Comparative analysis between mass and volume of catalysts as a criterion to determine the optimal quantity of Nafion ionomer in catalyst layers

Highlights

• Evaluation of catalyst mass and volume as criterion to adjust ionomer quantity in catalyst layer.

• Catalyst volume as criterion to determine the quantity of Nafion ionomer in catalyst layers.

• Ionomer quantity, catalyst layer thickness, and their effects in MEAs performances.

Abstract

Studies in the proton exchange membrane fuel cell (PEMFC) have evaluated different catalyst systems, using fixed mass percentages as the criterion to prepare catalyst layers. Some studies presented masses between 20 and 40% of ionomer in the catalyst layer as best composition; however, they also showed that any modification in the catalyst structure, such as support material or metal percentage, changes remarkably the membrane electrode assembly (MEA) performances. Thus, the volume of a catalyst used changes the amount of Nafion ionomer required to prepare catalyst layers with high efficiency. Consequently, to compare different catalysts in their highest performance conditions, it has become necessary to develop a volumetric criterion to calculate the quantity of Nafion ionomer necessary for each catalyst. In this work, the masses and the volumes of catalysts were compared to three other catalysts as the criterion to transpose the adjustment of the catalyst layer composition made to Pd/C 20%. The use of catalyst volume as the criterion to calculate the quantity of Nafion ionomer in the catalyst layer resulted in MEAs significantly more efficient than those prepared according to the fixed mass percentage.

Introduction

Membrane electrode assemblies (MEAs) are structures in which electro-chemical reactions occur between a fuel and an oxidant to generate electrical and thermal energy in a proton exchange membrane fuel cell (PEMFC) [1], [2]. Over the years, studies have been performed to make the PEMFC systems surpass their demands of efficiency, durability, and cost [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], [73], [74], [75], [76], [77], [78], [79], [80], [81], [82]. Many of these studies [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35] were conducted using the platinum commercial catalyst with 20 percent of metal mass and supported on Vulcan XC72 (Pt/C 20%) sold by BASF (previously by E-tek [28]). Some of these studies have evaluated the relationship between the amount of Nafion ionomer applied to the catalyst layer of MEAs and the respective performance obtained [3], [4], [5], [6], [7], [8], [9], [10] and found some criteria to formulate efficient electrodes, such as ratios between the masses of catalyst and Nafion ionomer [3], [4], [5], [6], [7], ratios between the masses of support material and Nafion ionomer [8] and ratios between the masses of platinum and Nafion ionomer [9], [10].

The formulations obtained in these studies resulted in values between 20 and 40 percent of Nafion ionomer (%NI) in relation to the total masses of the catalyst layers as the best setting for electrodes containing values between 0.05 and 0.50 mg of platinum per square centimeter (mgPt·cm⁻²) [3], [4], [5], [6], [7], [8], [9], [10]. However, the same studies found that changes in the mass of catalyst applied to the electrodes [9] and changes in the ratio between metal and support [8] modified the required masses of Nafion ionomer which resulted in higher performances. On the other hand, a recent study [83] verified that catalyst with the same support material (carbon) and the same metal loading (20%) can present different volumes due to modifications induced by heat treatment in the support material. It was also shown that these different volumes significantly modify the amount of ionomer needed to prepare catalyst layers with high performance, i.e., the formation of triple phase boundaries with high efficiency [83]. From this conclusion, it is expected that a volumetric criterion should be developed to determine the amount of ionomer to be used in the preparation of catalyst layers for different catalysts in order to make comparisons between them at each one's conditions of greatest efficiency [83].

In this present work, a volumetric criterion to transpose the better adjustment of ionomer quantity in the catalyst layer obtained from Pd/C 20% [12] to three catalysts with other compositions was proposed and evaluated. To verify the efficiency of this criterion without the interference of other factors, MEAs were prepared with catalysts using the same metal and the same support material. Thus, the differences between the catalysts were only the use of metal masses of 10, 20, 40, and 60%. For comparison, electrodes prepared with ionomer quantity determined according to the fixed mass percentage criterion were also evaluated in this study.

In the related previous works [12], [34] were performed the comparison between platinum and palladium electrodes, and it was verified that palladium catalyst using carbon Vulcan XC72 as support materials (Pd/C 20%) presented it best performance in catalyst layers with 59%NI [12]. Recently [83], it was verified that this ionomer quantity (59%NI) is higher than usual because the carbon used as support material in these studies is less dense than the carbon used in the commercial catalyst.