Preparation of electrocatalysts by reduction of precursors with sodium citrate † Stein Trygve Briskeby, a§ Mikhail Tsypkin, a‡ Reidar Tunold, a¶ and Svein Sunde a,b Received Xth XXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX First published on the web Xth XXXXXXXXXX 200X DOI: 10.1039/b000000x In this work synthesis of Pt/C catalysts by reduction of H 2 PtCl 6 with sodium citrate has been investigated. The strong pH– dependence of citrate as a reducing and stabilizing agent has been explored, and an optimum pH range for production of well dispersed catalysts is proposed. To achieve stabilizing and reducing conditions, the presence of both citrate anions and protonated citrates are required. This is achieved in an intermediate pH range between pK a2 and pK a3 (4.76 and 6.4) of citric acid, where both C 6 H 5 O 3– 7 (denoted CA 3– ) and C 6 H 7 O – 6 (denoted H 2 CA – ) are present. At pH 5.3–5.4 a catalyst with particles around 3 nm was thus successfully prepared. At high pH (∼12) the reduction of Pt is limited, whereas at low pH reduction is fast, but the stabilizing ability of the citrate in solution is poor resulting in large cubic Pt particles. CO-stripping voltammetry indicate that Pt(111) faces are the dominating crystal plane in the nanoparticles formed when citrate anions are used as stabilizing agent. This effect is presumably caused by the distance between oxygen groups in citrate correlating well with the Pt–Pt distance on (111) faces. 1 Introduction PEM fuel cells have over the last decades proven to be the chosen technology for conversion of chemical energy stored as hydrogen into electrical energy for e.g. vehicle propulsion. A major focus has been on increasing the lifetime of the PEM cells while at the same time reducing the amount of active materials, typically Pt or Pt- containing alloys. From early PEM fuel cell electrodes consisting of PTFE-bonded platinum black applied by hot-press to the ionomeric membrane, sub- stantial reduction in noble metal loading (20–40 times) has been achieved by the introduction of supported catalysts 1 . Typical support materials are carbon blacks like Vulcan XC72 with noble metal loadings ranging from 20–60 wt. % in or- der to achieve thin active electrode layers minimizing mass transport limitations and ohmic resistance 2,3 . Today’s state of the art PEMFC typically have noble metal loadings of 0.05– 0.1 mg cm -24 . Supported catalysts can be prepared by various methods in- cluding ion-exchange, homogenous deposition precipitation † Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/b000000x/ a Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway. b Fax: +47 7359 1105; Tel: +47 7359 4051; E-mail: svein.sunde@ntnu.no § Present address: Statoil ASA, Herøya Forskningspark, Hydroveien 67, 3936 Porsgrunn, Norway ‡ Present address: NEL Hydrogen AS, Heddalsvegen 11, 3674 Notodden, Norway ¶ Professor Reidar Tunold, 1933 – 2013 (HDP), impregnation and deposition of colloidal particles. 5,6 The former two methods are limited to low loading catalysts 2 . High loadings are readily achieved by impregnation. How- ever, it is challenging to achieve high loading and simultane- ously maintain high dispersion of Pt. 7 . Colloidal methods have proven able to achieve both load- ing and dispersion. Here, the noble metal precursor is chem- ically reduced in the presence of a protective agent. A narrow particle size distribution is achieved by stabilization of the nanoparticles by either steric hindrance or electro- static charges. Common protecting ligands include NR + 4 , triphenylphosphine (PPh 3 ), polyvinylpyrrolidone (PVP) and polyvinyl alcohol (PVA). One drawback with such methods, however, is the presence of the protective agents which can reduce the catalytic function of the nanoparticles. Removal of such agents can be done by washing in appropriate solvents or by thermal decomposition in inert atmosphere 5 . Routes which can omit additional stabilizers would be favourable, though. Such methods include the polyol method, where reduction and stabilization are achieved with ethylene glycol 8–11 . Citric acid is well known for its dual functions as a reduc- ing and stabilizing agent, and was used by Turkevich in the preparation of Pt nanoparticles in 1986 12 . Since then sev- eral researchers have used citrate stabilized methods. Gou et al. 13,14 prepared Pt/C and PtRu/C electrocatalysts by re- duction of chloroplatinic acid and ruthenium chloride with sodium borohydrate. Citrate was used as stabilizing agent in ammonium hydroxide solutions of pH 11–13. The ratio cit- rate:noble metal was varied, and optimum ratios of 2:1 and 1:1 1–10 | 1