Operando high-temperature near-ambient pressure X-ray photoelectron spectroscopy and impedance spectroscopy study of NiCe0.9Gd0.1O2δ solid oxide fuel cell anode

https://doi.org/10.1016/j.ijhydene.2020.06.228Get rights and content

Highlights

  • Operando NAP-XPS study of SOFC anode at different electrochemical conditions.

  • Depth probing of operating anode showed variations in the electron spectra.

  • Photoelectron spectra of Ni and Ce depend on pH2 and electrode polarization.

  • Electronic structure and impedance of Ni(O)-GDC were monitored simultaneously.

  • Used setup enables operando quantification of the level of redox processes.

Abstract

In this study we present the results of operando high temperature near-ambient-pressure x-ray photoelectron spectroscopy (HT-NAP-XPS) measurements of a pulsed laser deposited thin film NiCe0.9Gd0.1O2δ model electrode. In our measurements, we have used the novel three electrode dual-chamber electrochemical cell developed in our previous work at different H2 pressures and at different electrochemical conditions at around 650 °C.

The possible redox reactions on the anode surface (Ni2+Ni0,Ce4+Ce3+) were investigated by HT-NAP-XPS technique simultaneously with electrochemical impedance spectroscopy measurements. The oxygen partial pressure in counter and reference electrode compartment was controlled at 0.2 bar. Changes in electronic structure of the Ce3d and Ni2p photoelectron spectra caused by electrode potential and H2 pressure variations were observed and estimated by curve fitting procedure. The O1s and valence band photoelectron signals were used for depth probing of the chemical composition and redox changes at Ni-GDC and for studying the influence of the electrochemical polarization on the chemical state of Ni-GDC surface atoms.

As a result changes in oxidation state of electrode surface atoms caused by electrode polarization and oxide ion flux through the membrane were detected with simultaneous significant variation of electrochemical impedance.

Introduction

Electrochemical conversion device such as solid oxide fuel cell (SOFC) with a high electrical efficiency (up to 70 %) can be considered as a potential future energy source [1]. However, characteristics such as reversibility of the thermal and redox cycles, temporal durability of the cell, and its production price need to be optimized for the successful large-scale commercialization. Detailed information of the physical and chemical properties of the electrode surfaces is important for the rational development of SOFC electrodes’ materials. Due to the complicated operational conditions of the SOFCs (high temperature, fuel gas atmosphere etc.), the experimental information about reactions and thermodynamics on the electrode surfaces is not easily accessible ([[1], [2], [3], [4], [5], [6]]).

Spectro-electrochemical approach - simultaneous electrochemical characterization and control of electrodes together with spectroscopic techniques such as Raman spectroscopy ([[7], [8], [9]]), X-ray absorption spectroscopy ([10], and X-ray photoelectron spectroscopy ([[11], [12], [13], [14]]) - is a very promising way to study the surfaces of the SOFC electrodes. The important surface properties, like its coverage by adsorbates and reaction intermediates, oxidation states of chemical element on the electrode, and surface polarization, can be estimated by above-mentioned experimental methods close to the realistic working conditions of SOFC.

Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) has been already succesfully used for in situ studies of single-chamber 2-electrode electrochemical systems at (doped) ceria/gas interfaces [[15], [16], [17], [18]]. Also electrochemical processes on yttria-stabilized zirconia with nickel electrode is extensively studied by NAP-XPS method [[19], [20], [21]]. Electrochemical splitting of H2O and oxidation of H2 on the AuCeO2δ electrode at 700 °C has showed that the charge separation at the gas-solid interface in the working conditions affects reaction kinetics on the electrochemically active region of the electrode [11]. Single-sided setups with ceria electrode have also been used for studies of mass transport and reaction kinetics depending on the Ce3+/Ce4+ ratio on the three phase boundary region [[22], [23], [24]]. The single-chamber SOFC prototype with Ni/NiO anode and AuMnO2 cathode material at an O2/methane atmosphere has been studied by combining the XPS and XAS techniques [12]. The same methods are used for studying the chemistry of a Mn-oxide electrode in H2O and CO2, CO2/H2O environments [13]. Similarly, operando NAP-XPS studies of nickel-based cermet electrodes during steam electrolysis in single cell configuration are executed [[25], [26], [27]]. The NAP-XPS technique is used to reveal the mechanism of carbon soot oxidation in ceria-based catalysts [28]. The NAP-XPS results of the perovskite electrodes La0.6Sr0.4CoO3δ and La0.6Sr0.4FeO3δ at varied electrode polarization and fuel gas atmosphere conditions have been reported lately [3]. Also electrochemical XPS study of perovskite anode materials controlling oxygen partial pressure in ultra-high vacuum has been published [29].

However, thorough NAP-XPS studies of SOFC anodes in the realistic working conditions at a certain electrochemical potential (compared with well-defined reference electrode such as Pt/PtO at pO2=0.2 bar) are still missing. The most common electrolyte materials are gadolinia-doped ceria (GDC) and yttria-stabilized zirconia (YSZ). A nickel-containing cermet is widely used material for SOFC anode. For example, very recently a detailed insight of microstructure, physicochemical properties and impedance characteristics about Ni/GDC anode was published [30]. The transition metal Ni with excellent catalytic characteristics has no good non-noble metal replacement for SOFC anode in operating conditions.

This study focuses on simultaneous exploration of the electrochemical properties and electronic structure of the Ni-GDC anode depending on the fuel gas (H2) concentration in the anode compartment and different well-controlled electrode potentials. The oxygen partial pressure in the cathode section was controlled and kept constant at 0.2 bar. The experimental work is done by using 3-electrode dual-chamber electrochemical cell designed for operando NAP-XPS measurements. The changes of impedance characteristics and electronic structure composition of Ni, Ce and O on the anode surface depending on the hydrogen partial pressure and electrode polarization were studied carefully.

Section snippets

Preparation of the solid oxide fuel cell

The examined NiGDC electrode (Fig. 1(a)) is fabricated by pulsed laser deposition (PLD) with 60 % NiO and 40 % Ce0.9Gd0.1O2δ in weight). A schematic layout of the membrane and electrode configuration is given on Fig. 1(b). The supportive scandia-ceria-stabilized zirconia (10Sc1CeSZ) (from Kerafol GmbH) electrolyte between the anode and cathode compartments is 0.25 mm thick. The La0.6Sr0.4CoO3δ (LSC) cathode is aligned very carefully with the NiGDC anode and prepared using screen-printing

Results and discussion

The spectroscopic response of Ni, Ce and O surface atoms - different ions, their vacancies and adsorbates, depending on the electrochemical polarization and the concentration of the hydrogen fuel gas at the Ni-GDC anode-gas interface and beneath of it will be discussed in this section. In this study we used NAP-XPS in combination with electrochemical impedance spectroscopy (EIS) technique.

We used the SOFC HT-NAP-XPS cell prototype with separated anode and cathode compartments (see also Fig. 1

Conclusion

The first dual-chamber operando NAP-XPS study of SOFC anode was performed at different electrochemical conditions. Changes in electronic structure and electrode impedance of Ni-GDC electrode were recorded simultaneously. Also oxygen partial pressure was monitored and controlled. Near ambient XPS study of Ni/NiOGDC anode revealed several details of its surface chemistry at different H2 concentration on anode side and at constant partial pressure of the O2 on the cathode compartment depending on

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the EU through the European Regional Development Fund under projects TK141 Advanced materials and high-technology devices for energy recuperation systems (2014-2020.4.01.15–0011), by the Estonian Research Council (PUT551, PUT735), by Estonian target research project IUT20-13 and by the Academy of Finland. S.U. acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme, Project SURFACE (Grant

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      The variation of surface orientation has the valuable feature that other important parameters, such as doping and strain, are kept constant, while influencing the surface reducibility and oxygen vacancy formation. Electrochemical kinetic studies making use of model nanostructured electrodes in the form of thin films, combined with operando spectroscopic techniques [250], are well-suited for the above discussed purposes. Physical vapor deposition (PVD) techniques such as pulsed laser deposition (PLD) offer a means to vary the parameters in a controlled and systematic manner.

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