A. Vadivel Murugan

Nanostructured electrode materials for electrochemical energy storage and conversion

Nanostructured materials play an important role in advancing the electrochemical energy storage and conversion technologies such as lithium ion batteries and fuel cells, offering great promise to address the rapidly growing environmental concerns and the increasing global demand for energy. In this review, we summarize some of the recent progress and advances in our laboratory on nanostructured electrode materials for lithium ion batteries and platinum-based and platinum-free nanoalloy electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFC). Materials design, novel chemical synthesis and processing, advanced materials characterization, and electrochemical evaluation data are presented.

Surface Modification of High Capacity Layered Li [ Li0.2Mn0.54Ni0.13Co0.13 ] O2 Cathodes by AlPO4

Layered Li[Li 0.2 Mn 0.54 Co 0.13 Ni 0.13 ]O 2 cathode, which is a solid solution between layered Li[Li 1/3 Mn 2/3 ]O 2 and Li[Ni 1/3 Mn 1/3 Co 1/3 ]O 2 , has been surface modified with various amounts (0-4 wt %) of AlPO 4 and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and electrochemical measurements in lithium cells. Annealing the surface-modified samples at 400 and 700°C leads to the formation of Li 3 PO 4 on the surface and an incorporation of some Al 3+ into the layered oxide lattice. More importantly, surface modification with AlPO 4 (<4 wt %) increases the discharge capacity and drastically reduces the irreversible capacity loss in the first cycle compared to the values observed with the pristine (unmodified) sample. For example, the irreversible capacity loss decreases from 75 to 27 mAh/g and the discharge capacity increases from 253 to 279 mAh/g on surface modification with 2 wt % AlPO 4 . The results are explained based on the retention of more oxide ion vacancies in the layered lattice after surface modification.

High capacity double-layer surface modified Li[Li0.2Mn0.54Ni0.13Co0.13]O2 cathode with improved rate capability

Layered Li[Li0.2Mn0.54Ni0.13Co0.13]O2, which is a solid solution between layered Li[Li1/3Mn2/3]O2 and Li[Ni1/3Mn1/3Co1/3]O2, has been surface modified by single-layer coating with 2–5 wt.% AlPO4, CoPO4, and Al2O3 and double-layer coating with 2 wt.% AlPO4 or 2 wt.% CoPO4 inner layer and 2–3.5 wt.% Al2O3 outer layer. The samples have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements in lithium cells. The double-layer coated samples are found to exhibit lower irreversible capacity loss (Cirr) and higher discharge capacity values than both the pristine and the single-layer coated samples, which are due to the retention of a higher number of oxide ion vacancies in the layered lattice after the first charge. The double-layer coated composite cathodes exhibit Cirr values as low as 26 mAh g−1 with a high discharge capacity of ∼300 mAh g−1, which is two times higher than that achieved with layered LiCoO2. Moreover, the double-layer coated samples exhibit higher rate capabilities than the pristine and single-layer coated samples, which are attributed to the suppression of undesired SEI layers and the fast charge transfer reaction kinetics as indicated by the EIS data.

Low cost Pd–W nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells

Carbon supported Pd100 − xWx (0 ≤ x ≤ 30) nanoalloy electrocatalysts have been synthesized by simultaneous thermal decomposition of palladium acetylacetonate and tungsten carbonyl in o-xylene in the presence of Vulcan XC-72R carbon, followed by annealing up to 800 °C in H2 atmosphere. Characterization of the Pd–W samples by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) energy dispersive spectroscopic (EDS) analysis in scanning electron microscopy (SEM), and transmission electron microscopy (TEM) indicates the formation of single phase face centered cubic (fcc) solid solutions for 0 ≤ x ≤ 20 with controlled composition and morphology. The TEM data indicate an increase in particle size with increasing heat treatment temperature. Cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements reveal that the alloying of Pd with W enhances the catalytic activity for the oxygen reduction reaction (ORR) as well as the stability (durability) of the electrocatalyst compared to the unalloyed Pd. The composition Pd95W5 exhibits the maximum activity for ORR in the Pd–W system. The Pd–W catalysts show high tolerance to methanol electro-oxidation compared to a Pt catalyst, which may offer important advantages in employing the Pd-based electrocatalysts for ORR in direct methanol fuel cells.

Pt-encapsulated Pd-Co nanoalloy electrocatalysts for oxygen reduction reaction in fuel cells.

Pt-encapsulated Pd(x)Co(100-x) nanoalloy electrocatalysts supported on carbon have been synthesized by a rapid microwave-assisted solvothermal (MW-ST) method within 15 min at as low as 300 degrees C. Subsequently, the samples have been heat treated at 900 degrees C in a reducing gas atmosphere to obtain Pt-Pd-Co nanoalloys. X-ray diffraction (XRD) analysis of the as-synthesized and 900 degrees C heat-treated samples reveals interesting changes in phase compositions and degree of alloying with Co and Pt contents and heat treatment. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) data of the as-synthesized samples confirm Pt enrichment on the surface of the Pd-Co nanoparticles. Rotating disk electrode (RDE) and single cell proton exchange membrane fuel cell measurements reveal that the as-synthesized Pt-encapsulated Pd(80)Co(20) (i.e., 75 wt % Pd(80)Co(20) + 25 wt % Pt) with 20 wt % total metal loading on carbon or 5 wt % Pt exhibit higher catalytic activity for the oxygen reduction reaction (ORR) compared to Pt with 20 wt % Pt loading on carbon. Significant changes in the catalytic activity for ORR occur on heat treatment at 900 degrees C as a result of changes in the phase composition and increase in particle size. This study demonstrates that the encapsulation of Pd-Co alloys with Pt offers a significant enhancement in activity for ORR per unit mass of Pt, offering a significant cost savings.