Oxygen Reduction in Alkaline Media—a Discussion

Authors: Anna Ignaczak Renat Nazmutdinov Aleksej Goduljan Leandro Moreira de Campos Pinto Fernanda Juarez Paola Quaino Gustavo Belletti Elizabeth Santos Wolfgang Schmickler

Publish Date: 2017/03/10

Volume: 8, Issue: 6, Pages: 554-564

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Abstract

We propose a complete reaction sequence for oxygen reduction in alkaline solutions in which the first two steps occur in the outer sphere mode The oxygenoxygen bond is broken in the third step which involves adsorption of OH which is desorbed in the last step We have investigated the sequence by quantumchemical methods and determined the energies of activation Whether the reaction follows a four or a twoelectron mechanism depends critically on the energy of adsorption of OH We surmise that our mechanism holds on all electrodes which interact weakly with oxygen in particular on gold silver and graphite We explain why Au100 is a better catalyst than Au111 why at high overpotentials the reaction on Au100 reverts to a twoelectron mechanism and why this does not happen on silverWe propose a mechanism for oxygen reduction in alkaline solution and support each step by theoretical calculations based on DFT and on our own theory In particular we explain the central role of OH adsorption in the breaking of the oxygenoxygen bondFinancial support by the Deutsche Forschungsgemeinschaft FOR 1376 is gratefully acknowledged ES and WS thank CONICET for continued support E S acknowledges PIPCONICET 112201000100411 and PICT 20122324 Agencia Nacional de Promoción Cientíífica y Tecnolóógica FONCYT prééstamo BID for support LMCP thanks the Conselho Nacional de Desenvolvimento Científico e Tecnológico CNPq/CsF 203178/20149 for a fellowship A generous grant of computing time from the BadenWürttemberg grid is gratefully acknowledged PQ and GB thank PICT20141084 CONICET and UNL for supportOxygen on Silver Platinum and Gold Periodic density functional theory DFT calculations were performed using the DACAPO 43 code with implemented Vanderbilt 44 ultrasoft pseudopotentials for the representation of the atomic cores A PBE Perdew Burke Ernzerhof 45 functional and a set of plane waves with a cutoff energy of 350 eV 400 eV for the density were chosen to describe the valence electrons Brillouin zone integration 46 was performed using 4 kpoints in the x and ydirections respectively and 1 kpoint in the zdirection The surface was represented by 3 layers of metal atoms and a 3×3 unit cell was employedOHonGold All calculations were performed using the VASP code 47 48 The correlation and exchange functionals were described within the generalized gradient approximation GGA in the Perdew Burke and Ernzerhof PBE flavor 45 The electronion interactions were represented through ultrasoft pseudopotentials 44 and a plane wave basis set was used to describe the valence electrons The basis set was expanded to a kinetic energy cutoff of 500 eV Brillouin zone integration was performed using 10x10x1 kpoint MonkhorstPack 49 grid We used a dipolecorrection scheme 50 to avoid slabslab interactions To investigate the adsorption of OH as a function of the coverage both surfaces Au100 and Au111 were modelled by a 2 x 2 supercell with four metal layers In all the calculations 15 A of vacuum were considered For the relaxations the two bottom layers were fixed at the calculated nearestneighbour distance corresponding to bulk and all the other layers plus the OH were allowed to relax To mimic the aqueous media a water layer was considered on the previous systems at each coverage In order to take into account van der Waals interactions the DFTD3 51 approach of Grimme 52 was used which consists of adding a semiempirical dispersion potential to the conventional Kohn–Sham DFT energyReaction 6 The DFT calculations were performed using the b3lyp functional as implemented in the Gaussian 09 program suite 53 the standard 6311++gd p basis set was employed to describe the O and H atoms The spinpolarized formalism was used to treat open shell molecules Local hydration was considered explicitly by including of several water molecules into the nearest solvation sheath of the ions while longrange solvent effects were addressed by the Polarized Continuum Model PCM taking a value of 78 as static dielectric constant The molecular geometry in initial and final states was fully optimized without any restrictionsBreaking of the OxygenOxygen Bond These calculations were performed using the Gaussian 09 suite 53 The Au100 surface was modeled a the metal cluster composed of two layers of gold atoms 16+8 arranged according to the fcc structure typical for gold with the nearestneighbor distances AuAu fixed at the experimental value of 288 Å and was kept unchanged in all calculations The H 2O OOH − system was first optimized in the bulk solution and then placed above the Au 24 cluster as shown in Fig 6 The potential energy surface presented in Fig 7 was then obtained by systematically stretching the O–O bond of the OOH − ion and optimizing other parameters of the system undergoing adsorption In the potential energy scan some constraints were applied the metal cluster was kept rigid and the O–O bond of the OOH − anion was kept always perpendicular to the surface at the central bridge site of the Au24 cluster In all calculations the PBE1PBE hybrid functional 45 was used together with the 631++Gdp basis set for the H 2O OOH − system the pseudopotential LANL1DZ for the metal cluster and the polarizable continuum model PCM for the solvent waterTo calculate the PMF canonical ensemble constant NVT steeredmolecular dynamics simulation was conducted for 1 ns at 298 K on a simulation box containing a Ag100 slab with three metal layers thickness 409 Å an ensemble of 470 water molecules and the O 2 or O 2 species Previously an equilibration run of 700 ps was carried out Periodic boundary conditions were set in the x and y directions and the Ewald summation method was used to handle with long range electrostatic interactionsWellknown 126 LennardJones pairwise potential was used to model the interactions between the species For the water we used the SPC/E extended simple point charge model and the corresponding parameters for the oxygen and hydrogen were taken from Yoshida et al 54 The LennardJones parameters for silver were taken from Agrawal et al 55 and for the O 2 and the O 2 species the parameters were taken from Poling et al 56 and Shen et al 57 respectively The cross interactions were computed through the LorentzBerthelot mixing rules epsilon ij = sqrt epsilon iiepsilon jj and σ ij = σ ii + σ jj /2All simulations were performed using the LAMMPS largescale atomic/molecular massively parallel simulator code 58 with a time step equal to 20 fs The average temperature of 298 K was maintained by using a NoseHoover thermostat with a relaxation time of 01 ps

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