Towards an optimal synthesis route for the preparation of highly mesoporous carbon xerogel-supported Pt catalysts for the oxygen reduction reaction
Abstract: Pt particles were supported on a highly mesoporous carbon xerogel and used as catalysts for the oxygen reduction reaction (ORR) in direct methanol fuel cells (DMFCs). Different synthesis routes were followed in order to study their influence on the characteristics and the performance of Pt electrocatalysts, therefore determining the optimal synthesis method for the preparation of these carbon xerogel supported catalysts, leading to the highest catalytic activity. The highest active catalyst was compared to a Pt catalyst supported on commercial carbon support, Vulcan, synthesized in the same conditions. Synthesis methods studied were impregnation, following two different reduction protocols (sodium borohydride and formic acid), and microemulsion, used for the first time for carbon xerogels. The electrochemical characterization proved that the catalysts’ synthesis method strongly influenced the catalytic behavior. The impregnation method and reduction with formic acid lead to the highest active catalyst towards ORR. When compared to an analogously prepared Vulcan carbon black-supported catalyst, the carbon xerogel-based one still showed enhanced performance, in spite of higher ohmic loss, due to the lower electrical conductivity of this carbon material.
Summary (1 min read)
1. Introduction 21
- The oxygen reduction process is a limiting step in the development of highly 22 efficient low temperature fuel cells, due to the large overpotential needed to achieve 23 high current densities .
- Considerable efforts are being thus made whether to obtain Pt-free catalysts , 30 whether to improve the electrocatalytic performance of Pt catalysts .
- Besides, when 63 used as catalysts supports, their three-dimensionally interconnected uniform pore 64 structure allows a high degree of dispersion of the active phase and an efficient 65 diffusion of reagents .
- 76 Upon the optimization of the synthesis method, the catalytic activity of the carbon 77 xerogel-supported electrocatalyst has been compared to that of an analogously prepared 78 carbon black-supported one, in order to analyze the particular influence of the nature of 79 the carbon support.
2.2. Synthesis of the Pt-catalysts 94
- Synthesis routes included: impregnation and 96 reduction with two different reducing agents: sodium borohydride (i-SBM) and formic 97 acid (i-FAM) and a microemulsion based method (ME).
- The corresponding histogram shows 2 peaks, one centered in sizes 266 about 3.2 nm, the other at about 4.5 nm, pointing to the presence of both isolated 267 particles of lower size together with some others which tend to agglomerate to a higher 268 extent.
- The catalyst 383 supported on CXG, prepared following the formic acid route, Pt/CXG-i-FAM, 384 evidences higher performance than the catalyst supported on the commercial support, 385 Pt/CB-Vulcan-i-FAM, prepared by using the same method at 30ºC, Figure 11a.
- A highly mesoporous carbon xerogel was used as the carbon support in the preparation of Pt-catalysts, following several synthesis routes: two impregnation methods using both sodium borohydride (SBM) and formic acid (FAM) as reducing agents, and a microemulsion method.
- The catalyst prepared through the formic acid impregnation method showed the lowest crystal size and slightly higher amount of reduced Pt on its surface.
- TEM analysis pointed to a better dispersion of the metallic particles in this case.
- Though polarization and power density curves evidenced a better performance of the carbon xerogel-based catalyst, impedance studies showed that ohmic losses are more important when using this material as support, than when using the commercially available carbon black.
- This can be due to the higher electrical conductivity of Vulcan, in comparison to the carbon xerogel.
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Q1. What contributions have the authors mentioned in the paper "Towards an optimal synthesis route for the preparation of highly mesoporous carbon xerogel-supported pt catalysts for the oxygen reduction reaction" ?
In this paper, a simple procedure to obtain Pt catalysts with high metal loading and optimal dispersion is proposed.