3M NSTF

The break-in process of 3M NSTF films involves voltage cycling to create a smooth polycrystalline Pt surface on the whiskers. The surface area enhancement factors of resulting structures are from 10 to 25, determined from cyclic voltammo-grams in the potential region of hydrogen underpotential deposition. The surface area enhancement factor (or roughness factor or real-to-apparent surface area ratio) of a... 

Figure 3.23 shows SEM images of a platinized NSTF layer before (a) and after (b) transfer to the PEM surface. Excellent power densities have been obtained with Pt loadings as low as 0.1 mg cm . A further asset of 3M NSTF layers is their dramatically improved resistance to ECS A loss. Superior durability and longevity of MEAs fabricated with 3M NSTF layers have been clearly demonstrated (Debe et al., 2006). 

It is also widely known that MEAs utilizing 3M NSTF technology face severe water management challenges. They show poorer performance than conventional CCLs at low RH, presumably caused by poor proton transport in insufficiently hydrated layers. Moreover, NSTF MEAs exhibit an increased propensity for flooding... 

The nanostructured thin-film electrode was first developed at 3M Company by Debe et al. [40] and Debe [41], who prepared thin films of oriented crystalline organic whiskers on which Ft had been deposited. The film was then transferred to the membrane surface using a decal method, and a nanostructured thin-film catalyst-coated membrane was formed as shown in Figure 2.10. Interestingly, both the nanostructured thin-film (NSTF) catalyst and the CL are nonconventional. The latter contains no carbon or additional ionomer and is 20-30 times thinner than the conventional dispersed Pt/ carbon-based CL. In addition, the CL was more durable than conventional CCMs made from Pt/C and Nation ionomer [40]. 

The 2007 cost of 67/kW is based on a new design by the 3M Company, which utilizes 3M s nanostructured thin film (NSTF) catalyst support for the cathode (Ahluwalia et ah, 2007 Lasher et ah, 2007). The cathode uses the bulk of the platinum. NSTF (apparently a carbon fabric ), in conjunction with vacuum deposition of an iron-cobalt-carbon-nitrogen cathode catalyst followed by a heat treatment, has apparently been successful in cutting the platinum requirement by more than half while increasing performance (3M Company, 2007). The research team at 3M is also optimistic about production costs. 

In order to be able to properly examine the inherent activity of minute amounts of OER catalysts, one needs a substrate with minimal interference, extremely slow OER kinetics of its own and extraordinary stability at high positive electrode potentials. The unique featiues of 3M s Pt-NSTF (nanostructured thin film) catalyst [12] such as superior durability, electrochemical inertness at high potentials, and the absence of corrosion interference due to exposed carbrui, made it a logical choice as a support [13, 14]. It is well known that pure platinum has a high overpotential for OER. For instance, at a current density of 1 mA/cm, the OER on platinum proceeds at a potential that is 0.47 V higher than oti single crystal ruthenium oxide [15]. Thus, the OER partial current density oti the Pt-NSTF substrate wiU be orders of magnitudes lower than on ruthenium, iridium, and other similar OER-active materials. 

The last three chapters are dedicated to improving the durability of the catalyst/ electrode. Chapter 22 reports the development and evaluation of bimetallic Pt-Ru (Ir) oxygen evolution catalysts on 3M s nanostructured thin film (NSTF). This type of catalyst may significantly reduce carbon corrosion and Pt dissolution during transient conditions of fuel cells. Chapter 23 discusses the unique properties of carbide-modified carbon as the support for fuel cell catalysts. The final chapter gives a comprehensive review of novel materials other than carbon black as catalyst support. The interactions between the supports and catalysts are intensively discussed in the last two chapters. 

A new catalyst systan developed by 3M called Nanostructured Thin Film (NSTF) is the first practical example that managed to find a delicate balance by utilizing the... 

In general, thin catalyst layers will have the lower proton resistance. However, they may also fill up more easily with water as is, for example, the case with the very thin NSTF electrodes by 3M, which only seem to function well at non-saturated conditions. Also for start-up under freezing conditions, a thicker electrode, or at least a higher pore volume, seems to be an advantage as complete filling with ice is even more detrimental. 

Non-nano-sized Pt is not immune for oxide formation or dissolution, but the equilibrium potential of oxide formation is higher compared to nano sized particles. Pt black and NSTF electrodes have either much larger Pt particles or even a continuous phase Pt. This results in lower specific surface area but more stable activity as was convincingly demonstrated by the life-time studies carried out on NSTF electrodes by 3M [49]. 

Another solution to both the carbon support and the ionomer contact issue is to use a Pt catalyst that has no support and is embedded in the membrane such as the Nano-structured thin film (NSTF) catalyst being developed by 3M [5, 81]. A SEM of the NSTF-Pt catalyst is shown in Fig. 17.13. hi addition to not having a carbon support to corrode, this catalyst system is much less susceptible to Pt dissolution because the small whiskers are coated with a continuous layer of Pt, not Pt nanoparticles, and so behaves more like bulk Pt. MEAs made with these electrodes... 

Figure 19.12. Comparison of mass activity of Pt/MWNTs thin film with Pt loading of 6 pg Pt/cm (A) Pt/MWNTs thin film with Pt loading of 12 pg Pt/cm ( ) 3M Corporation, Pt/NSTF with Pt loading of 26 pg Pt/cm ( ) Los Alamos National Lab, Pt/C with Pt loading of 120 pg Pt/cm (A) [61], (Reprinted with permission from J Phys Chem C 2007 111 17901-4. Copyright 2000 American Chemical Society.)...

A current baseline performance standard or 3M catalyst with 0.15 mg Pt/cm total per MEA is shown in Figure 20.17. The combined effect of the reduced microstructured feature size of the NSTF substrate and the reduced anode loading allows over 500 mV at 2 A/cm and 150 kPa inlet pressure for the first time. The 3M catalyst demonstrates a ten-fold gain in catalyst specific activity < 0.3 g Pt/kW, equivalent or higher mass activity combined with durability. 

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. 

An initial evaluation of the nanopore model can be conducted by comparison with electrochemical performance data for MEAs that utilize UTCLs on the cathode side. The two types of experimental materials considered are nanoporous gold leafs plated with varying amounts of Pt (Pt-NPGL) (Zeis et al., 2007) and Pt NSTF layers of 3M (Pt-NSTF) (Debe et al., 2006). 

The 3M nanostructured thin film (NSTF) catalyst support has also been well developed, which consists of oriented, nanometer-sized crystalline organic whiskers, synthesized by sublimation and subsequent annealing of an organic... 

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