Microscale Evolution of Surface Chemistry and Morphology of the Key

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Microscale Evolution of Surface Chemistry and Morphology of the Key Components in Operating Hydrocarbon-Fuelled SOFCs Benedetto Bozzini, Matteo Amati, Luca Gregoratti, Majid Kazemian Abyaneh, Mauro Prasciolu, Alexander Trygub, and Maya Kiskinova J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 10 Oct 2012 Downloaded from http://pubs.acs.org on October 17, 2012

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The Journal of Physical Chemistry

Microscale Evolution of Surface Chemistry and Morphology of the Key Components in Operating Hydrocarbon-Fuelled SOFCs Benedetto Bozzini1*, Matteo Amati2, Luca Gregoratti2, Majid Kazemian Abyaneh2, Mauro Prasciolu3, Alexander L.Trygub2 and Maya Kiskinova2

1

Dipartimento di Ingegneria dell'Innovazione, Università del Salento, via Monteroni s.n., 73100

Lecce, Italy 2

Sincrotrone Trieste S.C.p.A., ELETTRA, s.s. 14 km 163.5 in Area Science Park, 34012

Basovizza,Trieste, Italy 3

CNR-INFM TASC National Laboratory, S.S. 14, km 163.5 in Area Science Park, 34012 Trieste-

Basovizza, Italy

*Corresponding author: telephone +39-0832-297323, fax +39-0832-297733, e-mail [email protected]

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ABSTRACT Replacement of hydrogen with hydrocarbon fuels in solid-oxide fuel cells (SOFCs) is an appealing alternative for reducing the implementation costs of SOFCs technology, but the electrode stability and susceptibility to carbon deposition still remain important issues to be solved. The present in-situ photoelectron microscopy study of a prototype hydrocarbon-fuelled SOFC, operated at 650 °C in C2H4 + H2O gas mixture and voltages in the range 0 - 3 V provides insights in morphologychemistry changes of the Ni electrodes and Cr interconnects with decisive impact on the electrochemical activity and durability. The results reveal the combination of thermal and electromigration of Ni across the electrode-electrolyte interface that can cause sensible material losses and structural changes responsible for the deterioration of device performance. The C 1s spectra evidence deposition of C and formation of carbides on the Ni electrodes and Cr interconnects at 650 °C as result of C2H4 dissociation, the process being promoted applying cathodic potential and reversed by switching to anodic potential. Following the attenuation of the C signal under anodic potential, the effect of the stability of different carbides on the reaction rate was observed.

KEYWORDS: SOFC, hydrocarbon fuel electrochemistry, mass transport, C deposition, XPS microscopy.

1) INTRODUCTION Fuel cells are one of the most appealing environmentally friendly devices for effective conversion of chemical energy into electricity and heat, but still there are key barriers to their broad commercialization1,2,3,4. One is the use of hydrogen as the fuel5, that has to be generated (mainly by hydrocarbon reforming)3, 6 and stored. Skipping the reforming and using directly hydrocarbon fuels is highly desirable since this will increase the overall efficiency and simplify the system. In this 2 Environment ACS Paragon Plus

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respect, SOFCs are the ones realistically expected to be directly fuelled with hydrocarbons that undergo reforming at the anode1,3,5,7,8 SOFCs use ionic (YSZ) or mixed ionic-electronic (e.g. Ce(III)/Ce(IV) oxides) electrolytic conductors and metal-containing cermets (most often Ni/YSZ) as porous anodes, where the catalytic electrochemical oxidation takes place1,

9,10

. The Ni/YSZ

anodes have several distinct advantages: they are efficient catalysts for the electrochemical oxidation of H2 and hydrocarbons, robust and relatively cheap and easy to fabricate by co-sintering NiO-YSZ composites or infiltration methods followed by NiO reduction

5,6,11,12,13

. However, there

are several drawbacks of the Ni/YSZ anodes, among which: (i) the low redox stability due to structural changes when cycling between Ni and NiO, (ii) decomposition of hydrocarbons, the undesired side reaction catalysed by Ni resulting in carbonaceous deposits14,15. Another critical issue with SOFCs operating at intermediate temperatures (600−800 °C) is that, on the one hand the interconnect material of choice is ferritic stainless steel - essentially for its low cost compared to the traditional ceramic materials and thermal expansion coefficient matching ptimally the other fuel cell stack components -, on the other hand the use of stainless steel is not devoid of drawbacks since its high Cr content leads to the formation of volatile Cr-based compounds that poison the cell and cause brittleness16,17,18,19,20,21,22,22,23,24.

Inventing and implementing novel materials to solve durability and reactivity issues when using hydrocarbon fuels is still severely impeded on the one hand by the poor understanding of the electrochemical and mass-transport processes, occurring at the interfaces between the SOFC components, and on the other hand by the reactions resulting in corrosion or catalyst deactivation. Most of the currently available information is based on post-mortem characterization of the SOFC components. A notable step ahead is the use of model systems simulating the materials and operation conditions of the SOFCs, which has allowed in-situ monitoring of processes at the key components (interconnects, electrodes and electrolyte) a3nd correlation to the operating conditions6,Error! Bookmark not

defined.,16,17

. Among the characterization methods capable of providing



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selective information about the electrochemical surface processes, X-ray photoelectron spectroscopy (XPS) with high spatial resolution18 is a unique tool for monitoring simultaneously the morphology, surface composition and chemical state of the SOFC components26, the lateral distribution of the component constituents and the local potential10,Error! 20,21,22,23,24,25,26

Bookmark not defined.,19,

. Using the cell configuration sketched in Figure 1(a), we focused in our previous

study on the evolution of chemical state and lateral distribution of interconnect and electrode metal constituents occurring in pure O2 ambient as a function of temperature, reaction time and electrochemical polarization.Error! Bookmark not defined. The three key components of this model cell are represented by Cr-interconnect/Ni-electrode (catalyst)/YSZ-electrolyte stacks. Each Cr/Ni bilayer can act either as a working electrode (WE) or simultaneously as a reference (RE) and counter electrode (CE), and the potentials reported below refer to the applied cell bias. In the present study the cell was operated in C2H4 + H2O gas mixture, which is a prototype of hydrocarbon-fuelled SOFC, representing steam reforming of unsaturated hydrocarbons, which are more prone to C deposition. We report new findings about two undesired processes occurring in operating cells under different electrochemical conditions with decisive impact on the electrochemical activity and durability. These are: (i) the mass transport events due to combination of thermomigration and electromigration of constituent atoms and related to restructuring of the key components and (ii) carbon deposition on the electrode catalyst and interconnects due to catalytic cracking of the hydrocarbons.

2) EXPERIMENTAL All experiments were carried out with the scanning photoelectron microscope (SPEM), at the ESCAmicroscopy beamline at the Elettra laboratory in Italy. The SPEM uses Fresnel zone plate focusing optics and operates in imaging and spectroscopic modes with lateral resolution down to 100 nm27. In the imaging mode, the lateral distribution of each element was mapped by collecting core level photoelectrons emitted within the relevant kinetic energy window while raster-scanning 4 Environment ACS Paragon Plus

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the specimen with respect to the microprobe. By combining elemental mapping with micro-XPS we followed the evolution of the morphology, composition and chemical state of the SOFC components and interfaces, involved in faradaic processes. The model cell is placed inside the SPEM chamber on a suitable sample-holder equipped with proper electrical connections for controlled electrochemical polarization and a heater, capable of reaching high temperatures. The present experiments were run at 650 °C in reactive gas ambient and under electrochemical control.

3) RESULTS AND DISCUSSION 3.1) Morphology, chemical state and electrochemical activity of SOFC components The elemental Ni 2p, Cr 2p and Zr 3d large area maps in Figure 1(a) confirm that the as-fabricated cell has well-defined Cr and Ni patches. In addition to the constituent elements, oxygen (due to partial Cr and Ni oxidation) and adventitious carbon contaminants (introduced during fabrication) were also present in the spectra. The cell was operated inside the SPEM chamber under various reaction conditions. It was first exposed to 10-6 mbar O2 at 650°C at open circuit potential (OCP) in order to attain the realistic morphology and chemical state of the components working at high temperature and remove the C contaminants via gasification to CO or CO2. The asymptotic OCP attained in our symmetrical cell after overnight exposure under these conditions was 0±11 mV, a value matching very closely that reported in previous cognate work.35 The changes induced by similar treatments have already been reported in ref. Error! Bookmark not defined.. The previous results showed that at high operating temperature Ni and Cr atoms can become mobile and diffuse away from their original locations, which results in specific morphology-composition changes of the Ni, Cr and YSZ areas. The most relevant to the present study are: (i) the conversion of the initially uniform Ni patch morphology into an island-type microstructure, similar to that of the commonly used Ni-YSZ cermet anodes5 and (ii) the aggregation of the Ni diffusing inside the YSZ into isolated Ni islands. The following in-situ electrochemical studies were carried out in 10-6 mbar



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1:1 C2H4:H2O ambient at 650°C under controlled polarization, where the possible reactions are reduction of water protons and metal oxides at the cathode and oxidation of C2H4 and metals at the anode. The measured OCP value was 0±6 mV and, since our cell is symmetric, no changes are to be expected upon changing the gas environment. The first notable result is that mass transport and restructuring processes were still observed after changing the gas ambient and applying potentials, which means that these processes are driven by the temperature and current density, whereas the gas ambient is of minor importance. In fact, as reported below, the gas ambient can affect only the chemical state of the component constituents. The dramatic changes in the morphology of the cell components after all treatments and prolonged operation in C2H4:H2O ambient under polarization are even visible in the large area elemental maps of the final state of the cell in Figure 1(a). Figure 1(b) shows the cell voltage vs. cell current plots corresponding to all reported experiments with the respective standard deviations. Due to the symmetric nature of the cell, the anodic and cathodic curves closely match. The functional I-V dependence has been fitted with the classical Tafel law.35 More detailed information about the chemical state and structural changes of Ni electrode is obtained from high-resolution elemental maps centred at the Ni/YSZ interface and Ni photoelectron spectra from selected micro-spots (Figures 2 and 3). For the Ni chemical state we used the well known assignments of the Ni 2p binding energy (BE): a peak at 852.6 eV for metallic Ni (Ni0) - and shifts to higher BE for oxidized Ni (oxides, hydroxide and oxo-hydroxides) by 2.0 eV and 3.5 eV for Ni2+ and Ni3+, respectively28. It should also be noted that the oxidation is accompanied by spectral broadening and, owing to to multiplet contributions, the extended Ni 2p satellite structures become very intense so the exact assignment of the oxidation state can be compromised. The results obtained when the cell was operated in C2H4:H2O ambient applying 1 V on the anodic side are summarized in Figure 2. The Ni 2p maps in panel (a) show the island-like structure of the Ni electrode and the YSZ regions close to the Ni patch, where Ni has diffused. The Ni islands farther from the original Ni patch edge appear isolated, while those within the Ni patch area are still connected by a thin Ni film, which is transparent for the photoelectrons emitted from the underlying 6 Environment ACS Paragon Plus

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YSZ substrate, as one can assess from the contrast levels of the Zr 3d map and the measured Zr 3d spectra of Zr oxide (not shown). As noted above, an island-like morphology has already been attained at OCP by the pre-treatment at 650°C in O2 when Ni was also fully oxidized to NiO (see the top Ni 2p spectrum in panel (b)). However, the mass transport and restructuring process was not terminated by the subsequent change to C2H4:H2O ambient, which, at OPC, results in reduction of NiO to Ni. Here we also address the spatial variation of electrochemical activity related to different local morphology and overpotentials. 30 min after the application of a potential of 1 V the Ni 2p spectra measured inside the regions with connected Ni islands clearly show that the reduced state is preserved only on the cathode side (bottom spectrum), while oxidation takes place on the anode side (spectra C - F), in full coherence with the electrical and chemical actions of electrode polarization. The relative weight of the oxide components increases moving progressively from anode areas closer to the interface with the electrolyte (spectra C, D) towards regions closer to the interconnects (spectra E, F). This lateral variation in oxidation rate under anodic polarization correlates with the voltage drop that reaches ~ 0.6 V moving from position C to F. This is elucidated from the observed BE energy shifts of the Ni 2p spectra, since the local potential affects the kinetic energy of the emitted photoelectrons. However, this is not the case with the developed disconnected Ni islands: apparently, not being polarized due the lack of electrical contact, their chemical state remains unchanged since it is entirely controlled by the reducing gas environment (spectrum A). In order to get further insight into the reversibility of local electro-catalytic activity we followed the evolution of the Ni 2p spectra inside the electrode with connected islands, which evidenced the reduction of Ni oxide by switching from anodic to cathodic polarizations, corresponding to cell voltages of -1, -2 and -3 V. The Ni 2p spectra in Figure 3(a), measured ~ 30 min after polarization, confirm the expected increase of the reduction rate with increasing the applied potential. Another notable finding is that along with the higher reactivity, the current flowing through the electrode when applying polarization promotes the Ni mass transport, owing to surface electromigration29. This is clearly seen comparing the Ni 2p images in Figure 3(b): they show progressive movement of 7 Environment ACS Paragon Plus


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the boundary between disconnected and connected Ni island regions in the direction of the current. Following the space-dependence of the Ni 2p peaks and evaluating the local integrated signal along the mass transport direction, we could estimate that the total loss of Ni from the imaged region is ca. 10%. Such material losses induced by the combination of heat and current density, can be regarded as one of the causes for degradation of the SOFC devices. In fact, this directed motion of atoms at solid surfaces, grain boundaries and interfaces, caused by an electric current in the bulk of the material, is well known reason for electromigration-induced failure in solder interconnections of electronic devices29.

3.2) C adsorption-desorption dynamics in hydrocarbon-fuelled SOFC Carbon deposits exposing Ni anodes to saturated and unsaturated hydrocarbons at temperatures higher than 700°C and different polarization has already been a subject of previous research: thermal program desorption,30 and Raman spectroscopy31 studies reported the structural dependence of the deposited C residues on the type of the hydrocarbons and the cleaning effect applying overpotentials. In the present study the electrochemical conditions leading to deposition of carbon residues are explored with surface sensitivity, monitoring the evolution of the C 1s spectra on both the electrodes and interconnects and the results provide information for the actual chemical state of the C residue and its evolution upon applying potentials. It should be noted that under our operation conditions (gas pressure and temperature), the gas-phase pyrolysis of hydrocarbons is negligible so that the formation of C deposits is a pure catalytic process occurring at the electrode and interconnect surfaces. Figure 4 shows the C 1s spectra, measured on the Ni and Cr patches under different electrochemical conditions. Let us remind that under cathodic polarization both Cr and Ni can be reduced to their metallic stateError! Bookmark not defined. . The top two C 1s spectra in Figure 4(a), measured inside the Ni and Cr patches, and the middle one, measured at the Cr-Ni interface, evidence the presence of C deposited on the cathodic side. The relatively narrow C 1s spectra measured inside the patches indicate a single C bonding configuration with C 1s binding energies 8 Environment ACS Paragon Plus

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within the range of Cr carbides ~ 282.8-283.0 eV32,33, and Ni3C ~ 281.5 eV34. The broader C 1s spectrum taken at the Ni/Cr interface apparently has contributions of both carbides. The bottom ‘spectrum’, measured on the anodically polarized Cr and Ni patches confirms that C is completely removed, in accordance with the anode oxidative activity. Figures 4(b) and (c) summarize the evolution of the C 1s spectra from the Ni and Cr patches upon systematic changes of the electrochemical reaction conditions. Starting from the steady state, reached on the anode and cathode sides and reported in Figure 4(a), the potential was switched off and the sample was left overnight in C2H4:H2O environment at OCP conditions. The C 1s spectra in Figure 4(b) show that the OCP treatment results in a similar C deposit on the patches at both sides. At the cathode side only on the Ni patch the C 1s intensity has decreased and the broadening of the peak towards higher binding energies indicates the formation of other NixC species39. This indicates that the water in the gas phase has partially reacted off only some C from the Ni patch, which is in accordance with the lower stability of Ni carbide compared to Cr carbide. Under OCP some C deposition occurs on the initially C-free Ni patch at the anode side which means that the cleaning effect of H2O is limited: indeed the C 1s spectrum is similar but less intense than the one on the partially cleaned cathode side. The C content on the Cr patch at the cathode side remains intact and almost the same C amount was deposited on the initially clean Cr patch at the anodic side. However, even the more stable Cr carbide can be reacted-off under strongly oxidising electrochemical conditions. As illustrated in Figure 4(c), the C 1s intensity drops rather fast after switching to anodic polarization of +3 V and the shift of the C 1s binding energy with time indicates the formation of C-C species, most likely with sp2 type bonding, at the Cr surface.

CONCLUSIONS The present in-situ electrochemical SPEM study of model hydrocarbon-fuelled SOFC, carried out at 650°C in operando conditions, reveals dynamically evolving morphology and chemistry of the Ni electrodes and Cr interconnects and identifies the formation conditions and chemical state of carbon 9 Environment ACS Paragon Plus


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deposits on their surfaces. The role of the electrochemical polarization is demonstrated by (i) the remarkable difference in the oxidation rate, observed comparing the behaviour of the networks with electrically disconnected and connected Ni islands on the anodic and cathodic side, and (ii) the lateral variations of the degree of Ni oxidation in concert with and the local anodic potential. The chemical mapping of Ni electrodes has also evidenced that the polarization promotes a significant directional Ni mass transport towards the electrolyte region. The loss of Ni and the resulting changes in the morphology of the electrode promoted by the surface electromigration can be one of the reasons for Ni catalysts degradation, which rises critical reliability issues for the interconnects with decreasing the dimensions of the SOFCs. The local C 1s spectra, measured on the Ni electrodes and Cr connectors under operating conditions at different potentials, have shown that C deposition in the form of carbides occurs only at OCP and is promoted by cathodic polarization. The formed carbides have different stability but can be removed by switching to strongly oxidizing anodic potentials. We reckon that the present results are shedding light on the status and evolution of Ni electrodes under operating conditions at a microscale and demonstrate the potential of our approach to directly link the local structure and composition of the chief SOFC components to electrochemical activity. This approach is opening an opportunity for systematic high-throughput studies under realistic operando conditions liable to pave the road towards future developments of SOFC devices.

ACKNOWLEDGMENTS The authors are indebted with one of the anonymous referees for insighftul comments that have given them a chance to prepare a stronger and more legible paper, with additional experimental data.


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Figure 1 – (a) A sketch of the cell (thickness of Ni and Cr layers is 70 nm) and Cr 2p, Ni 2p and Zr 3d maps (750×50 µm2), highlighting the Cr, Ni and the YSZ electrolyte regions of the initial (25°C, 10-10 mbar) and final state (after successive exposure at 650°C to in 10-6 mbar O2 and 1:1 C2H4:H2O and electrochemical polarization for total 120 hours). (b) Current-voltage curves recorded at 650°C to in 10-6 mbar O2 and 1:1 C2H4:H2O. The absolute values of the current are reported, the error bars correspond to 1 standard deviation, the lines were estimated by fitting with the Tafel law.35



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Figure 2 (a) Ni 3p and Zr 3d images centred at the Ni/YSZ interface. (b) Ni 2p spectra measured under different conditions: (top) at OCP after the pre-treatment in O2 environment (middle) under anodic polarization of 1 V in points A-F indicated in the Ni 2p maps, and (bottom) under cathodic polarization of – 1 V. The dotted lines indicate the BE energy position of the Ni0 and oxide components, measured under OCP conditions and indicate the position-dependent rigid energy shifts of the spectra induced by the polarization, in order to illustrate better the variations in the oxidation state, all Ni 2p spectra are aligned to the BE energy position of the Ni0 component, measured under OCP conditions.


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Figure 3 – (a) Ni 2p spectra measured in operando conditions starting from anodic polarization (+1 V) and switching successively to cathodic polarization (-1, -2, -3 V). (b) Ni 2p images taken at polarization of -1 and - 2 V showing the movement of the boundary between connected and disconnected islands as a result of potential-promoted Ni diffusion in the direction indicated by the arrows on top.



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Figure 4 C 1s spectra, measured inside the Ni and Cr patches at the anodic and cathodic sides under different electrochemical conditions. (a) The top three spectra are measured at the cathode side (-3 V), whereas the bottom spectrum stands for both patches on the anode side (+3 V); (b) After exposure to OCP conditions overnight: the least intense C 1s spectrum refers to the Ni patch on the anode side, which was initially C free. (c) Time evolution of the C 1s spectra on the Cr patch: 0, 5, 10 and 15 min after switching to anodic polarization of + 3 V.


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Table of Contents Graphic

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