1 Quasi-in-Situ Single-Grain Photoelectron Microspectroscopy of Co/ 2 PPy Nanocomposites under Oxygen Reduction Reaction 3 Patrizia Bocchetta, † Matteo Amati, ‡ Benedetto Bozzini,* ,† Massimo Catalano, § Alessandra Gianoncelli, ‡ 4 Luca Gregoratti, ‡ Antonietta Taurino, § and Maya Kiskinova ‡ 5 † Dipartimento di Ingegneria dell’Innovazione, Universita ̀ del Salento, via Monteroni, 73100 Lecce, Italy 6 ‡ Elettra-Sincrotrone Trieste S.C.p.A., Strada Statale 14, km 163.5 in Area Science Park, 34012 Basovizza, Trieste, Italy 7 § Institute for Microelectronics and Microsystems, IMM-CNR, via Monteroni, 73100 Lecce, Italy 8 * S Supporting Information 9 ABSTRACT: This paper reports an investigation into the aging of 10 pyrolyzed cobalt/polypyrrole (Co/PPy) oxygen reduction reaction 11 (ORR) electrocatalysts, based on quasi-in-situ photoelectron 12 microspectroscopy. The catalyst precursor was prepared by 13 potentiostatic reverse-pulse coelectrodeposition from an acetonitrile 14 solution on graphite. Accelerated aging was obtained by quasi-in- 15 situ voltammetric cycling in an acidic electrolyte. Using photo- 16 electron imaging and microspectroscopy of single Co/PPy grains at 17 a resolution of 100 nm, we tracked the ORR-induced changes in the 18 morphology and chemical state of the pristine material, consisting of 19 uniformly distributed ∼20 nm nanoparticles, initially consisting of a 20 mixture of Co(II) and Co(III) oxidation states in almost equal 21 amounts. The evolution of the Co 2p, O 1s, and N 1s spectra 22 revealed that the main effects of aging are a gradual loss of the Co present at the surface and the reduction of Co(III) to Co(II), 23 accompanied by the emergence and growth of a N 1s signal, corresponding to electrocatalytically active C-N sites. 24 KEYWORDS: cobalt, polypyrrole, oxygen reduction, electrocatalyst, nanocomposites, photoelectron microscopy, 25 X-ray photoelectron spectroscopy 1. INTRODUCTION 26 Nonprecious metal-nitrogen-carbon (M/N/C) catalysts have 27 been extensively studied as possible substitutes of Pt-based 28 materials for air cathodes, since the discovery of the 29 electrochemical activity of cobalt phthalocyanine and of 30 transition metal macrocycles, in general, toward the oxygen 31 reduction reaction (ORR). 1 Intense research activities were 32 dedicated to the fabrication of pyrolyzed transition metal 33 macrocycles 2-5 and to the identification of the catalytic centers 34 in the O 2 reduction mechanism. 6-11 M/N/C-based materials, 35 synthesized by employing a multiplicity of metal, nitrogen, and 36 carbon precursors, have been reported to exhibit promising 37 electrocatalytic performance. 9,2,12-17 In particular, conjugated 38 heterocyclic conducting polymers have been used as convenient 39 nitrogen sources for the fabrication of M/N/C ORR 40 electrocatalysts. 18,19,7,20-24 Among them, polypyrrole (PPy) is 41 considered the most promising one 25 for the following reasons: 42 (i) high electrical conductivity, 26 (ii) excellent support 43 capability for catalysts such as metal oxides 27 and metal-based 44 complexes, 28,29 and (iii) easy chemical 30 and electrochemical 31 45 synthesis routes. Currently, the most common methods to 46 prepare Co/PPy electrocatalysts are (i) functionalization of 47 carbon nanoparticles by chemical polymerization of pyrrole, 48 followed by the chemical reduction or impregnation of a cobalt 49 salt into the polymer; 22,32-34 (ii) fluidized-bed electropolyme- 50 rization, 35 leading to the formation of a cobalt-modified PPy 51 film supported on carbon nanoparticles; (iii) spraying or 52 printing of carbon-supported Co/PPy incorporated into an ink; 53 and (iv) direct electrodeposition using anodic polarization to 54 electropolymerize pyrrole as a film and cathodic polarization to 55 deposit the metal into the polymer structure. 36-39 However, 56 the catalyst stability in acidic solution under oxygen reduction is 57 the main problem hindering the practical replacement of the 58 traditional Pt/C catalysts with M/N/C-based ones and has not 59 yet been thoroughly investigated. Heat treatments have been 60 found to be beneficial for electrodeposited polypyrrole-metal 61 compounds, and in particular, both catalytic activity and 62 durability have been found to increase drastically after 63 pyrolysis, 22,37,40-45 the enhanced stability being attributed to 64 the formation of active sites favoring the four-electron-transfer 65 ORR pathway, thus reducing the peroxide generation rate. 46,47 66 In the present paper we combine electrochemistry and quasi- 67 in-situ X-ray photoelectron microspectroscopy to shed light on 68 the morphology and composition of pyrolyzed Co/PPy Received: June 25, 2014 Accepted: November 4, 2014 Research Article www.acsami.org © XXXX American Chemical Society A dx.doi.org/10.1021/am504111s | ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX pubsdm_prod | ACSJCA | JCA10.0.1465/W Unicode | research.3f (R3.6.i5 HF05:4232 | 2.0 alpha 39) 2014/10/10 09:17:00 | PROD-JCAVA | rq_4107189 | 11/10/2014 01:44:36 | 9 | JCA-DEFAULT