Sputter-deposited, Pt film

Keeping in mind that the dc sputter-deposited Pt films have the completely electrochemical-active surface, the fractal dimensions of the rough film surfaces were calculated from Eq. (27) according to the peak-current method by taking the slopes of the (log 7peak - log v) plots within the scan rate range of v0 to... 

The supremacy of such layers in terms of catalyst utiliza tion is demonstrated by recent MEA development of the company 3 M [96]. In this design, nanos-tructured films of oriented crystalline organic whiskers form the substrate for a sputter-deposited Pt film. The activity per total mass of Pt of these ultrathin films is about a factor six greater than the mass activity of conventional three-phase CCLs, which affirms the much better catalyst utilization. 

Here, A max is the voltage loss tolerance due to finite electronic conductivity, b = RgT/ aeffF) is the Tafel parameter with the effective electronic transfer coefficient Ueff of the ORR, and Jq is the operating current density. For instance, at Icl= 10 qm, Jo = 1 A cm , Ueff = I, T = 333 K, and Ai max = 1 mV, the electronic conductivity requirement of the CL is Oei > 0.01 S cm In an ultrathin catalyst layer (UTCL) with thickness L = 100 nm, this bound on Oei is lower, namely, uei > lO- Scm-i. This estimate explains why, in CLs fabricated with the NSTF of the company 3M, a thin film of sputter-deposited Pt provides sufficient electronic conductivity. UTCLs are much less sensitive to the support conductivity. 

FTM and atom-probe studies of thin films of Ni, Au, Pt, a-Ge H, a-Si H and WO3, etc., on various substrates were reported by Krishna-swamy et a/.81 First, field ion tips each with a field evaporated surface were prepared. They are placed in an MRC model 8502 r.f. sputtering system. Tips were mounted on a recessed and shielded structure behind the sputtering surface which is bored with small holes about 1 to 2 mm in diameter. The very end of the tips came out of the holes to approximately the same level of the sputtering surface. Films were sputtered at about 20 mTorr Ar at an r.f. power of about 50 W. Thickness of a deposited thin film was controlled by both the r.f. power and the deposition time. Film thickness in the range of a few hundred to a few thousand A were studied. These tips were then imaged with Ne in the field ion microscope, or analyzed in the flight-time-focused ToF atom-probe. 

The larger nucleation density of a Pt film on the BST also appears to have some relationship with the incorporation of the F at interfaces. It was observed that the top and bottom electrode interfaces came to have a noticeable amount of F impurity because the chemical bonding of Ba and Sr with F is quite strong and the fluorides are not volatile at the deposition temperature. Interestingly, the dielectric constant and leakage current density of the BST film having F impurity by the Pt top electrode are better than those of the film having a sputtered Pt top electrode. 

The TiOo thin films were first cleaned in UHV by Ar+ ion sputtering followed by oxygen annealing (1100 K, 10-6 mbar 0 ) to restore surface stoichiometry. A Pt film of approximately ID A (based on the attenuation of the Ti XPS signals) was vapor deposited on this defect free TiO2 surface. The Pt film was then sintered by heating to 875 K (2 hr., 10"6 mbar 0o). The size of the Pt particles formed in this manner are estimated to be 2 to 5 nm in diameter based on comparison with microscopy studies in the literature.(18)... 

Suzuki T, Honda N, Ouchi K (1997) Preparation on magnetic properties of sputter-deposited Fe-Pt thin films with perpendicular anisotropy. J Magn Soc Jpn 21-S2 177-180... 

Cyclic voltammetric studies indicated that the activity of the Pt-Ru films increased with operating temperature just as in conventional catalyst layers produced from unsupported catalyst inks. Membrane electrode assemblies were fabricated from Pt-Ru films of the most active compositions, and a power density of 800 mW/mg was realized for anodes that were deposited with about 0.1 mg/cm of Pt-Ru (see Figure 1). Applying the catalyst layers by sputter deposition on the electrode was found to yield better performance than applying them on the membrane. This was attributed to the enhanced electrical connectivity achieved when the catalyst layer is applied on the electrode. However, this is only true for very thin films. When thicker composite films are produced, such as those planned later in this project, good electrical connectivity may be achieved even with membrane deposition. 

SEM photographs of sputtered films show that the layers are fairly dense and appear to crack into platelets when subjected to MEA fabrication. The dense films do not lend themselves to high surface areas therefore, there is substantial scope for enhancement of performance if the surface area can be increased. This may be achieved by producing porous 3-D Pt-Ru layered structures. One such method for creating such 3-D structures, that seem to be extremely promising, involves the pre-treatment of the membrane surface by ion-beam etching, which is then followed by sputter-deposition of the metal. This results in substantially enhanced surface area and very rough nanostructures. Next year s effort will include characterization of such films. 

High catalyst activity and utilization of sputtered thin films was demonstrated in operating fuel cells. Optimal sputter-deposition conditions for platinum-ruthenium alloys have been determined. The effect of composition on the performance of Pt-Ru films was studied, and optimal composition has been determined. Novel methods of enhancing surface area and improving porosity have been identified. Co-sputtered ruthenium oxide has been demonstrated not to have any significant beneficial effect on the activity of the catalyst layers. While cost presents a major obstacle to commercialization of DMFCs for mobile applications, this project demonstrates novel means to reduce the catalyst costs in DFMC fuel cells. Efficiency enhancements that are also necessary for DMFCs to be viable will be addressed... 

Two further improvements in cell performance have occurred using polypyrrole films. One noted by Fan et al (41) precoats an n-Sl surface with a thin film of Au( 15A) followed by a thicker film of polypyrrole (3300A). Such a combined electrode substantially Improves stability in aqueous electrolytes containing the couple. The other improvement is due to Cooper et al (44) who sputter deposited a 72A "film" of Pt on the surface of a polypyrrole film covering a Ta substrate. (Ta is known to form surface oxides like n-Si). They were then able to sustain O2 evolution by water electrolysis in a manner indistinguishable from the use of naked Pt electrodes. Their results show that small deposits of Pt at the polymer/electrolyte lower the overvoltage necessary for water oxidation compared to that needed using non-platinized polypyrrole. 

Figures 1.34-1.36 compares the performance of these two electrode types. The sputtered film electrodes show good activity towards Oj reduction. This shows that the sputter-deposited layer are as active as the highly dispersed Pt in the ETEK electrodes. However, no enhancement is observed at high current densities. This behavior is understood as follows. The diffusion of O2 to the surface in the thin film electrode occurs through the layer of Vulcan XC-72 carbon which is quite similar to that through the ETEK electrodes. Therefore, eliminating this sub-layer of carbon is a better approach to implementing the thin film electrodes.

M. Alvisi, G. Galtieri, L. Giorgi, R. Giorgi, E. Serra, M.A. Signore, Sputter deposition of Pt nanoclusters and thin films on PEM fuel cell electrodes. Surf. Coat. Technol. 200, 1325—1329 (2005)... 

FIGURE 7.5. Evolution of the optical properties (a,b) and film thickness(c) as a function of time for a polypyrrole film grown potentiostatically on a Pt sputter-deposited film electrode at +0.60 V (SCE) using an aqueous solution of pyrrole (0.025 M) and KNO3 (0.5 M). (From Ref. 19). 

Figure 1.4 Microreactors with sputter-deposited catalysts (a) chip (63 X 25 mm) with Ag film for oxidative dehydrogenation of 3-methyl-2-buten-1-ol to aldehyde (464 °C) (b) chip (3x1 cm) containing Pt, Fe or Co for (de)hydrogenation of cyclohexene (up to 250°C) and synthesis gas methanation (up to 300°C). Reprinted from [31], Copyright 2004, and [35], Copyright 2003, with permission from Elsevier.

Physical processes may also be used to deposit Pt onto various types of supports. An example of this type of approach is the preparation of Pt-metal monolayers supported on low-cost transition metal carbides, prepared by magnetron sputtering of Pt onto thin films of W and Mo carbides. While Pt monolayers were achieved on the thin-film electrode geometry used in this study, uniform deposition of Pt onto high surface area particulate materials or mesoporous structures by these methods remains challenging [48]. 

Atanasoski RT, Atanasoska LL, Cullen DA, Haugen GM, More KL, Vemstrom GD (2012) Fuel cells catalyst for start-up and shutdown conditions electrochemical, XPS, and STEM evaluatitm of sputter-deposited Ru, Ir, and Ti on Pt-coated nanostmctured thin film supports. Electrocatalysis. Electrocatal 3 284—297... 

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