Electrooxidation of formic acid on carbon supported PtxPd1−x (x = 0–1) nanocatalysts
Introduction
Formic acid is a potential liquid fuel for the direct formic acid fuel cell (DFAFC) application due to its high theoretical open circuit potential (1.45 V) and low fuel crossover resulted from the repulsion between the partially dissociated form of HCOOH (formate anion) and sulfonic groups in Nafion® membrane [1], [2], [3], [4], [5].
The research on the electrocatalysts for formic acid oxidation is crucial for the development of DFAFC. In the past several decades, many well-defined model catalysts, such as single crystal Pt [6], [7], [8], [9], polycrystalline Pt [10], [11], [12], [13], Pt-based bimetallic electrodes [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25] and Pt-group metals [10], [26], [27] have been investigated. A “dual path” mechanism, involving a “direct path” and a “CO path”, is widely accepted for formic acid oxidation [28], [29]. In the “direct path”, HCOOH directly dehydrogenates to form CO2 via one or more active intermediates. The “CO path” involves the dehydration of HCOOH to CO which, depending on the applied potential, may poison the electrode or be further oxidized to produce CO2.
Among those model catalysts investigated, Pt modified with Pd [19], [20], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], Bi [15], [16], [18], [19], [21], [40], [41], [42] and Sb [15], [18], [42], [43], [44], [45], [46], [47], which can reduce the catalyst poisoning and/or increase the catalytic activity, shows good performance. In particular in Pt/Pd, Pd not only exhibits activity for formic acid oxidation, but also provides a synergistic electronic effect to Pt. This effect orients Pt/Pd in favor of formic acid oxidation via a “direct path”. The catalytic enhancement of Pt by Bi was interpreted either via an electronic effect (Pt(1 1 1)), or third body effect (Pt(1 0 0)), or via the combination of both, dependent on the surface structure of Pt electrodes. Bi could also demonstrate the catalytic activity for formic acid oxidation. Sb modified Pt electrodes show similar catalytic effects as the Bi counterparts. However, the poisoning in the Sb modified electrodes cannot be totally eliminated.
Like the Pt/C and PtRu/C, the state of the art catalysts of PEMFCs and DMFCs, high-dispersion catalysts supported on high surface area carbon black are necessary for the DFAFC development by improving the utilization efficiency and reducing the cost of the catalysts [48]. So far, to the best of our knowledge, very few studies have been published in the direction of synthesis and investigation of carbon supported catalysts for formic acid oxidation [49].
In this study, we synthesized PtxPd1−x/C (x = 0–1) catalysts by a surfactant-stabilized method previously developed in our group [50], [51], and compared the performance of the catalysts with the corresponding commercial one. Furthermore, we investigated the influence of Pt/Pd deposition sequence and atomic ratio on the catalytic properties of PtxPd1−x/C for formic acid oxidation.
Section snippets
Synthesis of PtxPd1−x/C (x = 0–1) catalysts
PtxPd1−x catalysts, supported on Vulcan XC-72 carbon with 20 wt% metal loading, were synthesized with a surfactant-stabilized method. The Pt/Pd ratio in the catalysts was controlled by the initial amount of Pt and Pd precursors. Chloroplatinic acid (H2PtCl6) and ammonium chloropalladate ((NH4)2PdCl6) were used as the precursors of Pt and Pd, respectively. The zwitterionic surfactant 3-(N,N-dimethyldodecylammonio) propanesulfonate (SB12, linear formula: CH3(CH2)11N(CH3)2(CH2)3SO3), was used as
Comparison of SB12-stabilized Pt0.50Pd0.50/C with the commercial catalyst
Fig. 1, Fig. 2 show the typical TEM images and the particle size distribution histograms of SB12-stabilized (SS) Pt0.50Pd0.50/C and the commercial (E-TEK) Pt0.50Pd0.50/C. It is evident that Pt/Pd dispersion in SS catalyst is better than that of E-TEK catalyst. The width of the size distribution of the former is smaller compared to the latter. And the average size of SS catalyst is 2.7 nm, which is 40% lower of the 4.5 nm E-TEK catalyst. It is noted here that other SB12-stabilized catalysts also
Conclusions
PtxPd1−x/C (x = 0–1) catalysts with higher electrochemical surface area and catalytic performance for formic acid oxidation than the commercial catalysts were prepared by the SB12-stabilized method. Physicochemical and electrochemical characterizations show that co-deposited PtxPd1−x/C has higher catalytic activity than the sequential-deposition catalysts, due to a probable synergistic effect between Pt and Pd. Furthermore, it was revealed that at a lower potential, formic acid oxidation current
Acknowledgement
The authors gratefully acknowledge the Innovation and Technology Commission of Hong Kong SAR Government (ITS/176/01A and ITS/069/02) for the funding support.
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