Arman Bonakdarpour

Reaction of Li with Alloy Thin Films Studied by In Situ AFM

Many intermetallic materials deliver poor capacity retention when cycled vs. Li. Many authors have attributed this poor capacity retention to large volume expansions of the active material. Here we report the volume changes of continuous and patterned films of crystalline Al, crystalline Sn, amorphous Si (a-Si), and a-Si 0 . 6 4 Sn 0 . 3 6 as they reversibly react with Li measured by in situ atomic force microscopy (AFM). Although these materials all undergo large volume expansions, the amorphous phases undergo reversible shape and volume changes. The crystalline materials do not. We attribute this difference to the homogeneous expansion and contraction that occurs in the amorphous materials. Inhomogeneous expansion occurs in the crystalline materials due to the presence of coexisting phases with different Li concentrations. Thin films of a-Si and a-Si 0 . 6 4 Sn 0 . 3 6 show good capacity retention with cycle number.

Impact of Loading in RRDE Experiments on Fe–N–C Catalysts: Two- or Four-Electron Oxygen Reduction?

We have investigated the impact of electrocatalyst loading on rotating ring-disk electrode (RRDE) experiments for the oxygen reduction reaction on Fe-N-C catalysts (ORR) in acid medium. In particular, the fraction of H 2 O 2 produced as a function of catalyst loading was studied. A dramatic increase in H 2 O 2 release was observed as the catalyst loading was decreased. For the same non-noble metal catalyst (NNMC), the fraction of produced H 2 O 2 varied between less than 5% and greater than 95%, depending on the catalyst loading. These observations suggest that oxygen reduction occurs stepwise, via H 2 O 2 , and if the catalyst is sparsely loaded, the produced H 2 O 2 cannot be efficiently reduced to H 2 O before it escapes. These studies have important implications for fundamental studies of ORR on NNMCs.

Studies of Transition Metal Dissolution from Combinatorially Sputtered, Nanostructured Pt1 − x M x (M = Fe, Ni; 0 < x < 1 ) Electrocatalysts for PEM Fuel Cells

The dissolution of Fe and Ni from Pt 1 - x M x (M = Fe, Ni; 0 0.6, the lattice constant expands indicating that transition metals dissolve also from the bulk. X-ray photoelectron spectroscopy results show complete removal of surface Ni (Fe) after acid treatment at 80°C for all compositions. The results of the acid treatments compare well to the composition changes that occur when a Pt 1 - x Fe x or Pt 1 - x Ni x combinatorial catalyst library is used in an operating PEM fuel cell.

Anomalous, High-Voltage Irreversible Capacity in Tin Electrodes for Lithium Batteries

Anomalous, high-voltage irreversible capacity is observed in electrodeposited and sputtered Sn films when used as working electrodes in electrochemical lithium cells. Sn electrodes can function as expected in Li/Sn cells provided the upper cutoff potential, while removing lithium from the Sn electrode, is kept below about 1.4 V. However, when the upper cutoff potential is increased to about 1.5 V and above, an anomalous irreversible high-voltage plateau in the voltage-capacity curve is often observed during subsequent lithiation. We discuss the relationship between the recharge cutoff voltage and the presence of anomalous high-voltage irreversible capacity. The dependence of the amount of anomalous, high-voltage irreversible capacity vs. discharge rate of the cell is investigated. A model is proposed to explain the anomalous high-voltage irreversible capacity, including strategies to avoid it. One of the strategies is to alloy Sn with Cu.

Co – C – N Oxygen Reduction Catalysts Prepared by Combinatorial Magnetron Sputter Deposition

Thin-film libraries of CO x C 1-x-y N y . (0 < x < 0.107, 0.003 < y < 0.389) were prepared by combinatorial magnetron sputter deposition in an Ar/N 2 gas mixture followed by subsequent heat-treatment at 700, 800, or 1000°C in N 2 atmosphere. By increasing the nitrogen partial pressure during sputtering, the nitrogen content was increased significantly in the as-sputtered libraries and more nitrogen remained in the libraries after heat-treatment. The catalytic activities of the libraries towards the oxygen reduction reaction (ORR) were studied using the rotating ring-disk electrode (RRDE) technique. CO x C 1-x-y N y libraries heat-treated at 800°C with 0.004 < x < 0.105 and 0.026 < y < 0.097 showed good catalytic activities towards ORR in 0.1 M HClO 4 solution at room temperature. The heating temperature that induces the onset of catalytic activity coincides with the temperature at which both substantial nitrogen release from the originally amorphous films and the formation of graphitic carbon and β-Co occurs. This temperature varies significantly with both the Co and N content in the films. The production of H 2 O 2 and the corrosion stability of the libraries are also discussed.

Fe-C-N oxygen reduction catalysts prepared by combinatorial sputter deposition

Thin-film libraries of Fe x C 1-x-y N y libraries (0 < x < 0.12; 0 < y < 0.5) have been prepared by combinatorial sputter deposition. The libraries were subsequently annealed at 700, 800, and 1000°C to induce structural and compositional changes. Respectable catalytic activity was achieved with libraries having 0 < x < 0.035 and 0.01 < y < 0.16 that were annealed at 800°C. This is explained in terms of nitrogen content and the degree of graphitization of the annealed catalysts.

Thermal Evolution of the Structure and Activity of Magnetron-Sputtered TM–C–N ( TM = Fe , Co ) Oxygen Reduction Catalysts

Thin-film libraries of TM x C 1-x-y N x (TM = Fe, Co; 0 < x < 0.09; 0 < y < 0.5) have been prepared by combinatorial sputter deposition. The libraries were subsequently annealed at 700-1000°C to induce structural and compositional changes. Using grazing-incidence X-ray diffraction and scanning electron microscopy, structural changes were followed as a function of annealing temperature. At temperatures above 700°C, the previously homogeneous and amorphous thin films became a heterogeneous mixture of (partially) graphitized nitrogen-containing carbon and either Fe 3 C or β-Co. The onset of this transformation is accompanied by a rapid decrease in N content and occurs as a function of both transition metal content and temperature. Catalytic activity for oxygen reduction is at its maximum partway through this transformation.

Combinatorial investigations of advanced Li-ion rechargeable battery electrode materials

Future advances in Li-ion rechargeable battery performance are strongly linked to improved electrode materials. Candidate materials for the negative electrode of the future generally contain multiple elements and broad composition ranges. There are surprisingly few published accounts of combinatorial investigations of Li-ion rechargeable battery electrode materials. This paper describes the combinatorial infrastructure of the Dahn group at Dalhousie University as it relates to other published accounts in the search for advanced Li-ion rechargeable battery negative electrode materials. Sample data sets are provided for various material systems. Special attention is paid to start-up and operational costs to encourage other groups to adopt combinatorial methods in this and other fields.

Characterization and PEMFC Testing of Pt1 − x M x ( M = Ru , Mo , Co , Ta , Au , Sn ) Anode Electrocatalyst Composition Spreads

Pt 1-x M x (M = Ru,Mo,Co,Ta,Au,Sn) random alloy samples, covering most of the binary composition range, have been prepared via magnetron sputtering. The alloys were deposited through shadow masks onto 3M nanostructured thin-film catalyst support for testing in a 64-electrode polymer electron membrane fuel cell (PEMFC). CO stripping voltammograms and hydrogen oxidation polarization curves with pure hydrogen and with reformate containing up to 50 ppm CO were measured on all the samples. In agreement with reports in the literature, Ru, Mo, and Sn were found to improve the CO tolerance of Pt, although the intrinsic hydrogen oxidation activity of Pt decreased significantly as the Sn content increased. The addition of Co to Pt had no impact on CO tolerance, possibly because of loss of surface Co through dissolution in the fuel cell. The addition of Au to Pt led to an increase in hydrogen oxidation overpotential when CO was present. Small amounts of Ta gave a small reduction in hydrogen oxidation overpotential in the presence of CO, but the overpotentials were still too high for practical application in a reformate-fed fuel cell.

Magnetron Sputtered Fe – C – N , Fe – C , and C – N Based Oxygen Reduction Electrocatalysts

Thin-film libraries of Fe x C 1-x-y N y (0 < x < 0.06, 0 < y < 0.5) have been prepared by combinatorial sputter deposition in a 40/60 N 2 /Ar process gas. The libraries were subsequently annealed between 800 and 1000°C to induce structural and compositional changes. These libraries contained/retained more nitrogen than those sputtered in lower N 2 partial pressures. Physical and electrochemical properties of these combinatorial libraries were studied using scanning electron microscopy, X-ray photoelectron spectroscopy, and rotating ring-disk electrode cells. Maximum catalytic activity for oxygen reduction reaction in acid was achieved for the libraries annealed at 800°C and approaches the best activity of Fe-C-N-based electrocatalysts reported in the literature to date.