The hydrogen solution? Polymer network for hydrogen storage
A polymer network using polyaniline could be the solution to enhanced hydrogen storage. Dr Balasubramanian from the Defence Institute of Advanced Technology (DIAT) in Pune, India, explains how this could provide an alternative to traditional energy systems.
See bottom of article for diagrams and charts
The use of hydrogen as a fuel and the development of hydrogen economy have been suggested as means to decrease both the worldwide dependency on oil products and emission of greenhouse gases. Ease of manufacture and ability to perform a large number of fast adsorption and desorption cycles are two of the many requirements placed on hydrogen storage materials that should easily be met by the nanoporous materials.
The most common nanoporous materials are carbon, glass, zeolites, aluminosilicates, oxides, metals and polymers. The two methods available for synthesis of nanoporous polymers are direct copolymerisation of mixtures containing high percentages of crosslinking monomers and porogen solvents, and hypercrosslinking. Highly crosslinked polymers have a permanent porous structure that is formed during their preparation and persists in their dry state. The internal porous morphology is characterised by interconnected channels that permeate the rigid, extensively crosslinked polymer matrix. Permanently porous materials are often synthesised in the form of uniform beads by suspension polymerisation.
One polymeric network is an interpenetrating polymer network (IPN), which is a composite of at least two polymers. This is obtained when at least one polymer network is synthesised or crosslinked independently in the presence of the other. Microporous materials are exemplified by crystalline framework solids such as zeolites, whose crystal structure defines channels and cages such as micropores, of strictly regular dimensions. The large internal surface area and void volumes with extremely narrow pore size distribution, as well as functional centres homogeneously dispersed over the surface, make microporous solids highly active.
The use of porous polyaniline as a material for hydrogen storage has been suggested. It has potential in a variety of applications such as sensors, electrodes, actuators and supercapacitors, where a porous structure might enhance conductivity and other electronic properties of the polymer. A lot of attention has been directed towards the development of porous polyaniline structures.
Hydrogen adsorption by porous lightweight materials can occur by two routes, physisorption and chemisorption. Physisorption is a non-activated process. Reversible and fast hydrogen uptake and delivery can be expected, which is a major advantage of porous adsorbents. Porous polymers provide the optimum combination of safety, low cost, convenient storage and release criteria. The polymers also have ease of synthesis along with numerous possibilities for molecular design and pore size variation. High surface area and microporosity of these materials have been established and increased crosslinking in the polymer introduces rigidity and enables the polymer to withstand applied stress.
The amounts of hydrogen adsorbed by porous materials at ambient temperatures and high pressures are much lower (0.5wt%) compared to other porous materials. Polyaniline with a density of 1.360g/cc showed an increase in the hydrogen storage capacity of up to 6-8wt%. For most porous hydrogen storage materials, the affinity for hydrogen decreases rapidly as the surface gets covered. The opposite trend is observed with the styrene maleic anhydride copolymer when combined with transition metals. Increasing the crosslinking density of the copolymer leads to an increase in the hydrogen storage capacity.
Aromatic rings are important sites for hydrogen adsorption. Aromatic rings containing electrondonating groups adsorb hydrogen more readily than electron withdrawing functions. The amine functionality is a strong electron-donator, so introducing these into a porous copolymer matrix can lead to an overall increase in the hydrogen adsorption capacity without changing the density of the polymer matrix.
Storing hydrogen on board vehicles for use as a fuel has always been a big challenge. Though many techniques, such as compressed or cryogenic tanks, metal hydride and organic hydride, are available to store hydrogen, none of them provide adequate safety, cost efficiency and convenience to store enough hydrogen on board vehicles for a reasonable driving range. Porous materials that store hydrogen via the physisorption mechanism could potentially meet all these requirements. Ease of manufacture and ability to perform a large number of fast adsorption and desorption cycles are two of the requirements met easily by porous polymers. The high surface area of such materials also satisfies one of the prerequisite conditions for physisorption of hydrogen.
The major advantages are that such materials are lightweight, and rapid complete adsorption and desorption occurs with no appreciable hysteresis. The porous polymers also give optimal heat of adsorption, well-interconnected pore structure and high mechanical and thermal stabilities. Porous organic polymers have versatile synthetic approaches and could be conveniently designed and prepared with different functionalities and pore structures. Using a variety of well-known reactions, the synthetic polymers can be easily modified and their adsorption ability tuned. The porous polymers also provide the opportunity for topological engineering of pore shape and insertion of other adsorbate surfaces inside the pores. Since modification of synthetic polymers is a well-charted field, different functionalities can be incorporated. The ease with which hydrogen adsorbing or other functional moieties can be attached to these polymers is one of the primary advantages.
The main challenges in research of porous polymers include the control of homogenous surfaces with regular distribution of active sites for optimised performance. However, major challenges are to increase the storage capacity and the heat of adsorption to enable adsorption at higher temperatures. There are still some barriers to overcome in order for porous materials to achieve high performance in terms of hydrogen adsorption capacity such as charging time, life cycles, and safety issues.
Crosslinked and encapsulated IPNs
An example from work conducted at DIAT involves using polyaniline with a porous polymer matrix to achieve reasonable hydrogen uptake at room temperature. Polyaniline was selected because excess aromatic rings and the amine functionality are effective for hydrogen uptake. Encapsulation of polyaniline by in-situ polymerisation on crosslinked styrene maleic anhydride (SMA) illustrates the significant role of the rings and the high propensity of polyaniline for hydrogen uptake.
The interpenetrating networks of SMA in the form of spherical beads and varied degree of crosslinking were synthesised through suspension polymerisation using a free radical initiator. Encapsulation of polyaniline was achieved by in-situ polymerisation of aniline over the SMA matrix within 24 hours. The formation of polyaniline was confirmed from the colour change of the IPN and appearance of characteristic peak in the Fourier transform infrared (FTIR) spectrum. The encapsulated SMA showed the presence of ~2.8 mole% polyaniline irrespective of the degree of crosslinking in the SMA networks.
When encapsulated, the IPNs showed homogeneous smoothening of the surface as revealed from the SEM micrographs. The presence of about 2.8 mole% of polyaniline does not drastically change the density of the networks, thus maintaining the material’s light weight, required for hydrogen storage.
The organic solvent vapour uptake by the encapsulated IPNs showed the presence of a substantial amount of polyaniline. The encapsulation of polyaniline does not alter the porous nature of the networks. The hydrogen uptake by the porous networks suggests the slow micropore opening mechanism is involved, where the bigger pores are filled up first. The pressure then created in these pores allows slow opening of micropores, which are connected through channels to the bigger pores. Prolonged exposure to a hydrogen adsorption atmosphere results in an increase in hydrogen uptake without reaching saturation limits.
Thus the encapsulation of polyaniline enhances the surface properties and hydrogen uptake, and adds negligible mass to the IPNs. This illustrates the possibility of creating a safe and efficient process through the use of nanoporous materials for hydrogen capture in an environmentally friendly energy storage system.
(A1 - 10% crosslinked polymer, A3 - 23% crosslinked polymer, A5 - 30% crosslinked polymer, PANI A1 - polyaniline encapsulated A1, PANI A3 - polyaniline encapsulated A3, PANI A5 - polyaniline encapsulated A5)
Thanks to co-author Miss Renuka Gonte