DEVELOPMENT OF POLYMER ELECTROLYTE MEMBRANES FOR FUEL CELL APPLICATIONS

Abstract

The demand for sustainable energy across the world is gradually increasing, which is driven by the increasing global population, limited supply of fossil fuels, the emission of greenhouse gases, the onset of global warming and climate changes, and the standard of living. Fuel cell technology has emerged as the most promising power source for producing clean energy, where chemical energy stored in liquid/gas/solid fuels gets directly converted to electrical energy without being combusted. A typical fuel cell comprises an electrolyte (e.g., polymeric membrane, solid oxide layer, acid solution, buffer solution, alkali solution, etc.), one anode, and a cathode. The anode and cathode layers are made up of catalyst and support materials forming the main support structure of the fuel cell, which contribute to the improvement of fuel cell performance. In the case of the hydrogen fuel cell, ionization of the hydrogen atom (fuel oxidation) occurs at the anode, releasing the electron and proton; the electron travels through an external circuit and combines with the oxygen available at the cathode, while the proton travels through the electrolyte to combine with oxygen forming water and electricity. Depending on the nature of the electrolyte used, fuel cells are classified as (i) proton exchange membrane fuel cells, (ii) solid oxide fuel cells, (iii) molten carbonate fuel cells, (iv) phosphoric acid fuel cells, (v) alkaline fuel cells, etc. Based on the nature of the fuel used, fuel cells can be further classified as (i) hydrogen fuel cells, (ii) natural gas fuel cells, (iii) methanol fuel cells, (iv) ethanol fuel cells (v) biofuel cells (vi) hydrocarbon fuel cells, (vii) ammonia fuel cells. Hydrogen is the most common and well-known fuel, hydrogen fuel cell is the most popular. The electrolyte layer in a fuel cell is usually sandwiched between two electrodes in which other essential components, i.e. the bipolar plate, catalyst, and gas diffusion layer are separately adhered. In the case of a proton exchange membrane fuel cell (PEMFC) transport of proton (through anodic oxidation of hydrogen) occurs through the electrolyte (polymer membrane), to react with oxide anions formed at a cathode (reduction of oxygen by ‘e’ released during oxidation of H atom). Rapid industrialization and population growth may soon put the globe at risk of depletion of non-renewable energy resources, as energy demand has increased at such a level that it can be met with the limited supply of conventional resources. Additionally, the combustion of conventional resources using IC engines releases harmful greenhouse gases, causing air pollution and global warming. With this concern, various research groups have been trying to explore alternative sources of energy generation for the last few decades. Fuel cells appear to fulfill this crisis due to their high energy conversion efficiency compared to conventional energy IC engines. Additionally, the combustion products out of fuel cells are also not harmful to the environment, besides forming clean energy. Among various fuel cells, PEMFCs are considered the most popular ones for their high energy conversion efficiency, low greenhouse gas emissions, negligible noise pollution compared to IC engines, the possibility of higher capacity to generate sustainable/clean energy, lightweight, quick startup time, low operating temperature, simple design, etc. are several significant advantages with the PEMFC. A thin proton exchange membrane (PEM, made of polymer containing fixed charged groups) acts as the electrolyte in PEMFC, which transports H+ ions from one electrode to another and prevents the mixing of fuel and oxidant. PEMs experience multiple challenges, such as low proton conductivity, poor hydrolytic/oxidative stability, low thermal stability, insufficient durability, high cost, etc., preventing their successful commercialization. Therefore, efforts are being made to develop a reliable PEM that is affordable (cost-wise), possesses long durability (good physico-mechanical properties), and shows excellent proton conductivity. The proton exchange membrane should have low fuel permeability, high ionic conductivity, mechanical strength, and excellent chemical stability, meeting the required demand of electrical output. The most commonly used proton exchange membranes in fuel cells are sulfonated poly(ether ketone), sulfonated polysulfone, sulfonated polyimide, polystyrene, quaternized poly(phenylsulfone), poly(ether ether ketone), poly(benzimidazole), poly(phenylene), and perfluoro sulfonic acid-based polymer. Among these proton exchange membrane materials, perfluoro sulfonic acid-based polymer membranes (Nafion) are the most often utilized in low-temperature fuel cell applications because of their outstanding mechanical, chemical, and thermal stability with higher proton conductivity under-hydrated conditions. However, Nafion membranes need further improvements in chemical stability, proton conductivity, thermal stability, and fuel cell performances for the real-world applications of fuel cells, which seek tremendous research in the development of suitable alternative polymer electrolyte membrane materials with excellent durability, proton conductivity, and other properties. The objective of this research was to fabricate suitable polymer composite membranes with high proton conductivity and mechanical, chemical, and thermal stability as fuel cell membranes using sulfonated poly(ether ether ketone) and PFSA polymer, SPBI as a matrix, and sulfonic acid functionalized inorganic fillers (e.g., sulfonic acid functionalized graphene oxide, sulfonic acid functionalized silica network, and acid functionalized metal-organic frameworks) as proton-conducting additives. Based on the basic understanding developed in the broad area of PEMFCs and gaps identified, the following objectives were identified for exploration in this report: 1. Synthesis of PEMs using different polymer matrices, i.e., SPEEK, PFSA, and sulfonated PBI, for improved understanding of their performance and durability in fuel cell applications. 2. Synthesis of composite membranes with sulfonated GO, sulfonated hydrolyzed TEOS, UiO-66-SO3H, sulfonated silica, and propylsulfonic acid functionalized GO, and evaluation of their performances in fuel cell.3. Incorporation of chemical crosslinking to improve hydrolytic, thermal, and mechanical properties of both pristine and composite PEMs to make them better performing in fuel cell applications. 4. To synthesize the SPEEK/SPBI (80/20) membrane and its proton, conducting performance evaluation without/with propylsulfonic acid functionalized GO ~0.5-6% (w/w) filler. 5. Evaluation of the glass transition temperature and mechanical properties (elastic and shear modulus) of pristine and composite membranes using MD simulation (Materials Studio, MS). Additionally, the structural properties of pristine and composite (filler-loaded) membranes were evaluated using MD simulation to gain better insight into the distribution of ‘S’ atoms across the membranes that facilitate proton conductivity. Sulfonated poly(ether ether ketone) (SPEEK) was synthesized as the matrix for the preparation of polymer composite membranes. The thermal, mechanical, and oxidative stability, dimensional, and hydration properties of pristine SPEEK membranes are not suitable for fuel cell applications. As a result, several tactics like covalent crosslinking and the insertion of inorganic fillers in the polymer matrix were used to improve the performance of polymer composite membranes, allowing them to be used in fuel cells. In addition, the electrochemical properties, particularly the proton conductivity of the polymer composite membranes, were improved by the acid functionalization of the incorporated inorganic additives. The experimental and model-based analysis of the perfluorosulfonic acid (PFSA) polymerbased composite membranes were prepared to incorporate sulfonic acid functionalized graphene oxide (SGO) as an inorganic filler. The experimental and fully atomistic model-based studies have shown the incorporation of SGO in the PFSA polymer matrix significantly influenced the structural, mechanical, and electrochemical properties of PFSA/SGO composite membranes. Due to the strong interfacial interaction between SGO and PFSA polymer chains, the elastic and shear modulus of the PFSA/SGO composites rose significantly with SGO loading. The glass transition temperature of PFSA/SGO nanocomposite with 2 wt% SGO loading is approximately 142.2˚C, which is higher than the glass transition temperature of the pristine PFSA polymer system, confirming strong interactions between SGO and the PFSA polymer. The elastic and shear modulus of the PFSA/SGO (with 4 wt.% SGO) composite was 961.7 MPa, whereas the elastic modulus of the PFSA polymer system without filler loading was observed ∼310.5 MPa at similar test conditions. The proton conductivity and fuel cell performance of PFSA/SGO composite membranes improved significantly with SGO loading because of the hygroscopic nature of SGO, the creation of interconnected proton-conducting channels, and the reduction of fuel permeability through the prepared composite membranes. Proton conductivity of the SGO composite membrane was observed around 0.167 S/cm at 100% RH, 95 °C at 2 wt.% SGO loading, which is 1.51 times that of recast Nafion membrane at similar test conditions. The PFSA polymer composite, incorporating 2% by weight of SGO, displayed the best performance, yielding 1.02 mg-eqv/g IEC and an OCV of 0.97 V. A strategy to enhance the properties of SPEEK membranes was applied by crosslinking and loading them with grafted sulfonic acid-functionalized tetraethyl orthosilicate derivative. Tetraethyl orthosilicate (TEOS) was hydrolyzed in an acidic environment provided by sulfonated poly(ether ether ketone) (SPEEK) solution to form a self-cross-linked partially hydrolyzed TEOS (HTEOS). After the hydrolysis, glycerol was grafted to the ethoxy group of HTEOS, which will act as a cross-linker between the HTEOS and SPEEK. The cross-linked SPEEK material, specifically the one prepared with a 30% loading of TEOS, exhibited peak performance. TGA showed that the XSPEEK-TEOS membranes exhibited enough thermal stability, with no visible degradation of XSPEEK-TEOS membranes below 200 °C, making them suitable for fuel cell applications. This included an elastic modulus of 3061 MPa, an IEC of mg-eqv/g, an open-circuit voltage (OCV) of 0.97 V, a proton conductivity of 0.18 S/cm, and a current density of 1.3 A/cm2 at 0.5 OCV when tested at 80 °C and 90% RH.Propylsulfonic acid-functionalized graphene oxide (PrSGO) was introduced into the sulfonated poly(ether ether ketone) (SPEEK)-sulfonated poly(benzimidazole) (SPBI) blend in this study to understand the impact on glass transition temperature structural properties, increase mechanical properties, proton conductivity, and single-cell performance. At different loadings of PrSGO, atomistic molecular dynamics (MD) simulations of SPEEK/SPBI, crosslinked SPEEK/SPBI (XSPEEK/SPBI), and XSPEEK/SPBI/PrSGO composite systems were performed. The glass transition temperature of the SPEEK/SPBI (80/20) is 535.24 K, whereas the glass transition temperature of the SPEEK/SPBI (90/10) was observed at 491.51 K, which is significantly higher than the glass transition temperature of SPEEK. The mechanical properties, chemical and thermal stability of the XSPEEK/SPBI/PrSGO nanocomposite membranes significantly increased PrSGO loading because of the strong interfacial interaction between PrSGO and the XSPEEK/SPBI matrix. XSPEEK/SPBI/PrSGO nanocomposite membrane at 4 wt % PrSGO loading demonstrated a considerable improvement in proton conductivity up to 0.17 S/cm at 100% relative humidity (RH) at 90 °C. XSPEEK/SPBI/PrSGO composite membrane sample with 4 wt% PrSGO showed thermal stability at 340 °C without significant degradation, chemical stability up to 330 min, and excellent OCV of 0.99 V. Incorporating PrSGO nanofillers in the polymer matrix considerably improved proton conductivity of the membranes, other significant properties, and overall FC performance due to their hygroscopic nature, a higher number of sulfonic acid groups, and excellent interaction of the acid functionalized fillers with the cross-linked SPEEK/SPBI-based matrix. Hybrid PEMs were created using sulfonic acid functionalized MOF UiO-66-SO3H and SPEEK/SPBI matrix. Because of the porous, hydrophilic characteristics of the hybrid filler and the presence of proton conducting sites covalently connected to UiO-66-SO3H, incorporation of MOF UiO-66-SO3H in an acid-base polymer blend was expected to improve the proton conductivity of the hybrid membranes. XSPEEK/SPBI/S-UiO-66 composite membranes, incorporating 1.5% by weight of S-UiO-66, demonstrated outstanding results, including an IEC of 1.84 mg-eqv/g, robust thermal stability up to 290 °C without notable deterioration, and a proton conductivity of 0.19 S/cm at 90 °C and 100% RH. Overall, sulfonated poly(ether ether ketone) (SPEEK) and perfluorosulfonic acid (PFSA) polymer, sulfonated polybenzimidazole (SPBI) as a matrix, and sulfonic acid functionalized inorganic fillers (such as acid-functionalized graphene oxide, silica, metal-organic frameworks) as proton-conducting additives fillers were used to fabricate and evaluate the properties of polymer composite membranes using for fuel cell applications. Chemical crosslinking of acid-functionalized polymers has improved the elastic and shear modulus and chemical and thermal stability of PEMs. In addition, the electrochemical properties, particularly the proton conductivity, thermal, and chemical stability, along with the fuel cell performance of the polymer composite membranes, were improved by the acid functionalization of the incorporated filler materials.