb the water physically on the surface either in the form of H.sub.2O or OH.sup.−. Apart from that, the pore solutions have extremely high concentration of OH.sup.− ions (pH 10-13). Thus water molecules, constructive nanopores and the solution in the pores will provide continuous conductive path (Grothuss mechanism) for migration of ions.

[0047] Usually polymeric membrane loses its mechanical stability on excessive water content. But the ceramic membrane described here on the present invention is expected to acquire high mechanical stability and ionic conductivity with increasing water content which is a decisive factor for an essential transport process.

[0048] Thus all these paths contribute to the ionic transport and thereby electrolytic conduction through this heterogeneous medium.

[0049] The impedance spectrum of the membrane is shown in FIG. 1 where imaginary versus real component of the impedance is reported (Nyquist plot). It is found from the plot that the ionic conductivity of the membrane is 10.sup.−3 S/cm. EDX (FIG. 2) shows that ratio of Ca/Si is high and infers that the surface charge will be positive. This has been confirmed by zeta potential analyzer (FIG. 3). TGA/DTA (FIG. 4) are taken under nitrogen atmosphere. It shows that the membrane is more thermally stable and decomposes around 450° C. The stress-strain graph (FIG. 5) of the membrane indicates that it has high mechanical strength and ultimate tensile strength of 6.3 MPa and 26% of elongation at break. FIG. 6 illustrates a cross sectional view of in-situ MEA of the embodiment of the present invention. This MEA constitutes an in-situ formation of catalyst layers (Nickel) and invented anion exchange membrane, where membrane (b) is in between the anode catalyst (a) and cathode catalyst (c). This new design (in-situ MEA) avoids the use of electrode binders, catalyst ink preparation, membrane/electrode attachment schemes (hot pressing, decal methods) and minimizes membrane-electrode contact resistance losses, condensed-phase transport losses. Fig.7 (A) & (B) represent the schematic cross sectional view of water electrolysis and fuel cell system respectively. The symbol 1, 2, 3, and 4 represent the titanium anode, Gasket and in-situ MEA and titanium cathode respectively and x, y, and z represent the Water, Hydrogen, and Oxygen gases respectively. The water electrolysis performance of the membrane is given in FIG. 8 (i) & (ii). The linear sweep voltammetry graph (FIG. 8(i)) indicates that the invented membrane exhibits a current density of 5 mA/cm.sup.2 at 2.0 V at a scan rate of 5 mV/s and oxygen and hydrogen gases have evolved at 1.6 V and −0.3 V respectively. For checking the stability, chronoamperometry (FIG. 8 (ii)) has been conducted for 15 h. It shows a constant current density of 5 mA/cm.sup.2 at 2.0 V. This primary stability test infers that there is no obvious performance degradation. FIG.9 describes the H.sub.2/O.sub.2 fuel cell performance curve obtained with invented membrane. It shows peak power density of 4.7 mW/cm.sup.2 and an open circuit potential of 0.975 V.

[0050] The following examples are given by way of illustration of working of the invention in actual practice and therefore should not be construed to limit the scope of the invention.

Example 1

[0051] Ordinary Portland Cement with the cement to water ratio of 1:0.45 is mixed. The additional usage of water will lead to pores formation (Yun-Yong Kim, Kwang-Myung Lee, Jin-Wook Bang, and Seung-Jun Kwon, Advances in Materials Science and Engineering Volume 2014, Article ID: 273460), which will allow mixing of hydrogen and oxygen gases and lead to explosion in fuel cell and water electrolysis. So, we have taken minimal water/cement ratio. After initial setting time, the membrane undergoes curing in different percentages of calcium chloride solution like 1%, 3%, 5%, 7%, 9%, and 11%. Repeated washing has been done to remove the surface chloride ion and its corresponding ionic conductivity has been given in Table 1.

TABLE-US-00003 TABLE 1 Ionic conductivity of the membrane For different percentages of calcium chloride curing solution Calcium chloride Ionic conductivity curing solution (%) (×10.sup.−4 S/cm) 1 1.3 3 23 5 35 7 64 9 58 11 61

[0052] From the table, we can infer that the membrane's ionic conductivity becomes saturated after 7% of calcium chloride curing solution. It may be due to the saturation of Ca.sup.2+ sites. Thus 7% of calcium chloride solution has been taken as an optimum concentration for curing solution and taken for further studies.

Example 2

[0053] Ordinary Portland Cement with the cement to water ratio of 1:0.45 is mixed. Lower water/cement ratio has been used to avoid the porosity. After initial setting time, the membrane undergoes curing in 7% calcium chloride solution. Then the Cl.sup.− ions in the membrane is converted to OH.sup.− form by immersing into saturated Ca(OH).sub.2 solution with pH 11.5 (pH is adjusted using 1 M KOH). Repeated washing has been done to remove the excess alkali.

Example 3

[0054] Ordinary Portland Cement with the cement to water ratio of 1:0.45 is mixed. Lower water/cement ratio has been used to avoid the porosity. After initial setting time, the membrane undergoes curing in 7% calcium chloride solution. Then the Cl.sup.− ions in the membrane is converted to OH.sup.− form by immersing into saturated Ca(OH).sub.2 solution with pH 14 (pH is adjusted using 1 M KOH). Repeated washing has been done to remove the excess alkali.

TABLE-US-00004 TABLE 2 Membrane Properties Example 2 3 Ionic Conductivity (mS/cm) 6.4-32  57-80 Ion exchange capacity IEC (mequiv/g) 0.22-0.33 0.36-0.45 Percentage of Conversion (Cl.sup.− into OH.sup.− ions) 100% 100% Water Uptake (%) 37-42 43-45 Swelling No No Static Transport Number 0.89-0.93 0.94-0.97 Dynamic Transport Number 0.61-0.63 0.67-0.77

TABLE-US-00005 TABLE 3 Membrane stability in alkaline media Example-Ionic Conductivity Treatment (mS/cm) None KOH 1M KOH 2M 1 1 day 3 days 6 days 9 days 1 day 3 days 6 days 9 days 2 6.4-32 6.0-30  0.21-12   0.02-0.5  0.004-0.13  6.2-29 0.1-19  0.005-0.09  0.0007-0.05  3  57-80 53-79   36-78.5 13-78 0.9-78   55-77 23-74 0.5-72  0.3-72 

[0055] Table 2 and 3 demonstrate the high and stable ionic conductivity of the invented anion exchange membrane. Since there will not be any Hofmann elimination reaction that usually occurs in an organic membranes, the invented membrane shows highly stable ionic conductivity when immersed in alkaline medium. On comparing Example 2 and Example 3, the membrane prepared with pH 14 (example 3) shows higher and stable ionic conductivity than membrane prepared with pH 11.5 (example 2). The favorable effect at high pH is due to more chemical binding of Ca.sup.2+ ion occurs. This would result in strong positive surface charge density and hence more hopping sites for conduction. Thus the anion exchange membrane prepared with pH 14 is taken for further studies.

[0056] Example 4

[0057] A further aspect of the present invention provides a Membrane Electrode Assembly (MEA) comprising the described membrane and a method for preparing such a MEA. The disclosed MEA can be incurred by a very simple process. The following procedure is used for the specimen preparation:

[0058] Ordinary Portland Cement with the cement to water ratio of 1:0.45 is mixed. Lower water/cement ratio has been used to avoid the porosity. Nickel deposited carbon cloth (reaction catalyst layer) is placed on both sides of the membrane before drying. After initial setting time, this sandwich-type membrane undergoes curing in 7% calcium chloride solution. Then the Cl.sup.− ions in the membrane is converted to OH.sup.− form by immersing into saturated Ca(OH).sub.2 solution with pH 14 (pH is adjusted using 1 M KOH). Repeated washing has been done to remove the excess alkali.

[0059] This integrated contiguous layer (anode-electrolyte-cathode body) in the form of slab results in enhancing the thorough use of active components by providing intimate interfaces between the electrolyte and either electrode. Thus it reduces the interfacial resistance, provides excellent mechanical integrity and good handling ability.

[0060] A water electrolysis and fuel cell system is used to evaluate the performance of the prepared MEA. The water splitting behavior of the membrane is studied in a water electrolyzer system where the electronic conductive metal electrodes (Titanium) are separated from one another by the invented sandwich type membrane. Water is passed at the cathode and three electrodes system is used to evaluate the performance with Ti metal as an electrode (anode and cathode) and Hg/HgO as the reference electrode in 0.1 M KOH under the potential of 2.0 V. The invented membrane exhibits a current density of 5 mA/cm.sup.2 at 2.0 V and oxygen and gases have evolved at 1.6 V and −0.3 V respectively. For checking the stability, chronoamperometry has run for 15 h, which shows a constant current density of 5 mA/cm.sup.2 at 2.0 V. This primary stability test infers that there is no obvious performance degradation.

[0061] The fuel cell performance of the membrane is investigated in a MEA where H.sub.2 and O.sub.2 gases are passed at anode and cathode respectively. FIG. 9 describes the H.sub.2/O.sub.2 fuel cell performance curve obtained with the invented membrane. It shows peak power density of 4.7 mW/cm.sup.2 and an open circuit potential of 0.975 V.

ADVANTAGES OF THE INVENTION

[0062] The advantages of the present invention are the developed solid anion exchange membrane is environmentally benign, cheap, earth abundant and the second most substance used. Since the membrane does not possess any organic moieties, it is well suited to operate at elevated temperatures and pressures.

[0063] This ion exchange membrane can also be used in electro dialysis, salt splitting, and other electrochemical applications.