CoO_xthin film deposited by CVD as efficient water oxidation catalyst: change of oxidation state in XPS and its correlation to electrochemical activity

Abstract

To reduce energy losses in water electrolysers a fundamental understanding of the water oxidation reaction steps is necessary to design efficient oxygen evolution catalysts. Here we present CoOx/Ti electrocatalytic films deposited by thermal and plasma enhanced chemical vapor deposition (CVD) onto titanium substrates. We report electrochemical (EC), photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) measurements. The electrochemical behavior of the samples was correlated with the chemical and electronic structure by recording XPS spectra before and after each electrochemical treatment (conditioning and cyclovoltammetry). The results show that the electrochemical behavior of CoOx/Ti strongly depends on the resulting electronic structure and composition. The thermal deposition leads to the formation of a pure Co(II)Ox which transforms to a mixed Co(II)Co(III)Ox during the OER. This change in oxidation state is coupled with a decrease in overpotential from η = 0.57 V to η = 0.43 V at 5 mA cm(-2). Plasma deposition in oxygen leads to a Co(III)-dominated mixed CoOx, that has a lower onset potential as deposited due to a higher Co(III) content in the initial deposited material. After the OER XPS results of the CoOx/Ti indicate a partial formation of hydroxides and oxyhydroxides on the oxide surface. Finally the plasma deposition in air, results in a CoOxOH2 surface, that is able to completely oxidizes during OER to an oxyhydroxide Co(III)OOH. With the in situ formed CoOOH we present a highly active catalyst for the OER (η = 0.34 at 5 mA cm(-2); η = 0.37 V at 10 mA cm(-2)).

An Operando Investigation of (Ni–Fe–Co–Ce)Ox System as Highly Efficient Electrocatalyst for Oxygen Evolution Reaction

“…75 In agreement with the core level spectroscopy, the VB Co(III) feature is weaker at the OCP and post mortem conditions due to the partial reductive conversion of Co (II,III) 3O4 and Co (II,III O(OH) phase should lead to a higher negative shift of the OH BE component (~1.1 eV) compared to that observed in this study. 75 This observation is in line with the previous discussion regarding the partial oxidative conversion of Ni and Co at low overpotential/low current density operation (0.55 V vs Ag/AgCl/Cl -(sat.) and ~ 1 mA cm -2 ).…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 14 In the first VB region of the pristine material and under operando OER conditions it is possible to observe the (t2g) 5 final state feature of Co(III) (d 6 , centered at 1.5 eV below the Fermi level), confirming the results discussed above about the core level photoemission of Co. 75,100 This is also in line with the observations of Weidler et al on Co-based electrocatalysts prepared using a CVD procedure. 75 In agreement with the core level spectroscopy, the VB Co(III) feature is weaker at the OCP and post mortem conditions due to the partial reductive conversion of Co (II,III) 3O4 and Co (II,III O(OH) phase should lead to a higher negative shift of the OH BE component (~1.1 eV) compared to that observed in this study. 75 This observation is in line with the previous discussion regarding the partial oxidative conversion of Ni and Co at low overpotential/low current density operation (0.55 V vs Ag/AgCl/Cl -(sat.)…”

“…These changes are a clear fingerprint of the partial oxidation of Co (II) (OH)2 into a sub-stoichiometric cobalt oxyhydroxyde (Co (II,III) Ox(OH)y) species. 75 Previous studies of Co and Ni oxide-based materials have established a correlation between OER catalytic activity and the concomitant oxidation of Co (II) and Ni (II) to Co (III) and Ni (III) , creating intrigue into the coexistience of these species in this low current density operation of Ni0.3Fe0.07Co0.2Ce0.43Ox, which is discussed below. [53][54][55] To further understand the active species in this complex oxide catalysts, we perform complementary XAS experiments, which probe transitions from core level electronic states to higher-energy unoccupied electronic states, showing sensitivity to oxidation state (similar to XPS), while also revealing details of the electronic structure of the material being observed.…”

Understanding the Oxygen Evolution Reaction Mechanism on CoOx using Operando Ambient-Pressure X-ray Photoelectron Spectroscopy

“…While either of these spectral assignments is physically reasonable based on the composition of the film and prior mechanistic studies, it is not currently possible to discriminate between these spectral components. Previously, Weidler et al also observed an additional small intensity O 1s component at low BE from a CoO x catalyst surface, ex situ after exposure to highly oxidizing electrochemical conditions, but they did not attribute it to any related Co chemistry (56). Since electronic decoupling or Co leaching can be observed only while running the electrochemical measurements, this prior finding supports our hypothesis that the observed BE shift is indeed correlated to a chemical shift of a new Co-based species developing on the catalyst surface under sustained OER conditions.…”

“…Analysis of VB spectra obtained under pristine and hydrated conditions confirms the presence of both Co 2+ and Co 3+ , centered at 2.1 eV and 1.2 eV below the Fermi level, respectively. These two spectral features are due to the 3d band photoemission of octahedral Co 2+ (from Co(OH) 2 ) and Co 3+ (Co 3 O 4 spinel structure), which leads to the (t 2g ) 6 and (t 2g ) 5 final states, respectively, that contribute to the valence band maximum (VBM) (56,61 …”