Liverpool team develops better material for PEM fuel cells; porous organic cage solids with 3D protonic conductivity
Proton conduction is key to devices such as proton exchange membrane fuel cells (PEMFCs); the performance-limiting component in PEMFCs is often the proton exchange membrane (PEM). In the search for more effective PEMs, reseachers have looked to porous solids such as metal-organic frameworks (MOFs) or covalent organic frameworks. With these, the proton conduction properties can be fine-tuned by controlling crystallinity, porosity and chemical functionality. To maximize proton conduction, three-dimensional conduction pathways are preferred over one-dimensional pathways, which prevent conduction in two dimensions.
Researchers led by a team at the University of Liverpool (UK) now report in an open-access paper in the journal Nature Communications that they have developed crystalline porous molecular solids where the proton transport occurs in 3D pathway by virtue of the native channel structure and topology. The development could lead to the design of more effective fuel cell materials, including high-temperature PEMFCs.
In principle, the rational design of architecture in crystalline porous molecules allows us to tune proton conductivity and improve our understanding of proton conduction mechanisms, as relevant to both materials science and biology. However, there are few examples of proton conduction in porous organic molecular solids. … One limitation of proton conduction in MOFs is the tendency for directional proton transport, which in turn arises from the low-dimension pore structures in most frameworks tested2. Even in the few 3D proton-conducting MOFs that are known, the protons were found to be transported in 1D channels in most cases. 3D proton transport is more favourable for application in PEMs, and hence there have been attempts to enhance proton mobility in MOFs by introducing defects or by decreasing the crystallinity.
Here we present an alternative strategy, which is to develop crystalline porous molecular solids where the proton transport occurs in 3D pathway by virtue of the native channel structure and topology. We demonstrate this concept for a range of crystalline porous organic cages. For a neutral imine cage, CC3, the proton conductivity is relatively low under humid conditions, despite the hydrated 3D diamondoid pore network in the material. However, when a related amine cage, RCC1 was transformed into its crystalline hydrated salt (H12RCC1)12+·12Cl−·4(H2O), the proton conduction was improved by a factor of over 150. Indeed, the proton conductivity of 1 is comparable to pelletized proton-conducting MOFs. This was rationalized using both computer simulations and quasi-elastic neutron scattering (QENS) to elucidate the proton transport mechanism. We also explain the influence of the counter anions in the protonated cage salts, which act to ‘gate’ the proton conduction.—Liu et al.
The researchers synthesized molecules that enclose an internal cavity, forming a porous organic cage into which other smaller molecules can be loaded, such as water or carbon dioxide. When the cages form solid materials, they can arrange to form channels in which the small guest molecules can travel from one cage to another.
The material forms crystals in which the arrangement of cages is very regular. This allowed the researchers to build an unambiguous description of the structure using crystallography. The molecules are also soluble in common solvents, which means they could be combined with other materials and fabricated into membranes (Nafion, for example).
The Liverpool researchers measured the protonic conductivity of these porous organic cages after loading the channels with water, to assess their viability as PEM materials. The cages exhibited proton conductivities of up to 10-3 S cm-1, which is comparable to some of the best porous framework materials in the literature.
In collaboration with researchers from the University of Edinburgh, Center for Neutron Research at National Institute of Standards and Technology (NIST), and Defence Science and Technology Laboratory (DSTL), they used a combination of experimental measurements and computer simulations to build a rich picture of how protons are conducted by the cage molecules.
Two distinctive features of the proton conduction in organic cage crystals were highlighted as design principles for future PEM materials. First, the cages are arranged so that the channels extend in three dimensions. This means that the movement of the protons is not limited to a particular direction, as in the case of many porous materials tested so far.
Second, the cages direct the movement of the water molecules, which means that protons can be passed between them quickly. Also, the cages are flexible enough to allow the water to reorganize, which is also important when protons are transported from one water molecule to the next over longer distances.
In addition to introducing a new class of proton conductors, this study highlights design principles that might be extended to future materials. For example, the ‘soft confinement’ that we observe in these hydrated solids suggests new anhydrous proton conductors where a porous cage host positions and modulates the protonic conductivity of guest molecules other than water. This would facilitate the development of high temperature PEMFCs, as water loss would no longer be a consideration.—Dr Ming Liu, who led the experimental work
Ming Liu, Linjiang Chen, Scott Lewis, Samantha Y. Chong, Marc A. Little, Tom Hasell, Iain M. Aldous, Craig M. Brown, Martin W. Smith, Carole A. Morrison, Laurence J. Hardwick & Andrew I. Cooper (2016) “Three-dimensional protonic conductivity in porous organic cage solids” Nature Communications 7, Article number: 12750 doi: 10.1038/ncomms12750