SCIENCE and TECHNOLOGIES applied to

MANUFACTURED NANOMATERIALS at

ENEA

Printed for the OECD Working Party on Manufactured Nanomaterials (WPMN), Workshop and face-to-face meeting. Organised by the SG9 Project on Environmentally Sustainable Use of Manufactured Nanomaterials, hosted by ENEA. Rome 14-15 September 2011. www.enea.it

Nanomaterials for Polymer Electrolyte Fuel Cells Polymer electrolyte fuel cells (PEFC) are non-polluting and efficient energy conversion devices that are expected to play a dominant role in future energy solutions. They attain the direct conversion of the chemical energy of a fuel into electricity by the hydrogen reaction with oxygen over a proper catalyst with water as its only by-product. PEFC operate with a polymer electrolyte membrane, a material that allows charged molecules or ions to move through it, that separates the fuel (hydrogen) from the oxidant (air or oxygen).

The catalyst normally employed consists of Pt and Pt-alloy nanoparticles, while the catalyst supports are usually made of high specific surface carbons such as Vulcan carbon, ordered porous carbon, hollow graphitic particles, etc. Current PEFC still face significant technological critical points which must be overcome before becoming economically viable. The high cost of platinum, together with its limited reserves in nature, has been shown to be the major drawback to mass

market fuel cell for commercial applications. Moreover, PEFC suffer from insufficient performance stability, due mainly to catalyst oxidation, migration and CO induced poisoning, loss of electrode active surface area and corrosion of the carbon support. In the following, an overview will be given on the applications of carbon nanomaterials as electrocatalyst support for PEFC and on the replacement of traditional chemical reduction method for catalyst dispersion with electrodeposition (ELD) and sputtering deposition (PVD). Both these techniques offer the advantage of easy catalyst preparation (absence of reducing and de-flocculating agents, no heat treatment in hydrogen) and easy transfer of the process to the industrial level. Nanomaterials can hold new prospects in the effort to increase the catalyst activity and utilization. Nanostructured materials are intrinsically materials with very high specific surface, which is a basic requirement for enhanced catalytic activity. Recently, the use of carbon nanotubes (CNT) has been proposed to this purpose but it has been reported that CNT has relative small specific surface areas and weak interactions with the supported metals. Carbon nanofibers (CNF) with larger specific surface area and more available edge atoms are thought to be more efficient from this point of view. CNF can have different morphology depending on the angle of graphene layers with respect to the fiber axis. Graphene layers are perpendicular to the filament axis in platelet CNF and parallel to the fiber axis in the tubular CNF. Since the properties of graphite are anisotropic, its orientation strongly affects the resulting material properties, thus exercising control over the morphology and layers stacking of CNF is of considerable importance for

Printed for the OECD Working Party on Manufactured NanomateOrganised by the SG9 Project on Environmentally Sustainable Use of Manufactured NanomaterialsRome 14-15 September 2011.

applications. Depending on the morphology, CNF can exhibit laand more available edge atoms than CNT. CNF with different controlled morphology have been grown on carbon paper by methane decomposition over Ni clusters through plasma enhanced CVD. In conventionally prepared electrusing the impregnation-reduction method, where the reduction is chemically performed. The minimum platinum load obtained by this method is 0.1 mg cmvalue still raises the cost of the fuel cell production. In the last years, to reduce the platinum load and increasing at the same time the utilization, techniques able to localize the metal on the surface of the electrode have been employed. By electrodeposition metal particles are deposited by means of an electric field that ensures the catalyst will be located only in regions that have access to electrons and protons. With this method the catalyst loading has been highly reduced. More recently sputter deposition technique has been examined as a means to obtain ultracatalyzed with this technique have shown electrochemical performance comparable with that of the conventionally prepared catalyst, combined with an outstanding decrease of the Pt load down to 0.06 mg cm-2. A considerable reduction of the catalyst load has been also achieved by the electrodeposition of Pt clusters onto CNF. Moreover, it was found that electrochemical performance and stability of Pt/CNF electrodes was strongly resheets arrangement. The platelet CNF showed the highest activity attributed to the higher ratio of edge to basal atoms and, as a consequence, larger number of active sites for electron transfer, a very large working surface area, a mosmaller size and a higher performance for the methanol oxidation reaction. Therefore, the platelet CNF are good candidate for catalyst support in PEFC with better performances with respect to commercial supports. An additional problem hindering the real penetration of PEFC into the market is the CO induced poisoning of the platinum, which implies the need of high platinum load in order to obtain reasonable performances. The development of more tolerant catalysts is required and the research is focused on the development of binary alloys, where the second component is thought to inhibit the CO adsorption on the catalyst surface. To this aim, electrodes have been catalyzed with sputtered PtAu clusters. The catalyst localized only on the uppermost surface, combining low loading levels with higher electrochemical performance and stability compared to PtAu deposited onto commercial carbon supports.

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applications. Depending on the morphology, CNF can exhibit larger specific surface area and more available edge atoms than CNT. CNF with different controlled morphology have been grown on carbon paper by methane decomposition over Ni clusters through plasma

In conventionally prepared electrodes, Pt is usually deposited onto the carbon support reduction method, where the reduction is chemically performed.

The minimum platinum load obtained by this method is 0.1 mg cm-2. Even though low, this of the fuel cell production. In the last years, to reduce the

platinum load and increasing at the same time the utilization, techniques able to localize the metal on the surface of the electrode have been employed. By electrodeposition metal particles are deposited by means of an electric field that ensures the catalyst will be located only in regions that have access to electrons and protons. With this method the catalyst loading has been highly reduced. More recently sputter deposition technique

examined as a means to obtain ultra-low levels of catalyst loading. Electrodes catalyzed with this technique have shown electrochemical performance comparable with

conventionally prepared catalyst, combined with an outstanding decrease of the .

A considerable reduction of the catalyst load has been also achieved by the electrodeposition of Pt clusters onto CNF. Moreover, it was found that electrochemical performance and stability of Pt/CNF electrodes was strongly related to the graphene

The platelet CNF showed the highest activity attributed to the higher ratio of edge to basal atoms and, as a consequence, larger number of active sites for electron transfer, a very large working surface area, a more uniform dispersion of Pt particles with smaller size and a higher performance for the methanol oxidation reaction. Therefore, the platelet CNF are good candidate for catalyst support in PEFC with better performances with respect to commercial supports.

An additional problem hindering the real penetration of PEFC into the market is the CO induced poisoning of the platinum, which implies the need of high platinum load in order to obtain reasonable performances. The development of more tolerant

is required and the research is focused on the development of binary alloys, where the second component is thought to inhibit the CO adsorption on the catalyst surface. To this aim, electrodes have been catalyzed with sputtered PtAu clusters. The catalyst resulted localized only on the uppermost surface, combining low loading levels with higher electrochemical performance and stability compared to PtAu deposited onto commercial carbon supports.

MANUFACTURED NANOMATERIALS

rials (WPMN), Workshop and face-to-face meeting. by the SG9 Project on Environmentally Sustainable Use of Manufactured Nanomaterials, hosted by ENEA.

www.enea.it

rger specific surface area and more available edge atoms than CNT. CNF with different controlled morphology have been grown on carbon paper by methane decomposition over Ni clusters through plasma

odes, Pt is usually deposited onto the carbon support reduction method, where the reduction is chemically performed.

. Even though low, this of the fuel cell production. In the last years, to reduce the

the catalyst loading has been highly reduced. More recently sputter deposition technique low levels of catalyst loading. Electrodes

catalyzed with this technique have shown electrochemical performance comparable with conventionally prepared catalyst, combined with an outstanding decrease of the

A considerable reduction of the catalyst load has been also achieved by the electrodeposition of Pt clusters onto CNF. Moreover, it was found that electrochemical

lated to the graphene The platelet CNF showed the highest activity attributed to the higher

ratio of edge to basal atoms and, as a consequence, larger number of active sites for electron re uniform dispersion of Pt particles with

smaller size and a higher performance for the methanol oxidation reaction. Therefore, the platelet CNF are good candidate for catalyst support in PEFC with better performances with

An additional problem hindering the real penetration of PEFC into the market is the CO induced poisoning of the platinum, which implies the need of high platinum load in order to obtain reasonable performances. The development of more tolerant

is required and the research is focused on the development of binary alloys, where the second component is thought to inhibit the CO adsorption on the catalyst surface. To this aim, electrodes have

resulted localized only on the uppermost surface, combining low loading levels with higher electrochemical performance and stability

100 nm

SCIENCE and TECHNOLOGIES applied to

MANUFACTURED NANOMATERIALS at

ENEA

Printed for the OECD Working Party on Manufactured Nanomaterials (WPMN), Workshop and face-to-face meeting. Organised by the SG9 Project on Environmentally Sustainable Use of Manufactured Nanomaterials, hosted by ENEA. Rome 14-15 September 2011. www.enea.it

ENEA personnel involved in the research activity: Rossella Giorgi*, Elena Salernitano*, Theodoros Dikonimos*, Serena Gagliardi*, Nicola Lisi*, Marco Alvisi**, Maria Federica de Riccardis** * ENEA-UTTMAT-SUP, C.R. Casaccia, via Anguillarese 301 00123 Roma, Italy ** ENEA-UTMATB, C:R: Brindisi, S.S. 7 Appia km 706, 72100 Brindisi, Italy Contact person: rossella.giorgi@enea.it, tel. +39 06 30484754 References: [1] L. Giorgi, R. Giorgi, S. Gagliardi, E. Salernitano, Th. Dikonimos, N. Lisi, M.F. De Riccardis, M. Alvisi, Pt alloys on carbon nanostructures as electrocatalysts for direct methanol fuel cell, Advances in Science and Technology, 2010 (72) 277 [2] L. Giorgi, E. Salernitano, S. Gagliardi, Th. Dikonimos, R. Giorgi, N. Lisi, M.F. De Riccardis, V. Martina, Electrocatalysts for methanol oxidation based on Platinum/Carbon Nanofibers Nanocomposite, Journal of Nanoscience and Nanotechnology, 2011 (11) (in press) [3] R. Giorgi, L. Giorgi, S. Gagliardi, E. Salernitano, M. Alvisi, Th. Dikonimos, N. Lisi, D. Valerini, M.F. De Riccardis, E. Serra, Nanomaterials-based PEM electrodes by combining chemical and physical depositions, Journal of Fuel Cells Science and Technology, 2011 (8) 041004 [4] L. Giorgi, R. Giorgi, S. Gagliardi, E. Serra, M. Alvisi, M.A. Signore, E. Piscopiello, Platinum-Gold nanoclusters as catalyst for direct methanol fuel cells, Journal of Nanoscience and Nanotechnology, 2011 (11) (in press)