S. R. Narayanan

Advances in direct oxidation methanol fuel cells

Fuel cells that can operate directly on fuels such as methanol are attractive for low to medium power application in view of their low weight and volume relative to other power sources. A liquid feed direct methanol fuel cell has been developed based on a proton-exchange membrane electrolyte and Pt/Ru and Pt-catalyzed fuel and air/O2 electrodes, respectively. The cell has been shown to deliver significant power outputs at temperatures of 60 to 90 °C. The cell voltage is near 0.5 V at 300 mA/cm2 current density and an operating temperature of 90 °C. A deterrent to performance appears to be methanol crossover through the membrane to the oxygen electrode. Further improvements in performance appear possible by minimizing the methanol crossover rate.

Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent

Easy to prepare solid materials based on fumed silica impregnated with polyethylenimine (PEI) were found to be superior adsorbents for the capture of carbon dioxide directly from air. During the initial hours of the experiments, these adsorbents effectively scrubbed all the CO(2) from the air despite its very low concentration. The effect of moisture on the adsorption characteristics and capacity was studied at room temperature. Regenerative ability was also determined in a short series of adsorption/desorption cycles.

Electrochemical Conversion of Carbon Dioxide to Formate in Alkaline Polymer Electrolyte Membrane Cells

This paper is about the continuous electrochemical conversion of carbon dioxide to formate in a polymer electrolyte membrane cell using an alkaline ion-exchange membrane sandwiched between two catalyzed electrodes. This type of cell configuration allows carbon dioxide conversion to occur at high efficiencies and is particularly attractive for large-scale implementation. Formate was produced at high efficiency, and hydrogen evolution was suppressed with lead and indium as catalysts. The production of formate was monitored by UV-visible spectroscopy. During short experimental runs, the faradaic efficiency of formate production was as high as 80%. The faradaic efficiency was strongly dependent on the concentrations of carbon dioxide, bicarbonate, and carbonate at the surface of the electrodes. Low concentrations of carbon dioxide in the reactant feed led to the mass transport limitations and hence low faradaic efficiencies. The results show that mass transport limitations can be mitigated and high efficiencies can be realized by conducting the electrolysis in a pulsed mode. An alkaline membrane-based flow cell that ensures abundant availability of carbon dioxide to the electrodes can be a cost-effective and efficient approach for the continuous production of fuels from sunlight, storing of renewable energy, and lowering carbon dioxide levels in the atmosphere.

Performance of Direct Methanol Fuel Cells with Sputter‐Deposited Anode Catalyst Layers

Performance of direct methanol fuel cells with sputter-deposited Pt-Ru anodes was investigated. The thin film catalyst layers w ere characterized using X-ray diffraction, energy dispersive X-ray analysis, Rutherford backscattering spectroscopy, and X-ray photoelectron spectroscopy. Different catalyst loadings and membrane electrode assembly (MEA) fabrication processes were tested. The maximum power density achieved at 90°C was 100 mW/cm 2, and almost 75 mW/cm 2 was attained with a loading of only 0.03 mg/cm2. The results demonstrate that a catalyst utilization of at least 2300 mW/mg can be achieved at current densities ranging from 260 to 380 mA/cm2. The application of the sputter-deposition method for MEA fabrication is particularly attractive for commercialization of direct methanol fuel cell technology.

Electrochemical Impedance Spectroscopy of Lithium‐Titanium Disulfide Rechargeable Cells

The two‐terminal alternating current impedance of lithium‐titanium disulfide rechargeable cells has been studied as a function of frequency, state‐of‐charge, and extended cycling. Analysis based on a plausible equivalent circuit model for the cell leads to evaluation of kinetic parameters for the various physicochemical processes occurring at the electrode/electrolyte interfaces. To investigate the causes of cell degradation during extended cycling, the parameters evaluated for cells cycled five times have been compared with the parameters of cells that have been cycled over 600 times. The findings are that the combined ohmic resistance of the electrolyte and electrodes suffers a ten‐fold increase after extended cycling, while the charge‐transfer resistance and diffusional impedance at the interface are not significantly affected. The results reflect the morphological change and increase in area of the anode due to cycling. The study also shows that overdischarge of a cathode‐limited cell causes a decrease in the diffusion coefficient of the lithium ion in the cathode. The study demonstrates the value of electrochemical impedance spectroscopy in investigating failure mechanisms. The approach and methodology followed here can be extended to other rechargeable lithium battery systems.

Analysis of redox additive-based overcharge protection for rechargeable lithium batteries

The overcharge condition in secondary lithium batteries employing redox additives for overcharge protection, has been theoretically analyzed in terms of a finite linear diffusion model. The analysis leads to expressions relating the steady-state overcharge current density and cell voltage to the concentration, diffusion coefficient, standard reduction potential of the redox couple, and interelectrode distance. The model permits the estimation of the maximum permissible overcharge rate for any chosen set of system conditions. Digital simulation of the overcharge experiment leads to numerical representation of the potential transients, and estimate of the influence of diffusion coefficient and interelectrode distance on the transient attainment of the steady state during overcharge. The model has been experimentally verified using 1,1-prime-dimethyl ferrocene as a redox additive. The analysis of the experimental results in terms of the theory allows the calculation of the diffusion coefficient and the formal potential of the redox couple. The model and the theoretical results may be exploited in the design and optimization of overcharge protection by the redox additive approach.

Electrocatalytic Properties of Nanocrystalline Calcium-Doped Lanthanum Cobalt Oxide for Bifunctional Oxygen Electrodes.

Calcium-doped lanthanum cobalt oxide is a promising electrocatalyst for oxygen evolution and oxygen reduction in rechargeable metal-air batteries and water electrolyzers operating with alkaline electrolyte. Nanocrystalline perovskite of composition La0.6Ca0.4CoO3 with a unique cellular internal structure was prepared at 350 °C and then annealed in air at progressively higher temperatures in the range of 600-750 °C. The samples were characterized by electrochemical techniques and X-ray photoelectron spectroscopy. The area-specific electrocatalytic activity for oxygen evolution/oxygen reduction, the oxidation state of cobalt, and the crystallite size increased with annealing temperature, while the Tafel slope remained constant. These trends provide new insights into the role of the cobalt center in oxygen evolution and oxygen reduction, and how preparation conditions can be altered to tune the activity of the cobalt center for electrocatalysis. We expect these findings to guide the design of electrocatalysts for bifunctional oxygen electrodes, in general.

Polymer solid acid composite membranes for fuel-cell applications

A systematic study of the conductivity of polyvinylidene fluoride (PVDF) and CsHSO4 composites, containing 0 to 100% CsHSO4, has been carried out. The polymer, with its good mechanical properties, served as a supporting matrix for the high proton conductivity inorganic phase. The conductivity of composites exhibited a sharp increase with temperature at 142°C, characteristic of the superprotonic phase transition of CsHSO4. At high temperature (160°C), the dependence of conductivity on vol % CsHSO4 was monotonic and revealed a percolation threshold of ~10 vol %. At low temperature (100°C), a maximum in the conductivity at ~80 vol % CsHSO4 was observed. Results of preliminary fuel cell measurements are presented.

Investigation of Direct Methanol Fuel Cell Electrocatalysts Using a Robust Combinatorial Technique

A combinatorial approach to batch fabricating and evaluating fuel cell catalyst surfaces is described. The well-known binary Pt/Ru alloy and two compositional regimes of a novel quaternary Ni/Zr/Pt/Ru system were examinedin detail. Catalyst films no thicker than 10 nm were deposited onto an array of 36 gold electrodes 0.5 cm 2 in area that were microfabricated on a 12.5 × 12.5 cm glass substrate. The catalyst films had identical bulk and surface compositions, a result of the atom-level mixing that occurred during the room-temperature cosputtering method used. A multichannel pseudopotentiostat was implemented for electrochemical screening. Compositions with promising and/or contrasting catalytic activities were also studied using X-ray diffraction. X-ray energy-dispersive spectroscopy, and X-ray photoelectron spectroscopy. A low-Pt-content Ni 3 1 Zr 1 3 Pt 3 3 Ru 2 3 film was found to exhibit nominally the same activity (at 0.45 V vs a normal hydrogen electrode in 1 M H 2 SO 4 , 1 M CH 3 OH) as the best PtRu alloys studied. This material had a fundamentally different crystal and electronic structure than that observed in the Pt/Ru films and exhibited a significantly higher degree of Pt site utilization. These results were consistent with the existence of a catalytic reaction pathway different than that reported for Pt/Ru.

Easily Regenerable Solid Adsorbents Based on Polyamines for Carbon Dioxide Capture from the Air

Adsorbents prepared easily by impregnation of fumed silica with polyethylenimine (PEI) are promising candidates for the capture of CO2 directly from the air. These inexpensive adsorbents have high CO2 adsorption capacity at ambient temperature and can be regenerated in repeated cycles under mild conditions. Despite the very low CO2 concentration, they are able to scrub efficiently all CO2 out of the air in the initial hours of the experiments. The influence of parameters such as PEI loading, adsorption and desorption temperature, particle size, and PEI molecular weight on the adsorption behavior were investigated. The mild regeneration temperatures required could allow the use of waste heat available in many industrial processes as well as solar heat. CO2 adsorption from the air has a number of applications. Removal of CO2 from a closed environment, such as a submarine or space vehicles, is essential for life support. The supply of CO2-free air is also critical for alkaline fuel cells and batteries. Direct air capture of CO2 could also help mitigate the rising concerns about atmospheric CO2 concentration and associated climatic changes, while, at the same time, provide the first step for an anthropogenic carbon cycle.