Arturo Gamonal

Chen Sun

A Lorenzo

Estefania Fernandez-Bartolome

Elena Guerrero-Sanvicente

Bess Vlaisavljevich

Javier Castells-Gil

Carlos Martí-Gastaldo

Roberta Poloni

Reinhold Wannemacher

] A Lorenzo

Elena Sanvicente

Javier Castells

Carlos Marti-Gastaldo

Juan Cabanillas-Gonzalez

Jose Sanchez Costa

Divergent Adsorption-Dependent Luminescence of Amino-Functionalized Lanthanide Metal-Organic Frameworks for Highly Sensitive NO 2 Sensors

Keywords: Lanthanide Metal-Organic Framework, NO2 sensing, Divergent luminescent behaviour, DFT calculations

come

Introduction

Monitoring the emissions of toxic chemicals represents an important element in preventing and reducing pollution and in ensuring a high level of protection of the environment taken as a whole. Among the most common pollutants, nitrogen dioxide (NO2) is a toxic gas which is generated by combustion processes at high temperatures and can cause inflammation of the airways and even death at high concentrations. [1] It also plays a pivotal role in the formation of smog and acid rain as well as being central to the formation of ground level ozone, all being associated with adverse health effects. [2] NO2 detection is a challenging task since the majority of the commercial techniques and sensors currently used are affected by interferences, are costly or difficult to implement in the field. [3], [4], [5] Recently, metal-organic frameworks (MOFs), have emerged as excellent candidates for chemical sensing applications. [6,7] Such porous materials can surpass some of the inherent limitations of the materials used in actual state-of-the-art sensing technologies, such as their required high temperature operating conditions, associated with a poor selectivity and high energy consumption, or their low selectivity and long response times. [8], [9] Moreover, the reticulated network of porous MOFs provides adjustable functionalities and unique optical, [10] electronic [11], [12] and magnetic [13] properties. In this general context lanthanide-metal organic frameworks (Ln-MOFs) have attracted great interest mainly due to the large changes of their optical properties upon inclusion of analytes. [14], [15] More precisely, it has been reported that analyte hosting may alter the emission profiles and/or luminescence lifetimes of the lanthanide centre, either through enhancement, quenching or shifting of photoluminescence. [16] Furthermore, the right selection of the organic linker and its functional groups within the Ln-MOF can be used to further promote preferred guest binding for selective and sensitive detection. All this, together with their rich geometric topology, makes Ln-MOFs excellent candidates for chemosensing. [15] In the present work three isostructural Ln-MOFs have been synthesized from 2-amino-1,4-benzene dicarboxylic acid (H2N-BDC) and europium, terbium and gadolinium salts. [17] H2N-BDC, already described as an antenna-like ligand in previous work, [18] also functions as an excellent recognition centre for detection of small molecules [19] such as cations and nitro-aromatics, [20] solvent molecules, [21] or even some inert gases like CO2 or H2. [22] The NO2 sensing capacity of the Ln-MOFs is studied through the changes in the luminescence intensities, resulting in divergent luminescence behavior between Tb +2 and Eu +2 based-MOF. Combining experimental studies of the adsorption/desorption of NO2 molecules with density functional theory and multiconfigurational wave functionalbased methods we show that NO2 is bound via hydrogen bonding to the ligand leading to energy down shift of the lowest triplet state (T1) of the photosensitizing ligand. This gas induced downshift, yields to a modulation of the energy transfer from the triplet of the ligand to the Ln ion, thus providing a powerful new approach for the design of a novel generation of divergent-luminescent sensors.

Results and Discussion

STRUCTURAL DESCRIPTION. Three porous Ln-MOFs were synthesized according the method reported by Gamez and co-authors [23] , {[Tb2(N-BDC) S11a. The PL spectrum (solid green line) shows the characteristic sharp line emission from Tb 3+ ions corresponding to 5 D4→ 7 FJ (J = 2-6) transitions located at 642, 622, 586, 544 and 489 nm, respectively. Simultaneous emission from the ligand is manifested as a broad underlying band in the 400-550 nm range with a maximum at 445 nm. The PLE spectrum of 1 (dashed green line), obtained by monitoring the emission at 544 nm ( 5 D4→ 7 F5 transition in Tb 3+ ), exhibits a broad band due to the ligand indicating photosensitization of Tb. Regarding 2•2DMF, the PL spectrum exhibits sharp peaks at 580, 593, 617, 651 and 697 nm assigned to 5 D0→ 7 FJ (J = 0-4) transitions in Eu 3+ (solid red line in Figure S11b), in agreement with previous reports. [24] Similar to the case of 1•2DMF, the PLE spectrum monitored at 617 nm ( 5 D0→ 7 F2) exhibits a broad band covering the 300 -450 nm range associated to the organic ligand. Concerning 3, the PL and PLE spectra are summarized in Figure S11c. The PL spectrum (solid purple line) shows a broad band in the 400-700 nm range with a maximum at 454 nm, corresponding to the ligand emission. Note that the lowest energy level of Gd 3+ (32200 cm -1 ) is well above the S1 and T1 levels of NH2-BDC (as confirmed below) and therefore no quenching of the ligand emission by energy transfer to Gd 3+ occurs. The PLE spectrum of 3 (Figure S11c dashed purple line) exhibits a broad band assigned to the organic ligand.

In order to understand the Ln 3+ sensitization mechanisms and the nature of the excited states in 1•2DMF and 2•2DMF, the S1 and T1 ligand energy levels were determined by PL measurements of 3, as well as the pure ligand in solution, at 65 K (see SI for further information). Compound 3 permits the determination of the T1 level because the lowest energy level of Gd 3+ (32200 cm -1 ) is well above the S1 and T1 levels of NH2-BDC (as confirmed below) and therefore only phosphorescence of the ligand can be observed.

The singlet (S1) and triplet (T1) energy levels of the ligand were determined as the onsets of the fluorescence and phosphorescence spectra at about 405 nm (24700 cm -1 ) and 479 nm (20900 cm -1 ), respectively (see Figure S15 and explanation within). The energy position of T1 with respect to the 5 DJ states of Ln 3+ is one of the most relevant factors to determine the lanthanide luminescence efficiency in coordination compounds is the energy of T1 with respect to the 5 DJ states of Ln 3+ . [25] Previous studies have shown that for efficient room temperature Ln luminescence, the energy gap between the lowest T1 energy level and the emitting state of the lanthanide ion ΔE ( 3 ππ*-5 D0) should be larger than 2500 cm -1 . [26], [27] To ensure a fast and irreversible energy transfer. If the offset energy is lower, a back-transfer from the Ln 3+ ion to the ligand may take place thus reducing the efficiency of the sensitized PL.

The T1 of the ligand in 2 is located 3660 cm -1 above the 5 D0 state of Eu 3+ (17240cm -1 ) (see Figure 5 for further details), which confirms that back-transfer is efficiently hindered at room temperature. Conversely, the energy gap between T1 and the emitting level of Tb 3+ ( 5 D4, 20430 cm -1 ) is 470 cm -1 , too small to block back-transfer at room temperature, which explains the residual ligand emission in the PL spectrum of 1 [START_REF] Blasse | Spectra and Chemical Interactions. Structure and Bonding[END_REF] De-activation of Tb 3+ levels by back-transfer to the ligand is further indicated by the significantly faster PL decay of 1 (197 µs) with respect to 2 (1.05 ms) monitored at room temperature at the respective Ln 3+ emission peaks, (Figure S18). The exposure of 1 to 5 ppm NO2 results in a decrease in PL intensity by about 20% monitored at the peak of the emission (543 nm) and reaching a plateau after 2 min. Subsequent exposure of 1 to PSA leads to recovery of the emission intensity within a similar time frame. Contrary to the behavior of 1, exposure of 2 to 5 ppm of NO2 leads to an increase in PL by about 20% during 90 seconds until it reaches a plateau. Upon PSA exposure, the PL intensity returns to the original level. Sample 2 was cycled three times in order to show the reversible behavior of the sample. In order to further assess the gas sensitivity degree, both materials were exposed to diluted NO2 flow (from 5 to 0.5 ppm) while keeping similar measurement conditions. The results shown in Figure S17 demostrate that both samples maintain the same PL behaviour even at low pressure: the lower sensing detection with our setup for 1 is 0.5 ppm (LOD of 1.8 ppm) and for 2, 1 ppm (LOD of 2.2 ppm).

Importantly, exposure of 3 to NO2 did not lead to significant changes in PL intensity, confirming that NO2 has negligible effect on the ligand emission, this reversible change in the luminescence intensity of 1 and 2 must be associated to the NO2 adsorption/desorption process inside the Ln-MOF.

As previously proposed for a closely-related Ln-based porous compound, the NO2 molecule could interact within the MOF via three different mechanism; i) bonding to the metal centers, [START_REF] Sato | [END_REF], [30] ii) reacting with pendant groups of the ligand, [31], [32] or iii) interacting non-covalently with the ligand. [33], [34], [35] All three options are discussed below.

Figures 2a and2b already show no noticeable spectral shifts or changes in the crystal field splitting in the emission lines assigned to the 5 D0→ 7 FJ, J=1...4 and 5 D4→ 7 FJ, (J=2-6) transitions from either Tb 3+ or Eu 3+ ions, upon gas absorption. There are also no changes in the relative intensity between the specific transitions upon NO2 uptake, although, as discussed above, the 5 D0→ 7 F2 transition of Eu 3+ is known to be hypersensitive to the environment, in contrast to the magnetic dipole-allowed 5 D0→ 7 F1 transition. Furthermore, PL lifetimes measured at the respective Ln 3+ emission peaks in 1 and 2 upon exposure to NO2 confirm that the lifetime of 1 drops from 197 to 126 µs, whereas no significant effect of NO2 adsorption on the PL lifetime is observed in 2 (see Figures S18 to S20). These observations strongly suggests that NO2 adsorption does not lead to changes in the first coordination sphere around the Ln 3+ ion. [36] Thus, a molecular absorption of NO2 by the ligand should be expected.

AB INITIO CALCULATIONS. To understand the nature of the NO2 interactions with the Ln-MOF, DFT calculations [37], [38][39][40] have been performed. A total of 8 different configurations of the NH2 groups within the MOF were considered for 1 and 2 in order to account for the fractional occupation of the amino site reported experimentally [23] (Fig. S22). The electrostatic potential energy map of the bare MOF was computed for each configuration in order to find the NO2 binding location.

The NO2 molecule was placed in regions of highly attractive potential within the unit cell and a full geometrical optimization was performed (see section S11). This procedure has allowed us to correctly predict in the past the location of adsorbate CO2 molecules within a MOF in absence of experimental guidance. [41] Remarkably, despite the fact that the starting NO2 is near the metal site (see section S9 and Figure S25 for information), the final configuration always converges to the NO2 being located within the pore near the NH2 groups.

Specifically, the NO2 binds via a network of hydrogen bonds, involving the amino groups of the neighbouring N-BDC ligands and CH3 from one coordination molecule of DMF (Figure S25, S27-28 shows the NO2 binding sites determined computationally). For both MOFs, a significant charge transfer from the ligands to NO2 is predicted (Table S5), ranging from 0.4 to 0.7 electrons, depending on the amine configuration. This charge transfer mostly distributes on the amino groups close to the guest molecule and to a minor extent to DMF, while negligibly on the Ln ions (Table S6). The cooperative binding mechanism between samples 1 and 2 and the NO2 molecules, attributed to multiple hydrogen bonds, electrostatics and van der Waals interactions, yield to binding energies of NO2 ranging from 35 kJ/mol to 65 kJ/mol, depending on the specific amine configuration. This noncovalent adsorption is consistent with reversible sorption of NO2 by the MOF. We note that binding energies (and consequently charge transfer) are determined by the specific amino group configuration, and negligibly by the specific Ln atom (Table S5).

Previous studies [27,42], [43] have shown that a modulation in the luminescence quantum yield occurs, depending on the position of the ligand triplet state (T1) relative to the lanthanide 5 Dj levels. In order to determine whether the shift of the T1 state of the ligand, upon binding, can explain the experimental results, multireference CASSCF/CASPT2 [44] calculations were performed for the NH2-BDC ligand bound to NO2 (see Fig. 3c). As the NO2 molecule is rigidly displaced from the binding configuration to larger distances from the ligand, the triplet state shifts to higher energies. The first excited triplet state, T1, exhibits a dominant π to π* (HOMO to LUMO) character and is predicted to be at 22398 cm -1 (Figure S28). When the NO2 is hydrogen bonded to the ligand the triplet is at 22137 cm -1 (see Figure 3b) corresponding to a downshift in energy by approximately 260 cm -1 . The computed downshift of T1 as a function of the distance between NO2 and the ligand is found to linearly correlate with the ligand-to-molecule charge transfer (see Figure S27). By making use of this linear regression and the computed charge transfer within the MOF, the T1 downshift is estimated to be in the 230-410 cm -1 range, approximately, in the MOF, depending on the binding configuration.

Summarizing, the downshift of T1, upong NO2 adsorption, leads to opposed luminicence scenarios in 1 and 2 (see Figure 3a). The results are rationalized as follows. In 1, the ligand T1 level is pushed only about 200 cm -1 above the 5 D4 level of Tb 3+ after adsorption of NO2, leading to a detrimental effect on the PL quantum yield. The two states are close in energy and the back transfer is enhanced thus decreasing the luminescence quantum yield [45,46], [47] . Conversely, NO2 binding in 2 results in a smaller T1 -5 D1 energy offset, (from 1900 cm - 1 to 1640 cm -1 ), promoting triplet-to-Eu 3+ transfer rates as well as the total PL quantum yield. [27] These theoretical predictions are in accordance with our experimental findings.

Conclusion

We introduce a proof-of-concept sensor based on two lanthanide-based MOF that efficiently monitor (or detect), in a straightforward manner, the inclusion/exclusion of NO2 molecules within the MOF. The reversible inclusion of NO2 molecules in the porous framework at low concentration level triggers an increase of the intensity of the Eu 3+ luminescence and a decrease of the Tb 3+ luminescence intensity. Experimental spectroscopy and computational studies were combined to deliver the signal transduction mechanism.

Based on the divergent luminescence change upon NO2 exposure in 1 and 2, the lack of changes in the relative intensities of the 5 DJ→ 7 FJ transitions, and the computational predictions we conclude that the modification of the luminescence intensity can be ascribed to changes in the relative positions of the ligand triplet and the Ln energy levels upon NO2 adsorption. In spite of the weak interaction (physisorption) between NO2 and the MOF, the affinity of the amino groups for NO2 makes this MOF sensitive enough for detection of NO2 in conventional luminescence displays without requiring expensive electronics. This study herein suggests a powerful new approach for the design of future luminescent sensors of noxious volatile compounds.

Experimental Section

Experimental Details. Compounds 1, 2 and 3 were synthesized according to the literature procedures (see S1-S2). TGA, FT-IR, PXPD are detailed in S3 to S5. Photo-luminescence measurements, sorption measurements and Ab initio calculation details are also described in S6 to S11 sections.

Figure 1 .

 1 Figure 1. 2-amino-1,4-benzene dicarboxylic acid (H2N-BDC) (a); dinuclear cluster of Ln 3+ (in green) illustrated with balls and polyhedrons. Each metal ion is coordinated to a total of nine oxygen atoms from three chelating and two bridging N-BDC ligands, as well as from two coordinated DMF molecules (b); Ln-MOF views showing the molecular cages and the different occupancies of amino groups found by X-ray single crystal diffraction (c) and (d).

  NO2 SENSING. Prior to the sensing experiment, 1 and 2 were activated by exposing them to a flow of pure synthetic air (PSA) for five minutes at room temperature, with the aim of removing the non-coordinated DMF molecules inside the lattice. This activation process was confirmed by FTIR measurements (FigureS1 and S6). Then, the samples were exposed to consecutive cycles of pure PSA and 5 ppm of NO2 in PSA. During exposure the variation of the luminescence intensity was recorded (see SI for further details). The time duration of each cycle (5 minutes) was long enough to reach equilibrium conditions. The results for both Ln-MOF are plotted in Figure2a-b.

Figure 2 .

 2 Figure 2. Evolution of PL spectra in 1 (a) and 2 (b) during three NO2 / PSA cycles. The PL intensity evolution at 543 nm for 1 (c) and at 615 nm for 2 (d) are plotted vs time (the NO2 exposition is denoted by the shaded areas). A single cycle sensing evolution illustrating the divergent behaviour for 1 (e) and 2 (f).

Figure 3 .

 3 Figure 3. Jablonski diagram featuring the ground singlet (S0), first excited singlet (S1) and lowest triplet (T1) states of the ligand together with the relevant atomic levels of Gd 3+ , Tb 3+ and Eu 3+ . Values for the energy levels are given in cm -1 (a). Calculated energy of T1 vs charge transfer (b). Illustration of the ligand / NO2 configuration used for the CASSCF/CASPT2 calculation (c).

Acknowledgements

JSC is grateful to the Spanish MINECO for financial support through National Research Project (CTQ2016-80635-P), the Ramon y Cajal Research program (RYC-2014-16866) and the Comunidad Autónoma de Madrid (PEJD-2017-PRE/IND-4037) for funding support. IMDEA Nanociencia acknowledges support from the 'Severo Ochoa' Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). This research used resources of the Advanced Light Source, which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. We would like to thank to XALOC-ALBA synchrotron source under the project no. (2018012561). JC-G and RW acknowledge funding from the European Union structural funds and the Comunidad de Madrid MAD2D-CM (S2013/MIT-3007) and NMAT2D-CM (S2018/NMT-4511) projects. JC-G acknowledges funding from MINECO through projects RTI2018-097508-B-I00 (AMAPOLA) and PCIN-2015-169-C02-01/02. RW acknowledges funding from MINECO through project MAT2015-71879-P. IMDEA Nanociencia acknowledges support from the "Severo Ochoa" Programme for Centres of Excellence in R&D (MINECO, Grant SEV-2016-0686). Computational resources were granted by GENCI under CINES grant A0060907211 and by froggy platform of the CIMENT infrastructure. This work benefited from the support of the project ANR-15-CE06-0003-01 funded by the French National Agency for Research.

RESEARCH ARTICLE

Supramolecular interactions are key for highly sensitive detection of NO2 using porous amino-linked lanthanide metal-organic framework. Tuning of ligandto-metal energy transfer causes an unprecendented divergent luminescence response to NO2 sorption in Ln-MOFs rationalized using computational and photoluminescence method


Jonathan Riffaud

Philippe Cassette

Margot Corbel

Marie-Christine Lepy

Valérie Lourenço

Mark A Kellett

Marie-Christine Lépy

Measurement of the absolute gamma-ray emission intensities from the decay of 103 Pd

Introduction

Conclusions and perspectives

• 6 point sources for gamma-and X-ray spectrometry Dilution to 10 MBq.g -1 using 3 mol•L -1 HCl with 10 µg.g -1 of stable ruthenium to ensure chemical stability of 103m Rh in equilibrium with 103 Pd.

• 6 volume sources for liquid scintillation (LS) New dilution by a factor of 20. Each source contained about 5 kBq of 103 Pd in Ultima Gold scintillator, and about 100 µL of water was added for stabilization purposes.

The impurity checking on the initial solution did not reveal any impurity and the detection limit was less than 2�10 -4 Bq•Bq -1 . A complementary measurement was performed one month after the activity measurement, to benefit from the 103 Pd decay: Rhodium-101 (half-life = 3.30 (10) a) was detected with an activity relative to that of 103 Pd equal to 10 -5 Bq•Bq -1 .

Activity measurement

The palladium chloride solution was standardized by liquid scintillation, using the Triple-to-Double Coincidence Ratio (TDCR) method.

The LS sources were measured using the RCTD1 counter of LNHB. Each source was measured 10 times for 1 minute, allowing relative counting uncertainty of 6•10 -4 .

The detection efficiency was calculated taking into account the asymmetry of the photomultiplier tubes, by considering the three individual TDCR values, i.e. T/AB, T/BC and T/AC. The absorption of the photons was calculated by Monte Carlo simulation with PENELOPE

The relative standard uncertainty due to the source dispersion was 0.13% and the global uncertainty calculation was achieved by a global Monte Carlo simulation, following the recommendations of the supplement 1 of the Guide to the expression of uncertainty in measurement.

Reference activity: 9 472 (24) kBq.g -1 (Reference date: 19/12/2017, 12h00 UTC)

Half-life

The half-life of 103 Pd was measured by gamma-ray spectrometry on a high-purity germanium (HPGe) detector, using an aliquot of the diluted solution, for 50 days. The measurement was carried out by following the 103 Pd main line (357.43 keV).

The absolute photon emission intensities were derived from gamma-and X-ray spectrometry using HPGe calibrated detectors, with different measuring conditions to cross-reference the results.

Spectrum of 103 Pd in the high energy range (8 keV -550 keV)

For each energy 𝐸𝐸 𝑖𝑖 , the photon emission intensity, 𝐼𝐼 𝑖𝑖 , is derived from the count rate in the relevant peak, 𝑛𝑛 𝑖𝑖 : The 357.43-keV photon emission intensity is 0.02464 ( 16) per 100 disintegrations, in agreement with the tabulated data and obtained with a lower uncertainty.

In addition, the emission intensities of four other gamma rays are also derived with a significant reduction of the uncertainties. Three gamma emissions (62.43 keV, 295.00 keV and 497.08 keV) from the third, fourth and fifth excited levels have significantly higher intensities than the tabulated data. Conversely, for the emission from the metastable level (39.75 keV), the present result is 7% lower than the tabulated value. Also, the L-and K-X-ray emission intensities are weaker (around 10%) than the tabulated data. This would suggest that the intensity of the electron capture towards the first excited level should be weaker, while the electron capture branches to levels 3, 4 and 5 should be more intense. Another possibility would be an electron capture branch towards the stable level.

The present results provide new information and should give some useful clues to be exploited in a future evaluation of the 103 Pd decay scheme.

Absolute photon emission intensities in the decay of 103 Pd

Fit of two Gaussian functions to the experimental data of the 62-63 keV doublet

  Photon emission intensities

  𝜀𝜀 𝑖𝑖 : FEP efficiency for energy 𝐸𝐸 𝑖𝑖 , 𝐼𝐼 𝑖𝑖 = 𝑛𝑛 𝑖𝑖 � ∏ 𝐶𝐶 𝑖𝑖𝑖𝑖 𝜀𝜀 𝑖𝑖 �𝐴𝐴 𝐴𝐴: source activity (Bq), 𝐶𝐶 𝑖𝑖,𝑗𝑗 represents different correction factors.

in the photon emission intensities Question on the decay scheme equilibrium

	Energies are quoted in keV
								LARA (2019)	Present study
							Energy (keV)	Photon emission intensity (%)	Relative uncertainty (%)	Photon emission intensity (%)	Relative uncertainty (%)
							XL	8.73 (23)	2.6	8.61 (43)	5
							20.07 (Kα 2)	22.05 (22)	1.0	19.59 (21)	1.1
	Authors	Year	Half-life (d)	Absolute uncertainty (d)	Relative uncertainty (%)	Number of half-lives	20.22 (Kα 1) K alpha total 22.81 (Kβ1) 23.20 (Kβ2)	41.70 (40) 63.75 (46) 11.34 (15) 1.88 (7)	1.0 0.7 1.3 3.7	39.48 (42) 59.07 (47) 10.27 (11) 1.781 (19)	1.1 0.8 1.1 1.1
	Rietjens et al. Panontin et a. Grunditz et al. Czock et al.	1954 1968 1969 1975	17.5 16.9 18.4 16.961	0.5 0.1 0.5 0.016	2.86 2.72 0.09 0.59	2 4	62.41 53.285 39.756 K beta total K total	7.8 (23) E-4 7.2 (20) E-6 0.0698 (35) 13.22 (17) 76.97 (49)	29 28 5.0 1.3 0.6	0.001128 (16) < 8 E-6 0.0647 (7) 12.05 (11) 71.1 (6)	1.4 -1.1 0.9 0.9	Inconsistencies
	Vaninbroukx et al.	1981	16.991	0.019	0.11	2-3	241.88	4.9 (5) E-7	10	-	-
	Popov et al.	2004	16.8	0.6	3.57	5.9	294.98	0.00297 (24)	8	0.00315 (7)	2.3
	Present study	2017	17.106	0.058	0.34	2.9	317.72	1.50 (19) E-5	13	1.37 (17) E-5	13
							357.43	0.0246 (6)	2.4	0.02486 (17)	0.68
							443.8	1.60 (12) E-5	7.5	2.1 (8) E-5	33
							497.08	0.00411 (11)	2.7	0.00439 (7)	1.6

* Corresponding author: marie-christine.lepy@cea.fr


Alexandre Nau

Clément Comminges

Nicolas Bion

Operando isotopic exchange on SOFC cells: oxygen transport dependency on applied potential

Keywords: Isotopic exchange, oxygen electrochemistry, SOFC, Operando methods, Platinum

The oxygen isotopic exchange technique is a powerful tool to investigate the oxygen transport kinetics in an oxide solid. In a solid oxide fuel cell, isotopic surface exchange and diffusion coefficients are classically determined by using the Isotopic Exchange Depth Profiling method followed by ex situ SIMS characterizations. Despite its relevance, the utilization of in situ or operando techniques to measure the isotopic exchange under an electrical bias remains marginal. We developed here a set-up which enables operando monitoring of oxygen exchange in SOFC type cells under polarization. The system has been used for studying the oxygen mobility dependency upon polarization on a symmetrical Pt/YSZ/Pt cell. Homomolecular and heterolytic exchange reactions were undertaken to investigate the oxygen activation step and discriminate the limiting step among the sequence of elementary steps which constitute the oxygen transport process in the SOFC system. Oxygen ions incorporation into the dense ionic conductor was identified to be the rate determining step, and its first order rate constant dependency on applied potential was established.

Introduction

Oxygen activation and diffusion are of paramount importance for various applications such as catalytic oxidation reactions, [1,[START_REF] Bion | Metal Oxides in Heterogeneous Catalysis[END_REF] membrane reactors, [START_REF] Sammells | [END_REF]4] solid oxide fuel cells [5] , electrolyzers, [6] electrochemical promotion of catalysis [7,8] ... The fine understanding of mechanisms involved in oxygen activation and diffusion is therefore critical in the quest of efficient materials adapted to the aimed application, and its use requires knowledge in oxygen reactivity, and more specifically on its mobility. Oxygen reduction reaction (ORR) in solid oxide fuel cells is known to be the half reaction that limits SOFC's performances. In the cathodic half cell (where the ORR occurs), the mechanism can be split into five elemental steps: i) dissociative adsorption of molecular oxygen at the catalyst surface, ii) surface diffusion of oxygen atoms from the catalyst surface to the triple phase boundary (TPB), iii) charge transfer to produce oxygen ions, iv) oxygen ions incorporation into the ionic conductive electrolyte and finally v) diffusion of ionized oxygen atoms in the bulk of the electrolyte. The rate of each step is depending on several parameters such as temperature, oxygen partial pressure and on the magnitude of the polarization. Oxygen Isotopic Exchange (OIE) is classically undertaken to study the oxygen diffusion through an oxide powder. [9] Although several studies report on the effect of temperature and pressure, [START_REF] Duprez | Isotopes in Heterogeneous Catalysis[END_REF][START_REF] Bouwmeester | [END_REF] very little is known on the effect of electrode polarization on oxygen exchange mechanism.

Destructive ex-situ methods such as isotope exchange depth profile (IEDP) method have been employed to probe the oxygen mobility by using TOF-SIMS after OIE without polarization [12][13][14] and with polarization. [15] However, experimental methods that enable operando monitoring of oxygen exchange in SOFC type cells is highly needed in order to decipher the effect of electrode polarization on oxygen mobility. Early works of Sobyanin et al. demonstrated the feasibility of such an experiment by performing homomolecular oxygen isotope exchange on a Pt/8-YSZ/Pt cell. [16] They concluded that at a moderate temperature of ca. 500°C, electrode polarization has no effect on the rate of oxygen homomolecular exchange with respect to its open circuit value. More recently, Wachsman et al. developed an operando setup for monitoring oxygen surface exchange on LSM, LSCF, LSC and LSF cathode powders while applying a cathodic bias. By applying a two-step mechanism involving dissociative adsorption (i.e. steps i) to iii) in the global mechanism discussed above) followed by incorporation of the oxygen ion in the oxide lattice (iv), they concluded that oxygen exchange coefficient (kex) increases exponentially with applied cathodic overvoltage. [17] This experimental setup based on cathode powders is however differing from real electrochemical cells where the microstructure plays a significant role in performances. Khodimchuk et al. have developed an isotopic exchange setup to study the effect of the polarization on the reaction. By using a symmetrical cell Pt/YSZ/Pt, they have shown that kinetics of oxygen exchange depend on the electrode charge. [18] The main goal of the present study is to identify the relationship between oxygen exchange/mobility and applied electrical bias on a Pt/8-YSZ/Pt as a model symmetrical cell, and shed light on the rate determining step.

Results and Discussion

In order to monitor precisely the influence of electrode polarization on the rate of oxygen exchange, one has to choose an adequate working temperature where the YSZ electrolyte is conductive enough to enable the O 2-diffusion from the cathodic to the anodic compartment, but not too high temperature so as to not activate too much both electrochemical and diffusion processes. Fig. 1 displays polarization curves with various temperatures at the atmospheric pressure (a) and various oxygen pressures at the selected temperature of 600°C (b).

[a] Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), University of Poitiers, CNRS, 4 rue Michel Brunet, TSA51106, F86073 Poitiers Cedex 9, France. Email: clement.comminges@univ-poitiers.fr; nicolas.bion@univ-poitiers.fr Supporting information for this article is given via a link at the end of the document.

The obtained voltammograms (Figure 1.a)) between 500°C and 700°C shows that decreasing the temperature leads to a significant decrease of the current density, a diffusion limiting current can be observed below 500°C. Moreover, it can be seen that anodic and cathodic branches are symmetrical thus confirming that gas flux in the two compartments is homogeneous. This is also confirmed by the open circuit voltage of c.a. 40 mV which is very close to the theoretical value of 0 V, indicating a fair gas and temperature homogeneity between the two compartments. The working temperature of 600°C has been defined for this work. The obtained voltammograms (Figure 1.b)) between 50 mbar up to an oxygen flow of 100 cm 3 .min -1 at atmospheric pressure shows that decreasing oxygen pressure leads to a slight decrease of the current density but no diffusion limiting current is observed. This confirms that activation control is maintained even when operating the cell at low oxygen pressure. The voltammogram clearly displays two distinct zones. A first faradaic zone between -0.5 and +0.5 V (zoom in figure S1) which is characterized by two symmetrical waves whose intensities are dependent on the oxygen pressure. Electrochemical impedance spectroscopy (EIS) was employed to characterize further these two distinct regions. At OCV (fig. 2a), a semi-circle whose diameter is dependent on oxygen pressure with a characteristic frequency of 200 Hz is attributed to the O2/O 2- redox couple in agreement with the data of Mitterdorfer et al. [19] These waves are related to a two electron transfer with dissociatively adsorbed oxygen on Pt at the three phase boundary (TPB). According to Mizusaki et al., at the temperature of 600°C, there is a competition between two parallel pathways for charge transfer: molecular oxygen surface diffusion to the TPB (pathway a) or dissociative adsorption of O2 near the TPB (pathway b). [20] In the situation presented in fig. 1, both regimes are observed. At low oxygen pressure (i.e. 50 mbar), a diffusion limited current is observed (the plateaus) corresponding to the molecular oxygen surface diffusion to the TPB prior electron transfer. As PO2 increases, this diffusion current drastically decreases thus favoring the second pathway (dissociative adsorption of O2 near the TPB) as can be seen on figure S1. This second pathway is dominating at cell voltages below -0.5 V and above + 0.5 V. The Nyquist plot shown in figure 2a (at OCV) relates this evolution. A Gerisher impedance at high frequency mostly observed at OCV (diffusion process coupled to an electrochemical reaction: 45° straight line at the beginning of the charge transfer semicircle) [21] showing the atomic oxygen surface diffusion. When PO2 increases at OCV., the polarization resistance of this process increases as well. It shows that this pathway is disfavored compared to the dissociative adsorption of O2 directly at the TBP. This is also in agreement with data of Mitterdorfer et al. who demonstrated that the surface diffusion coefficient of molecular oxygen rapidly increases at low oxygen surface coverage (θ<0.3). [19] This suggests that when PO2 decreases, the surface coverage decreases as well leading to an increase of the surface diffusion coefficient, and finally a smaller polarization resistance. Below -0.5 V ORR occurs and an additional faradaic current appears. Symmetrically, oxygen evolution reaction (OER) onset at ca. +0.5 V draws the corresponding anodic faradaic current. This means that the Pt/YSZ/Pt symmetrical cell can be run at the low oxygen pressure required for the isotopic exchange. Additional evidences are provided by EIS. At the working temperature of 600 °C, it is observed that the electrolyte resistance is 50 Ω as can be seen on the high frequency intercept in the real axis of the Nyquist representation in Figure 2 a, c and e. This value of 50 Ω is rather high and is a consequence of the chosen temperature (600°C) that induces a moderate ionic conductivity. Therefore, the contribution of YSZ ohmic resistance is rather important at high current densities as it is shown on the corresponding voltammetries at 600°C corrected for ohmic drop (figure S2). This ohmic penalty corresponds to almost doubling the cell voltage at the highest current densities investigated. It seems also quite clear that the electrode kinetics for oxygen reduction and oxygen evolution reactions remains fast, despite the low ionic conductivity of the YSZ electrolyte. Another faradaic contribution is also observed at higher frequencies (around 30 kHz) and clearly visible at 0.6 V (figure 2d), less at 1.5 V, but not at OCV. This is attributed to the dissociative adsorption of O2 near the TPB (pathway b). When this capacitive loop at 30 kHz appears, there is also the appearance of an inductive loop at low frequency.

Both loops seem to be related. This low frequency inductive loop can be attributed to adsorbed oxygen species. [22] . Van Hassel et al. demonstrated that the transient species Oad -surface coverage is strongly potential dependent and concluded that inductive effects may occur whenever a stepwise electron transfer takes place towards adsorbed intermediates. [23] On the other hand, Chen et al. concluded that the occurrence of low frequency inductive loops is primarily determined by the ability of the electrode materials to supply atomic oxygen for the reaction at the interface, and proposed a mechanism of competitive atomic oxygen supply and dissociative oxygen adsorption and diffusion. [24] Both models leads to the appearance of low frequency loops and implies multiple transient adsorbates. From this EIS data at 600°C and 0.6 V (Figure 2c), we can determine the different resistances of 50 Ω for the solid electrolyte, 20-25 Ω for pathway a, and 20-25 Ω for pathway b. This means that polarization resistances are ca. 40-50 Ω, comparable to that of the solid electrolyte. Therefore the total cell resistance is equally split between the solid electrolyte conductivity and the polarization resistances.

Mass transfer limitation is a critical issue when assessing kinetic parameters. As can be observed on the Nyquist representations, no Warburg impedance is observed at low frequencies (finite diffusion: a 45° straight line should have appeared). The sole diffusion process that might be observed in this data can be seen at higher frequencies with a Gerisher behavior as discussed above. This is also naturally confirmed on the voltammetric measurements (figs. 1a and 1b) where no current limitation appears at high cell voltage. The study of oxygen isotopic exchange was performed at T=600 °C, where all phenomenon (oxygen surface diffusion, electron transfer and electrolyte conductivity) can be observed without any clear dominant contribution. Under these experimental conditions (600°C and PO2 = 20 mbar), the oxidizing atmosphere on both cathodic and anodic compartments prevents the YSZ reduction. Indeed, extremely low O2 pressures as well as high overvoltage are required to trigger electrochemically the YSZ reduction and introduce a significant electronic conduction in the electrolyte. [25] We first investigated the homomolecular exchange. On oxide-supported Pt sample, the reaction occurs according to a mechanism of adsorptiondesorption and gives useful information on the oxygen activation process at the surface of metallic particles. [26] Huang et al. have shown that at high temperature due to the mass transfer limitation the surface exchange has a low apparent activation energy (8 kJ.mol -1 ), when at low temperature (with no mass transfer limitation) the value increases to 42.3 kJ.mol -1 . [27] With our experimental setup we calculated the activation energy of the initial rate of exchange (rq) of 50 kJ.mol -1 (for experiment between 300 and 700°C, figure S3), we can therefore assume that our system has no mass transfer limitation.

rq is defined as :

𝑟 𝑞,𝑡=0 = 2𝑁 𝑔 𝑃 0 [ 𝑑𝑃 34 𝑑𝑡 ] (1)
Where Ng is the number of 18 O+ 16 O atoms in the gas phase; P0 is the total pressure, t is the time of equilibration and P34 is the partial pressure of 18 O 16 O.

As it can be seen in Figure 3, there is no influence of the electrode polarization on the rate of equilibration. It reinforces the previous result of Sobyanin et al. [16] who reported the same conclusion at 510 °C and P(O2)= 0.5 mbar. In this range of temperature, there is no effect of the polarization on the oxygen activation process which is likely limited by the dissociative adsorption energy. The heterolytic exchange was first undertaken at OCV (Figure 4a). A decrease in the partial pressure of 18 O2 (m/z=36) takes place as a function of increasing time during which the partial pressures of the two other oxygen isotopomers increase. The production 18 O 16 O (m/z=34) is distinctly predominant at the initial points of the exchange while the presence in the gas phase of 16 O2 (m/z=32) is noticeable after 15 min. This evolution of the isotopic distribution is indicative of a simple mechanism of the exchange process which involves only one single O atom from the solid (eqn ( 12)). Because of the closed recycle system, 16 O2 is produced later on according to a consecutive simple reaction (eqn ( 13)). The simple mechanism was already observed for the exchange of powder YSZ sample [28] and is consistent with the ability of Pt to dissociate O2. [START_REF] Duprez | Isotopes in Heterogeneous Catalysis[END_REF] However another explanation is proposed by Huang et al. who described how the measured values of the oxygen isotopologues are related to the surface concentrations of 16 O and 18 O. [27] In their explanation the production of 34 only depends on the surface 18 O coverage and could not inform on the nature of heterolytic exchange mechanism.

When a similar experiment as above is performed in which 1.5 V cell potential is substituted for OCV, the mechanism of the reaction remains unchanged with preponderance of the simple exchange but an important increase of the kinetic is noticed (Figure 4b). Indeed it can be seen that the curves corresponding to the consumption of the 18 O2 and to the production of 16 O 18 O intersect after 50 min of reaction at OCV against only 30 min when the cell is polarized. This result is reinforced by the suppression of the polarization at t=118 min which is concomitant with a decrease of the rate of exchange (slope discontinuity on the curves of the figure 4b). Closer inspection of the isotopic distribution shows that the production of 16 O2 was quicker under polarization than at OCV. Figure 5a illustrates the evolution of the gas-phase 18 O atomic fraction (hereafter denoted as g) as a function of time at different magnitudes of cell polarization.

The general trend is that increasing the cell voltage systematically increases the exchange rate as compared to the OCV baseline. It is also worth noting that between OCV and 1V, the decrease of g seems to be more linear than that observed above 1V.

Figure 6a illustrates the evolution of the gas-phase 18 O atomic fraction (hereafter denoted as αg) as a function of time at different magnitudes of cell polarization. The general trend is that increasing the cell voltage systematically increases the exchange rate as compared to the OCV baseline. It is also worth noting that between OCV and 1V, the decrease of αg seems to be more linear than that observed above 1V. This tends to show that two different regimes are observed, the first one corresponding to the pathway a) where oxygen diffusion to the TPB dominates, and the second one to the pathway b) O2/O 2-faradaic region where g follows an exponential decrease. Assuming that oxygen exchange is a first order reaction, the rate constant of the rate determining O in the gas phase with Pt/YSZ/Pt cell at different potentials (a), and evolution of the kinetic rate of exchange at different potentials (black: OCV, red: 0.6V, blue: 1V, orange: 1.5V, and green: 2V). The first order rate constant is determined from exponential fit using:  g =  0 exp(-kt) step (r.d.s) can be extracted by fitting the data in Figure 5a with eqn (2):

𝛼 𝑔 = 𝛼 0 exp (-𝑘𝑡) (2)
where k is the first order rate constant of the r.d.s and t is time. An exponential increase of k with the applied cell voltage (Figure 5b) is observed, highlighting that cell polarization has an important effect in the rate of oxygen exchange. When looking at this evolution, one can draw the parallel with the Butler-Volmer law which describes an exponential increase of the rate constant for electron transfer with the applied overvoltage. The two electrode setup along with the short circuiting of gas atmospheres allows observing all elementary steps happening from the cathode to the anode. Therefore, the reaction sequence can be described as follows:  Dissociative adsorption of molecular oxygen in the cathodic side (eqn (3)):

18 O2,g + Pt*  2 18 OPt ads

 Surface diffusion from the Pt active site to the triple phase boundary (TPB) (eqn (4)):

18 OPt ads  18 OTPB ads + Pt*

 Charge transfer at the cathodic TPB (eqn ( 5)): 18 OTPB ads + 2 e - 18 O 2-TPB cathode (

 Incorporation of oxygen ions into YSZ (eqn ( 6)):

18 O 2-TPB cathode  18 O 2-YSZ (6)
 Oxygen diffusion in the bulk of YSZ to the anode TPB (eqn (7))

18 O 2-YSZ  18 O 2-TPB anode (7) The thick YSZ electrolyte implies that the amount of 18 O atoms incorporated is negligible compared to the number of 16 O atoms present in the electrolyte. Therefore, an accumulation of 18 O atoms at the interphase cathode/ electrolyte is created. [29] The respect of the charge balance in YSZ implies that 16 O 2-ions at the YSZ/ anode interphase are pumped out to the anodic TPB and undergoes the charge transfer at the anode (eqn (8)) 16 O 2-TPB anode  16 OTPB anode + 2 e -

 Surface diffusion from anode TPB to PtO active site (eqn (9)) 16 OTPB anode  16 OPtO anode (9)  Desorption (eqn (10)):

2 16 OPtO anode  16 O2,g

However, It can be seen on Figure 4 that at initial times the molecule 16 O 18 O (m/z=34) is preferentially formed instead of 16 O2 (m/z=32), indicating that a competitive reaction to eqn.

10 is involved during the oxygen desorption at the anode side. Therefore, rapid surface exchange takes place at the anode surface between the 18 O2 which is also present in the gas phase (identical atmosphere for both anodic and cathodic compartments) and the 16 OPtO anode before the desorption reaction. This tends to further argue that surface exchange is a much faster reaction than incorporation or desorption.

When polarizing the cell in the O2/O 2-faradaic region (ΔE>1V, Figure 4b), additional oxygen atoms are transferred at the anode side at a rate determined by the Faraday's law (i.e Q/2F). This additional amount is drove by the rate of the charge transfer reaction which is exponentially activated with overvoltage (eqn ( 5) and ( 8)). It is clearly evidenced by comparing the amount of oxygen atoms exchanged determined independently from the isotopic exchange experiment with the amount of oxygen atoms exchanged calculated from the Faraday's law (Figure S4 to S7). When subtracting the amount of exchanged oxygen atoms at 1V to the amount exchanged at OCV, one finds the amount of exchanged oxygen atoms predicted from the faradaic current. This is valid at short times (t < 15 min) when the 18 O2 partial pressure in the gas phase is close to its initial value.

In other words, the heterolytic exchange (observed at OCV) and the faradaic exchange are cumulative processes. The amount of 18 OPt ads depends mainly on the molecular oxygen partial pressure and the reaction temperature. In our experimental conditions, PO2 as well as T are constant in all experiments. This means that under steady state conditions, the oxygen coverage, θ, is constant as well. Mitterdorfer et al. shown that rate constants for adsorption and desorption (eqn. (4) and eqn. (11), respectively) decreases exponentially when θ increases. [19] Upon polarization, the rate of the two faradaic reactions (eqn (6) and eqn ( 9)) increases exponentially with the overvoltage according to the Butler Volmer kinetics. This induces an oxygen depletion at the cathodic TPB. This is the driving force that accelerates by the same proportion the rate of the surface diffusion reactions (eqn ( 5) and ( 10)). The direct consequence is an exponential increase in the rate of surface diffusion as it was proposed by Mitterdorfer et al. [19] for the Pt/YSZ system as well as Horita et al. for the LSF/YSZ [15] and shows that neither adsorption/desorption nor surface diffusion are rds. As the charge transfer reaction cannot be rds as well [30] and as the O 2-diffusion in the bulk of YSZ is very fast, [28] the sole realistic possibility is that oxygen ions incorporation at the TPB is the rds (eqn (11)). This is in total agreement with Bouwmeester et al. who showed that oxygen incorporation is the rate limiting step on YSZ powders below ca. 800°C. [START_REF] Bouwmeester | [END_REF] It is worth noting that with our measurements O 2-dis-incorporation from YSZ cannot be excluded as the rds, since our setup cannot formerly discriminate between incorporation or disincorporation.

Conclusions

A novel experimental setup that enable operando monitoring of oxygen exchange in SOFC type cells under polarization was developed and has been used for studying the oxygen mobility dependency upon polarization on a Pt/YSZ/Pt symmetrical cell. Dissociative adsorption or associative desorption was identified to be the rate determining step. These two steps cannot be distinguished with the presented experimental setup due to the two electrodes configuration. Its first order rate constant increases exponentially with the applied overvoltage in the faradaic region. Additionally, homolytic exchange is observed to be independent of the polarization magnitude whereas heterolytic and Faradaic exchanges are cumulative processes. The polarized isotopic exchange method is a powerful and versatile tool for studying the mobility of reactive species for relevant energy conversion systems such as Solid Oxide Fuel Cells, Solid Oxide Electrolyser Cells or membrane reactors.

Experimental Section

A setup classically used for OIE experiments on powder oxide catalysts [31] was adapted for investigating OIE on SOFC type cell. In the experimental setup developed for that purpose (Figure 6) the U-form reactor used for powder samples was replaced by a two compartment SOFC test bench (ProboStat A normal system, SS/S; NorECs) connected to a multi Autolab (Metrohm PGSTAT302N) for electrochemical measurements. This equipment was then placed in the closed recycle system and connected on one side to vacuum pump and on the other side to a mass spectrometer (Pfeiffer Vacuum QMS 200 Prisma TM ).

The gas inlet and outlet of the anodic and cathodic sides are short circuited together in order to get a single atmosphere for both compartments. The total volume of cathodic and anodic chambers is 199.2 cm 3 .The masses 32, 34 and 36 m/z were monitored every 3 s. The mass 28 m/z was also recorded to detect a possible leak. A pressure of 10 -6 mbar is maintained in the ionization chamber, while the total pressure in the test bench was around 20 mbar (modifications of pressure and temperature are discussed in the results and discussion part). A recycling pump (170 cm 3 s -1 ) avoids any diffusion issues in the gas phase which would limit the changes of the isotopic distribution monitored by the mass spectrometer.

The gas fluxes are slightly different in the anodic and cathodic compartments due to the different geometry of these two chambers.

During the isotopic exchange different reactions can occur:

 The homomolecular exchange (also called equilibration reaction) in which a mixture of labeled oxygen molecules, typically 18



The heterolytic exchange in which 18 O2 is scrambled with the "lattice" oxygen species of the oxide solid. In this case, the exchange reaction can imply only one atom (simple exchange described by eqn ( 12) and ( 13)) or two atoms from the solid (multiple exchange described by eqn ( 14)). From the partial pressure of each isotopomer, one can calculate the atomic fraction of 18 O in the gas phase (αg) using the eqn ( 15)

𝛼 𝑔 = 1 2
𝑃 34 +𝑃 36 𝑃 32 +𝑃 34 +𝑃 36 (15) where P32, P34, P36 are the partial pressures of 16 O2, 16 O 18 O and 18 O2 respectively.

A symmetrical cell composed of 8-YSZ (from Tosoh) as electrolyte (diameter: 18.8 mm; thickness: 2.1 mm, density around 95%) and Pt electrodes (brush printed with ink from METALOR Technologies (UK) Ltd; diameter: 10 mm, 3 layers) has been used. The cell was submitted to an in-situ standard pretreatment consisting in an oxidation step at 700°C under a flow of 50 cm 3 .min -1 of pure 16 O2 (Alpha 2; Air Liquide) during 1 hour, a decrease of the temperature at 600°C under the same flow and an outgassing at 600°C for 15 min. A 20 mbar dose of pure 18 O2 (99%, Shipper: Sigma-Aldrich Chemie GmbH) was then introduced into the reactor.

Figure 1 .

 1 Figure 1. Evolution of the voltammogram for Pt/YSZ/Pt cell: a) at atmospheric pressure (flow of 100 cm 3 min -1 ) with change on the temperature (black: 500 °C; red: 600 °C; and blue: 700 °C); b) at 600 °C with change on the pressure of 16 O2 (black: 50 mbar; red: 100 mbar; blue: 250 mbar; orange: 400 mbar; and green: flow of 100 cm 3 min -1 at atmospheric pressure). Scan rate = 20 mV s -1 .

Figure 2 .

 2 Figure 2. Nyquist and Bode representations for the Pt/YSZ/Pt cell at 600°C at OCV (a, b), 0.6 V (c, d) and 1.5 V (e, f). (black: 50 mbar; red: 100 mbar; blue: 250 mbar; green: 400 mbar). The shoulder at 1 MHz on Bode plots is due to impedance of cables

Figure 4 .Figure 5 .

 45 Figure 4. Evolution of the gas-phase O2 isotopic distribution, during isotopic exchange experiment on Pt/YSZ/Pt at 600°C at OCV (a); 1.5V (b) (blue: m/z=32 16 O2, red: m/z=34: 16 O 18 O, black: m/z=36: 18 O2, green: current density)

ARTICLE

  Polarized isotopic exchange revealed that ionized oxygen incorporation into YSZ is exponentially accelerated with applied overpotential Alexandre Nau, Clément Comminges* and Nicolas Bion* Page No. -Page No. Operando isotopic exchange on SOFC cells: oxygen transport dependency on applied potential

  O2 and16 O2, is scrambled over a surface (eqn(11)).

	18	𝑂 2,𝑔	+ 𝑂 2,𝑔 18	⇌ 2 𝑂 18	16	𝑂 𝑔

Acknowledgements

Figure 6. Isotopic exchange apparatus adapted for measurement on SOFC type cells.

The authors acknowledge financial support from the European Union (ERDF) and "Région Nouvelle Aquitaine".