What makes a Ru-based compound a viable Li-ion insertion material?

Density functional theory (DFT) can be used to calculate the properties of Li-ion battery materials, including insertion potential In this paper, we compare Li + intercalation in the layered compound Li 2 RuO 3 We use DFT to fully relax the crystal structure at each Li + concentration and for several different Li + ion configurations where possible. Choosing the lowest energy configuration, we calculated the intercalation potential vs. a Li metal anode. The details of the electronic structure were examined to determine the microscopic origin of properties relevant to good electrochemical performance, such as structural stability and electrical conductivity. The calculations are consistent with measurements predicting that Li 2 RuO 3 is a good electrical conductor with an intercalation potential around 3.1 V and a flat charge/discharge profile. Calculations also show that Li + combines with RuO 2 by two processes, either at 1.9 V by the intercalation of Li + into the rutile 2c sites to form Li x RuO 2 (figure 3), or by the formation of an inhomogeneous mixture of Li 2 O and Ru metal at 2.1 V. The latter process is energetically favored by 0.2 V, explaining why it is the practical product of the electrochemical interaction of RuO 2 and Li

Direct-write planar microultracapacitors by laser engineering

We have successfully employed laser direct write and micromachining to fabricate high capacity hydrous ruthenium oxide (RuO x H y or RuO 2 • xH 2 O) microultracapacitors. A laser direct-write process is used to deposit uniform pads of RuO 2 • 0.5H 2 O in sulfuric acid under ambient temperature and atmospheric conditions. Ultraviolet laser micromachining is used to tailor the shape and size of the deposited material into planar electrodes. The specific capacitance of the laser-deposited materials is comparable to reported values of ϳ720 F/g. The microultracapacitors demonstrate linear charge and discharge behavior at currents below 1 mA, as expected for an ideal capacitor. By studying the charge storage and power output as a function of discharge current, the power can be successfully modeled assuming only simple ohmic losses. Parallel and series combinations of these microultracapacitor cells provide the expected addition of capacitance. Maximum discharge currents of 50 mA are applied to two cells in parallel without damage to the microultracapacitor cells. The microultracapacitors exhibit high specific power and specific energy with over 1100 mW/g at approximately 9 mWhr/g for an 80 g cell with a footprint of 2 mm 2 and a thickness of 15 m

Lithium-Ion Cell Fault Detection by Single-Point Impedance Diagnostic and Degradation Mechanism Validation for Series-Wired Batteries Cycled at 0 °C

The utility of a single-point impedance-based technique to monitor the state-of-health of a pack of four 18650 lithium-ion cells wired in series (4S) was demonstrated in a previous publication. This work broadens the applicability of the single-point monitoring technique to identify temperature induced faults within 4S packs at 0 °C by two distinct discharge cut-off thresholds: individual cell cut-off and pack voltage cut-off. The results show how the single-point technique applied to a 4S pack can identify cell faults induced by low temperature degradation when plotted on a unique state-of-health map. Cell degradation is validated through an extensive incremental capacity technique to quantify capacity loss due to low temperature cycling and investigate the underpinnings of cell failure

Fabrication Method for Laboratory-Scale High-Performance Membrane Electrode Assemblies for Fuel Cells

Fabrication Method for Laboratory-Scale High-Performance Membrane Electrode Assemblies for Fuel Cell

High-Activity, Durable Oxygen Reduction Electrocatalyst: Nanoscale Composite of Platinum−Tantalum Oxyphosphate on Vulcan Carbon

A new oxygen reduction electrocatalyst for proton-exchange membrane fuel cells (PEMFCs) is synthesized by dispersing nanoscale Pt on a nanoscale tantalum oxyphosphate phase on a Vulcan carbon support, designated as Pt/[TaOPO₄/VC]. Electrocatalytic activity is measured by the thin-film rotating disk electrode methodology in 0.1 M HClO₄ electrolyte. The Pt/[TaOPO₄/VC] electrocatalyst has a high mass-specific activity of 0.46 A mg⁻¹_Pt compared to 0.20 A mg⁻¹_Pt for a Pt/Vulcan carbon standard and has met the 2015 DOE goal of 0.44 A mg⁻¹_Pt. This tantalum-containing electrocatalyst is twice as durable as the standard Pt/carbon in terms of its loss of Pt electrochemical surface area after aggressive electrochemical cycling

The Role of Compressive Stress on Gas Diffusion Media Morphology and Fuel Cell Performance

Understanding the respective morphology changes with compression of the gas diffusion layer (GDL) and microporous layer (MPL) in unitized gas diffusion media (GDM) is critical for polymer electrolyte fuel cell (PEFC) high-power performance, as the compression affects the ohmic resistance and the porosity that influences mass-transport resistance. We present a comprehensive study of morphology of two types of GDM (paper-type SGL 29BC and felt-type Freudenberg H2315 C2) under varied levels of compression using X-ray computed tomography (CT) to link GDM microstructure to fuel cell performance. The SGL 29BC morphology evolves more significantly with compression in ways that we expect to occlude oxygen diffusivity, while transitions in the Freudenberg H2315 C2 are more gradual. Upon compression by 0–34% its initial thickness, the 29BC pore-size distribution (PSD) shifts from bimodal (12.6 and 34.9 μm average pore radii) to unimodal (9.67 μm), extensive MPL surface cracks decrease in surface area and depth (5–2.2% crack surface area), and void volume fraction decreases from 0.45 to 0.18. Freudenberg H2315 C2 GDM maintains a unimodal PSD (10.5 to 8.33 μm average pore radii), has minimal surface cracking in its discrete MPL layer, and maintains a larger void volume (0.54 to 0.35) upon compression from 0 to 28% its initial thickness. As a result, PEFCs operated in hot and humid conditions (80 °C, 100% RH) with SGL 29BC applied as cathode GDM lose performance beyond 14% compression; the current density at 0.6 V decreases from 827.8 to 795.9 mA cm^–2 as 29BC compression increases from 14 to 28% the uncompressed GDM thickness. Alternatively, PEFCs with Freudenberg H2315 C2 GDM at the cathode increase in current density at 0.6 V as compression increases from 14 to 28% (1007 to 1098 mA cm^–2)

Effective Strategy for Improving Electrocatalyst Durability by Adhesive Immobilization of Catalyst Nanoparticles on Graphitic Carbon Supports

We have found that Ta-based additive films in our catalyst system act as adhesives, which improves electrocatalyst durability by immobilizing the catalyst Pt NPs on the graphitic Vulcan carbon support. Furthermore, we suggest that this can be a general design principle in producing higher-durability electrocatalysts on graphitic supports. By electrochemically probing the contributing roles of the tantalum oxide (Ta₂O₅) and the polyphosphate (PPA) components in separate samples, we show that these combine to produce the observed improvement in activity and durability of our best catalyst, the tantalum polyphosphate (TaOPO₄)-treated sample. To control variables for a valid electrochemical comparison, such as dissimilar catalyst particle size distributions and variations in surface coverage, four new catalyst samples closely matched in every way were prepared: (1) Pt/VC, (2) Pt/[PPA/VC], (3) Pt/[Ta₂O₅/VC], and (4) Pt­[TaOPO₄/VC]. We present HR-TEM/HAADF-STEM, EDS elemental mapping, PXRD, XPS, and electrochemical activity and durability evidence, showing that the TaOPO₄ and Ta₂O₅ additives act as adhesives, effectively tethering the NPs to the VC graphitic support surface. Pt/[Ta₂O₅/VC] exhibited 3× better durability as compared with the Pt/VC control because of better catalyst nanoparticle immobilization by the Ta₂O₅ adhesive. Pt­[TaOPO₄/VC] is the overall best performer, exhibiting both a high MA of 0.82 A mg_Pt^–1, the highest ORR MA after heat treatment, as well as 1.75× greater durability over the Pt/VC control