Nanocomposites and nanomaterials

Oxygen reduction and hydrogen evolution on Co-N-C nanocomposite electrocatalysts, derived from poly-2,6-diaminopyridine 
Pariiska O.O., Mazur D.O., Kurys Ya.I., Koshechko V.G., Pokhodenko V.D.
L.V.Pysarzhevsky Institute of Physical Chemistry of the NAS of Ukraine. Prospect Nauki, 31, Kyiv-03028, Ukraine. 
E-mail: pariiska@ukr.net

By means of pyrolysis of poly-2,6-diaminopyridine (PDAP), as a nitrogen source, together with Со(II) nitrate and multilayered graphene (MLG) [1] or acetylene black (AB) precious metals free nanocomposite Со-N-CMLG and Со-N-CAB electrocatalysts for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) were obtained. Selection of PDAP was caused by the high nitrogen content in it as well as the ability of the corresponding monomer to effective oxidative polymerization. The latter made it possible to use in the formation of nanocomposites not the previously prepared PDAP, but the system 2,6-diaminopyridine (monomer)/(NH4)2S2O8 (oxidant), which simplified the synthesis procedure. 
It is shown, that activity in 0.5 М H2SO4 of Со-N-CMLG in both ORR (ORR starting potential ~ 0.78 V vs. RHE; halfwave potential ~ 0.68 V; Tafel slope ~ 71 mV/dec) and HER (HER starting potential ~ -0.18 V; overpotential at 10 mA/cm2 ~ 0.25 V; exchange current ~ 1.14*10-5 A/cm2) exceeds analogue derived from traditionally used AB. Functional advantages of Со-N-CMLG electrocatalyst in ORR are high tolerance to methanol or carbon monoxide, as well as small contribution of 2-electron process (~ 6%) in dominant direct 4-electron oxygen reduction to water. At the same time, in 0.1 М NaOH activity of obtained electrocatalysts in ORR is comparable, but in HER Со-N-CAB have better performance. One of the possible reasons of observed differences in electrocatalytic properties of Со-N-CMLG and Со-N-CAB may be a different nature of the dominant active sites (Co9S8, CoNx, C/Nx) in the composition of such catalysts, wherein the same sites in Co-N-CMLG and Co-N-CAB, apparently involved both in ORR and HER.

1.	Kurys Ya.I., Ustavytska O.O., Koshechko V.G., Pokhodenko V.D. Structure and Electrochemical Properties of Multilayer Graphene Prepared by Electrochemical Exfoliation of Graphite in Presence of Benzoate Ions // RSC Adv.-2016.-6, N 42-P. 36050-36057.