Submitted, accepted and published in Applied Catalysis B: Environmental 147 (2014) 947-957.
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1
Towards an optimal synthesis route for the preparation of
highly mesoporous carbon xerogel-supported Pt catalysts for
the oxygen reduction reaction
C. Alegre
1,2
, M.E. Gálvez
1
, R. Moliner
1
, V. Baglio
2,*
, A.S. Aricò
2
and M.J. Lázaro
1,*
1
Instituto de Carboquímica (CSIC), Miguel Luesma Castán 4, 50018 Zaragoza, Spain
2
Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano”, CNR.
Via Salita S. Lucia sopra Contesse, 5, 98126, Messina (Italia)
*Corresponding authors:
María Jesús Lázaro, Instituto de Carboquímica, CSIC, Miguel Luesma Castán 4,
50018, Zaragoza, Spain. Fax: +34 976733318; Tel: +34 976733977; E-mail:
mlazaro@icb.csic.es;
Vincenzo Baglio, Istituto di Tecnologie Avanzate per l’Energia “Nicola Giordano”,
CNR, Via Salita S. Lucia Sopra Contesse 5, Messina, Italy. Fax: +39 090 624247; Tel:
+39 090 624237; vincenzo.baglio@itae.cnr.it;
Submitted, accepted and published in Applied Catalysis B: Environmental 147 (2014) 947-957.
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Abstract: Pt particles were supported on a highly mesoporous carbon xerogel and used
1
as catalysts for the oxygen reduction reaction (ORR) in Direct Methanol Fuel Cells
2
(DMFCs). Different synthesis routes were followed in order to study their influence on
3
the characteristics and the performance of Pt electrocatalysts, therefore determining the
4
optimal synthesis method for the preparation of these carbon xerogel supported
5
catalysts, leading to the highest catalytic activity. The highest active catalyst was
6
compared to a Pt catalyst supported on commercial carbon support, Vulcan, synthesized
7
in the same conditions. Synthesis methods studied were impregnation, following two
8
different reduction protocols (sodium borohydride and formic acid), and microemulsion,
9
used for the first time for carbon xerogels. The electrochemical characterization proved
10
that the catalysts’ synthesis method strongly influenced the catalytic behavior. The
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impregnation method and reduction with formic acid lead to the highest active catalyst
12
towards ORR. When compared to an analogously prepared Vulcan carbon black-
13
supported catalyst, the carbon xerogel-based one still showed enhanced performance, in
14
spite of higher ohmic loss, due to the lower electrical conductivity of this carbon
15
material.
16
17
Keywords: Pt; catalysts; carbon xerogel; synthesis method; ORR.
18
19
Submitted, accepted and published in Applied Catalysis B: Environmental 147 (2014) 947-957.
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1. Introduction
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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 [1]. Due to their intrinsic activity and stability in acidic solutions,
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Pt/C electrocatalysts are, at present, the most widely used materials as cathodes [2] in
25
proton conducting electrolyte-based low temperature fuel cells, such as direct methanol
26
fuel cells (DMFCs), operating with methanol, a liquid fuel presenting great advantages
27
in terms of handling [3]. Nevertheless, there is still great interest in developing more
28
active, selective and less costly electrocatalysts for the oxygen reduction reaction (ORR)
29
[4]. Considerable efforts are being thus made whether to obtain Pt-free catalysts [5],
30
whether to improve the electrocatalytic performance of Pt catalysts [6]. Regarding the
31
first issue, several papers have been published in the last years describing the use of
32
carbon–supported iron-based catalysts with active sites containing iron cations
33
coordinated by pyridinic nitrogen functionalities, which show similar activities to those
34
prepared using Pt [7-9]. On the other hand, efforts to improve the electrocatalytic
35
performance of Pt catalysts have focused on improving catalytic effectiveness of Pt by
36
dispersing catalyst materials onto an electrode support with high surface area and
37
conducting properties, such as carbon materials and metallic oxides [10-14], or by
38
synthesizing Pt and Pt-based advanced nanomaterials [14], such as core-shell catalysts
39
[15-17].
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Regarding the use of carbon supports, it is generally recognized that a high Pt wt.
41
% on the carbon substrate will significantly decrease the thickness for the same Pt
42
loading per geometric electrode area. Thus, it is possible to enhance mass transport
43
through the electrode and, at the same time, considerably reduce the ohmic losses. The
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synthesis of a highly dispersed electrocatalyst phase in conjunction with a high metal
45
loading on carbon support is one of the goals of the recent activity in the field of
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DMFCs [18]. In this sense, one of the main requirements for an optimal electrocatalyst
47
is its high dispersion. The mass activity (A g
-1
) of the catalyst for an electrochemical
48
reaction is directly related to the degree of dispersion, since the reaction rate is generally
49
proportional to the active surface area [3].
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Submitted, accepted and published in Applied Catalysis B: Environmental 147 (2014) 947-957.
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The success obtaining a highly dispersed catalyst will depend both on the
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synthesis method and the support. Among the main synthesis routes for the preparation
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of Pt/carbon electrocatalysts one can find impregnation, colloidal procedures, self-
53
assembling and Pt decoration methods [3, 19- 21]. The strong influence of the catalyst
54
preparation procedure on its properties makes it necessary to optimize each synthesis
55
method, taking into consideration the particular characteristics of the support, in order
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to obtain a properly dispersed active phase, with the most appropriate crystal size and
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chemical state.
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With respect to the carbon material, and among the various types of carbon
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supports considered in the last decades, carbon xerogels have been extensively studied
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and successfully employed in electrochemical applications, due to their unique and
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easily controllable properties [22, 23]. These materials offer high surface area,
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mesopore structure with tunable pore size distribution, and high purity. Besides, when
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used as catalysts supports, their three-dimensionally interconnected uniform pore
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structure allows a high degree of dispersion of the active phase and an efficient
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diffusion of reagents [24]. Moreover, several authors have described enhanced
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performance of catalysts supported on carbon xerogels when compared to conventional
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supports, such as Vulcan carbon black [25-28].
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However, in order to further increase the efficiency of carbon xerogel supported
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Pt catalysts, it is necessary to develop a simple procedure to obtain Pt catalysts with
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relatively high metal loading and optimal dispersion [29]. In this work, several synthesis
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methods have been considered with the aim of determining the optimal procedure to
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prepare highly active Pt catalysts supported on a highly mesoporous carbon xerogel. As
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a first screening, a metal loading of a 20 wt. % has been chosen to analyzed the optimal
74
method, in order to carry on in the future with higher Pt loadings, allowing enhancing
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mass transport through the electrode and, at the same time, reducing the ohmic drop.
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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.
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Submitted, accepted and published in Applied Catalysis B: Environmental 147 (2014) 947-957.
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2. Experimental details
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2.1. Synthesis of the carbon xerogel
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CXG was synthesized as described in [31] by the pyrolysis at 800 ºC of an organic
84
gel obtained by the polycondensation of resorcinol and formaldehyde in stoichiometric
85
ratio (2 moles of formaldehyde per mole of resorcinol). The gelation and curing process
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took place at an initial pH of 6.0 and using sodium carbonate as catalyst (0.04 mol %
87
with respect to total content of resorcinol + formaldehyde). Curing of the organic gel
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was carried out for 24 h at room temperature, 24 h at 50 ºC and 120 h at 85 ºC.
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Subsequently, remaining water was exchanged with acetone and the gel was dried under
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subcritical conditions before its pyrolysis. Pyrolysis took place at 800 ºC under a
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nitrogen atmosphere for 3 h.
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2.2. Synthesis of the Pt-catalysts
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Pt catalysts with a 20 wt. % loading were synthesized using CXG as support, by
95
means of different synthesis methods. Synthesis routes included: impregnation and
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reduction with two different reducing agents: sodium borohydride (i-SBM) and formic
97
acid (i-FAM) and a microemulsion based method (ME).
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For the impregnation method and reduction with NaBH
4
, a 3 mM aqueous
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solution of H
2
PtCl
6
(Sigma-Aldrich), was slowly added to a dispersion of the CXG in
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ultrapure water under sonication. pH was adjusted to 5 with a NaOH (Panreac) solution,
101
followed by addition of a 25 mM aqueous solution of NaBH
4
(Sigma-Aldrich),
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maintaining the temperature around 18ºC. The catalyst so obtained was named Pt/CXG-
103
i-SBM.
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In the case of using formic acid as reducing agent, the carbon material was first
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dispersed in a 2 M HCOOH (Panreac) solution at 80 ºC. Subsequently, a 4mM aqueous
106
solution of the metallic precursor, H
2
PtCl
6
(Sigma-Aldrich), was added stepwise. The
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catalyst obtained in such a way was named Pt/CXG-i-FAM. Finally both type of
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catalysts were filtered and thoroughly washed with ultrapure water, and dried overnight
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at 60 ºC.
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Pt nanoparticles were also synthesized by the water in oil microemulsion route (ME)
111
[31], that consists of preparing a microemulsion composed of a commercial surfactant
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