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E.I. Musina a. ,. Yu.H. Budnikova a. , A.A. Karasik a. , O.G. Sinyashin a a. A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, ...
ACCEPTED MANUSCRIPT New catalysts for PEM fuel cells M.K. Kadirova,b, T.I. Ismaevb, R.A. Safiullinb, I.R. Nizameeva, I.D. Strelnika, E.I. Musinaa, Yu.H. Budnikovaa, A.A. Karasika, O.G. Sinyashina a

A.E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Center, Russian

Academy of Sciences, Kazan, Russia b

Kazan National Research Technological University, Kazan, Russia

Address correspondence Marsil Kadirov. Email:[email protected] This research was supported by the Russian Foundation for Basic Researches (grant no.14-0300258A) and by a Program of the Presidium of Russian Academy of Sciences Abstract Fuel cell (FC) test results have been obtained for the first time with the bis-ligand nickel(ii) complex of 1,4-diaza-3,7-diphosphacyclootane heterogeneous catalyst. Using AFM technique the [Ni(PPh2Np-Tol2)2](BF4)2 self-assemblies morphology on pyrolytic graphite has been studied. By the method of rotating disk electrode the mechanism of FC cathode side reaction has been investigated

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ACCEPTED MANUSCRIPT Keywords fuel cell; organometallic catalyst, oxygen reduction reaction

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ACCEPTED MANUSCRIPT INTRODUCTION The development of fuel cell technology can receive a new impetus with the introduction of organic catalysts that can compete with traditional catalysts based on platinum group metals. The use of organic compounds as catalysts in FCs has several advantages. One of them is the possibility of quite easy change of physicochemical and mechanical properties varying the molecular design and synthesis techniques, and another one is the possibility of molecules immobilization on various substrates. Biomimetic approach to the development of new catalyst materials opened a big range of possibilities of replacement of platinum.1 One of the most successful achievements of the biomimetic approach involves a family of bis-ligand nickel(ii) complexes of 1,4 -diaza-3,7-diphosphacyclootanes.2 The electrochemical behavior of the nickel complexes, and their catalytic activities in electrochemical hydrogen evolution and hydrogen oxidation have been studied.3 The nickel(ii) complex with phenyl and p-tolyl substituents of nitrogen and phosphorus atoms correspondingly ([Ni(PPh2Np-Tol2)2](BF4)2) has been found as the oxygen reduction reaction (ORR) catalyst for hydrogen/oxygen polymer electrolyte membrane (PEM) FC. Morphological parameters of the complex on pyrolytic graphite surface, diagnostic characteristics of membrane electrode assembly (MEA) with the [Ni(PPh2Np-Tol2)2](BF4)2 catalyst on the cathode side and platinum-based catalyst on the anode side and possible ORR mechanism for the catalyst are discussed. RESULTS AND DISCUSSION [Ni(PPh2Np-Tol2)2](BF4)2 catalyst nanoscale self-assemblies Figure 1 shows tapping-mode AFM images on air for [Ni(PPh2Np-Tol2)2](BF4)2 selfassemblies on pyrolytic graphite with a cross-section profile along the line at 25 °C. The images

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ACCEPTED MANUSCRIPT indicate the almost parallel arrays of necklace-like linear chains of relatively large length scale of micrometer size. The widths of the bands vary from 30 to 90 nm and the repeat period is about 100 nm while their thickness is 5 ± 1 nm, according to the AFM cross-section profile. Diagnostic characteristics in H2/O2 FC Figure 2 shows the diagnostic characteristics of membrane electrode assembly with the [Ni(PPh2Np-Tol2)2](BF4)2 catalyst on the cathode side and platinum-based catalyst on the anode side: open circuit voltage = 330 mV, maximum current density = 16 mAcm-2, maximum power density = 0.664 mWcm-2.

Electrochemical data Oxygen reduction reaction on the cathode of FC plays a key role in its operation. ORR may proceed either via (I) - four-electron process of association on of oxygen with the electrons and protons in combination with the process of hydrogen oxidation on the anode with the production of water as the end product, or (II) less effective two-staged two-electron way with the creation of hydrogen peroxide ions as the intermediates.1 Electrochemical data show that during the ORR process 3.4 electrons are transferred, thus the ratio of 4- and 2-electron processes was 70/30. EXPERIMENTAL AFM Microscopic images were captured by scanning probe microscopy MultiMode V (Veeco instruments Inc., USA) using silicon cantilevers RTESP (Veeco instruments Inc., USA) with nominal spring constants of 40 N/m and tip curvature radius of 10−13 nm.

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ACCEPTED MANUSCRIPT FC tests The catalytic ink was prepared according to the following procedure: 0.5 mg of catalyst and 5 mg Vulcan XC-72 were added to 375 mL of IPA. The ink was first sonicated for 15 min then 46 µL of 5 wt% Nafion® solution (Aldrich) added to the ink and sonicated again. The obtained ink was deposited on carbon paper gas diffusion layer (GDL) Sigracet® 25CC. MEA was obtained by hot-pressing of GDLs on both sides of a Nafion® 212 membrane at 80°C with a load of about 200 lbs. Polarization curves were obtained with mechanical test station ElectroChem (USA) with a gas flow and pressure control system MTS-A-150 and electronic load unit ECL-150. Electrochemistry For electrochemical experiments potentiostat PI 50-1, programmer Pr 8 and custom-made set of rotating disc electrode have been used. CONCLUSIONS FC test results on the cathode side have been obtained for the first time with the bisligand nickel(II) complex of 1,4-diaza-3,7-diphosphacyclootane heterogeneous catalyst. Despite the low fuel cell performance the organometallic catalysts have great possibilities of increasing their working characteristics by a variation of complex structure and by a search of different combinations of ligands and counterions.

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ACCEPTED MANUSCRIPT REFERENCES 1. A. Morozan, B. Jousselme and S. Palacin Energy Environ. Sci, 2006, 128, 358. 2. A.D. Wilson, R.H. Newell, M.J. McNevin, J.T.Muckeman, M.Rakowski DuBois and D.L. BuBois J Am Chem. Soc., 2011, 4, 1238. 3. E.I. Musina, V.V. Khrizanforova, I.D. Strelnik, M.I. Valitov, Yu.S. Spiridonova, D.B. Krivolapov, I.A… Litvinov, M.K. Kadirov, P. Lönnecke, E. Hey-Hawkins,Yu.H. Budnikova, A.A. Karasik, O.G. Sinyashin Chem. Eur. Jnl 2014, 20, 3169.

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Figure 1 AFM images and a profile of the [Ni(PPh2Np-Tol2)2](BF4)2 catalyst structures on a pyrolytic graphite

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Figure 2 Diagnostic characteristics of MEA with the [Ni(PPh2Np-Tol2)2](BF4)2 catalyst on the cathode side and platinum-based catalyst on the anode side.

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Figure 3 A - Rotating-disk voltammograms for the reduction of O2 in an O2-saturated aqueous solution of the [Ni(PPh2Np-Tol2)2](BF4)2 (modified glassy carbon working electrode, Pt counter electrode, Ag/AgNO3 reference electrode, scan rate=50 mVs-1, disk area=0.0314 cm2, electrolyte= 0.5 М H2SO4, i=current). B - Koutecky–Levich plot of the inverse of the disk current (i-1) measured at 0 V versus Ag/AgNO3 as a function of the square root of the inverse of the rotation rate. The fitted line yields 3.4 electrons. The intercept is the inverse of the kinetically limited current. The parameters for the Koutecky–Levich plot: v = 0.01 cm2 s-1, D = 2.0 * 10-5 cm2 s-1, C* = 1.2 * 10-3M.

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