Rajankumar L. Patel

Stabilizing Nanostructured Solid Oxide Fuel Cell Cathode with Atomic Layer Deposition

We demonstrate that the highly active but unstable nanostructured intermediate-temperature solid oxide fuel cell cathode, La0.6Sr0.4CoO3-δ (LSCo), can retain its high oxygen reduction reaction (ORR) activity with exceptional stability for 4000 h at 700 °C by overcoating its surfaces with a conformal layer of nanoscale ZrO2 films through atomic layer deposition (ALD). The benefits from the presence of the nanoscale ALD-ZrO2 overcoats are remarkable: a factor of 19 and 18 reduction in polarization area-specific resistance and degradation rate over the pristine sample, respectively. The unique multifunctionality of the ALD-derived nanoscaled ZrO2 overcoats, that is, possessing porosity for O2 access to LSCo, conducting both electrons and oxide-ions, confining thermal growth of LSCo nanoparticles, and suppressing surface Sr-segregation is deemed the key enabler for the observed stable and active nanostructured cathode.

A 1-D coordination polymer route to catalytically active Co@C nanoparticles

Pyrolysis of a 1-D polymeric cobalt(II) coordination complex ([Co(BDC)(Mim)2]n, H2BDC = benzenedicarboxylic acid; Mim = N-methylimidazole) results in the formation of carbon embedded fcc cobalt nanoparticle composites, Co@C. The as-prepared Co@C shows an agglomerated secondary structure with a highly embedded carbon shell comprising of cobalt nanoparticles of 20–100 nm. These Co@C particles show excellent catalytic activity in the reduction of nitrophenol to aminophenol, studied as a model reaction, and evolves as a promising candidate for the gas phase reduction process.

Ionic and electronic conductivities of atomic layer deposition thin film coated lithium ion battery cathode particles

It is imperative to ascertain the ionic and electronic components of the total conductivity of an electrochemically active material. A blocking technique, called the “Hebb–Wagner method”, is normally used to explain the two components (ionic and electronic) of a mixed conductor, in combination with the complex ac impedance method and dc polarization measurements. CeO2 atomic layer deposition (ALD)-coated and uncoated, LiMn2O4 (LMO) and LiMn1.5Ni0.5O4 (LMNO) powders were pressed into pellets and then painted with silver to act as a blocking electrode. The electronic conductivities were derived from the currents obtained using the dc chronoamperometry mode. The ionic conductivities were calculated based on results of the electronic conductivities and the mixed conductivities obtained using the ac impedance method. The results showed that the ionic conductivities of the LMO and LMNO particles coated with CeO2 thin films were twice as much as those of the uncoated LMO and LMNO particles. Also, LMO particles coated with insulating materials, such as alumina and zirconia ALD films, were tested and compared. No significant effects of the substrates on the ionic conductivities of the coated and uncoated samples were noticed, although the electronic conductivities of the LMO samples were found to be higher than those of the LMNO samples. Indeed, the ionic conductivity of the CeO2 films and the optimal film thickness achieved by ALD helped overcome the trade-off between long cycle-life and the reduced initial capacity fade of the LMO when used as a cathode in lithium ion batteries.

Boosting the Electrochemical Performance of Li1.2Mn0.54Ni0.13Co0.13O2 by Atomic Layer-Deposited CeO2 Coating

It has been demonstrated that atomic layer deposition (ALD) provides an initially safeguarding, uniform ultrathin film of controllable thickness for lithium-ion battery electrodes. In this work, CeO2 thin films were deposited to modify the surface of lithium-rich Li1.2Mn0.54Ni0.13Co0.13O2 (LRNMC) particles via ALD. The film thicknesses were measured by transmission electron microscopy. For electrochemical performance, ∼2.5 nm CeO2 film, deposited by 50 ALD cycles (50Ce), was found to have the optimal thickness. At a 1 C rate and 55 °C in a voltage range of 2.0−4.8 V, an initial capacity of 199 mAh/g was achieved, which was 8% higher than that of the uncoated (UC) LRNMC particles. Also, 60.2% of the initial capacity was retained after 400 cycles of charge–discharge, compared to 22% capacity retention of UC after only 180 cycles of charge–discharge. A robust kinetic of electrochemical reaction was found on the CeO2-coated samples at 55 °C through electrochemical impedance spectroscopy. The conductivity of 50Ce was observed to be around 3 times higher than that of UC at 60–140 °C. The function of the CeO2 thin-film coating was interpreted as being to increase substrate conductivity and to block the dissolution of metal ions during the charge–discharge process.

Unveiling the Role of CeO2 Atomic Layer Deposition Coatings on LiMn2O4 Cathode Materials: An Experimental and Theoretical Study

An atomic layer deposition (ALD) coating on active materials of a lithium ion battery is a more effective strategy for improving battery performance than other coating technologies. However, substantial uncertainty still remains about the underlying physics and role of the ALD coating in improving battery performance. Although improvement in the stability and capacity of CeO2 thin film coated particles for batteries has been reported, a detailed and accurate description of the mechanism has not been provided. We have developed a multiphysics-based model that takes into consideration stress mechanics, diffusion of lithium ion, and dissolution of transition-metal ions of spinel LiMn2O4 cathode. The model analyzes how different coating thicknesses affect diffusion-induced stress generation and, ultimately, crack propagation. Experimentally measured diffusivity and dissolution rates were incorporated into the model to account for a trade-off between delayed transport and prevention of side reactions. Along with experimental results, density functional theory results are used to explain how a change in volume, due to dissolution of active material, can affect battery performance. The predicted behavior from the model is well-matched with experimental results obtained on coated and uncoated LiMn2O4-Li foil cells. The proposed approach and explanations will serve as important guidelines for thin film coating strategies for various battery materials.

Employing Synergetic Effect of Doping and Thin Film Coating to Boost the Performance of Lithium-Ion Battery Cathode Particles

Atomic layer deposition (ALD) has evolved as an important technique to coat conformal protective thin films on cathode and anode particles of lithium ion batteries to enhance their electrochemical performance. Coating a conformal, conductive and optimal ultrathin film on cathode particles has significantly increased the capacity retention and cycle life as demonstrated in our previous work. In this work, we have unearthed the synergetic effect of electrochemically active iron oxide films coating and partial doping of iron on LiMn1.5Ni0.5O4 (LMNO) particles. The ionic Fe penetrates into the lattice structure of LMNO during the ALD process. After the structural defects were saturated, the iron started participating in formation of ultrathin oxide films on LMNO particle surface. Owing to the conductive nature of iron oxide films, with an optimal film thickness of ~0.6 nm, the initial capacity improved by ~25% at room temperature and by ~26% at an elevated temperature of 55 °C at a 1C cycling rate. The synergy of doping of LMNO with iron combined with the conductive and protective nature of the optimal iron oxide film led to a high capacity retention (~93% at room temperature and ~91% at 55 °C) even after 1,000 cycles at a 1C cycling rate.

Enhanced cycle life and capacity retention of iron oxide ultrathin film coated SnO2 nanoparticles at high current densities

Tin oxide (SnO2) has a high theoretical capacity (∼782 mA h g−1), but it experiences large volume changes during charge and discharge cycles that cause rapid capacity fade, which limits its practical use as an anode material. In an attempt to solve this, we coated these particles with ultrathin electrochemically active iron oxide (FeOx) films that act as an artificial solid electrolyte interphase layer, thus stabilizing the SnO2 particles for better longevity of significantly improved performance at high current densities in a practical voltage window. Since there exists a tradeoff between species transport and protection of particles (expecting long life), a film with an optimum thickness was achieved by atomic layer deposition (ALD) of FeOx on SnO2 particles. With an optimum thickness of about 0.24 nm after 20 cycles of iron oxide ALD (20Fe), an initial capacity of ∼658 mA h g−1 was achieved at a high current density of 1250 mA g−1. After 1000 cycles of charge/discharge at 1250 mA g−1, the 20Fe sample showed a capacity retention of 94% as compared to 52% of the uncoated sample when cycled at room temperature; at 55 °C, the capacity retention of the 20Fe sample was 93% compared to 33% of the uncoated sample.

The ubiquitous paddle-wheel building block in two-dimensional coordination polymers with square grid structure

Abstract This work describes design of a series of new paddle-wheel binuclear clusters containing 2-D coordination polymers based on ditopic carboxylate linkers, 1,4-benzenedicarboxylate (BDC) or 2-amino,1,4-benzenedicarboxylate (Am-BDC). The strategic use of strongly coordinating base/solvent as blocking ligand to restrict the structure in 2-D space is explored, and the role of organic base on the overall structure formation is further elaborated. The isostructural [Zn(BDC)(Py)]n (1) and [Co(BDC(Py)]n (2) were formed by the use of strong base pyridine (Py) as a blocking ligand whereas reaction using N-methylimidazole (Mim) in place of pyridine gives [Co(BDC)(Mim)]n (3) with similar topology and coordination environment. The use of weak/non-coordinating base such as 2-chloropyrimidine, pyrazine, and tetramethylammoniumhexafluorophosphate [(CH3)4 N(PF6)] gives the DMF-coordinated 2-D frameworks, [Cu(BDC)(DMF)]n (4), [Zn(BDC)(DMF)]n (5), and [Zn(AmBDC)(DMF)]n (6). All the structures crystallize in monoclinic crystal system yielding 2-D nets with square grid 44 topology and solid state 3-D structure via extensive non-covalent supramolecular interactions. Surface area analysis via N2 adsorption of three representative 2-D coordination polymers, 1, 4, and 6, indicate that 4 has a surface area of 450 m2 g−1 which is a signature of microporosity, while 1 and 6 have moderate (161.6 m2 g−1) and negligible (33 m2 g−1) surface areas, respectively.

Significant improvement in TiO2 photocatalytic activity through controllable ZrO2 deposition

ZrO2 was deposited on anatase TiO2 nanoparticles using 5–80 cycles of atomic layer deposition (ALD). The photocatalytic activity of all samples was evaluated based on the degradation of methylene blue (MB) solution under UV light. The TiO2 sample with 45 cycles of ZrO2 deposition (45c-Zr/TiO2, 1.1 wt% ZrO2) was proved to be the most efficient catalyst with a degradation kinetic constant 10 times larger than that of the pure TiO2 sample. All samples were characterized using inductively coupled plasma atomic emission spectroscopy (ICP-AES), nitrogen adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra analysis (UV-DRS), Raman and photoluminescence (PL) techniques. The high photocatalytic activity of 45c-Zr/TiO2 can be attributed to stronger adsorption in the ultraviolet region and a reduction in the recombination rate of electron/hole pairs.