Rut Fernández-Marín

Jalel Labidi

María Ángeles Andrés
email: marian.andres@ehu.eus

Susana C M Fernandes
email: susana.fernandes@univ-pau.fr

María Ángeles Andrés

María Ángeles

Using α-chitin nanocrystals to improve the final properties of poly (vinyl alcohol) films with Origanum vulgare essential oil

Keywords: Chitin nanocrystals, poly(vinyl alcohol), oregano oil, nanocomposite films, mechanical properties, antioxidant activity

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

One of the big challenges for our society in this century is to decrease the enormous amount of non-biodegradable plastics waste. Every year, 150 million tons of plastic waste are estimated to reach the oceans [START_REF] Zhao | Analysis of suspended microplastics in the Changjiang Estuary : Implications for riverine plastic load to the ocean[END_REF].

To replace part of these non-biodegradable plastics for biodegradable and/or 'bioplastics', i.e. plastics prepared from renewable resources (biopolymers), in recent years, many by-products from both agricultural and marine companies have gained interest in different industry sectors such as cosmetics, medical and food packaging due to their sustainable and eco-friendly attributes. Therefore, numerous scientific studies have been done in this sense. This include the use of natural polymers or synthetic biodegradable polymers that can be use as matrices and/or reinforcing agents; and bioactive compounds, which are used to improve the biological properties of the final materials.

Among the polymers, chitin (poly(β-(1-4)-N-acetyl-D-glucosamine), is a biopolymer found in the exoskeleton of crustaceans like crabs, lobster and shrimps; and is one of the biggest residues in fishing industry [START_REF] Rinaudo | Chitin and chitosan : Properties and applications[END_REF][START_REF] Ruiz-Navajas | In vitro antibacterial and antioxidant properties of chitosan edible fi lms incorporated with Thymus moroderi or Thymus piperella essential oils[END_REF]. However, it can also be found in the cell walls of fungi and in cuticle of insects. Being a supporting material in nature, chitin presents a highly organized micro-and nano-fibrillated structure. Under acid hydrolysis it is possible to obtain chitin nanocrystals (CHNC) with dimensions that can range from 6-60 nm in width and 100-800 nm in length [START_REF] Zeng | Chitin Whiskers : An Overview[END_REF][START_REF] Salaberria | Role of chitin nanocrystals and nanofibers on physical, mechanical and functional properties in thermoplastic starch films1[END_REF]. These chitin nanocrystals present excellent properties namely low density, biocompatibility, low toxicity, biodegradability and antimicrobial properties. In addition, its use as reinforcement agents in biocomposite films, improves the thermal stability and the mechanical properties [START_REF] Salaberria | Processing of α-chitin nanofibers by dynamic high pressure homogenization: Characterization and antifungal activity against A. niger[END_REF][START_REF] Salaberria | Chitin nanocrystals and nanofibers as nano-sized fillers into thermoplastic starch-based biocomposites processed by melt-mixing[END_REF]. A very interesting synthetic polymer, that can be used for the mentioned applications, is poly(vinyl alcohol), PVA. It is a biodegradable, non-toxic, biocompatible and soluble in water polymer presenting good film-forming and mechanical properties [START_REF] Morales | Industrial Crops & Products UV -vis protective poly ( vinyl alcohol )/ bio-oil innovative fi lms[END_REF][START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF][START_REF] Gaikwad | Development of polyvinyl alcohol and apple pomace bio-composite film with antioxidant properties for active food packaging application[END_REF].

Among the bioactive agents, essential oils (EOs) are one of the most important products of agriculture-based industry due to its bioactive properties that include antimicrobial, antiviral, antioxidant, anticancer and immunomodulatory [START_REF] Manso | Diminution of aflatoxin B1 production caused by an active packaging containing cinnamon essential oil[END_REF][START_REF] Asbahani | Essential oils : From extraction to encapsulation[END_REF][START_REF] Ribeiro-Santos | ScienceDirect Application of encapsulated essential oils as antimicrobial agents in food packaging[END_REF]. Their production is superior to 70,000 tons per year. EOs are aromatic compounds with low molecular weight extracted from plants namely barks (cinnamon), leaves (eucalyptus), flowers (lavender) and peels of fruits (orange) [START_REF] Asbahani | Essential oils : From extraction to encapsulation[END_REF][START_REF] Ribeiro-Santos | Use of essential oils in active food packaging: Recent advances and future trends[END_REF]. They possess various applications in health, agriculture, cosmetic, food and medicine industries. Moreover, are recognized by the Generally Recognized as Safe (GRAS) by the US Food and Drug Administration (USFDA) as safe foods [START_REF] Manso | Diminution of aflatoxin B1 production caused by an active packaging containing cinnamon essential oil[END_REF]. Due to its medicinal and culinary properties, oregano essential oil (Origanum vulgare), which belongs to the Lamiaceae family, is one of the most employed oil since ancient times. Its properties are attributed to the presence of monoterpenes among which carvacrol and thymol stand out [START_REF] Baydar | Antibacterial activity and composition of essential oils from Origanum , Thymbra and Satureja species with commercial importance in Turkey[END_REF][START_REF] Hosseini Fakhreddin | Two-step method for encapsulation of oregano essential oil in chitosan nanoparticles : Preparation , characterization and in vitro release study[END_REF].

The use of EOs is quite limited because of their high volatile properties and easily decomposition during handling, heating, exposure to oxygen or ultraviolet light, etc. [START_REF] Perdones | Food Hydrocolloids Physical , antioxidant and antimicrobial properties of chitosan e cinnamon leaf oil fi lms as affected by oleic acid[END_REF][START_REF] Sahraee | LWT -Food Science and Technology Effect of corn oil on physical , thermal , and antifungal properties of gelatin-based nanocomposite fi lms containing nano chitin[END_REF]. For these reasons, EOs need to be encapsulated/incorporated in materials that could present the form of films, coatings, nanocapsules and emulsions, in order to preserve their bioactive activities. Therefore, different polymeric matrices have been used such as chitosan, polylactic acid (PLA), soy protein, fish gelatine among others [START_REF] Duman | International Journal of Biological Macromolecules Crayfish chitosan for microencapsulation of coriander ( Coriandrum sativum L .) essential oil[END_REF][START_REF] Chi | Scientia Horticulturae E 32 ff ect of PLA nanocomposite fi lms containing bergamot essential oil , TiO 2 nanoparticles , and Ag nanoparticles on shelf life of mangoes[END_REF][START_REF] Chen | International Journal of Biological Macromolecules Effects of adding corn oil and soy protein to corn starch on the physicochemical and digestive properties of the starch[END_REF][START_REF] Ahmad | Food Hydrocolloids Physicomechanical and antimicrobial properties of gelatin fi lm from the skin of unicorn leatherjacket incorporated with essential oils[END_REF]. For instance, Munhuweyi et al. [START_REF] Munhuweyi | Postharvest Biology and Technology In vitro and in vivo antifungal activity of chitosanessential oils against pomegranate fruit pathogens[END_REF] used chitosan as matrix with oregano oil, and Wu J. et al. [START_REF] Wu | ScienceDirect Properties and antimicrobial activity of silver carp ( Hypophthalmichthys molitrix ) skin gelatin-chitosan films incorporated with oregano essential oil for fish preservation, Food Packag[END_REF] used a mixture of gelatine with chitosan by incorporating this oil to make films. Also, PVA films were developed by mixing this polymer with clove oil and apple pomace as bioactive agents [START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF][START_REF] Gaikwad | Development of polyvinyl alcohol and apple pomace bio-composite film with antioxidant properties for active food packaging application[END_REF][START_REF] Singh | International Journal of Biological Macromolecules Antimicrobial and antioxidant properties of polyvinyl alcohol bio composite films containing seaweed extracted cellulose nano-crystal and basil leaves extract[END_REF][START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF][START_REF] Hashemi | Progress in Organic Coatings Characterization of novel basil-seed gum active edible fi lms and coatings containing oregano essential oil[END_REF]. Hashemi et al. [START_REF] Hashemi | Progress in Organic Coatings Characterization of novel basil-seed gum active edible fi lms and coatings containing oregano essential oil[END_REF] prepared a basil seed gum coating with oregano oil and Fraj et al. [START_REF] Fraj | Industrial Crops & Products A comparative study of oregano ( Origanum vulgare L .) essential oil-based polycaprolactone nanocapsules / microspheres : Preparation , physicochemical characterization , and storage stability[END_REF] obtained polycaprolactone nanocapsules using this oil as an active ingredient. On the other hand, Hosseini et al. [START_REF] Hosseini Fakhreddin | Two-step method for encapsulation of oregano essential oil in chitosan nanoparticles : Preparation , characterization and in vitro release study[END_REF] studied emulsions of chitosan with oregano oil and Ribes et al. [START_REF] Ribes | E ff ect of oregano ( Origanum vulgare L . ssp . hirtum ) and clove ( Eugenia spp .) nanoemulsions on Zygosaccharomyces bailii survival in salad dressings[END_REF] prepared an emulsion with clove and oregano oil mixed with xanthan gum.

Nonetheless, there are few works in which nanocrystals or nanofibers like nanocellulose or nanochitin, have been used to reinforce the final mechanical and biological properties of the films. Some examples are the research done by Luzi et al.

[31] where the authors used PVA and chitosan, as a matrix adding carvacrol, as bioactive agent and cellulose nanocrystals as reinforcing agent; or by Ardekani et al. [START_REF] Torabi | International Journal of Biological Macromolecules Evaluation of electrospun poly ( vinyl alcohol ) -based nano fi ber mats incorporated with Zataria multi fl ora essential oil as potential wound dressing[END_REF] that assessed the properties of PVA, Zataria multiflora oil and mats nanofiber films; or by Jahed et al. [32] that studied the physicochemical properties of Carum copticum essential oil loaded chitosan films containing organic nanoreinforcements.

In this context, the aim of this research was to developed bioactive nanocomposite films using two different sources of alpha-chitin nanocrystals (α-CHNC) as reinforcing agents to improve the final properties of PVA and Origanum vulgare essential oil (OEO) films. To the best of our knowledge, chitin nanocrystals have not been studied as reinforcing agents in PVA with EOs, in particular with oregano oil. The effect of the different concentrations of EO and the origin of chitin nanocrystals on the final properties of the nanocomposite films, were assessed. Therefore, their thermal, mechanical and antioxidant properties were studied in order to evaluate their potential use in food packaging applications.

Materials and methods

Materials

Two different chitin samples were used, one was kindly provided by Mahtani Chitosan Pvt. Ltd. (India), and the other, was extracted in-house from lobster exoskeletal waste that was kindly supplied by Antarctic Seafood S.A. (Chile). The two chitin nanocrystal samples were isolated in-house with the mentioned chitin samples by microwave irradiation technique (CEM-Discover) at 125 w for 10 min using a 1 M HCl solution (37 % w/w, ACS reagent) in order to hydrolyse the amorphous regions of the chitin. The ratio chitin: HCl was of 1:30 w/v as described by Salaberria et al. [START_REF] Salaberria | Role of chitin nanocrystals and nanofibers on physical, mechanical and functional properties in thermoplastic starch films1[END_REF]. After, the suspensions were cooled at room temperature, filtered, washed with distilled water until neutral pH and centrifuged in order to remove the excess of water. The two likegel samples were finally stored in a refrigerator at 4 °C. These two chitin samples were then characterized. The degrees of acetylation (DA), determined by solid 13 C NMR using a Bruker Advance III 400 spectrometer (USA), determined according Kasaai et al. [START_REF] Kasaai | Determination of the degree of N -acetylation for chitin and chitosan by various NMR spectroscopy techniques : A review[END_REF] and were found to be 85% and 90% for shrimp and lobster chitin nanocrystals, respectively. The crystallinity index (CI%), was 89% for shrimp chitin nanocrystals and 94% for lobster chitin nanocrystals, as obtained by X Ray Diffraction (XRD) spectra (Philip X'pert Pro Automatic diffractometer, Amsterdam, Netherlands), using the published Sagheer et al. [START_REF] Al Sagheer | Extraction and characterization of chitin and chitosan from marine sources in Arabian Gulf[END_REF] equation.

Oregano oil FCC (OEO, Origanum vulgare, with Food Grade) was supplied by Sigma-Aldrich. Origanum vulgare essential oil composition was determined using gas chromatography and mass spectrometry (GC/MS, Agilent GC 7890A, MS 5975C, USA). Briefly, 10 μL of oregano oil were dissolved in 1 mL of ethyl acetate and injected in a capillary column HP-5MS ((5 %-Phenyl) -methylpolysiloxane, 30 m x 0.25 mm). The GC/MS conditions were: split/splitless method in split mode (10:1), injector at 280 °C and Helium was used as carrier gas at a flow rate of 0.7 mL/min. The rate temperature began at 50 °C and was increased until 120 ˚C at 10 K/min and held for 5 min. In a second time, the temperature was raised to 280 ˚C at 10 K/min and holds this temperature during 8 min. And, finally, the temperature was increased to 300 ˚C at 10 K/min and held for 2 min. The identification of the different compounds was carried out by using the National Institute of Standards Library (NIST). The obtained data is summarized in the Table 1. Only compounds with areas superior to 0.4 were accounted for. 14 compounds of OEO were identified representing 97.4% of the total essential oil.

The main components were carvacrol representing 75% and ϒ-terpinene at around 5.96%. These results are in agreement with those obtained previously by Munhuweyi et al. [START_REF] Munhuweyi | Postharvest Biology and Technology In vitro and in vivo antifungal activity of chitosanessential oils against pomegranate fruit pathogens[END_REF] and Hosseini et al. [START_REF] Hosseini Fakhreddin | Two-step method for encapsulation of oregano essential oil in chitosan nanoparticles : Preparation , characterization and in vitro release study[END_REF]. and Folin-Ciocalteu reagent were purchased from Scharlau. Sodium carbonate anhydrous (general-purpose grade) was purchased from Fischer.

Preparation of the nanocomposite films

PVA solutions (2% w/v) were prepared by dissolving PVA in a mixture of ethanol (60% v/v) and distilled water under stirring overnight at 55 ˚C. The oregano essential oil (OEO) concentration varied from 0.5 to 2% v/v with respect to PVA solution. The nanocomposite films were prepared by adding 0.5% w/v of chitin nanocrystals (α-CHNC, from shrimp or lobster). The dispersion of α-CHNC in the PVA solutions was done using an Ultra-Turrax (Heidolph Silent Crusher M., Germany) at 13 500 rpm for 10 min. A second set of samples was prepared without α-CHNC. All suspensions or solutions were degassed in order to remove entrapped air. The films were then prepared by casting at room temperature using 50 mm-diameter silicon molds for 72 hours. The ensuing materials were kept in a conditioning cabinet at 50 ± 5% relative humidity (RH) and 25 °C.

In order to assess the impact of the ultraviolet (UV) irradiation on the nanocomposite films, the samples were exposed to UV light into a dark room with an UV lamp at 364 nm during 3 days according to the method described by Morales et al. [START_REF] Morales | Industrial Crops & Products UV -vis protective poly ( vinyl alcohol )/ bio-oil innovative fi lms[END_REF]. The effect of the UV irradiation was evaluated by measuring the mechanical properties of the films.

The identification and composition of the nanocomposite films in terms of α-CHNC and OEO are listed in Table 2.

Table 2. Identification and composition of the nanocomposite films.

Samples

Characterization of nanocomposite films

Physico-chemical characterization

A UV-Vis spectrophotometer (Jasco V-630 UV-VIS spectrophotometer, JASCO, Germany) was used to assess the transmittance of the films. Spectra were recorded at RT in steps of 1 nm in the 400 -750 nm range. The measurements were made in triplicate.

The opacity of each films was determined according to the method described by Priyadarshi et al. [START_REF] Priyadarshi | Food Hydrocolloids Chitosan fi lms incorporated with Apricot ( Prunus armeniaca ) kernel essential oil as active food packaging material[END_REF]. The opacity was determined by equation (Eq. 2):

= (Eq. 2)
Where Abs 600 represents the absorbance obtained at 600 nm and x is the thickness of the film (mm). The thickness of the nanocomposite films was measured using a digital micrometer (Ultra Präzision Messzeuge GmbH, Glattbach, Germany) with an accuracy of 0.001 mm in six random different points of the each film. The average thickness was then calculated.

Attenuated Total Reflection-Fourier Transform Infrared Radiation (ATR-FTIR)

analysis was used to study the chemical interactions between PVA, OEO and α-CHNC using a Spectrum Two FTIR Spectrometer (Perkin Elmer Inc., Waltham, MA, USA) with a Universal Attenuated Total Reflectance accessory. The range of transmittance was between 600 and 4000 cm -1 using 64 scans and 4 cm -1 of resolution.

The moisture content of the nanocomposite films was determined by adapting the method described by [START_REF] Sahraee | LWT -Food Science and Technology Effect of corn oil on physical , thermal , and antifungal properties of gelatin-based nanocomposite fi lms containing nano chitin[END_REF]. Prior to testing, all samples were cut in 1.5 x 3 cm 2 models. Afterwards, the samples were weighed and dried at 106 ˚C for 24 h in an oven (Memmert UN160 plus Twindisp, Germany) to attain a constant weight. The moisture content was determined according to the following equation:

Moisture content (%) = ( ) 100 (Eq. 1)
Where W1 is the initial weight of the sample (g) and W2 is the weigh after drying (g).

Three measurements were conducted for each sample and their average was calculated.

Thermal stability and mechanical properties of the nanocomposite films

The thermal stability of the films was determined by thermogravimetric analysis (TGA) using a TGA/SDTA 851 Mettler Toledo instrument (New Castle, DE, USA), at a scanning rate of 10°C/min, from room temperature to 750 °C under a nitrogen atmosphere.

The mechanical properties of the nanocomposite films were performed using an Instron 5967 tester (Instron, Norwood, MA, USA). A load cell of 50 N was used with a cross head speed of 3 mm/min. Eight replicates of each sample (0.5 cm x 4.5 cm, L x W) were used to determine the average values of Young's modulus, tensile strength and elastic modulus.

Antioxidant activity

Determination of the total phenolic content (TPC)

The total phenolic content of the nanocomposite films was determined according to the method described by Ruiz-Navajas et al. [START_REF] Ruiz-Navajas | In vitro antibacterial and antioxidant properties of chitosan edible fi lms incorporated with Thymus moroderi or Thymus piperella essential oils[END_REF]. 25 mg of each film were cut and dissolved in 3 mL of methanol and centrifuged at 300 rpm for 12, 24, 48 and 72h. Then, the supernatant was collected and for each extract 300-μL solution was employed.

Gallic acid was used as reference standard and the measurement was done at 760 nm using a UV-Vis spectrophotometer (Jasco V-630, Pfungstadt, Germany). The results were expressed as mg gallic acid equivalents (GAE)/ g of dried film. Each sample was tested in triplicate.

DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay

The antioxidant activity of the nanocomposite films was evaluated using a free radical scavenging assay DPPH (2, 2-diphenyl-1-picrylhydrazyl) according to the method described by Gullon et al. [START_REF] Gullón | Industrial Crops & Products Optimization of solvent extraction of antioxidants from Eucalyptus globulus leaves by response surface methodology : Characterization and assessment of their bioactive properties[END_REF] with slight modifications. 25 mg of each film were cut and dissolved in 3 mL of methanol and centrifuged at 300 rpm for 12, 24, 48 and 72h. Then, the supernatant was collected. Afterwards, 300 μL of film extracts were mixed with 3 mL of 6x10 -5 M methanol solution of DPPH. After 15 min, the decrease in absorbance was measured at 515 nm using a UV-Vis spectrophotometer (Jasco V-630, Pfungstadt, Germany). Trolox was applied as standard reference and the results were expressed in μmol of Trolox equivalent (TE)/ g of dried film. Each sample was measured in triplicate.

Statistical analysis

The statistical analysis was performed using one-way analysis of variance (ANOVA) by SPSS software (Version 24, Inc. Chicago, IL, USA). The values of the significant differences were determined by Duncan's multiple range test. The experiments were carried out at least three times for each condition. The results are expressed as Mean±SD and values of p<0.05 were considered to be statistically significant.

Results and discussion

Physico-chemical characterization of the nanocomposite films

Our main aim in this study was to developed bioactive nanocomposite films using two different sources of alpha-chitin nanocrystals (α-CHNC) as reinforcing agents to improve the final properties of PVA and Origanum vulgare essential oil (OEO) films.

Herein, we specifically focus on the effect of the different concentrations of OEO and the origin of chitin nanocrystals on the final properties of the PVA-based nanocomposite films. As a first step, we focused on the general aspect of the films and on their physicochemical characterization. Afterwards, their thermal and mechanical properties, as well as their stability against UV light and their antioxidant properties were assessed in order to evaluate their potential use for packaging-food applications.

One of the most important parameters in films for food applications is their transparency because it has a direct impact on the consumer acceptance. To evaluate the impact of the incorporation of OEO and chitin nanocrystals on the transparency of the final materials, pictures were taken and the transmittance and opacity of the nanocomposite films were measured and calculated, respectively. As demonstrated in Figure 2, in the range of the visible light (400 to 750 nm), the highest transmittance were ascribed to the unfilled PVA film (P-0, around 90 % at 600 nm) and to the films prepared with low concentrations of OEO. However, our data showed that the incorporation of high concentration of OEO (P-2) decreases the transmittance of the films. On the other hand, as expected, the addition of chitin nanocrystals decreases the transmittance of the nanocomposite films (PNCS-0 and PNCL-0), as previously demonstrated with other matrices [START_REF] Salaberria | Role of chitin nanocrystals and nanofibers on physical, mechanical and functional properties in thermoplastic starch films1[END_REF][START_REF] Salaberria | Preparing valuable renewable nanocomposite films based exclusively on oceanic biomass -Chitin nanofillers and chitosan[END_REF]. Interestingly, the incorporation of OEO into the nanocomposite films showed a decrease of the transmittance of the final materials. Previous studies, have demonstrated the influence of the addition of EOs on different matrices, namely the work described by Sahraee et al. [START_REF] Sahraee | LWT -Food Science and Technology Effect of corn oil on physical , thermal , and antifungal properties of gelatin-based nanocomposite fi lms containing nano chitin[END_REF] and Gomes et al. [START_REF] Gomes | Bionanocomposites of chitosan / montmorillonite incorporated with Rosmarinus o ffi cinalis essential oil : Development and physical characterization, Food Packag[END_REF], where the authors prepared materials using chitosan as matrix with different concentrations of essential oils; or described by Kanatt et al. [START_REF] Kanatt | Food Hydrocolloids Active chitosan e polyvinyl alcohol fi lms with natural extracts[END_REF],

where chitosan and PVA were blended with plants extract. To underline these data, the opacity of the films was also calculated (equation 2, that take in account the Absorbance of the films at 600 nm and the thickness). The results are listed in Table 3. The lowest opacity value was observed in the pure PVA film (P-0) presenting an opacity of 0.65 ± 0.01. Similar to the data obtained by Chen et al. [START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF], by incorporating clove essential oil or apple pomace into PVA films, the opacity of our nanocomposite films increases with the incorporation of high concentrations of OEO (P-1.5 and P-2). In addition, the addition of nanocrystals resulted in a consequent increase of the opacity of the films. No significant differences were observed between the 2 types of chitin nanocrystals: 2.16 to 2.99 and 2.43 to 2.84 for films prepared with shrimp and lobster chitin nanocrystals, respectively. Another important aspect for food-packaging materials is their water absorbance capacity, because being in contact with food can cause the damage of the product. Table 3 reports the results of the moisture content of the nanocomposite films. The data listed in Table 3 show that the incorporation of OEO decreases the moisture content of the PVA films. The lowest value of moisture content was observed for the P-2 films (5.1 ± 0.2%). Interestingly, even for the nanocomposite films that have an important amount of OH groups, the incorporation of OEO induces the decrease of moisture content. These results are attributed to the hydrophobic properties of the OEO, and good interaction between OEO, α-CHNC and PVA leaving least possibility for them to interact with water molecules [START_REF] Chen | Development of moistureabsorbing and antioxidant active packaging film based on poly ( vinyl alcohol ) incorporated with green tea extract and its effect on the quality of dried eel[END_REF].

In Table 4 are listed the ATR-FTIR band assignments of OEO and PVA. For instance, the OEO spectra presented a broad peak at 3397 cm -1 corresponding to the O-H stretching which was attributed to carvacrol, and the phenolic ring was assigned to the peaks between 1622 and 1424 cm -1 . The peak at 941 cm -1 showed C-H bending and at 810 cm -1 revealed the characteristic peak of thymol [START_REF] Fraj | Industrial Crops & Products A comparative study of oregano ( Origanum vulgare L .) essential oil-based polycaprolactone nanocapsules / microspheres : Preparation , physicochemical characterization , and storage stability[END_REF][START_REF] Torabi | International Journal of Biological Macromolecules Evaluation of electrospun poly ( vinyl alcohol ) -based nano fi ber mats incorporated with Zataria multi fl ora essential oil as potential wound dressing[END_REF].

Regarding the PVA, for example, the peaks at 1732 cm -1 and 1573 cm -1 were attributed to ester C=O stretching vibration and C=C stretching vibration, respectively.

Other representative bands of PVA are the C-C stretching vibration at 1090 cm -1 and 1022 cm -1 , and C-C and C-O stretching vibration at 843 cm -1 [START_REF] Torabi | International Journal of Biological Macromolecules Evaluation of electrospun poly ( vinyl alcohol ) -based nano fi ber mats incorporated with Zataria multi fl ora essential oil as potential wound dressing[END_REF][START_REF] Yu | Silica in situ enhanced PVA / chitosan biodegradable fi lms for food packages[END_REF]. In addition, the C-H2 bending vibration and C-CH3 deformation vibration were observed at 1428 cm -1 and 1377 cm -1 , respectively [START_REF] Morales | Industrial Crops & Products UV -vis protective poly ( vinyl alcohol )/ bio-oil innovative fi lms[END_REF][START_REF] Singh | International Journal of Biological Macromolecules Antimicrobial and antioxidant properties of polyvinyl alcohol bio composite films containing seaweed extracted cellulose nano-crystal and basil leaves extract[END_REF][START_REF] Fan | A novel electrospun β -CD / CS / PVA nano fi ber membrane for simultaneous and rapid removal of organic micropollutants and heavy metal ions from water[END_REF].

Thermal and mechanical properties of the nanocomposite films

The effect of the incorporation of chitin nanocrystals into the PVA/OEO films was evaluated in terms of thermal and mechanical properties. The TGA serves as a proof for chemical interactions and the formation of hydrogen bonds in the final materials. Figure 4 shows the thermogravimetric analysis (TGA) and derivative (dTGA) curves of the PVA and PVA/OEO films and of the nanocomposite films. The results from TGA and dTGA, clearly showed that the PVA film displayed two-stages of degradation a first step with a maximum degradation at 319 °C attributed to the degradation of the amorphous parts of the polymer, and a second step at around 420 °C that corresponds to the degradation of the higher thermal stability crystalline parts. The PVA/OEO films depicted 4 degradation steps and presented lower thermal stability compared to control. The first step occurred at around 100 °C and was attributed to the loss of water; the second step was observed at 180 °C and was ascribed to the degradation of the OEO [START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF]. The third and fourth steps were attributed to the PVA degradation, but their maximum temperature of degradation decreased between 15-20 °C. Nevertheless, it was observed that the incorporation of both chitin nanocrystals showed an improvement of the thermal stability of the PVA/OEO films. This fact was attributed to the high temperature of degradation of chitin nanocrystals (around 380 °C, degradation of chitin macromolecules) and to the good homogeneity and interactions between the chitin nanocrystals and the PVA, the increasing of crystallinity of PVA molecules in the presence of the chitin nanocrystals is also a possibility. Like for PVA/OEO films, the nanocomposite's dTGA curves exhibited 4 degradation steps.

However, the maximum temperature of degradation of the materials increased (for instance, 318 °C for PNCL-1 and -2, Figure 4). The data of the mechanical properties of the nanocomposite films is summarized in Table 5. The results showed no significant differences (p> 0.05) regarding the TS values of the unfilled PVA films (without α-CHNC) with the increasing of the concentration of OEO (P-0 to P-2). However, the TS of the nanocomposite films decreased significantly (p<0.05) with the concentration of OEO. These results are in accordance with those obtained by Gaikwad et al. [START_REF] Gaikwad | Development of polyvinyl alcohol and apple pomace bio-composite film with antioxidant properties for active food packaging application[END_REF], in which the TS values decreased with the concentration of apple pomace in PVA films; or with Chen et al. [START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF] that observed a decrease in TS values in PVA films with clove oil. Nonetheless, the incorporation of the chitin nanocrystals allowed increase the TS with respect to PVA/OEO films making the nanocomposite films more resistant, as demonstrated before in other matrices [START_REF] Salaberria | Role of chitin nanocrystals and nanofibers on physical, mechanical and functional properties in thermoplastic starch films1[END_REF][START_REF] Zubillaga | Chitin Nanoforms Provide Mechanical and Topological Cues to Support Growth of Human Adipose Stem Cells in Chitosan Matrices[END_REF][START_REF] Salaberria | Preparing valuable renewable nanocomposite films based exclusively on oceanic biomass -Chitin nanofillers and chitosan[END_REF]. As it could be seen in Table 5, like for TS, no significant differences were observed regarding Young's modulus of PVA films with the OEO concentration.

However, the Young's modulus of the nanocomposite films decreased with the OEO concentration; but the incorporation of chitin nanocrystals made that the Young's modulus values are higher PVA/OEO films. Similar results were found by Ardekani et al. [START_REF] Torabi | International Journal of Biological Macromolecules Evaluation of electrospun poly ( vinyl alcohol ) -based nano fi ber mats incorporated with Zataria multi fl ora essential oil as potential wound dressing[END_REF], the authors used wound dressing of PVA with Zataria multiflora oil reinforced with nanofibers of mats, and used new mats fibers consisting of blend electrospun chitosan / poly (ε-caprolactone) with oregano essential oil.

On the other hand, the addition of the OEO improved the elongation (E %) of the nanocomposite films and consequently their flexibility due to the OEO plasticizing effect [START_REF] Chen | Properties , vapour-phase antimicrobial and antioxidant activities of active poly ( vinyl alcohol ) packaging fi lms incorporated with clove oil[END_REF][START_REF] Bonilla | Food Bioscience Antioxidant potential of eugenol and ginger essential oils with gelatin / chitosan fi lms ☆[END_REF][START_REF] Yoon | Journal of Photochemistry & Photobiology , B : Biology Preparation and antibacterial activities of chitosan-gallic acid / polyvinyl alcohol blend fi lm by LED-UV irradiation[END_REF].

Effect of the UV irradiation on the nanocomposite films

The effect of the UV irradiation on the nanocomposite films was assessed by measuring the mechanical properties of the films after irradiation. The samples were exposed to UV light radiation (364 nm) for 72 h and the data are listed in Table 5. No significant changes were observed in the general aspect of the films after the 72 h (images not showed).

In general, comparing the results of the films exposed to UV irradiation with those not irradiated (Table 5), a decrease in mechanical properties was observed. The TS values decreased between 25-50% for the films without α-CHNC and 25-30% for the nanocomposite films.

Rodriguez-Felix et al. [START_REF] Rodríguez | Current encapsulation strategies for bioactive oils: From alimentary to pharmaceutical perspectives[END_REF] observed this trend in chitosan and polyethylene films. With this work, the authors demonstrated that TS values decrease between 80-90 % during 135 days. Other researchers such as Bai et al. [START_REF] Bai | UV-shielding alginate films crosslinked with Fe3+ containing EDTA[END_REF], showed that after 30 days of exposure to UV light, the values of TS decrease in EDTA/Fe 3+ /Alginate.

Regarding the Young's modulus results, it was observed that films irradiated during 72 h had lost stiffness since they showed lower values than the films that have not been irradiated. These results are in accordance with Morales et al. [START_REF] Morales | Industrial Crops & Products UV -vis protective poly ( vinyl alcohol )/ bio-oil innovative fi lms[END_REF], which demonstrated that Young´s modulus decreased to 1750 MPa in the bio-oil film when irradiated during 60 h. Furthermore, the Young's modulus values of the films PNCL (between 468-373 MPa with the content of OEO) were higher than the films PNCS (between 487-186 MPa with OEO concentration). The data also showed that the PNCL nanocomposite films were the ones that showed the highest reduction when they were irradiated with respect to the non-irradiated, observing a drop between 33-67% in their Young´s modulus.

Summarizing, the addition of both α-CHNC as reinforcing agents, for one hand, allowed the increase in mechanical properties of the unfilled PVA/OEO films; and for the other hand, causes an increased of the stability of the final materials when they were submitted to UV radiation.

Antioxidant activity of the nanocomposite films

In order to avoid the use of synthetic compounds to protect food from oxidation, today there is an increasing demand for active packaging to release natural compounds, in particular essential oils. In this work, to assess the antioxidant activity of the nanocomposite films, two different assays were done: (i) total phenolic content (TPC) used to determine the total amount of phenolic compounds that are important constituents with redox properties responsible for antioxidant activity [START_REF] Hafsa | Physical, antioxidant and antimicrobial properties of chitosan films containing Eucalyptus globulus essential oil[END_REF]; and (ii) DPPH assay, in which the antioxidant activity of the samples was measured through the ability of the samples to donate hydrogen to the radical DPPH [START_REF] Jahed | Physicochemical properties of Carum copticum essential oil loaded chitosan films containing organic nanoreinforcements[END_REF].

Therefore, herein, the profile of total phenolic content and antioxidant release of the OEO from the films and nanocomposite films were studied by immersion in a methanol solution for 12 h, 24 h, 48 and 72 h. The results of the TPC and DPPH assays are shown in Figures 5 and6, respectively.

In both assays, the pure PVA film (P-0) did not show any antioxidant activity, as demonstrated earlier [START_REF] Singh | International Journal of Biological Macromolecules Antimicrobial and antioxidant properties of polyvinyl alcohol bio composite films containing seaweed extracted cellulose nano-crystal and basil leaves extract[END_REF]31]. However, the films prepared with OEO and α-CHNC showed antioxidant activity. This is consistent with other studies in which it was shown that the OEO contains antioxidant compounds such as carvacrol or thymol [START_REF] Baydar | Antibacterial activity and composition of essential oils from Origanum , Thymbra and Satureja species with commercial importance in Turkey[END_REF][START_REF] Hosseini Fakhreddin | Two-step method for encapsulation of oregano essential oil in chitosan nanoparticles : Preparation , characterization and in vitro release study[END_REF].

Furthermore, the data also demonstrated that the total phenolic content and antioxidant activity of the samples, gradually increased with the increasing of the immersion time and OEO concentration (Figure 5 and6).

More specifically, regarding to the TPC data (Figure 5), the P-2 film, which contains a higher concentration of OEO, showed the greater release of phenolic compounds with 36.47±0.04 mg GAE/g film value at 72 h. On the other hand, it was observed that in films without nanocrystals there was a huge increase in phenolic content from 12h to 72 h. For instance, for the films P-1, P-1.5 and P-2 an increase of 85%, 78% and 95% was observed, respectively. In Figure 5 when comparing the films with shrimp and lobster nanocrystals (PNCS and PNCL), it was observed that the films prepared with shrimp nanocrystals showed slightly higher values of TPC for all release times, in particular for PNCS-1 film. On the other hand, the release of phenolic compound over the time for the films PNCS-1.5 and PNCS-2 was practically constant, keeping its values constant around 20 and 21 mg GAE /g film, respectively. Interestingly, the materials made of shrimp nanocrystals showed better results than the unfilled films over the first 48 h, and then the nanocomposite films made of lobster shrimp over the 72 h.

Figure 6 shows the antioxidant activity of the films obtained by DPPH assay.

Among all materials, the antioxidant activity of P-2 film reflected the highest value after 72 h (2.12 ± 0.02 μmol TE/g film). In the films made of PVA and OEO, an important increase in the antioxidant activity was observed from 12 to 72h. The higher growth occurred in the P-1 film with an increase of around 86%. The nanocomposite films made of shrimp nanocrystals showed better results than the nanocomposites prepared with lobster nanocrystals, but just the samples PNCS-1.5 and PNCS-2 over the first 48h. In general, films containing nanocrystals (PNCS and PNCL) showed a slow release of antioxidant compounds over time (more constant release compared with unfilled films). There are different hypothesis that could explain this fact: (i) chitin nanocrystals create sinuous paths in the PVA matrix and makes that the antioxidant molecules of the OEO take longer to pass through these paths; (ii) the formation of strong interactions between nanocrystals and OEO compounds, making their release slow and constant;

(iii) by adding chitin nanocrystals, the porosity of the PVA matrix decreases and therefore, there is a reduction in the segmental mobility of the OEO; and (iv) chitin nanocrystals form a network and, in addition, increase the crystallinity of PVA molecules and this obstructs the mobility of OEO molecules [START_REF] Almasi | Food Additives & Contaminants : Part A Development of a novel controlled-release nanocomposite based on poly ( lactic acid ) to increase the oxidative stability of soybean oil[END_REF][START_REF] Jahed | Physicochemical properties of Carum copticum Essential Oil Loaded Chitosan Films Containing Organic Nanoreinforcements[END_REF].

Conclusion

The main aim of this study was to evaluate the effect of the incorporation of α-chitin nanocrystals from shrimp and lobster on the final properties PVA/OEO films made by solvent casting.

Our data demonstrated that the nanocomposite films were homogeneous and translucent even with a slight increase of the opacity. Nonetheless, the obtained nanocomposite films containing α-CHNC showed: (i) better thermal stability; (ii) better mechanical properties. Moreover, after exposure the nanocomposite films to UV radiation, it was demonstrated that the presence of α-CHNC had a retarding effect on their loss of mechanical properties; (iii) a more constant release of the antioxidant compounds over time for both TPC and DPPS assays; (iv) better antioxidant activity for the materials made of shrimp nanocrystals over the first 48h; and (v) no major effects were found regarding the source of chitin.

These results demonstrated that the nanocomposites films of PVA, α-CHNC and OEO could be promising biodegradable and active films for the food industry.

Figure 1 ,

 1 Figure 1, displays the general aspect of the final films. The obtained films are homogeneous and translucent. Figure 2 represents the transmittance (measured in the range between 400 -750 nm) profiles of the nanocomposite films and confirms the qualitative analyse of Figure 1.

Figure 1 .

 1 Figure 1. General appearance of the nanocomposite films. From left to right: a) P-O,

Figure 2 .

 2 Figure 2. Transmittance from 400 to 750 nm of the nanocomposites films.

Figure 3 ,

 3 Figure 3, displays the ATR-FTIR spectra of OEO, PVA, PNCS-2 and PNCL-2.All nanocomposite films (not showed) showed characteristics bands corresponding to each component, as demonstrated with the representative examples. For instance, when compared to OEO, P-0 and P-2 spectra, the PNCS-2 and PNCL-2 spectra, showed an increase of the intensity of the band corresponding to the O-H stretching vibrations (around 3400 cm -1 ); of the bands corresponding to the C-H stretching (around 2950 cm - 1 ); and of the bands at 1657 and 1620 cm -1 attributed to amide I and at 1556 cm -1 , corresponding to amide II, which is due to the presence of the chitin nanocrystals.

Figure 3 .

 3 Figure 3.FTIR spectrum of a) OEO, b) P-0, c) P-2, d) PNCS-2, e) PNCL-2.

Figure 4 .

 4 Figure 4. TGA and dTGA curves nanocomposite films: P-0, P-1, P-2, PNCS-0, PNCS-1,

Figure 5 .

 5 Figure 5. Total phenolic compounds (TPC) of the films at different time. The values

Figure 6 .

 6 Figure 6. DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging assay of the films

Table 1 .

 1 Composition of Origanum vulgare assessed by GC/MS.

	Area	Retention Time
	Component
	(%)	(min)

Table 3 .

 3 Data concerning the thickness, opacity and moisture content of the

	nanocomposite films.
	The values were average ± standard deviation (thickness n=6; moisture content and
	opacity n=3). Superscript letters depict significant differences (Duncan's test, p < 0.05)
	among OEO concentration with in each chitin nanocrystals treatment.
				Moisture
	Samples	Thickness
			Opacity	content
	Identification	(μm)
				(%)
	P-0	48.0± 1.8 a	0.65 ± 0.01 a	11.0 ± 0.2 a
	P-0.5	49.0 ± 3.5 a	0.66 ± 0.02 a	10.8 ± 0.8 a
	P-1	60.2 ± 3.6 b	0.68 ± 0.06 a	8.0 ± 0.1 b
	P-1.5	66.7 ± 2.3 c	0.75 ± 0.02 b	5.9 ± 0.7 c
	P-2	72.8 ± 2.0 d	0.93 ± 0.04 c	5.1 ± 0.2 d
	PNCS-0	48.2 ± 2.1 a	2.16 ± 0.12 a	13.2 ± 3.2 a
	PNCS-0.5	53.2 ± 2.0 b	2.36 ± 0.34 ab	10.0 ± 2.7 a
	PNCS-1	56.2 ± 3.0 b	2.38 ± 0.05 ab	9.1 ± 1.2 a
	PNCS-1.5	61.8 ± 3.8 c	2.44 ± 1.94 b	7.7 ± 2.9 a

Table 4 .

 4 ATR-FTIR band assignments of OEO and PVA.

	Band (cm -1 )	Functional group assignment
	3397	O-H stretching
	2960	symmetric C-H stretching
	2872	asymmetric C-H stretching
	1622 -1424	phenolic ring
	OEO
	1586	N-H bending
	1456	C-H2 bending
	1250 -1112	C-O-C stretching
	941	C-H bending
	3295	O-H tensile vibration
	2914 -2943	alkyl asymmetric stretching
	1732	ester C=O stretching vibration
	1573	C=C stretching vibration
	PVA

Table 5 .

 5 Mechanical properties of the nanocomposite films before and after UV

	radiation no irradiation and irradiation films (TS: tensile strength; YM: Young's
	modulus; and E: elongation). The values were average ± standard deviation (n = 10).
	Superscript letters depict significant differences (Duncan's test, p < 0.05) among oil
	concentration with in each chitin nanocrystals treatment.
		Not UV-irradiated samples	UV-irradiated samples
	Samples	TS	YM		TS	YM
				E (%)			E (%)
	Identification	(MPa)	(MPa)		(MPa)	(MPa)
		16.02 ±	575.61 ±	176.44 ±	17.68 ±	291.44 ±	117.00 ±
	P-0	7.73 a	41.89 a	64.11 a	3.23 ab	93.21 a	10.73 a
		13.74 ±	574.41 ±	202.85 ±	19.43 ±	287.52 ±	177.80 ±
	P-0.5	1.58 a	5.79 a	72.24 a	1.14 b	98.59 a	6.75 b
		14.11 ±	555.89 ±	224.38 ±	12.45 ±	260.45 ±	172.00 ±
	P-1	2.71 a	30.84 a	81.11 a	1.82 a	69.77 a	56.87 b
		14.44 ±	512.48 ±	295.34 ±	11.63 ±	243.65 ±	186.71 ±
	P-1.5	7.43 a	65.98 a	81.89 b	4.95 a	75.53 a	45.93 b
		13.06 ±	237.99 ±	298.97 ±	11.44 ±	170.89 ±	208.51 ±
	P-2	3.85 a	3.31 b	85.50 b	3.98 a	94.89 a	18.23 b
		33.39 ±	756.56 ±	139.45 ±	25.19 ±	468.96 ±	135.40 ±
	PNCS-0	2.93 a	197.89 a	82.56 a	4.21 a	179.09 a	38.71 a
	PNCS-0.5	27.22 ±	579.00 ±	128.73 ± 26.83 ± 454.99 ±	185.70 ±

Acknowledgments

The authors would like to acknowledge the financial support of the Department of Education of the Basque Government (IT1008-16). Rut Fernández-Marín would like to express her gratitude to the Department of Economic Development and Infrastructures of the Basque Government (scholarship of young researchers training) for supporting financially this research. The authors would like to thank for technical and human support provided by SGIker (UPV/EHU/ERDF, EU). SCMF is the recipient of an E2S UPPA Partnership Chair MANTA (Marine Materials) supported by the 'Investissements d'Avenir' French programme managed by ANR (ANR-16-IDEX-0002), by the Région Nouvelle-Aquitaine and by the Communauté d'Agglomération du Pays Basque.


Clement Nicollet
email: clement.nicollet@cnrs-imn.fr

Harry L Tuller

A perspective on the relationship between the acidity of perovskite oxides and their oxygen surface exchange kinetics

The oxygen exchange reaction kinetics of a large number of ternary mixed ionic and electronic conducting oxides were analyzed with respect to their calculated surface acidity factors derived from the Smith acidity factors of the corresponding binary oxides from which the ternary oxides are composed. The analysis suggests that the surface exchange kinetics have a direct link to the acidity of the mixed conducting oxide. The possible interplay between acidity and transport properties is discussed. The analysis is also applied to OER electrocatalysts but does not show a link between OER overpotentials and acidity, suggesting that the link is more relevant for gas/oxide interactions at high temperatures. The observations suggest that acidity impacts most directly the oxygen adsorption process while impacting the electron charge transfer only indirectly.

Introduction

The reduction of molecular oxygen at the surface of oxide materials is a key reaction in many energy storage, sensor, and memory applications [START_REF] Barsan | Metal oxide-based gas sensor research: How to?[END_REF][START_REF] Akinaga | Resistive random access memory (ReRAM) based on metal oxides[END_REF][START_REF] Kilner | Materials for intermediate-temperature solid-oxide fuel cells[END_REF][START_REF] Laguna-Bercero | Recent advances in high temperature electrolysis using solid oxide fuel cells: A review[END_REF][START_REF] Chueh | High-flux solar-driven thermochemical dissociation of CO₂ and H₂ O using nonstoichiometric ceria[END_REF] . In a previous article, we have shown that it is possible to tune the surface exchange kinetics of a given mixed conducting oxide, i.e. praseodymium substituted ceria (Pr 0.1 Ce 0.9 O 2-δ ), by many orders of magnitude by infiltrating single valence binary oxides with different acidities [START_REF] Nicollet | Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides[END_REF] , as defined in an acidity scale initially proposed by Smith et al. [START_REF] Smith | An acidity scale for binary oxides[END_REF] . In our previous work, the activation energy of the oxygen surface exchange coefficient of Pr 0.1 Ce 0.9 O 2-δ remained unchanged for the different surface infiltrated binary oxides, and only the pre-exponential factor changed, showing a linear dependence of the log of the pre-exponential factor on the Smith acidity factor. Based on work function and electrical conductivity trends, we explained this phenomenon by arguing that the infiltrated oxides act as electron donors or acceptors depending on whether the surface additive was more basic or acidic than Pr 0.1 Ce 0.9 O 2-δ , and thereby be able to tune the surface electron concentration in Pr 0.1 Ce 0.9 O 2-δ by up to many orders of magnitude either in accumulation or depletion. This pointed to the acidity of the infiltrated oxide relative to the host, i.e. Pr 0.1 Ce 0.9 O 2-δ , as the key relevant parameter to consider in manipulating the oxygen surface exchange kinetics. Here, we examine if 1) the relationship between acidity and oxygen surface exchange kinetics can be extended to a broader range of oxides, and 2) if the absolute value of acidity, rather than relative acidity in a two phase system, has importance for oxygen exchange kinetics. To perform this analysis, we propose a calculation of an acidity factor for perovskites, based on the Smith acidity for binary oxides, that allows ranking perovskite oxides on an acidity scale, from the most basic to the most acidic. Then, we use a selection of literature data for oxygen surface exchange kinetics, measured primarily by electrochemical impedance spectroscopy and O [START_REF] Chang | Electrical properties of GdBaCo 2 O 5+x for ITSOFC applications[END_REF] tracer diffusion experiments, for a wide variety of mixed conducting oxides, and analyze the relationship between their respective acidity and their catalytic activity.

Methods

Calculation of the Smith acidity factor for complex oxides

To be able to study the influence of acidity of ternary and quaternary compounds such as perovskites and double perovskites, commonly considered as efficient catalysts for the oxygen exchange reaction, their acidity must first be calculated. The method for determining acidity values of various binary oxides proposed by Smith relies on the availability of thermochemical data for those oxides [START_REF] Smith | An acidity scale for binary oxides[END_REF] . The calculation involves knowledge of the enthalpy of formation of a variety of salts for each oxide, which are not necessarily available for more complex oxides. An alternate approach was therefore developed to calculate an effective acidity of these more complex oxides based on available data. Here we chose to use a linear extrapolation of perovskite acidity based on the Smith acidity of the binary oxides that make up given perovskite compositions of nominal composition ABO 3 . This extrapolation is reasonable considering that the original calculation by Smith for binary compounds is also based on a linear mean value extracted from various reaction enthalpies [START_REF] Smith | An acidity scale for binary oxides[END_REF] . The Smith factor for a given binary oxide is normalized with the cation stoichiometry, for example, α(La 2 O 3 ) is divided by a factor of 2 to be used in perovskite of general composition LaMO 3 . As a perovskite has two moles of cations per unit formula, we choose to divide the sum of α by the overall number of cations.

The general expression of α for perovskite oxides of general formula is as follow:

(1)

The same approach is used for double perovskites, but because their unit formula is twice that of perovskite, the sum is divided by 4 instead of 2, which allows for comparison between the two different structures.

The main issue arising when attempting this calculation comes from the transition metal on the B site.

For the A site, the cations are primarily single-valent, and there is only one stable oxide from which because alkaline earth oxides are more basic, and increasing B cation oxidation state which would increase its acidity. On the other hand, oxygen deficiency in perovksites can also induce a lower oxidation state of the B cation. In summary, seeking more accuracy in the determination of the acidity factor for perovskites will inevitably make it lose its simplicity, which defeats the purpose of this analysis.

Selection of surface exchange kinetic data from the literature

To apply our analysis, we chose to use data for surface exchange coefficient k* (measured by isotopic exchange) and surface polarization resistance R p (measured by impedance) that were already gathered and averaged in the work of Lee et al. [START_REF] Lee | Prediction of solid oxide fuel cell cathode activity with first-principles descriptors[END_REF] on a large number of perovskites and double perovskites.

Measuring oxygen exchange kinetics at high temperature can often induce surface modification of the perovskite, such as cation segregation or contamination from the measurement setup. These effects can degrade the surface exchange kinetics and complicate our analysis. To minimize this issue, we included the work from Baumann et al. [START_REF] Baumann | Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes[END_REF] in this analysis, because it compares the surface polarization resistance R p of a range of perovskites, measured on thin films, prepared under the same conditions.

This study therefore serves to reduce the uncertainties arising from the collection of data from a large number of laboratories with a variety of different sample preparation and measurement conditions.

This analysis was also extended to the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) in alkaline solutions, often considered a rate limiting step for electrochemical water splitting. In this case, measurements of ORR activity by Suntivich et al. [START_REF] Suntivich | Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal -air batteries[END_REF] for a large number of oxide catalysts were extracted and plotted against their respective Smith acidity. In alkaline solution, the stability of the surface of the oxide catalyst can also be an issue that would complicate our analysis.

For example some materials such as Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ are known to amorphize during operation [START_REF] Fabbri | Dynamic surface self-reconstruction is the key of highly active perovskite nano-electrocatalysts for water splitting[END_REF] . To avoid those issues and complete our analysis, the theoretical OER activity data from Man et al. [START_REF] Man | Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces[END_REF] for a large number of perovskites was also extracted and analyzed with respect to their calculated Smith acidity.

Results

To analyze the possible relationship between acidity and surface exchange kinetics, values of polarization resistances R p and surface exchange coefficient k* taken from Lee et al. [START_REF] Lee | Prediction of solid oxide fuel cell cathode activity with first-principles descriptors[END_REF] were plotted against the calculated Smith acidity for a series of perovskites : LaMnO 1a shows the evolution of log R p as a function of the Smith acidity of these materials, and demonstrates that there is a clear linear relationship between surface exchange kinetics and acidity for most of the considered materials. However, the trend does not hold for the two manganites considered, i.e. LaMnO 3 and La 0.8 Sr 0.2 MnO 3 , which have significantly higher R p values than their cobalt and iron-based counterparts. The same trend is observed in Figure 1b comparing log k* values as a function of the corresponding materials acidity, with the exception that for this set of data, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ is also significantly lower than the linear trend observed with the other materials. Although this trend is remarkable for such a wide variety of materials, it is necessary to discuss the possible reasons that some materials clearly fall outside of it. For manganites, there are two hypotheses that can explain why their surface exchange rate is not correlated directly to their acidity. First, the manganites are not oxygen deficient at high pO 2 [START_REF] Mizusaki | Oxygen nonstoichiometry and defect equilibrium in the perovskite-type oxides La1-xSrxMnO3+d[END_REF] . This is a problem since the oxygen vacancy concentration is key to the oxygen exchange reaction, as oxygen vacancies are needed at the surface to incorporate freshly reduced oxygen ions generated at the surface into the bulk. They are also key for insuring sufficient oxide ion conduction, allowing for adsorbed oxygen to rapidly find incorporation sites and diffuse from the surface sites and become incorporated into the bulk of the oxide [START_REF] Wang | Oxygen exchange kinetics on solid oxide fuel cell cathode materials -general trends and their mechanistic interpretation[END_REF] . The acidity of the oxide material thus only becomes a highly relevant parameter if the oxide also possesses a sufficiently high oxygen vacancy concentration. Second, manganese in perovskites can be significantly more oxidized than its nominal 3+ state, especially at the surface. Wu et al. [START_REF] Wu | X-ray photoelectron spectroscopy of La 0.5 Sr 0.5 MnO 3[END_REF] measured the average oxidation state of Mn in La 0.8 Sr 0.2 MnO 3 at the surface by photoelectron spectroscopy to be +3.5. Although this might have implications for its catalytic activity toward the oxygen reduction reaction, it also means that using the acidity of MnO for calculating the Smith acidity of Mn based perovskites can significantly underestimate the overall acidity of the perovskite.

For Ba 0.5 Sr 0. [START_REF] Lee | Prediction of solid oxide fuel cell cathode activity with first-principles descriptors[END_REF] , initially collected from refs .

To overcome potential data inconsistencies collected from different research groups who utilized different measurement techniques, we found it useful to apply our analysis to results obtained in studies that examined different materials, but under the same conditions of preparation and measurement. Baumann et al. studied the surface exchange kinetics of different Co-Fe based perovskites, prepared by Pulsed Laser Deposition by impedance spectroscopy utilizing microelectrodes under near identical conditions [START_REF] Baumann | Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes[END_REF] . Figure 2a to what is observed in Fig. 1, the polarization resistance R p shows a strong correlation and dependence on the calculated Smith acidity. To assess whether this correlation is perhaps caused by third parameter that could be linked to both Acidity and surface exchange kinetics, it is necessary to study the evolution of other keys parameters with the Smith acidity. Previous studies of parameters affecting surface exchange kinetics have shown that fast kinetics can stem from high oxide ion conductivity [START_REF] Kilner | Surface exchange of oxygen in mixed conducting perovskite oxides[END_REF] , or high electronic conductivity [START_REF] Jung | A new model describing solid oxide fuel cell cathode kinetics: Model thin film SrTi 1-x Fe x O 3-δ mixed conducting oxides -A case study[END_REF] (regardless of n or p type character). Therefore, the oxide ion diffusion coefficient and the electronic conductivity of the materials considered in the Baumann study were also plotted as a function of the calculated Smith acidity. In Figure 2b, the tracer diffusion coefficient D* measured at 600 °C by various authors [START_REF] Berenov | Oxygen tracer diffusion and surface exchange kinetics in La 0.6 Sr 0.4 CoO 3-δ[END_REF][START_REF] Fullarton | Study of oxygen ion transport in acceptor doped samarium cobalt oxide[END_REF][START_REF] Wang | Surface Kinetics and Mechanism of Oxygen Incorporation Into Ba 1-x Sr x Co y Fe 1-y O 3-δ SOFC Microelectrodes[END_REF][START_REF] Steele | Properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-x (LSCF) double layer cathodes on gadoliniumdoped cerium oxide (CGO) electrolytes II. Role of oxygen exchange and diffusion[END_REF][START_REF] Song | Influence of alkaline-earth metal substitution on structure, electrical conductivity and oxygen transport properties of perovskite-type oxides La 0.6 A 0.4 FeO 3-δ (A = Ca, Sr and Ba)[END_REF] is shown to decrease with increasing Smith acidity for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , Sm 0.5 Sr 0.5 CoO 3-δ , La 0.6 Sr 0.4 CoO 3-δ , La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ , but that correlation does not hold for the most acidic material of the series, namely La 0.6 Sr 0.4 FeO 3-δ , which shows a higher D* than La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ . On the other hand, the electronic conductivity, also extracted from various studies [START_REF] Chen | Assessment of Ba 0.5 Sr 0.5 Co 1-y Fe y O 3-δ (y=0.0-1.0) for prospective application as cathode for IT-SOFCs or oxygen permeating membrane[END_REF][START_REF] Ishihara | Intermediate Temperature Solid Oxide Fuel Cells Using a New LaGaO 3 Based Oxide Ion Conductor: I. Doped SmCoO 3 as a New Cathode Material[END_REF][START_REF] Petrov | Crystal structure, electrical and magnetic properties of La 1-x Sr x CoO 3-y[END_REF][START_REF] Tai | Structure and electrical properties of La 1-x Sr x Co 1-y Fe y O 3 . Part 2. The system La 1x Sr x Co 0.2 Fe 0.8 O 3[END_REF][START_REF] Tripković | Surface recrystallizationan underestimated phenomenon affecting oxygen exchange activity[END_REF] (Figure 2c) also decreases when the Smith acidity increases, but the correlation does not hold for the most basic materials of the series, namely Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , which shows the lowest conductivity. These observations suggest that the surface exchange kinetics correlate better with the Smith acidity than they do with electronic and ionic transport properties. To look further into possible side correlations between Smith acidity and relevant properties for surface exchange kinetics, the oxygen vacancy concentrations were also compared with respect to the Smith acidity. Indeed, oxygen vacancies were previously shown to be involved in nearly all relevant rate determining steps in most of the relevant mixed conducting oxides [START_REF] Wang | Oxygen exchange kinetics on solid oxide fuel cell cathode materials -general trends and their mechanistic interpretation[END_REF] . Figure 2d shows the chemical capacitance of each materials as a function of the Smith acidity, extracted from the impedance measurements of Baumann et al. [START_REF] Baumann | Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes[END_REF] at 750 °C. In the case of thin films in which oxygen chemical potential is homogenous and controlled by the oxygen exchange reaction, the chemical capacitance is a good measure of the vacancy concentration [START_REF] Kawada | Determination of oxygen vacancy concentration in a thin film of La 0.6 Sr 0.4 CoO 3-δ by an electrochemical method[END_REF] . Again, although it scales nicely with the Smith acidity for Sm 0.5 Sr 0.5 CoO 3-δ , La 0.6 Sr 0.4 CoO 3-δ , La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ and La 0.6 Sr 0.4 FeO 3-δ , the correlation does not hold for Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ .

From these comparisons, it becomes possible to evaluate what appears to be the most relevant parameters relating to oxygen surface exchange in mixed conducting perovskites. First, although there is broad consensus that adequate ionic and electronic transport properties are mandatory for an oxide to be a good catalyst for oxygen exchange, those parameters, in and of themselves, are not sufficient to predict how fast the oxygen exchange kinetics will be. For example, Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ has a much lower electronic conductivity compared to some of the other transition metal perovskites, but still exhibits some of the fastest oxygen surface exchange kinetics. One can argue that oxide ion diffusivity is the most relevant property to describe oxygen exchange kinetics and the correlation between D* and k* demonstrated by Kilner et al. [START_REF] Kilner | Surface exchange of oxygen in mixed conducting perovskite oxides[END_REF] shows that. This still holds true here, except between La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ and La 0.6 Sr 0.4 FeO 3-δ for which the difference of D* is not significant enough to clearly invalidate the trend. Then, an important question is whether the acidity/k relationship is just a correlation, not a causation. However, even if this question remains open, we believe that because this link exists, it is interesting to analyse it in terms of the possible effects that acidity can have on the surface reaction, in an attempt to enrich the discussions on the matter. From this standpoint, the Smith acidity seems to exhibit a more direct relationship with oxygen exchange kinetics than the transport properties. This observation implies that the interaction of molecular oxygen with a more basic oxide surface is always favorable for fast kinetics. This statement naturally draws attention to adsorptionrelated steps involved in the overall reaction. If molecular oxygen was actually adsorbing preferentially on surface oxygen vacancies, one would expect a linear trend between the chemical capacitance (vacancy concentration), the Smith acidity and the surface exchange kinetics. Again, the chemical capacitance fails to explain the very fast kinetics observed with Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , while the Smith acidity does. This observation suggests that high oxygen vacancy concentration at the surface does not necessarily mean fast kinetics. This is consistent with the previous findings of Tsvetkov et al. [START_REF] Tsvetkov | Improved chemical and electrochemical stability of perovskite oxides with less reducible cations at the surface[END_REF] which aimed at lowering surface vacancy concentration to limit Sr segregation in La 0.6 Sr 0.4 CoO 3-δ . In that work, the authors proved that high enthalpies of vacancy formation at the surface do not decrease surface exchange properties. It also corroborates our previous observation [START_REF] Nicollet | Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides[END_REF] that infiltrating non redox active oxides on a mixed conducting oxide can induces dramatic changes in the surface exchange kinetics. Indeed, those oxides are not expected to modify the vacancy concentration at the surface because of their close shell electronic structure. [START_REF] Berenov | Oxygen tracer diffusion and surface exchange kinetics in La 0.6 Sr 0.4 CoO 3-δ[END_REF][START_REF] Fullarton | Study of oxygen ion transport in acceptor doped samarium cobalt oxide[END_REF][START_REF] Wang | Surface Kinetics and Mechanism of Oxygen Incorporation Into Ba 1-x Sr x Co y Fe 1-y O 3-δ SOFC Microelectrodes[END_REF][START_REF] Steele | Properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-x (LSCF) double layer cathodes on gadoliniumdoped cerium oxide (CGO) electrolytes II. Role of oxygen exchange and diffusion[END_REF][START_REF] Song | Influence of alkaline-earth metal substitution on structure, electrical conductivity and oxygen transport properties of perovskite-type oxides La 0.6 A 0.4 FeO 3-δ (A = Ca, Sr and Ba)[END_REF] . (c) Electronic conductivity measured at 600 °C reproduced from various authors [START_REF] Chen | Assessment of Ba 0.5 Sr 0.5 Co 1-y Fe y O 3-δ (y=0.0-1.0) for prospective application as cathode for IT-SOFCs or oxygen permeating membrane[END_REF][START_REF] Ishihara | Intermediate Temperature Solid Oxide Fuel Cells Using a New LaGaO 3 Based Oxide Ion Conductor: I. Doped SmCoO 3 as a New Cathode Material[END_REF][START_REF] Petrov | Crystal structure, electrical and magnetic properties of La 1-x Sr x CoO 3-y[END_REF][START_REF] Tai | Structure and electrical properties of La 1-x Sr x Co 1-y Fe y O 3 . Part 2. The system La 1x Sr x Co 0.2 Fe 0.8 O 3[END_REF][START_REF] Tripković | Surface recrystallizationan underestimated phenomenon affecting oxygen exchange activity[END_REF] . (d) Chemical capacitance measured by impedance spectroscopy at 750 °C by Baumann et al. [START_REF] Baumann | Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes[END_REF] . All parameters are plotted against the calculated Smith Acidity for each composition.

Although the link between an oxide's acidity and the associated oxygen surface exchange kinetics appears to be established, the main question remains: what reaction mechanism is so tightly bound to acidity, and in what way does changing acidity change the overall exchange kinetics?

In our previous work, we discussed the connection between acidity and surface electron concentration at the surface, although this change alone could not be responsible for the change of kinetics experienced on Pr-doped ceria. Here, the problem is different because we are no longer considering interfacial effects between two materials. It is interesting to discuss possible reactions steps that can be influenced by acidity.

For the majority of commonly used mixed conducting oxides, i.e. those with fast oxide ion conductivities, the rate limiting step of the oxygen surface exchange reaction is either a step related to adsorption of molecular oxygen, or to the charge transfer step, corresponding to the reduction of neutral oxygen species with electrons provided by the mixed conductor. Hence, we choose to limit the discussion to these two steps. We begin by considering the following: if the acidity of an oxide is affecting the charge transfer reaction of oxygen reduction, similar trends could be expected for electrocatalysts studied for the oxygen evolution reaction in water electrolysis at room temperature. To assess if this is the case, the calculation of the Smith acidity using Eq. ( 1) was repeated for a number of oxygen evolution reaction (OER) electrocatalysts. Then, their activity as an oxygen electrode is plotted as a function of the calculated Smith acidity and discussed. The overpotential first set of data is extracted from the theoretical work of Man et al. [START_REF] Man | Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces[END_REF] , in which they calculate an electrode overpotential based on the free energy of the rate limiting step. Although their approach can be considered over simplified as previously discussed [START_REF] Exner | Why approximating electrocatalytic activity by a single free-energy change is insufficient[END_REF] , their work has the advantage of gathering a large number of perovskites, which is convenient for looking at a potential trend with our analysis. To complete this analysis, experimental data of oxygen reduction reaction activity (ORR) was extracted from the work of Suntivich et al. [START_REF] Suntivich | Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal -air batteries[END_REF] . Figure 3a shows the theoretical OER overpotential of various perovskites as a function of their acidity, and Figure 3b shows the ORR potential at 25 µA.cm -2 of various perovskites, also as a function of their calculated Smith acidity. It becomes clear from this analysis that in the case of OER catalysts, no clear trend between their performance and the Smith acidity is evidenced. This independence between the two metrics strongly suggests that the acidity of the oxide does not have a direct link with the charge transfer step of the oxygen evolution reaction. It also points to a different behavior of materials between high temperature gas phase reactions and low temperature water reactions. There are a number of hypotheses that could explain this difference. First, at low temperature, oxygen vacancies in perovksites are not mobile which can have a direct effect on reaction rates. Second, electronic transport is also much different at low temperature. Most of these oxides being semi-conductors, the availability of electron at the surface is much lower than at high temperature (for gas phase reactions). On the other hand, the reactant molecule is different and their adsorption onto the active site is also fundamentally different (gas vs liquid phase). [START_REF] Man | Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces[END_REF] and (b) ORR potential measured at 25 µA.cm -2 from Suntivich et al. [START_REF] Suntivich | Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal -air batteries[END_REF]

Discussion

Assuming charge transfer is not affected by the acidity of the oxide (at least not directly), then the adsorption of molecular oxygen from the gas phase takes over as the most likely mechanism. In the case of alkaline electrolysis, the adsorption of reactants on active sites have fundamentally different energetics than for a gas phase at high temperature, being in the liquid phase and at room temperature.

For example, physisorption of gas molecules on a solid is not believed to maintain to high temperatures, so that only chemisorbed molecules can remain. That, in principle, sets aside the possibility of multilayer adsorption, adsorbate interactions, etc. The high temperatures at which solid oxide cells operate also decreases the surface coverage of adsorbates dramatically; so that it is often concluded that the adsorption sites are nearly empty (coverage 0) [START_REF] Fleig | The p(O 2 ) dependence of oxygen surface coverage and exchange current density of mixed conducting oxide electrodes: model considerations[END_REF][START_REF] De Souza | Limits to the rate of oxygen transport in mixed-conducting oxides[END_REF] . Considering a strong relationship between oxide acidity and adsorption kinetics is reasonable and is already well established in other fields of research. For example, CO 2 capture from the gas by solid sorbents relies on highly basic compounds such as alkaline oxides [START_REF] Dunstan | CO 2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances[END_REF] .

From this standpoint, we can speculate as to how the acidity of the mixed conducting oxide can change the overall rate of oxygen surface exchange. First, we can argue that the surface coverage increases with the basicity of the oxide, i.e. that the binding strength between molecular oxygen and the surface is increased on more basic oxides. That, on its own, does not appear to explain the orders of magnitude variations of k chem as measured in our previous work on Pr doped ceria [START_REF] Nicollet | Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides[END_REF] , given that surface coverage can only vary by so much. Second, through the same modulation of binding strength between molecule and solid, the adsorption/desorption equilibrium is shifted towards more adsorption.

This statement is essentially the same as increasing the surface coverage, in the sense that the latter is simply a consequence of the former according to the Langmuir theory of adsorption. Nevertheless, one can also interpret the equilibrium shift as a change of residence time for a molecule on the surface, i.e.

how long a molecule stays adsorbed before being desorbed. Expressing the adsorption as a residence time is essentially what originally led Langmuir to his pioneering expression of adsorption isotherms [START_REF] Langmuir | The constitution and fundamental properties of solids and liquids. Part I. Solids[END_REF][START_REF] Swenson | Langmuir's Theory of Adsorption: A Centennial Review[END_REF] . We think it is a useful interpretation that can help one understand the acidity/surface exchange relationship. Indeed, assuming that an adsorbed molecule is either reduced and incorporated into the solid or desorbed, if a molecule stays longer on the surface, then the probability of it being reduced by electrons before being desorbed increases accordingly. This interpretation reflects the fact that a change in adsorption/desorption equilibrium will not simply change the surface coverage of adsorbed species, but will also modify the attempt frequency for the reduction reaction. This analysis can be particularly helpful if the surface coverage stays close to 0, i.e. at high temperatures, because it could differentiate slow and fast kinetics by simply modifying the attempt frequency for charge transfer. It would also explain our previous observation that the activation energy for the overall reaction is independent of the surface acidity [START_REF] Nicollet | Acidity of surface-infiltrated binary oxides as a sensitive descriptor of oxygen exchange kinetics in mixed conducting oxides[END_REF] . This interpretation also resonates with what was previously discussed by Adler [START_REF] Adler | Mechanisms and rate laws for oxygen exchange on mixed-conducting oxide surfaces[END_REF] , who concluded that for electron rich perovskites, the reaction can by limited by the availability of sites or intermediates, and that this does not change the activation barrier for the reaction.

Conclusion

The relative Smith acidity of binary oxides used to infiltrate mixed conducting SOFC cathode materials was previously demonstrated to serve as a key descriptor of the oxygen exchange kinetics at the cathode/oxygen atmosphere interface. The analysis proposed in this work was aimed at examining if the Smith acidity of ternary perovskite mixed conducting cathode candidate materials could likewise be used to predict oxygen exchange behavior. Towards this end, we proposed a readily calculable acidity parameter derived from knowledge of the acidity of the individual binary oxides, i.e., AO & BO 2 , that together make up the ABO 3 perovskite structure. This acidity parameter is then used to examine if trends in published oxygen exchange behavior can be correlated with this calculated Smith acidity factor. The main conclusion that can be drawn from this analysis is that there is a clear link between this calculated surface acidity and the oxygen exchange kinetics, and that this link seems to hold best when the mixed conducting properties of the examined oxides are already favorable, i.e.

exhibiting both fast ionic and electronic transport. Because this correlation appears to apply only for high temperature gas/solid reactions, and not to the OER at reduced temperatures, we conclude that the impact of acidity lies more with the adsorption thermodynamics rather than the charge transfer reaction. Nevertheless, even though a link between acidity and surface exchange kinetics can be established, its implications at the mechanistic level appear rather complex and need further investigation. In particular, computation of accurate Smith acidity values for a large number of mixed conductors could provide a way to systematically predict the performance of mixed conductors utilized as the oxygen electrode in Solid Oxide Cells.

Figure 1 :

 1 Figure 1 : Polarization Resistance (a) and surface exchange coefficient k* (b) as a function of Smith acidity for materials measured at 1000 K. Data copied from[START_REF] Lee | Prediction of solid oxide fuel cell cathode activity with first-principles descriptors[END_REF] , initially collected from refs16-43 .

  compares the polarization resistance associated with the surface exchange kinetics (R p ) of Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , Sm 0.5 Sr 0.5 CoO 3-δ , La 0.6 Sr 0.4 CoO 3-δ , La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ and La 0.6 Sr 0.4 FeO 3-δ with respect to their Smith acidity. Similar

Figure 2 :

 2 Figure 2: Dependence on Smith acidity of surface exchange kinetics and transport properties. (a) Area specific resistance associated with oxygen exchange kinetics, R p, of thin films of various perovskites measured at 600 °C reproduced from Baumann et al 9 . (b) Oxygen diffusion coefficient D* measured at 600 °C reproduced from various authors[START_REF] Berenov | Oxygen tracer diffusion and surface exchange kinetics in La 0.6 Sr 0.4 CoO 3-δ[END_REF][START_REF] Fullarton | Study of oxygen ion transport in acceptor doped samarium cobalt oxide[END_REF][START_REF] Wang | Surface Kinetics and Mechanism of Oxygen Incorporation Into Ba 1-x Sr x Co y Fe 1-y O 3-δ SOFC Microelectrodes[END_REF][START_REF] Steele | Properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-x (LSCF) double layer cathodes on gadoliniumdoped cerium oxide (CGO) electrolytes II. Role of oxygen exchange and diffusion[END_REF][START_REF] Song | Influence of alkaline-earth metal substitution on structure, electrical conductivity and oxygen transport properties of perovskite-type oxides La 0.6 A 0.4 FeO 3-δ (A = Ca, Sr and Ba)[END_REF] . (c) Electronic conductivity measured at 600 °C reproduced from various authors[START_REF] Chen | Assessment of Ba 0.5 Sr 0.5 Co 1-y Fe y O 3-δ (y=0.0-1.0) for prospective application as cathode for IT-SOFCs or oxygen permeating membrane[END_REF][START_REF] Ishihara | Intermediate Temperature Solid Oxide Fuel Cells Using a New LaGaO 3 Based Oxide Ion Conductor: I. Doped SmCoO 3 as a New Cathode Material[END_REF][START_REF] Petrov | Crystal structure, electrical and magnetic properties of La 1-x Sr x CoO 3-y[END_REF][START_REF] Tai | Structure and electrical properties of La 1-x Sr x Co 1-y Fe y O 3 . Part 2. The system La 1x Sr x Co 0.2 Fe 0.8 O 3[END_REF][START_REF] Tripković | Surface recrystallizationan underestimated phenomenon affecting oxygen exchange activity[END_REF] . (d) Chemical capacitance measured by impedance spectroscopy at 750 °C by Baumann et al.[START_REF] Baumann | Quantitative Comparison of Mixed Conducting SOFC Cathode Materials by Means of Thin Film Model Electrodes[END_REF] . All parameters are plotted against the calculated Smith Acidity for each composition.

3 @ 25 mA.cm - 2 oxFigure 3 :

 32523 Figure 3: OER and ORR activity as a function of Smith acidity for various oxides at room temperature. (a) Calculated overpotential from Man et al.[START_REF] Man | Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces[END_REF] and (b) ORR potential measured at 25 µA.cm -2 from Suntivich et al.[START_REF] Suntivich | Design principles for oxygen-reduction activity on perovskite oxide catalysts for fuel cells and metal -air batteries[END_REF]

  we can use a Smith acidity factor. For transition metals such as Fe or Co, several stable oxides exist, sometimes with mixed valence (Co 3 O 4 for example), which makes the selection of a relevant Smith factor less obvious. It is tempting to consider that the transition metal in most perovskites is primarily in the 3+ valence state, and therefore one would choose the acidity factor of the M 2 O 3 oxide to better match the oxidation state in the perovskites. However, this strategy is less convenient because the acidity of the corresponding M 2 O 3 oxide is not available for all transition metals considered in this study. This is because the Smith factor for a given oxide is determined from the formation of oxoacid salts that are, for transition metals, primarily in the 2+ oxidation state. As there are more thermochemical data available for oxoacid salts with the 2+ oxidation state, the Smith acidity value for BO oxides is also more reliable because it is calculated from an average of more reaction enthalpies.

	Therefore, for comparison purposes, we chose to use the Smith acidity for the 2+ oxide of each
	transition metal oxide (when possible). Nevertheless, one should keep in mind that the Smith factor
	can change greatly with the oxidation state of a given cation. For example, α(MnO) is -4.8 whereas
	α(Mn 2 O 7 ) is 9.6, which illustrates the challenge in comparing a large number of different compositions
	with this analysis. Additionally, one should keep in mind that different levels of A site doping of rare
	earth 3+ cations with alkaline earth like Sr 2+ can have two opposite effects: decreasing A site acidity

  3 , La 0.8 Sr 0.2 MnO 3 , LaCoO 3 , La 0.6 Sr 0.4 CoO 3-δ , La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ , Sm 0.5 Sr 0.5 CoO 3-δ , Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , and double perovskites: PrBaCo 2 O 5+δ , and GdBaCo 2 O 5+δ . Figure

  Sr 0.5 Co 0.8 Fe 0.2 O 3-δ , stability issues and reactivity problems are well known to exist due to the formation of surface carbonates that could negatively impact the analysis. Another feature that renders the analysis of surface exchange kinetics with acidity more difficult than in our previous work, is that the activation energy for k* (or R p ) can change significantly between materials, especially when the 3d cation, believed to be the redox center, changes.

	BSCF : Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ	SSC: Sm 0.5 Sr 0.5 CoO 3-δ	LSC: La 0.6 Sr 0.4 CoO 3-δ
	PBCO: PrBaCo 2 O 5+δ	GBCO: GdBaCo 2 O 5+δ	LSCF: La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ
	LSM: La 0.8 Sr 0.2 MnO 3	LMO: LaMnO 3	LCO: LaCoO 3

5

Co 0.8 Fe 0.2 O 3-δ , the inconsistency between the k* and R p data raises the question of the reliability of surface exchange data in the literature. Especially in the case of Ba 0.5

Supporting information

The datasets used in this manuscript are available as an excel spreadsheet in the supplementary information section.

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