Platinum catalyst layers with Pt loadings w = 0.05-0.40 mg/cm 2 were deposited by magnetron sputtering from a variable deposition angle ␣ onto gas diffusion layer ͑GDL͒ substrates and tested as cathode electrodes in proton exchange membrane ͑PEM͒ fuel cells using Nafion 1135 membranes and Teflon-bonded Pt-black electrode ͑TBPBE͒ anodes. Layers deposited at normal incidence ͑␣ = 0°͒ are continuous and approximately replicate the rough surface morphology of the underlying GDL. In contrast, glancing angle deposition ͑GLAD͒ with ␣ = 87°and continuous substrate rotation yields highly porous layers consisting of vertically oriented Pt particles, 100-500 nm high and 100-300 nm wide, that are separated by 20-100 nm. The particle electrodes exhibit a higher ͑lower͒ mass-specific performance than the continuous-layer electrodes for a high ͑low͒ current density i. This is attributed to a higher porosity but lower overall electrochemically active surface area for the particles compared to the continuous layer. Increasing w in particle cells from 0.05 to 0.10 to 0.18 mg/cm 2 yields increasing potentials, but w = 0.40 mg/cm 2 causes a voltage drop at i Ͼ 0.4 A/cm 2 , associated with the reduced pore density at large w. Comparison cells with a TBPBE cathode exhibit comparatively low Tafel slopes but a lower Pt mass specific performance than the sputtered catalysts. Quantitative analyses of kinetic and mass-transport losses in the polarization curves suggest a competing microstructural effect, favoring mass-transport performance and an efficient oxygen reduction reaction for particle and continuous layer electrodes, respectively. The overall results suggest that in addition to the well-known promise of sputter-deposited Pt catalysts as an approach to increase Pt utilization at low loading, GLAD provides the unique ability to control Pt porosity and to achieve efficient reactant flow for high-currentdensity operation.
Platinum-coated chromium nitride electrodes are deposited onto gas diffusion layers by normal and glancing angle deposition and are tested as cathodes for proton exchange membrane ͑PEM͒ fuel cells. X-ray diffraction and scanning electron microscopy show that the CrN forms 111-oriented nanoparticles with ͕100͖ facets that are covered by 3.4 ϫ 10 11 Pt mounds/cm 2 , independent of Pt loading 0.05 to 0.25 mg/cm 2 . Polarization curves exhibit dE/d͑log i͒ slopes b Ϸ −100 and Ϫ150 mV/dec for high ͑E Ͼ 0.75 V͒ and low ͑E Ͻ 0.5 V͒ potentials, respectively, but show an anomalous drop with b Ϸ −420 mV/dec in the intermediate voltage range. This is attributed to poor proton conduction associated with a reversible dewetting of electrode pores during low current operation. Quantitative analyses of rate-dependent polarization curves and electrochemical impedance spectra show that the time scale for pore filling by process water is 10 3 s, and that the ionic resistance R C within the electrode increases by a factor of 4, from R C Ϸ 0.2 to 0.8 ⍀ cm 2 , as E increases from 0.5 to 0.8 V. The increasing electrode resistance is attributed to a low water production rate at low current, which allows the relatively hydrophobic CrN to expel water from the electrode pores, resulting in a higher resistance for ionic transport. These results show that even ultrathin sputtered catalyst layers can exhibit incomplete flooding.Proton exchange membrane ͑PEM͒ fuel cells hold much promise for portable and automotive applications due to their high conversion efficiency and power density. 1 One obstacle to the widespread commercialization of fuel cells is the high cost of the platinum catalyst. Therefore, much effort is devoted to replacing or reducing the Pt loading w. 2 Most of today's commercial fuel cells minimize w by using a network of 30-50 nm wide carbon particles that support 2-5 nm wide Pt nanoparticles. 3,4 A challenge associated with this Pt/C approach is a decreasing efficiency and limited lifetime due to sintering and growth of the Pt, 5 peroxide attack of the membrane, 6 and carbon corrosion. 7 Moreover, some reports suggest that the lower limit in reducing Pt loading in this approach has already been reached, as reducing w to below 0.40 and 0.05 mg/cm 2 for cathode and anode, respectively, decreases the output efficiency. 8,9 Considerable efforts have been directed at replacing the precious metal catalyst with other less costly materials. One possible material is CrN, which has a lower activity than Pt, 10 but is an attractive candidate to supplement or replace Pt in lower cost fuel cell electrodes because of its well-known wear and corrosion resistance. 11 Industrial CrN coatings are most commonly deposited by reactive magnetron sputtering. Sputter deposition has also gained interest for Pt catalyst deposition, 2,12-17 as it provides a pathway to fabricate very low Pt loading ͑w = 0.005 mg/cm 2 ͒ 18 electrodes with a high degree of control and reproducibility. We have recently reported that sputtering Pt from highly oblique incide...
Layers of 150 nm wide and 0.5-1.5 m long carbon nanorods were grown by glancing angle deposition on Si substrates, sputter-coated with 0.10 mg/cm 2 Pt, and transferred to polymer electrolyte membranes for testing as cathode electrodes in fuel cells. The rods were etched within fully assembled cells by applying a potential above the reversible H 2 /O 2 voltage, which leads to polarization curves that show a 4-7 times higher current at 0.40 V. The current increase is attributed to the opening of pores within the electrode, which facilitates easy oxygen transport and leads to a reduction in mass transport resistance by a factor of 360, as determined by electrochemical impedance spectroscopy. Etching sequences with increasing voltage V E indicate that V E Յ 1.6 V yields water electrolysis and Pt oxidation that facilitates Pt agglomeration and migration of Pt ions into the electrolyte, while V E = 1.7 V results in removal of C and the formation of pores within rods that facilitate oxygen transport to reaction sites, yielding a 400-700% increase in fuel cell output current at low potential. These results suggest that the controlled etching of temporary scaffolds to create pores in an operating fuel cell may be an effective approach to reduce mass transport limitations.
Arrays of 1 m long C nanorods were grown by glancing angle deposition on flat and patterned Si wafers, coated with 0.1 mg/cm 2 Pt catalyst by magnetron sputtering, removed from the substrates, and tested as cathode electrodes in proton exchange membrane ͑PEM͒ fuel cells. Deposition on flat substrates yields a nearly fully dense nucleation layer with Ͻ5 nm wide pores, followed by the formation of separated rods with an average width that strongly increases with rod height, from Ͻ30 to 190 nm. In contrast, deposition on a patterned surface results in regularly spaced 50 nm wide pores and a rod width that only moderately increases with height, from 95 to 155 nm. Polarization curves on pure H 2 and O 2 for the two sample types are identical at high potential E Ͼ 0.55 V. However, the cathodes deposited on the patterned substrates yield considerably higher currents at low potential, with a 2 times higher limiting current density i L = 0.73 A/cm 2 than those grown on flat substrates. The higher current in the mass-transport-limited regime is attributed to the 10 times wider engineered pores that facilitate O 2 transport to the active catalyst sites, resulting in a 5 times lower mass transport resistance R MT = 1.5 ⍀ cm 2 at E = 0.50 V, as quantified by electrochemical impedance spectroscopy.
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