AVS2012 Session EN+NS-ThM: Batteries and Fuel Cells

Proton exchange membrane (PEM) fuel cells are still the most desirable component of future zero emission, high efficiency automobiles fueled with hydrogen. However, their unit cost, ease of operation, and reliability must be reduced which includes eliminating the humidifier from the fuel cell system. Currently the commercial PEM is fabricated from a perfluorosulfonic acid (PFSA) polymer such as Nafion^®. Unfortunately PFSA ionomers must be fully hydrated to achieve practical levels of proton conductivity which can only be achieved in vehicles operating at an inlet RH of 85% which still necessitates the use of a humidifier and undesirable complex water management and recovery. To achieve the goal of a PEM that can operate at temperatures from freezing to 120ºC using dry inlet gases it will be necessary to develop new PEMs that are based on new chemistries or dramatically improved morphologies of existing chemistries. The versatility of the polymer electrolyte fuel cell could be expanded to more complex fuels with the use of an anion exchange membrane (AEM). An AEM fuel cell could potentially utilize less expensive metal catalysts and have the ability to oxidatively cleave carbon-carbo bonds.

Ionomers are generally perceived as being phase separated materials. However, the optimal morphology that an ionomer should adopt and that would be practical is still being debated. This situation is not helped by the uncertainty of the morphology adopted under operating fuel cell conditions of the incumbent material. Here I will describe a study that contrasts fully amorphous materials with materials designed to have phase separated morphologies of known symmetries and dimensions. While the designed materials allow us to probe certain consepts of ion conduction with pore shape and size, they do not necessarily out perform the amorphous materials. Curiously we show that the role of water has more to do with morphological changes in flexible materials than enhancing ion conduction in non-sulfonic acid based materials. This has implications for the fabrication of thin robust films that will be needed for an operating fuel cell. In the case of AEMs while a cation has yet to be found that is stable under hot and dry operation, water may be a necessary evil rather than an enhancer of anion conduction.

The large-scale commercial application of hydrogen fuel cells requires efficient and cost-effective electrocatalysts for the oxygen reduction reaction (ORR), which occurs on the cathode. We demonstrate that rational design can render electrocatalysts possessing both virtues. In this work, by unifying the knowledge on surface morphology, composition, electronic structure and reactivity, we find that tri-metallic sandwich-like structures are an excellent choice for optimization. Their constituting species are designed to couple synergistically rendering reaction-environment stability, cost-effectiveness and high catalytic activity. This cooperative-action concept enabled us to predict two Pt-free ORR electrocatalysts: Pd/Fe/W(110) and Au/Ru/W(110) [1]. Density functional theory calculations of the reaction free-energy diagrams indicate that these materials are more active toward ORR than the so far best Pt-based catalysts.

1. S. Stolbov and M. Alcántara Ortigoza; J. Phys. Chem. Letts. 3, 463 (2012)

Lithium (Li)-ion batteries have drawn much attention for their outstanding performance in portable electronics applications with the potentials to function as a power source for further minizaturized devices, including micro-systems through the utilization of 3-dimensional electrodes based on high aspect ratio pillars. To realize this potential, an ultra-thin and highly conformal electrolyte layer is needed to coat the 3D electrode array. The ionic conductor lithium aluminosilicate (LiAlSiO4) synthesized by atomic layer deposition (ALD) is a promising electrolyte material for 3D battery applications not only due to its high ionic conductivity along its c-axis resulting from channels formed by the alternating tetrahedra of aluminum-oxygen (Al-O) and silicon-oxygen (Si-O), but also expected to provide similar improved cell cyclability, as reported in the preliminary studies of ultra-thin metal-oxide ALD coatings on electrodes.

The self-limiting characteristic of ALD allows for precise control of thickness and composition of complex oxides and results in a highly conformal and pinhole-free coating suitable in 3D micro-battery applications or electrolyte surface coatings. The metal precursors used in this work are tetraethyl orthosilicate (TEOS), trimethylaluminum (TMA) and lithium t-butoxide (LTB). These precursors, along with water vapor as the oxidant, were used to deposit SiO2, Al2O3 and Li2O, with the deposition rates in the range of 0.8~2Å/cycle, respectively. The deposition rate of stoichiometric LiAlSiO4 was ~20Å/cycle at a temperature of 290°C. The concentration of each metal element in LixAlySizO (LASO) thin films was found to correlate closely to ALD cycles and the associated incubation times. The crystallinity of the films after post-deposition rapid thermal annealing (RTA) was a function of cation atomic percentage. Li-ionic conductivities and the activation energy of as-deposited LASO films with respect to lithium contents as well as their relation to the film thickness were studied. The LASO ALD coating on 3D features, such as NWs and nanopaticles (NPs), were confirmed to be conformal and uniform by transmission electron microscopy (TEM) imaging. The cell performance as well as cyclability enhancement from LixAlySizO was investigated for a silicon-nanowire 3D microbattery, where SiNW was used as an anode, to explore the potentials of a solid-state SiNW battery with a solid-oxide electrolyte.

With a specific capacity of almost 3580 mAh.g^-1 at room temperature (corresponding to the Li₁₅Si₄ alloy), silicon is a promising element for designing new efficient anodes in Li-ion battery technology. However, because of huge material volumic expansion (around 300%) during the lithiation process¹, leading to a quick pulverization of the electrode², silicon has been used only as an additive to graphite in commercial cells so far³. In order to improve Si-based electrodes cyclability, a better understanding of the lithium insertion mechanisms is among the key issues⁴. The study of the lithiation process in silicon particles is particularly challenging. Indeed, a complete amorphization of the material during cycling⁴ makes difficult the use of surface characterization techniques based on beam diffraction (e.g. X-rays or electrons). In this study, we have used Auger Electron Spectroscopy (AES) to study these mechanisms. Poor attention has been focused on AES in the battery research field so far^5,6. In this work we emphasize its interest, notably for the study of the lithiation mechanisms in silicon particles. The first part of this work is dedicated to the study of Li-Si alloys by using AES. Several crystalline Li_xSi alloys (Li₇Si₃, Li₁₃Si₄ and Li₂₂Si₅) have been synthesized and characterized by X-Ray diffraction and AES. It appears that the atomic relative concentrations obtained by AES for the elements Li and Si are accurate with a tolerance of approximately 10%. The effect of Ar⁺ sputtering has also been investigated. After this initial work, six electrodes based on silicon particles have been analyzed by using AES for different "State Of Charge" (SOC) within the first electrochemical cycle.

For each SOC, several silicon particles have been investigated individually. Li and Si depth concentration profiles have been achieved by following Li (KLL) and Si (LVV) Auger transitions. Thanks to the good spatial resolution (17 nm at 10kV/1nA), it is possible to detect inhomogeneities of Li concentration at the particle surface. Based on the results obtained on different silicon particles at several SOC, a lithiation model is proposed and discussed.

(1) Beaulieu, L. Y.; Hatchard, T. D. ; Bonakdarpour, A. ; Fleischauer, M. D.; Dahn, J. R. J. Electrochem. Soc. 2003, 150(11), A1457-A1464

(2) Kasavajjula, U.; Wang, C.; Appleby, A. J. J. Power Sources 2007, 163 (2), 1003-1039

(3) www.hitachi.com

(4) Zhang, W. J. J. Power Sources 2011, 196 (1), 13-24

(5) Morigaki, K. I.; Ohta, A. J. Power Sources 1998, 76 (2), 159-166

(6) Kim, Y. J., Lee, H.; Sohn, H. J. Electrochem. Comm. 2009, 11 (11), 2125-2128

Silicon continues to draw great interest as an anode material for lithium ion batteries due to its large specific capacity for lithium. Electrochemical etching silicon is one of several anode materials of interest, but its energy density is oftentimes limited due to its attachment to an unreactive silicon substrate. Here, we present a novel “liftoff” method by which a freestanding macroporous silicon film (MPSF) is electrochemically detached from the underlying bulk silicon and combined with pyrolyzed polyacrylonitrile (PAN), a conductive polymer. We report the performance of these silicon thin films with and without pyrolyzed PAN.