The aggregate size in fuel cell catalyst inks depends on the type of dispersion medium, particle concentration, and addition of stabilizing agents. In this work, ink stability and particle size of carbon black and carbon black/Nafion dispersions in four nonaqueous media, viz., methanol, ethanol, isopropanol and ethyl acetate are studied. Based on visual inspection, isopropanol is found to be the best medium for dispersion of carbon black inks. To rationalize this observation, a semi-empirical model based on diffusionlimited aggregation was developed to evaluate the rate of particle aggregation and predict the ink stability time for each dispersion medium. The proposed model supports the experimental observation by qualitatively predicting the same relationship between carbon stability and the dispersion media. The model also showed that the dielectric constant of the dispersion medium and the particle zeta potential are primarily responsible for the ink stability. Particle size for the different inks was determined by dynamic light scattering with and without dilution. Experimental results show that Nafion is a strong stabilizing agent, increasing the ink stability and decreasing the particle size of carbon aggregates. The beneficial effects of Nafion are independent of its concentration and are observed even at Nafion volume fractions of 10 wt%. The interaction energy is found to be a strong function of the surface potential for the dispersion medium with a higher dielectric constant. Most catalyst layer (CL) fabrication methods for polymerelectrolyte fuel cells (PEFCs) to date are based on wet deposition of a colloidal dispersion, i.e., the catalyst ink, onto either a membrane or a diffusion medium.1,2 The CL ink is usually a mixture of carbonsupported platinum particles, ionomer and a dispersion medium (DM). Ink stability, defined as the ability of particles to remain dispersed in the DM, and a reduced aggregate size are critical to deposition methods such as inkjet printing 3-6 and spray coating. 1,7,8 Inkjet printing allows for controlled spacial resolution, 6 however in order to create appropriate droplet sizes, the ink has to be ejected from micrometer size nozzles, 5 easily resulting in nozzle clogging. Inkjet printing of microporous layers using carbon ink has also been found to be extremely difficult unless ionomer solution, acting as a stabilizing agent, is added to the ink. 9 Similarly, spray coating requires inks to go through a nozzle, therefore clogging issues are likely to be a challenge when small nozzle sizes are used. Control over the aggregate size and the right selection of a DM to achieve the desired viscosity and surface tension therefore becomes critical in these deposition techniques. Further, the ink stability may also be important to larger commercial fabrication processes to increase the ink storage time.Even though the importance of appropriate ink recipes has been recognized, 1,10 very few studies have aimed at understanding the impact of DM on the ink stability. The type of DM, par...

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PEFC electrodes manufactured using inkjet printing are investigated. Cell performance, Tafel slope, reaction order and local oxygen transport resistance of electrodes with varying Pt loadings of 0.014 to 0.113 mg/cm 2 are studied in order to understand the nonlinear relationship between loading and performance. The performance increase with Pt loading was substantially reduced above Pt loadings of 0.08 mg/cm 2 . Electrochemical active area of the electrodes decreased from 66.4 to 40.4 m 2 /g as the Pt loading increased from 0.026 to 0.113 mg/cm 2 . Below the transition voltage of 0.8 V i R f ree , the Tafel slope was found to be a function of the Pt loading and oxygen partial pressure. The kinetic performance dependence on p O 2 was quantified by measuring the total reaction order. Oxygen transport resistance evaluated from limiting current experiments revealed its dependence on the inlet relative humidity of the reactants. The local oxygen transport resistance was found to drop from 5.93 ± 3.16 s/cm to 3.43 ± 1.67 s/cm as the humidity increased from 50% to 90%. Hydrogen polymer electrolyte fuel cells (PEFCs) are a zero emission and efficient energy conversion technology for transportation, stationary and electronic applications. The current generation of fuel cell vehicles provide quick start-up, long-range and increased durability. The high and unstable cost of platinum (Pt), which is the commonly used catalyst, however hinders its commercial prospects, especially when compared to the internal combustion engine. The cost of Pt is responsible for over 34% of the fuel cell stack cost.1 Less than 10-20% of the catalyst is however estimated to be utilized during the operation of a conventional catalyst layer (CL) due to mass and charge transport limitations.2 Re-designing the CL to improve these transport limitations has the potential to achieve better Pt utilization, i.e., generate more current per gram of catalyst. The CL fabrication process governs its utilization efficiency, microstructure and Pt loading in the electrodes.The effect of Pt loading on electrodes manufactured by spraying, using film applicators, sputtering as well as inkjet printing has been studied in the literature. [3][4][5][6][7][8][9][10][11][12][13][14][15] Results show that fuel cell performance is severely degraded at low Pt loading, 3,6,12,16 and that an optimal value of Pt loading exists above which the performance gains are not very significant. 4,12 The reason for the reduced performance of low loading electrodes has thus far mainly been attributed to a high oxygen mass transport resistance at the reaction site. 12,[17][18][19][20][21][22] The source of this resistance is still under debate. Oxygen dissolution in the ionomer, 23 densification of the ionomer layer near the ionomer/Pt interface, 24 and catalyst/oxygen interactions at the catalyst site 20 have been proposed as causes for this local mass transport barrier. These results indicate that in order to understand the performance degradation of low loading electrodes, and to comp...