In an interview, Dr Sujit Ghosh, Associate Professor - Chemistry at Indian Institutes of Science Education and Research (IISER) in Pune, India with Chemical Today magazine speaks about the need to create new materials, which can eventually replace the 50-year-old Nafion and his quest to develop cheap microporous crystalline materials that can be effectively used as PEM in fuel cell industries.
Current research for microporous crystalline materials (MCMs)
A major portion of the research in our lab revolves around the development of advanced microporous crystalline materials (MCMs) like Metal-organic Frameworks (MOFs), Covalent organic Frameworks (COFs) and more recently Hydrogen-Bonded Organic Frameworks (HOFs), which has shown promising potential in terms of energy and environmental applications. In the present work, we have dealt with the proton conduction aspects of HOF materials by careful design of purely organic moieties with specific functionalities (sulfonate groups and guanidinium groups). This work represents the first report of a new class of microporous crystalline materials ie. HOFs, as a potential proton exchange membrane (PEM) for fuel-cell applications.
For proton conduction measurements, we collaborated with a team of researchers from National Chemical Laboratory (NCL) Pune, led by Dr Sreekumar Kurungot who works on proton exchange membrane fuel cells (PEMFCs). Fuel Cell technology holds a promising candidature for serving as an alternate major energy source and addressing the issue of the energy crisis in coming future. One key feature of this aspect is further development of PEMs for usage in PEM fuel cells for widespread clean energy applications. Among several components, PEM is one of the most important parts of fuel cells for their efficient performance.
Currently, a polymeric compound Nafion (Developed by DuPont in the 1960’s) serves as a benchmark in this regard. But Nafion suffers a major drawback in terms of applicability at high-temperature range or low humidity, high production costs and gas leakage issues. So the development of new class of materials with potentials to function as PEM in fuel cells is desired. In the current research, we have developed a new class of MCM based proton conducting materials (here HOF), which has a potential to serve as PEM in fuel cell and can be a rational alternative to Nafion.
Research in connection to HOFs
HOFs are a relatively new class of compounds in the domain of microporous materials. Although H-bonded architectures are well known to the scientific community, it was not until 2011 that the systematic studies on a porous HOF were first started. However, considering the importance of these HOF compounds and the potential it has shown to be used in PEM fuel cells, we feel that industrial use of HOFs and related materials is a dream that can be achieved.
Advantages of HOFs
In our current work two HOFs, namely HOF-GS-10 and HOF-GS-11 were synthesised as crystalline materials by simple room temperature solution phase synthesis of the sulfonic acids (1,5-naphthalene disulfonic acid and 4,4'-biphenyl sulfonic acid in the present case) with guanidine hydrochloride using appropriate solvent systems. This strong hydrogen bonding complementarity between the sulfonate groups and the guanidinium groups in such HOF compounds have been utilised to design a proton conduction pathway for potential applications in PEM-fuel cells.
The salient features, which mark the importance of these types of compounds, are their ease of syntheses, solution processability, low energy consuming regeneration processes and pre-designed architectures by crystal engineering. Also, because of their very strong hydrogen bonding nature, these types of compounds are thermally robust - this is essential for real-time applications. Moreover, since these compounds are essentially crystalline, they provide the in-depth structural elucidation and would help to predict the exact proton conduction pathway.
Benefits of HOFs over other such materials
Various crystalline materials like MOFs, COFs etc have shown promising potential in terms of fuel cell applications. However, the quest for novel crystalline materials for such clean energy applications is the state-of-art. Our novel findings in terms of usage of such HOF compounds would eventually propel more researchers towards the development of these types of materials for clean energy applications. HOFs have some unique advantages over the conventional materials that are tried and tested for fuel cell applications; like, a) metal free syntheses, b) light-weight materials, and c) structural elucidation via single crystal x-ray diffraction studies. Being crystalline, HOFs can provide the atomic level insight and would help to predict the exact mechanism of proton conduction pathway.
Opportunities for HOFs research in India and abroad
In India, although some of the groups are currently working on the proton conduction applications of porous materials like MOFs and COFs, various other groups abroad are trying to develop new materials for PEM fuel cell applications. Given the enormous research devoted globally to the development of PEMs for fuel cells, our research on the proton conduction properties of these HOF materials presents a new platform towards the development of crystalline materials based PEM for fuel cell applications.
Sectors to benefit from the HOFs research
One of the key features in fuel cell industries is the further development of PEMs for fuel cells. Since Nafion, which is the only major commercialised PEM (developed by Du Pont about 50 years ago), material chemists have for a long time tried to develop new materials, which can eventually replace Nafion. Since PEM is the heart of fuel cell industries we feel our research can inspire various researchers from various sectors for the quest to develop alternate PEMs. PEMFCs have shown great promise to replace energy converting devices in internal combustion engines and are considered the top candidate for automotive power applications.
Among the various to be commercialised products that use fuel cell technology are miniature cellular phones, laptop computers and many other electronic gadgets. Various hospitals, credit card centres, police stations, and banks are all using fuel cells to provide power to their facilities. As one of the significant advancements, various fuel cell vehicles (FCVs) have been produced by major automobile manufacturers (eg. Daimler, Honda and Toyota). Thus we feel various energy-based industrial sectors can benefit from our research.
Exploring research potential related to fuel cells
Our present goal is to develop novel crystalline materials, which can be used as PEMs and can be a rational alternative for Nafion. Several other engineering perspectives must be taken care in order to develop an efficient fuel cell system. However, since no significant advancement has been made in the development of PEMs over the past 50 years, we are keenly interested in fabricating cheap and efficient ways to develop the same.
Further steps to microporous crystalline materials research
In our lab, our primary aim is to develop cheap microporous crystalline materials that can be effectively used as PEM in fuel cell industries but further technological research for the implementation of these types of materials in industries is underway. We would like to indulge in further development and optimisation of these types of crystalline materials before we can think of commercialising our technology. For future growth, e need to develop chemically stable cheap crystalline materials based efficient PEM, which can operate over a wide temperature range and at varying humidity.
Effective for removal of greenhouse gases
From gas adsorption studies done on the course of this work, it was observed that both the frameworks HOF-GS-10 and 11 showed CO2 selectivity over other gases like N2, H2 and O2 at low temperatures. Thus, the HOF compounds reported here do have the potential to remove greenhouse gases (here CO2) apart from their applications in fuel cells.
Carbon dioxide, generated mainly through combustion of fossil fuel, accumulates at an alarming pace due to the rapid expansion of the energy consumption worldwide. Porous crystalline materials like MOFs, COFs etc. have shown promising potential to trap these greenhouse gases from the environment over the years. Recently, HOFs have shown tremendous potential to remove CO2. Thus HOFs and related materials may have a bright prospect in the industries, for the separation of CO2 from combustion gases using carbon capture and sequestration (CCS) technology.
Challenges faced during research
Although various H-bonded architectures are reported in the literature, developing materials, which has high proton conduction and also can retain the crystallinity after guest removal is a challenge. We tried various systems and it was in most cases these criteria was not satisfied. We then strategically chose this type of arene disulfonates and guanidinium ions based architectures, which fulfilled our requirement satisfactorily. Again since these compounds have an ionic character so improving their water stability was a big challenge and we could overcome this by tuning the hydrophobicity by crystal engineering.
Read More: Moving a step ahead in fuel cell technology
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