Enhancing Electrochemical Water‐Splitting Kinetics by Polarization‐Driven Formation of Near‐Surface Iron(0): An In Situ XPS Study on Perovskite‐Type Electrodes

Opitz, Alexander Karl; Nenning, Andreas; Rameshan, Christoph; Rameshan, Raffael; Blume, Raoul; Hävecker, Michael; Knop‐Gericke, Axel; Rupprechter, Günther; Fleig, Jürgen; Klötzer, Bernhard

doi:10.1002/anie.201409527

Cited by 113 publications

(111 citation statements)

References 30 publications

Supporting

Mentioning

106

Contrasting

Order By: Relevance

“…On LSF, these effects were more pronounced, which is discussed in detail in ref (11). There the formation and reoxidation of metallic iron is highly reversible.…”

Section: Resultsmentioning

confidence: 69%

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

et al. 2016

Self Cite

View full text Add to dashboard Cite

The oxygen exchange activity of mixed conducting oxide surfaces has been widely investigated, but a detailed understanding of the corresponding reaction mechanisms and the rate-limiting steps is largely still missing. Combined in situ investigation of electrochemically polarized model electrode surfaces under realistic temperature and pressure conditions by near-ambient pressure (NAP) XPS and impedance spectroscopy enables very surface-sensitive chemical analysis and may detect species that are involved in the rate-limiting step. In the present study, acceptor-doped perovskite-type La0.6Sr0.4CoO3-δ (LSC), La0.6Sr0.4FeO3-δ (LSF), and SrTi0.7Fe0.3O3-δ (STF) thin film model electrodes were investigated under well-defined electrochemical polarization as cathodes in oxidizing (O2) and as anodes in reducing (H2/H2O) atmospheres. In oxidizing atmosphere all materials exhibit additional surface species of strontium and oxygen. The polaron-type electronic conduction mechanism of LSF and STF and the metal-like mechanism of LSC are reflected by distinct differences in the valence band spectra. Switching between oxidizing and reducing atmosphere as well as electrochemical polarization cause reversible shifts in the measured binding energy. This can be correlated to a Fermi level shift due to variations in the chemical potential of oxygen. Changes of oxidation states were detected on Fe, which appears as FeIII in oxidizing atmosphere and as mixed FeII/III in H2/H2O. Cathodic polarization in reducing atmosphere leads to the reversible formation of a catalytically active Fe0 phase.

show abstract

“…On LSF, these effects were more pronounced, which is discussed in detail in ref (11). There the formation and reoxidation of metallic iron is highly reversible.…”

Section: Resultsmentioning

confidence: 69%

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

et al. 2016

Self Cite

View full text Add to dashboard Cite

show abstract

“…In water-rich atmosphere these particles become oxidized and the ASR increases. 33,34 Hence, the surface reaction mechanism can be switched from oxide-mediated to Fe-metal catalysed by a small variation of the electrode overpotential or gas composition. The exsolution of Fe 0 , however, occurs only very close to the thermo-chemical stability limit.…”

Section: Discussionmentioning

confidence: 99%

The Electrochemical Properties of Sr(Ti,Fe)O_3-δfor Anodes in Solid Oxide Fuel Cells

Nenning

Volgger

Miller

et al. 2017

J. Electrochem. Soc.

Self Cite

View full text Add to dashboard Cite

Reduction-stable mixed ionic and electronic conductors such as Sr(Ti,Fe)O 3-δ (STF) are promising materials for application in anodes of solid oxide fuel cells. The defect chemistry of STF and its properties as solid oxide fuel cell (SOFC) cathode have been studied thoroughly, while mechanistic investigations of its electrochemical properties as SOFC anode material are still scarce. In this study, thin film model electrodes of STF with 30% and 70% Fe content were investigated in H 2 +H 2 O atmosphere by electrochemical impedance spectroscopy. Lithographically patterned thin film Pt current collectors were applied on top or beneath the STF thin films to compensate for the low electronic conductivity under reducing conditions. Oxygen exchange resistances, electronic and ionic conductivities and chemical capacitances were quantified and discussed in a defect chemical model. Increasing Fe content increases the electro-catalytic activity of the STF surface as well as the electronic and ionic conductivity. Current collectors on top also increase the electrochemical activity due to a highly active Pt-atmosphere-STF triple phase boundary. Furthermore, the electrochemical activity depends decisively on the H 2 :H 2 O mixing ratio and the polarization. Acceptor-doped mixed ionic and electronic conductors are a promising class of materials for application in solid oxide fuel cell (SOFC) electrodes and they are widely investigated as SOFC cathodes due to their low polarization resistance.1-3 Some of these materials are chemically stable and mixed conducting also in humidified hydrogen atmosphere, 4 which makes them applicable in SOFC anodes. Several mixed conductors, such as acceptor doped (La,Sr)(Cr,Mn)O 3 , [5][6][7][8] donor-doped SrTiO 3 9,10 and Sr(Ti,Fe)O 3-δ -Ce 0.9 Gd 0.1 O 2-δ (STF-GDC) composites 11 were investigated in form of porous SOFC anodes. Most of these studies revealed moderately low polarization resistances. In STF-GDC composites, for example, the anode polarization resistance was <0.2 cm 2 at 800• C, and the overall performance increased with increasing Fe content. However, the usually ill-defined geometry of porous electrodes makes the analysis of specific materials parameters such as electronic and ionic conductivity or oxygen exchange activity very challenging.Thin film model electrodes have well defined and controllable geometry and comparatively simple pathways of electron and oxygen ion migration. This strongly facilitates identification of reaction pathways and quantification of the oxygen exchange activity of the material's surface. [12][13][14][15][16][17][18] Mechanistic studies using thin film model electrodes have so far been performed mainly in oxidizing atmosphere. Thorough studies of the mechanisms of electrochemical oxygen exchange in reducing atmospheres are still largely missing, not only for STF, but for most reduction stable mixed conductors, except for (Gd or Sm) doped ceria [19][20][21][22][23][24] and a study on (La,Sr)FeO 3-δ. 25 The typically low electronic conductivity of accep...

show abstract

“…12,19,20 Studies of the dc current voltage characteristics, however, are rather scarce on thin mixed ionic electronic conducting (MIEC) thin films, since determination of the true electrode overpotential (voltage loss at the working electrode) is often difficult in solid-state ionics. 31,56,57 Two experimental approaches exist: Manufacturing sufficiently small working electrodes (microelectrodes) in order to ensure that the polarization resistance of the counter electrode is much smaller than that of the working electrode and can therefore be neglected. 17,25,45 However, in previous experiments we found that the resistance of La 0.6 Sr 0.4 FeO 3−δ (LSF) thin films can irreversibly increase by a factor of 10–100 during the photolithographic microstructuring step.…”

Section: Introductionmentioning

confidence: 99%

How To Get Mechanistic Information from Partial Pressure-Dependent Current–Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes

2018

Self Cite

View full text Add to dashboard Cite

The oxygen incorporation and evolution reaction on mixed conducting electrodes of solid oxide fuel or electrolysis cells involves gas molecules as well as ionic and electronic point defects in the electrode. The defect concentrations depend on the gas phase and can be modified by the overpotential. These interrelationships make a mechanistic analysis of partial pressure-dependent current–voltage experiments challenging. In this contribution it is described how to exploit this complex situation to unravel the kinetic roles of surface adsorbates and electrode point defects. Essential is a counterbalancing of oxygen partial pressure and dc electrode polarization such that the point defect concentrations in the electrode remain constant despite varying the oxygen partial pressure. It is exemplarily shown for La0.6Sr0.4FeO3−δ (LSF) thin film electrodes on yttria-stabilized zirconia how mechanistically relevant reaction orders can be obtained from current–voltage curves, measured in a three-electrode setup. This analysis strongly suggests electron holes as the limiting defect species for the oxygen evolution on LSF and reveals the dependence of the oxygen incorporation rate on the oxygen vacancy concentration. A virtual independence of the reaction rate from the oxygen partial pressure was empirically found for moderate oxygen pressures. This effect, however, arises from a counterbalancing of defect and adsorbate concentration changes.

show abstract

Enhancing Electrochemical Water‐Splitting Kinetics by Polarization‐Driven Formation of Near‐Surface Iron(0): An In Situ XPS Study on Perovskite‐Type Electrodes

Cited by 113 publications

References 30 publications

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

The Electrochemical Properties of Sr(Ti,Fe)O_3-δfor Anodes in Solid Oxide Fuel Cells

How To Get Mechanistic Information from Partial Pressure-Dependent Current–Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes

Contact Info

Product

Resources

About

Enhancing Electrochemical Water‐Splitting Kinetics by Polarization‐Driven Formation of Near‐Surface Iron(0): An In Situ XPS Study on Perovskite‐Type Electrodes

Cited by 113 publications

References 30 publications

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization

The Electrochemical Properties of Sr(Ti,Fe)O3-δfor Anodes in Solid Oxide Fuel Cells

How To Get Mechanistic Information from Partial Pressure-Dependent Current–Voltage Measurements of Oxygen Exchange on Mixed Conducting Electrodes

Contact Info

Product

Resources

About

The Electrochemical Properties of Sr(Ti,Fe)O_3-δfor Anodes in Solid Oxide Fuel Cells