Electrochimica Acta 54 (2009) 1954–1960 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Electrocatalytic reduction of oxygen at electropolymerized films of metalloporphyrins deposited onto multi-walled carbon nanotubes Ayodele Okunola a,1 , Barbara Kowalewska b,1 , Michael Bron a,1 , Pawel J. Kulesza b,1 , Wolfgang Schuhmann a,∗,1 a Anal. Chem. – Elektroanalytik & Sensorik, Ruhr-Universität Bochum, D-44780 Bochum, Germany b Department of Chemistry, University of Warsaw, Pasteura 1, PL-02-093 Warsaw, Poland article info Article history: Received 14 May 2008 Received in revised form 16 July 2008 Accepted 23 July 2008 Available online 6 August 2008 Keywords: Electropolymerized films Electrocatalytic activity Electrodeposition Electrocatalysts Metalloporphyrins Oxygen reduction abstract The electrocatalytic reduction of oxygen at electropolymerized films of a number of different manganese, iron and cobalt porphyrins supported on multi-walled carbon nanotubes (MW-CNTs) which have been pre-stabilized with ultra-thin layers of organic 4-(pyrrole-1-yl) benzoic acid (PyBA) is reported. Special emphasis has been on the multiple oxidation states of manganese as central metal ion and its potential advantages for the electron transfer process during the reduction of molecular oxygen in a 0.1M phos- phate buffer solution. Electropolymerization of metal porphyrins leads to catalytically active films on the electrode. The incorporation of MW-CNTs in the film leads to a significant decrease in the hydrogen per- oxide produced during the oxygen reduction reaction (ORR) and a significant positive shift of the oxygen reduction potential. Of the complexes studied, manganese tetratolyl porphyrin (MnTTP) has shown the best activity and stability towards oxygen reduction. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction There are many examples in nature demonstrating the inter- esting ability of macrocyclic organic N 4 -complexes to catalyze redox reactions involving molecules such as O 2 ,H 2 , and N 2 . These include reactions found in enzymatic systems such as sulfite reduc- tase [1], nitrate reductase, cytochrome c oxidase [2], blue copper oxidases, pseudocatalase [3], photosystem II [4], nitrogenase and hydrogenase [5]. In each case, the enzyme contains one or more metal atoms at the active site and in many cases the enzyme may also include active peripheral metal sites. N 4 -macrocyclic complexes containing first row transition metals have also been extensively applied for electrocatalytic redox reactions. Of these macrocycles, complexes of cobalt and iron have been mostly applied as reduction electrocatalysts [6], especially for oxygen reduction. Interests in the oxygen reduction electrocatalysis of macrocyclic organometallic complexes date back to at least four decades. In 1964, Jasinski reported the electrocatalytic oxygen reduction with cobalt phthalocyanine adsorbed on carbon and nickel electrodes ∗ Corresponding author. Tel.: +49 234 32 26200; fax: +49 234 32 14683. E-mail address: wolfgang.schuhmann@rub.de (W. Schuhmann). 1 ISE member. [7,8] which has inspired interests in the electrocatalytic properties of other macrocyclic organic N 4 -complexes such as metal por- phyrins and metal phthalocyanines containing a large variety of different transition metal atoms. These complexes were consid- ered promising as cathode electrocatalysts for fuel cells as they are relatively inexpensive, exhibit considerable activity in various het- erogeneous redox reactions and are thermally stable [6]. Co and Fe containing metalloporphyrins have been investigated as hybrids in combination with carbon nanotubes [9,10], methanol tolerant electrocatalysts [11], in polyaniline complexes [12] or as compos- ite catalysts along with other transition metal oxides [13]. While metalloporphyrins containing Co and Fe as central metal ions have been investigated in greater detail, especially for two- and four- electron oxygen reduction reactions (ORR), only a few reports on manganese-containing N 4 -macrocyclic complexes with respect to their role in oxygen reduction reaction [14,15] have been pub- lished. However, Mn complexes are known to catalyze ORR under physiological conditions [16]. Nature utilizes a Mn complex as a catalyst to facilitate oxygen evolution in green plant photosynthe- sis [17]. This oxygen-evolving step in the natural process can be described as the chlorophyll-photosensitized oxidation of water to O 2 requiring a four-electron exchange. Thus, presuming that radical intermediates are to be avoided, the Mn complex in its specific environment must be able to store up to four electrons intermediately. In addition, manganese-containing N 4 -macrocyclic 0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2008.07.077