Ramaiyan Kannan

Improved performance of phosphonated carbon nanotube–polybenzimidazole composite membranes in proton exchange membrane fuel cells

Development of thermally stable polymer electrolyte membranes with higher proton conductivity as well as mechanical stability is a key challenge in commercializing PEM fuel cells operating above 100 °C. Polybenzimidazole membranes are one of the promising candidates in this category although with limited mechanical stability and moderate proton conductivity. Here the incorporation of functionalized MWCNT is shown to increase both these key parameters of the polybenzimidazole membranes. Further, formation of a domain like structure after the incorporation of phosphonated MWCNTs (P-MWCNTs) in phosphoric acid doped polybenzimidazole membranes is demonstrated. The enhanced performance has been attributed to the formation of proton conducting networks that formed along the sidewalls of P-MWCNTs with a domain size of 17 nm as estimated from the small angle X-ray scattering measurements. Membrane electrode assembly (MEA) impedance measurements further reveal that the activation energy of oxygen reduction reaction (ORR) reduced for the composite membranes with enhanced proton conductivity. In addition, the mechanical strength measurements reveal a significant improvement in the yield strength and ultimate strength. Also, the mechanical strength of the composite membrane has been increased significantly as indicated by the improvement in the ultimate strength from 65 MPa to 100 MPa for the pristine and composite membranes, respectively. The optimum loading of P-MWCNTs is found to be 1% as inferred from the polarization measurements carried out using pure hydrogen and oxygen. Thus, this study provides a unique opportunity to tune the properties of polymer electrolytes to prepare application oriented hybrid membranes using CNTs with tailor-made functional groups.

Chemically Stable Proton Conducting Doped BaCeO3 -No More Fear to SOFC Wastes

Development of chemically stable proton conductors for solid oxide fuel cells (SOFCs) will solve several issues, including cost associated with expensive inter-connectors, and long-term durability. Best known Y-doped BaCeO3 (YBC) proton conductors-based SOFCs suffer from chemical stability under SOFC by-products including CO2 and H2O. Here, for the first time, we report novel perovskite-type Ba0.5Sr0.5Ce0.6Zr0.2Gd0.1Y0.1O3−δ by substituting Sr for Ba and co-substituting Gd + Zr for Ce in YBC that showed excellent chemical stability under SOFC by-products (e.g., CO2 and H2O) and retained a high proton conductivity, key properties which were lacking since the discovery of YBCs. In situ and ex- situ powder X-ray diffraction and thermo-gravimetric analysis demonstrate superior structural stability of investigated perovskite under SOFC by-products. The electrical measurements reveal pure proton conductivity, as confirmed by an open circuit potential of 1.15 V for H2-air cell at 700°C, and merits as electrolyte for H-SOFCs.

Facile construction of non-precious iron nitride-doped carbon nanofibers as cathode electrocatalysts for proton exchange membrane fuel cells

We demonstrate a facile construction of iron nitride-doped carbon nanofiber by effectively utilizing the existing slit pores and rough edges along the inner wall of the substrate as originated by virtue of its cup-stack structure for effectively increasing the number of active sites and consequently the oxygen reduction activity.

Effect of Zr substitution for Ce in BaCe0.8Gd0.15Pr0.05O3−δ on the chemical stability in CO2 and water, and electrical conductivity

In this paper, for the first time, we report the chemical stability of a highly proton conducting Gd+Pr-codoped BaCe0.8−xZrxGd0.15Pr0.05O3−δ (BCZGP) (0.01 < x < 0.3) as a function of Zr-doping in H2O vapour, 30 ppm H2S in H2, and pure CO2 along with its electrical conductivity in air, N2 + 3% H2O, H2 + 3% H2O and N2 + D2O. All prepared BCZGP compositions retain the original cubic perovskite-type structure in 30 ppm H2S in H2 at 600 °C. BCZGP with x = 0.3 shows significant stability under pure CO2 at 400 °C, while upon exposure to H2O vapor all compositions form Ba(OH)2·xH2O. The maximum electrical conductivity obtained with higher Zr-doping in BCZGP (x = 0.3) is 7.6 × 10−3 S cm−1 which is about 30% of that of the parent compound BaCe0.8Gd0.15Pr0.05O3−δ. Current work clearly shows that Zr-doping at x = 0.3 increases the stability of BCZGP under 30 ppm H2S and pure CO2 at intermediate temperatures (T ≤ 400 °C), and retains good proton conductivity in H2 containing atmosphere.

Synthesis and characterization of perovskite-type BaMg0.33Nb0.67−xFexO3−δ for potential high temperature CO2 sensors application

Mixed ionic-electronic conducting BaMg0.33Nb0.67−xFexO3−δ was successfully synthesized by conventional solid state method in air at elevated temperature. Fe-doping helped to increase the total conductivity, while the chemical stability under CO2 at elevated temperatures and under H2O vapour at boiling conditions decreased with increasing Fe content above x = 0.17 in BaMg0.33Nb0.67−xFexO3−δ. Maximum total conductivity of 10−3 S cm−1 at 300 °C was obtained for BMNF33 in air, and further increase in Fe content lowered the electrical conductivity, particularly at low temperatures. Activation energy, in the range of 300–700 °C, for total electrical conduction was found to decrease in air by the incorporation of Fe in BaMg0.33Nb0.67O3 (BMN) (BMN: 0.96 eV; BMNF17: 0.72 eV; BMNF33: 0.25 eV). Furthermore, the x = 0.17 composition demonstrated excellent CO2 sensing characteristics at 700 °C. The investigated perovskite-type metal oxides seem to be promising candidates for monitoring CO2 (ppm level in air) at elevated temperatures.

Bio-inspired catalyst compositions for enhanced oxygen reduction using nanostructured Pt electrocatalysts in polymer electrolyte fuel cells

Composites of Nafion with a class of bio-molecules viz., plant hormones, are explored as potential polymer electrolytes for improving the proton transport inside the catalyst layer of a H2/O2 fuel cell. Specifically, four nitrogenous plant hormones, two each from the class of auxins and cytokinins have been investigated, following preliminary characterization of the composite dispersions and membranes. Interestingly, the use of indole-3-acetic acid (an auxin) in the catalyst layer reveals a 30% enhancement in Pt catalyst utilization and improved fuel cell performance by 150 mW cm−2. The effect of these bio-molecules on the kinetic and mass transport parameters has been analyzed systematically using a combination of electrochemical and spectroscopic techniques.

Application of functionalized CNT-polymer composite electrolytes for enhanced charge storage in "all solid-state supercapacitors"

The ability of specifically functionalized carbon nanotubes to enhance proton transport in Nafion and polybenzimidazole membranes leading to improvement in the specific capacitance of an all solid-state supercapacitor is demonstrated. Cyclic voltammetry experiments reveal a 25% improvement (185 and 150 F per gram of RuO2 for composite and Nafion membranes respectively) in capacitance by a meager 0.05 wt% addition of sulfonated MWCNTs in Nafion membranes. On the other hand, an addition of 1% phosphonated MWCNTs results in ∼60% improvement in olybenzimidazole (PBI) based composites (from 160 to 260 F g−1). Further, composite membranes based on functionalized MWCNTs show increased cycle life which is attributed to the presence of electrostatically linked network structures due to functional moieties on the side walls of carbon nanotubes that increases the interfacial charge density and integrity of the membrane. The equivalent series resistance for the PBI and PBI phosphonated MWCNT (PBpNT) membranes is 470 and 89 milli ohm respectively suggesting improved proton conductivity with the composite membrane. Charge discharge measurements reveal a capacitance value of 500 F g−1 for PBpNT membrane based supercapacitors even after 1000 cycles of operation. Use of such nanocomposite membranes is expected to dramatically improve the life time as well as performance of supercapacitors which in turn would facilitate deployment in different applications such as hybrid electric vehicles.