Henrietta Essie Whyte
email: henrietta.whyte@imt-atlantique.fr

Cécile Raillard

Albert Subrenat

Valérie Héquet

Influence of operating parameters on the single-pass photocatalytic removal efficiency of acrylonitrile

Keywords:

Photocatalytic oxidation (PCO) is an advanced air cleaning technology that is used as a means to improve air quality in indoor environments and could also potentially be used in hospital operating rooms (ORs). However, when it comes to the feasibility of using PCO to remove VOCs, most studies have been on those that are commonly found in indoor environments like homes and schools. There are little or no studies on other indoor environments like hospitals. Therefore in this work, acrylonitrile, one of the hazardous compounds found in surgical smoke (a source of pollution in the OR) was chosen as a representative compound to evaluate the performance of a photocatalytic system in an OR. The experiments were performed in a 420-L multi-pass laboratory reactor. The performance of the system was based on the influence that three operating parameters (air velocity, light intensity and initial concentration) would have on the single-pass removal efficiency (SPRE). A mathematical model was used to enable the calculation of the SPRE from the experimental degradation profile. The influence of the operating parameters on the degradation of acrylonitrile as well as the possible intermediates formed and mineralization rates are discussed.

Introduction

Indoor Air Quality (IAQ) is an important issue in industrialized and developed countries due to the considerable time people spend indoors [START_REF] Sundell | Ventilation rates and health: multidisciplinary review of the scientific literature[END_REF]. In hospitals however, the IAQ is even more critical, because of the increased susceptibility of the patients [START_REF] Bivolarova | Bed-integrated local exhaust ventilation system combined with local air cleaning for improved IAQ in hospital patient rooms[END_REF]. Consequently poor IAQ can lead to nosocomial infections and occupational diseases [START_REF] Jung | Indoor air quality varies with ventilation types and working areas in hospitals[END_REF][START_REF] Hellgren | Complaints and symptoms among hospital staff in relation to indoor air and the condition and need for repairs in hospital buildings[END_REF].

The first concern when it comes to IAQ in the hospital is with microbial contamination because they cause nosocomial infections whilst contamination with chemical pollutants is rarely considered. However, hospital occupants can be exposed to a variety of chemical pollutants due to the diversity of activities that take place in the different zones in the hospital [START_REF] Bessonneau | VOC contamination in hospital, from stationary sampling of a large panel of compounds, in view of healthcare workers and patients exposure assessment[END_REF][START_REF] Leung | Control and management of hospital indoor air quality[END_REF][START_REF] Gangneux | Concomitant determination of indoor air microbiological and chemical contamination for a global approach of air quality in hospitals[END_REF]. One of the most demanding zones when it comes to IAQ is the operating room (OR). In the OR, the air can be contaminated by waste anesthetic gases, disinfection and sterilization products, surgical smoke and particles like lint, respiratory droplets and aerosols [START_REF] Dascalaki | Air quality in hospital operating rooms[END_REF][START_REF] Balaras | HVAC and indoor thermal conditions in hospital operating rooms[END_REF][START_REF] Barrett | Surgical smoke: a review of the literature[END_REF]. The literature reports that one of the main culprits of air pollution in ORs is surgical smoke [START_REF] Alp | Surgical smoke and infection control[END_REF]. Surgeons and operating theatre staff are routinely exposed to pollution from the surgical smoke. It hinders the vision of the surgeon, produces an unpleasant odor, and releases harmful substances into the air that are harmful to the staff [START_REF] -M. Fan | Surgical smoke[END_REF]. The chemicals present in greatest quantity in surgical smoke are hydrocarbons and nitrile compounds among which is acrylonitrile [START_REF] Barrett | Surgical smoke: a review of the literature[END_REF][START_REF] Hill | Surgical smokea health hazard in the operating theatre: a study to quantify exposure and a survey of the use of smoke extractor systems in UK plastic surgery units[END_REF][START_REF] Al Sahaf | Chemical composition of smoke produced by high-frequency electrosurgery[END_REF].

Acrylonitrile is a hazardous compound that has been classified as a Group-2B (possibly carcinogenic to humans) carcinogen by the IARC [START_REF]IARC Monographs on the Evaluation of carcinogenic risks to humans Classifications[END_REF]. The occupational exposure limit (OEL) for an 8-h time weighted average set by NIOSH is 2 ppm. Short-term exposure to acrylonitrile can cause eye irritation, nausea, vomiting, headache, sneezing, weakness and light headedness, whilst long-term exposure may cause cancer [START_REF] Hill | Surgical smoke -a health hazard in the operating theatre: a study to quantify exposure and a survey of the use of smoke extractor systems in UK plastic surgery units[END_REF][START_REF] Mowbray | Is surgical smoke harmful to theater staff? A systematic review[END_REF]. There are not a lot of studies that show the concentration levels of acrylonitrile in the OR. Barret and Garber [START_REF] Barrett | Surgical smoke: a review of the literature[END_REF] reported acrylonitrile concentrations in ORs to be between 1 and 1.6 ppm. Chung et al. [START_REF] Chung | Harmful gases including carcinogens produced during transurethral resection of the prostate and vaporization[END_REF] measured average acrylonitrile concentration of 15 ppm in a Korean OR.

In ORs, the IAQ is ensured by heating, ventilation and air conditioning (HVAC) systems through the dilution of indoor air pollutant concentrations by the supply of conditioned outdoor air [START_REF] Balaras | HVAC and indoor thermal conditions in hospital operating rooms[END_REF]. However this is not always sufficient thus air treatment devices are gaining popularity as complementary methods to ensure good air quality [START_REF] Bergeron | Decreasing airborne contamination levels in high-risk hospital areas using a novel mobile air-treatment unit[END_REF][START_REF] Le | Air purification equipment combining a filter coated by silver nanoparticles with a nano-TiO 2 photocatalyst for use in hospitals[END_REF][START_REF] Perraud | Air filtration and prevention of aspergillary pneumopathies. Preliminary comparative study of two mobile units for bacteriological air purification with recycling[END_REF][START_REF] Rosenbaum | Use of a portable forced air system to convert existing hospital space into a mass casualty isolation area[END_REF][START_REF] Schoenleber | Évaluation d'un dispositif mobile de décontamination pour la maîtrise de l'air au bloc opératoire[END_REF]. These devices are accepted by The Center for Disease Control and Prevention (CDC) in the US and the Société Française d'Hygiène Hospitalière (SF2H) in France for treating the air and also improving ventilation in ORs [START_REF]Guidelines for Environmental Infection Control in Health-Care Facilities: Recommendations of CDC and the Healthcare Infection Control Practices Advisory Committee (HICPAC)[END_REF][START_REF]Hospitalière, Qualité de l'air au bloc opératoire et autres secteurs interventionnels[END_REF]. These treatment devices are based on processes such as ozonation, cold plasma, UV disinfection and photocatalytic oxidation (PCO) to purify the air.

PCO is an efficient technology that exists for air treatment. It has the advantage of being cheap and a flexible solution for the degradation of a wide spectrum of volatile organic compounds (VOCs) [START_REF] Destaillats | Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions[END_REF][START_REF] Boyjoo | A review on photocatalysis for air treatment: From catalyst development to reactor design[END_REF][START_REF] Ren | Photocatalytic materials and technologies for air purification[END_REF][START_REF] Mo | Novel insight into VOC removal performance of photocatalytic oxidation reactors[END_REF]. It removes gaseous contaminants via chemical reactions on semiconductor (mostly TiO 2 ) catalyst surface under UV irradiation. There has been a lot of work published in the literature on the efficiency of PCO for the removal of several pollutants [START_REF] Zhao | Photocatalytic oxidation for indoor air purification: a literature review[END_REF][START_REF] Mo | Photocatalytic purification of volatile organic compounds in indoor air: a literature review[END_REF][START_REF] Zhong | Photocatalytic air cleaners and materials technologies -Abilities and limitations[END_REF]. The influence of several key parameters such as VOC concentration, UV light intensity, air velocity and relative humidity on the degradation of pollutants have also been extensively studied [START_REF] Raillard | Kinetic study of ketones photocatalytic oxidation in gas phase using TiO2-containing paper: effect of water vapor[END_REF][START_REF] Sleiman | Photocatalytic oxidation of toluene at indoor air levels (ppbv): towards a better assessment of conversion, reaction intermediates and mineralization[END_REF][START_REF] Ao | Photodegradation of formaldehyde by photocatalyst TiO2: effects on the presences of NO, SO2 and VOCs[END_REF][START_REF] Ye | Photocatalytic oxidation of aldehydes: byproduct identification and reaction pathway[END_REF][START_REF] Chen | Photocatalytic oxidation of multi-component systemsan References investigation using toluene/ethylbenzene, octane/decane/dodecane and formaldehyde/acetaldehyde[END_REF][START_REF] Yu | Indoor air purification using heterogeneous photocatalytic oxidation. Part I: experimental study[END_REF]. In the literature, the performance of photocatalytic systems is based on the use of descriptors like the kinetic rate constant, the clean air delivery rate, the single-pass removal efficiency and energy efficiency index [START_REF] Héquet | Determination of the Clean Air Delivery Rate (CADR) of Photocatalytic Oxidation (PCO) Purifiers for Indoor Air Pollutants Using a Closed-Loop Reactor[END_REF][START_REF] Palmiste | Gas-phase optical fiber photocatalytic reactors for indoor air application: a preliminary study on performance indicators[END_REF][START_REF] Kolarik | Can a photocatalytic air purifier be used to improve the perceived air quality indoors?[END_REF].

Most of the focus of PCO studies has been on pollutants that are typically found in homes, schools and offices without much attention being paid to specific pollutants found in other indoor environments like hospitals. Additionally, as PCO devices could be potentially used in the ORs as a complementary means of improving IAQ it is necessary to study the feasibility of PCO for the removal of a typical OR pollutant. The PCO of acrylonitrile has not been widely studied in the literature. Krichevskaya et al. [START_REF] Krichevskaya | Gas-phase photocatalytic oxidation of acrylonitrile[END_REF] studied the gas phase degradation of acrylonitrile by using Degussa P25 under UV irradiation at concentrations ranging from 10 to 100 ppm at temperatures ranging from 50 to 130 °C. They observed that degradation rates were improved at longer contact times and lower concentrations and decreased at higher temperatures. They also reported that the products of gas-phase acrylonitrile PCO included carbon dioxide, water and nitrogen dioxide and hydrogen cyanide. Joks et al. [START_REF] Jõks | Gas-phase photocatalytic oxidation of acrylonitrile on sulphated TiO2: continuous flow and transient study[END_REF] also studied gas-phase PCO of acrylonitrile on sulphated TiO 2 and showed improved performance at higher temperature and longer retention times under UV irradiation. They identified hydrogen cyanide and isocyanic acid as intermediates. Pang et al. [START_REF] Pang | Fluorine promoted and silica supported TiO2 for photocatalytic decomposition of acrylonitrile under simulant solar light irradiation[END_REF] in studying suspension phase (the catalyst was suspended in aqueous solution of acrylonitrile) acrylonitrile degradation on F-doped TiO 2 / SiO 2 catalyst under simulant solar light irradiation reported higher photocatalytic activity for the F-doped samples due to the increase of number and strength of surface acid sites. In another study using N-F codoped TiO 2 /SiO 2 under simulant solar light irradiation, Pang et al. [START_REF] Pang | Photocatalytic decomposition of acrylonitrile with N-F codoped TiO2/SiO2 under simulant solar light irradiation[END_REF] identified some organic acids (formic acid, acroleic acid, acetic acid) as the intermediates of suspension phase acrylonitrile degradation. These works however were performed at high initial concentrations (> 10 ppm) or under conditions not relevant to ORs. Thus, further studies on the efficiency of PCO of acrylonitrile at more realistic OR conditions are necessary.

This work therefore focuses on evaluating the efficiency of PCO of acrylonitrile under conditions that are relevant to actual OR applications with the objective of studying the influence of air velocity, light intensity and initial concentration on the single-pass removal efficiency. The study of the influence of these parameters will help to provide knowledge to improve the efficiency of the system and finally to highlight the performance of PCO in real OR conditions. The influence of initial pollutant concentration on the mineralization rates is also examined. The single-pass removal efficiency (SPRE) is chosen as an indicator because commercial air purification units that are available on the market mostly operate under single-pass configuration in which a volume of air passes through the reactor once for treatment. Thus it would be a representative way to evaluate the performance of a PCO device. The experiments were conducted in a closed loop reactor which has been designed to study low concentration air pollutants and has also been recently modeled [START_REF] Dumont | Determination of the clean air delivery rate (CADR) of photocatalytic oxidation (PCO) purifiers for indoor air pollutants using a closedloop reactor. Part I: theoretical considerations[END_REF][START_REF] Héquet | Photocatalytic oxidation of VOCs at ppb level using a closed-loop reactor: the mixture effect[END_REF]. This reactor also permits a better representation of PCO systems developed in dynamic mode and is realistic regarding air treatment systems working as standalone devices or implanted in HVAC systems.

Materials and methods

Generation of acrylonitrile concentrations

In order to generate the desired concentrations of the pollutant, gaseous acrylonitrile (C 3 H 3 N) supplied by Air products was used. It was supplied in a certified commercial gas cylinder at a concentration of 149 ppm and balanced in nitrogen at 150 bars. The generation process was started by first passing compressed air through Claind 2020 air zero generator to clean it. It was then humidified to a desired level by bubbling it through a water bottle at an average room temperature of 20 °C. Prior to injection into the reactor, acrylonitrile was mixed with the humid air zero to dilute it. The desired concentrations of acrylonitrile were achieved by adjusting the gas flow rates using mass flow controllers (Brooks).

Photocatalytic reactor

A 420-L closed-loop reactor operating in recirculating mode was used for the experiments. This reactor is presented in Fig. 1 and has been described in previous articles [START_REF] Batault | How chemical and physical mechanisms enable the influence of the operating conditions in a photocatalytic indoor air treatment device to be modeled[END_REF][START_REF] Raillard | Use of experimental designs to establish a kinetic law for a gas phase photocatalytic process[END_REF][START_REF] Maudhuit | Adsorption phenomena in photocatalytic reactions: the case of toluene, acetone and heptane[END_REF][START_REF] Whyte | Photocatalytic oxidation of isoflurane, an anesthetic gas: the influence of operating parameters[END_REF]. The air flow through the reactor is controlled by a variable speed which allows a flow rate from 28 to 300 Nm 3 . h -1 . Acrylonitrile is introduced into the reactor through a valve in the tranquilization chamber which is equipped with a honeycomb to enable a homogeneous flow distribution. Sampling is also done from the tranquilization chamber. The photocatalytic module houses the media and the lamps. The media used is QUARTZEL® PCO supplied by Saint-Gobain Quartz. It consists of SiO 2 fibers coated with TiO 2 deposited through a sol-gel method with average thickness of

⎟ = ⎛ ⎝ - - - ⎞ ⎠ C t τ kτ C exp exp [1 ( )] R P 0 (1)
In Eq. (1) τ R is the residence time in the reactor which is the complete closed loop system, C is the pollutant concentration at time t and C 0 is the initial pollutant concentration, τ P is the residence time in the photocatalytic module and k is the kinetic constant (which is a composite expression accounting for several chemical kinetics and reactor dynamics mechanisms).

The term kτ P (lumped as α) is the fractional yield of the treated flow rate of the PCO device. In other words, α is the single-pass removal efficiency and represents the fraction of the total flow treated during the time τ R . Then, Eq (1) can be rewritten as follows:

⎜ ⎟ = ⎛ ⎝ - - -⎞ ⎠ C t τ α C exp exp [1 ( )] R 0 (2)
The second assumption stated that since the volume of the photocatalytic module is small compared to the reactor, the α value is small with regards to the flow rate. In this case, Taylors theorem leads to the approximation that α ≈ exp(1 -α). Thus Eq. ( 2) can be simplified as Eq 3 :

⎜ ⎟ = ⎛ ⎝ - ⎞ ⎠ C t τ α C exp R 0 (3)
However, in this study Eq. ( 2) is used in the determination of α. Numerical resolutions were carried out using Excel® Solver which is based on the least-square method.

Results and discussion

Degradation profile of acrylonitrile

Acrylonitrile degradation profile is shown in Fig. 2. Under the experimental conditions of C 0 = 2 ppm, v = 1 m.s -1 and I =4.5 mW.m -2 the complete degradation of acrylonitrile was achieved after 1 h and it was characterized by an exponential profile. The experimental points were then fitted to the model (Eq. 2) and were shown to fit satisfactorily based on the regression co-efficient (R 2 ) of 0.99. Under these conditions, the single-pass removal efficiency (α) was then calculated as 0.012. This indicated that in one pass, 1.2% of acrylonitrile would be eliminated from the total flow that passed through the media.

To be able to study the influence of the three operating parameters on α two of the parameters were held constant whilst the third was varied. The reference experiment (C 0 = 2 ppm, v = 1 m.s -1 and I Fig. 2. Experimental degradation curve of acrylonitrile fitted to model developed by Dumont and Héquet under reference experimental conditions of C 0 = 2 ppm; v = 1 m.s -1 ; I =4.5 mW.cm -2 , RH = 50%. 17.5 ± 1.5 mm, a developed surface of 0.04 m 2 (20 cm x 20 cm) and a TiO 2 load of 94 g.m -2 . The BET specific surface area was measured at 112 ± 1 m 2 .g -1 with a Micrometrics ASAP 2020 by N 2 adsorption.

The media was irradiated by two 18-W UVC (254 nm) fluorescent tubes (Phillips TUV PL-L series). A Vilber Laurmat VLX-3W radiometer equipped with a calibrated CX-254 nm sensor was used to measure the irradiance over the entire photocatalytic media surface. The average light intensities were achieved by using a variable voltage supply to modify the power of the lamps. Prior to each experimental run, the reactor and photocatalytic media were cleaned by flushing with humid zero air under UV irradiation for several hours. Preliminary experiments were also carried out to evaluate the extent of adsorption process on acrylonitrile removal. The removal rates for adsorption were calculated at 0.1% per hour indicating that no significant loss of acrylonitrile was observed when exposed to the media without UV light.

Operating conditions

The air velocities varied from 0.5 to 1.5 m.s -1 and were determined as the ratio between the air flow rate (73 -216 Nm 3 .h -1 ) and the crosssectional area (0.04 m 2 ) of the photocatalytic module. The selected average light intensities (I) ranged from 1 to 4.5 mW.cm -2 . The ranges of velocity and light intensity were chosen based on the operating conditions typically used by a commercial air purifier that was studied. Initial concentrations (C 0 ) chosen for the experiments ranged from 0.5 to 10 ppm and would be representative of the concentration levels that could be found in the OR [START_REF] Barrett | Surgical smoke: a review of the literature[END_REF][START_REF] Chung | Harmful gases including carcinogens produced during transurethral resection of the prostate and vaporization[END_REF]. The relative humidity (RH) was maintained at 50% at 20 °C to be representative of average humidity levels.

Sampling and analytical methods

The concentrations of acrylonitrile were monitored by sampling onto sorbent tubes packed with Carbopack B (Perkin Elmer). The sampling was done at a rate of 200 ml.min -1 for 2.5 min, using an Automatic Clean Room Sampling System (ACROSS) by Tera Environnement. The analysis was carried out using a Thermal Desorber/Gas Chromatograph/Flame Ionization Detector/Mass Spectrometer (TD/GC/FID/MS). The column used in the GC was a polar column from Restek (Rxi-624Sil MS) which was connected to a FID for the quantification of compounds and a MS for the identification of compounds. The TD/GC/FID/MS analysis conditions are described in a previous article [START_REF] Whyte | Photocatalytic oxidation of isoflurane, an anesthetic gas: the influence of operating parameters[END_REF]. The limit of detection for acrylonitrile was calculated as 0.075 ppm for acrylonitrile calculated by multiplying the background noise by 3. The concentrations of CO 2 produced during the degradation were measured with an Ecotech EC 9820 T infra-red spectrometer.

Calculating the single-pass removal efficiency

As previously mentioned, the reactor used in this work is a multi pass reactor. This reactor is well adapted to provide information on the concentration degradation profiles over the entire duration of the experiment. It however does not provide direct information on the singlepass removal efficiency. This indicator has to be determined indirectly by employing mathematical models. In this work the model that is used has been developed by Dumont and Héquet [START_REF] Dumont | Determination of the clean air delivery rate (CADR) of photocatalytic oxidation (PCO) purifiers for indoor air pollutants using a closedloop reactor. Part I: theoretical considerations[END_REF]. In this work, the model that is used is based on one proposed by Walker and Wragg [START_REF] Walker | The modelling of concentration-time relationships in recirculating electrochemical reactor systems[END_REF] for the concentration-time relationship for a recirculating reactor system and adapted by Dumont and Héquet [START_REF] Dumont | Determination of the clean air delivery rate (CADR) of photocatalytic oxidation (PCO) purifiers for indoor air pollutants using a closedloop reactor. Part I: theoretical considerations[END_REF] for the photocatalytic system displayed at Fig. 1. Considering two assumptions, described in details in [START_REF] Dumont | Determination of the clean air delivery rate (CADR) of photocatalytic oxidation (PCO) purifiers for indoor air pollutants using a closedloop reactor. Part I: theoretical considerations[END_REF], the equation proposed by Walker and Wragg was simplified. The first assumption stated that as the volume of the photocatalytic module is small in relation to the volume of the reactor, the residence time in the photocatalytic module is small compared to that in the reactor. Thus the concentration-time relationship is given as: =4.5 mW.m -2 ) was repeated three times and the coefficient of variation of α was calculated as 1%. The results of the analysis of these 13 experiments are shown in Table 1. The reference experiment was carried out at regular intervals over the course of the experimental part to make sure that the photocatalyst was not deactivated. The results of the 3 experiments (Table 1) were quite similar and indicated that no deactivation occurred during experimental part.

Influence of the air velocity on the single-pass removal efficiency during photocatalytic degradation of acrylonitrile

In this work, the velocity is studied in order to determine the sensitivity of PCO performance to a change in velocity. The experiments were carried out at velocities ranging from 0.5 to 1.5 m.s -1 . This range was chosen based on the operating conditions typically used by commercial air purifiers.

For single-pass reactors, the effect of air velocity is known to present a dual antagonistic effect [START_REF] Mamaghani | Photocatalytic oxidation technology for indoor environment air purification: the state-of-the-art[END_REF]. As the velocity increases the mass transfer improves but the contact time between the pollutant and the photocatalytic media decreases. Some authors have observed a positive effect of the increase in velocities on the photocatalytic degradation of pollutants due to improvement in mass transfer [START_REF] Wang | The kinetics of photocatalytic degradation of trichloroethylene in gas phase over TiO2 supported on glass bead[END_REF][START_REF] Yu | Experimental study of the photocatalytic degradation of formaldehyde in indoor air using a nano-particulate titanium dioxide photocatalyst[END_REF] whilst others have observed negative effect of increasing velocities attributed to insufficient contact time [START_REF] Destaillats | Key parameters influencing the performance of photocatalytic oxidation (PCO) air purification under realistic indoor conditions[END_REF][START_REF] Quici | Effect of key parameters on the photocatalytic oxidation of toluene at low concentrations in air under 254+185nm UV irradiation[END_REF][START_REF] Zhong | Modeling and physical interpretation of photocatalytic oxidation efficiency in indoor air applications[END_REF]. Depending on the range of velocities studied, a transition between the effects can also be observed. For example, Yang et al. [START_REF] Yang | Performance analysis of a novel TiO2-coated foam-nickel PCO air purifier in HVAC systems[END_REF] studied the degradation of formaldehyde using a single pass tubular foam-nickel PCO reactor and observed that the removal efficiencies increased when velocities were increased from 0.47 to 0.66 m.s -1 , whilst removal efficiencies decreased when velocities ranged from 0.66 to 0.94 m.s -1. They explained this by the fact that when velocities were increased from low values (0.47 m.s -1 ) there was an enhancement in the mass transfer rate leading to an increase in removal efficiencies. However, once the air velocity reached 0.66 m.s -1 , the removal efficiencies declined due to a reduction in contact time between the pollutants and the photocatalytic media. At this stage, the process was no longer controlled by the mass transfer but by the surface reaction.

The Reynolds number has been described as a good indicator to set an appropriate regime to determine mass transfer limitations. Obee [START_REF] Obee | Photooxidation of sub-parts-per-million toluene and formaldehyde levels on titania using a glass-plate reactor[END_REF] demonstrated when he studied the degradation of 0.7 ppm of toluene in a TiO 2 -coated glass-plate reactor that with a Reynolds number over 500 the photocatalytic reaction was less influenced by mass transfer. In the present work, the photocatalytic module was inserted in a 20 cm × 20 cm square cross-section so that the hydraulic diameter was 20 cm. The Reynolds number thus ranged between 6500 and 19,800, depending on the air velocity value (0.5 to 1.5 m.s -1 ). This means that the mass transfer is not a limiting factor.

In multi-pass reactors, the velocity brings into play an additional effect which is the number of passes of the pollutant through the media. Therefore, when the velocity within the media is increased, the contact time decreases but then the number of passes of the pollutant through the media increases. The superposition of these two effects subsequently makes it difficult to highlight the influence of velocity on the reaction rate. Consequently, the global effect of the velocity on the degradation remains the same irrespective of the velocity. In studying the degradation of toluene in a multi-pass reactor, Batault et al. [START_REF] Batault | How chemical and physical mechanisms enable the influence of the operating conditions in a photocatalytic indoor air treatment device to be modeled[END_REF] observed that the velocity had no influence on the kinetic constant due to the superposition of the contact time and number of passes.

In this study however, the air velocity was shown to have an influence on the entire kinetic curve of acrylonitrile even though operating in a multi-pass system as shown in Fig. 3.

An assumption is made that there is competition created between the velocity of the molecules through the media and the amount of time needed for molecules to react with active species created by the media. Thus, increasing the velocity tends to not favor the occurrence of the chemical reaction. This would in turn emphasize the influence of air velocity on the kinetic degradation. In order to clearly bring into evidence the influence of the velocity on the degradation of acrylonitrile, the single-pass removal efficiency is chosen as the descriptor. The results obtained for acrylonitrile experiments are discussed below.

The initial concentration, C 0 and light intensity, I were maintained at 2 ppm and 4.5 mW.cm -2 respectively. The results of the experiments are shown in Fig. 4.

It can be observed that as the velocity increases from 0.5 to 1.5 m.s -1 , the α decreases from 0.04 to 0.007. This is in accordance Fig. 4. Influence of air velocity on the single-pass photocatalytic removal efficiency of acrylonitrile under experimental conditions of C 0 = 2 ppm; I =4.5 mW.cm -2 ; RH = 50%.

Table 1

Experimental conditions for PCO degradation of acrylonitrile and experimental results using the Dumont and Héquet [START_REF] Dumont | Determination of the clean air delivery rate (CADR) of photocatalytic oxidation (PCO) purifiers for indoor air pollutants using a closedloop reactor. Part I: theoretical considerations[END_REF] model.

The relationship between α and light intensity was well described by a power function and was non-linear. The power exponent was calculated as 0.5. This result is similar to what Raillard et al. [START_REF] Raillard | Use of experimental designs to establish a kinetic law for a gas phase photocatalytic process[END_REF] reported when they studied the degradation of toluene in a closed loop reactor at similar light intensities of 1-4 mW.cm -2 . This half-order relationship indicates that over the range of light intensities studied, the reactions were dominated by the electron-hole recombination effect. As previously mentioned, the efficiency increases with the light intensity since more hydroxyl radicals are generated. However, the recombination of the electron-hole pairs at high light intensity inhibits the rate of electron transfer and thus increasing the light intensity would not significantly improve or influence the removal efficiencies but would only impose unnecessary energy costs.

According to our results, for the range of light intensities studied, higher light intensities provide better degradation performance better removal efficiencies for acrylonitrile. However, due to reduction in quantum efficiency as a result of the domination of electron-hole recombination, increasing the light intensity above 5 mW.cm -2 would not significantly improve the removal efficiencies but would only impose unnecessary energy costs.

Effect of initial concentration on the single-pass removal efficiency during photocatalytic degradation of acrylonitrile

From literature, the pollutant initial concentration plays an important role on the performance of photocatalytic oxidation. For various classes and concentration ranges of VOCs, higher concentrations generally result in lower removal efficiencies [START_REF] Mamaghani | Photocatalytic oxidation technology for indoor environment air purification: the state-of-the-art[END_REF][START_REF] Mo | Effect of TiO2/adsorbent hybrid photocatalysts for toluene decomposition in gas phase[END_REF]. This can be explained from two perspectives. Firstly, due to the fixed active sites on the photocatalytic media surface the amount of molecules effectively participating in the photocatalytic reaction is not enhanced in the same ratio as an increase of the inlet concentration resulting in a decrease of removal efficiency [START_REF] Mamaghani | Photocatalytic oxidation technology for indoor environment air purification: the state-of-the-art[END_REF][START_REF] Lopes | Insights into UV-TiO2 photocatalytic degradation of PCE for air decontamination systems[END_REF]. Secondly, there could be the interference of intermediates as higher amounts of intermediates generated during PCO reactions could occupy part of the active sites and impede the degradation progress [START_REF] Mamaghani | Photocatalytic degradation of VOCs on various commercial titanium dioxides: impact of operating parameters on removal efficiency and by-products generation[END_REF][START_REF] Zhong | Performance of ultraviolet photocatalytic oxidation for indoor air applications: systematic experimental evaluation[END_REF]. Zhong et al. [START_REF] Zhong | Performance of ultraviolet photocatalytic oxidation for indoor air applications: systematic experimental evaluation[END_REF] studied the degradation of eight VOCs using a media made up of TiO 2 coated on fiber glass fibers placed in a open-loop PCO reactor. They found that the removal efficiency of all eight VOCs decreased when the initial concentrations were increased from 0.25 to 1 ppm. They attributed this behavior to the fixed active sites at the catalyst surface. The amount of active sites compared to pollutant molecules decreases and consequently the removal efficiency is decreased.

The concentration of the pollutants in the OR varies depending on the activities performed and could range from several ppb to some tens with what some authors observed in the literature. Zhong et al. [START_REF] Zhong | Modeling and physical interpretation of photocatalytic oxidation efficiency in indoor air applications[END_REF] studied the degradation of 8 VOCs (ethanol, 1-butanol, hexane, octane, acetone, MEK, toluene and p-xylene) at 0.5 ppm in a single-pass reactor. They observed that when the velocities were increased from 0.12 to 0.76 m.s -1 , the removal efficiencies of all of the compounds decreased. Similarly, Ginestet et al. [START_REF] Ginestet | Development of a new photocatalytic oxidation air filter for aircraft cabin[END_REF] found that the single-pass removal efficiencies of ethanol, toluene and acetone decreased in half when airflow rate increased from 40 m 3 .h -1 to 80 m 3 .h -1 . These authors attributed the decrease in single-pass removal efficiencies to insufficient contact time. In this work, as the velocity is increased, the residence time of acrylonitrile molecules within the media is decreased consequently leading to shorter contact time between the molecules and the photocatalytic media which then leads to a decrease in the removal efficiencies.

The study of the influence of velocity on the degradation of acrylonitrile showed that increasing the contact time between the molecules and the photocatalytic media is an important factor in improving the efficiency of the degradation system. Consequently, it can be said that, to be able to improve the effectiveness of removal, high single pass removal efficiencies are needed which can be achieved at lower velocities.

Influence of the light intensity on the single-pass removal efficiency during photocatalytic degradation of acrylonitrile

During PCO, the light intensity plays a crucial role on the degradation of pollutants as UV light generates photons that activate the photocatalyst to create the electron-hole pairs that will in turn lead to the photocatalytic oxidation of the pollutants [START_REF] Zhao | Photocatalytic oxidation for indoor air purification: a literature review[END_REF]. In photocatalysis, the influence of light intensity varies as a power function, I n where n approaches 1 at very low intensities and approaches 0 at very high intensities. According to what is commonly agreed in the literature, depending on the range of light intensities studied, the order "n" can transition into 3 regimes [START_REF] Herrmann | Heterogeneous photocatalysis: state of the art and present applications[END_REF][START_REF] Ollis | Photocatalyzed destruction of water contaminants[END_REF]. At low light intensity, the production of electron-hole pairs is low and so they are rapidly consumed by the chemical reactions resulting in a linear increase in the reaction rate (n = 1). At medium-high light intensity, rate of formation of electronhole pairs is higher than their consumption leading to recombination of the charges resulting half order increase in the reaction rate (n = 0.5). At very high light intensity, the electron-hole recombination is so fast that the degradation efficiency no longer depends on the light intensity and reaches a plateau (n = 0). Lim and Kim [START_REF] Lim | Photocatalytic degradation of trichloroethylene over TiO2/ SiO2 in an annulus fluidized bed reactor[END_REF] who studied the degradation of trichloroethylene, reported a transition of regimes from first order to half-order when light intensities were increased from 0.25 to 9 mW.cm -2 . They observed that when the light intensity increased from 0.25 to 2 mW.cm -2 , the degradation efficiency of TCE increased linearly (n = 1) due to the rapid consumption of the electron hole pairs. When the light intensity was increased from 2 to 9 mW.cm -2 , the degradation efficiency followed a half order regime (n = 0.5) which they attributed to faster electron-hole recombination. It should be mentioned that an increment in the light intensity leads to higher degradation rate; however excessive light intensity diminishes the quantum efficiency due to electron-hole recombination. When electronhole pairs created by the photons recombine, these photons are not utilized effectively. In this way, increasing the light intensity causes much more energy waste instead of much more degradation of VOCs consequently imposing unnecessary energy costs [START_REF] Wang | Photocatalytic degradation of gaseous benzene in air streams by using an optical fiber photoreactor[END_REF].

In this work, to evaluate the influence of light intensity on the degradation of acrylonitrile, experiments are performed with constant C 0 and v at 2 ppm and 1 m.s -1 respectively whilst light intensities are varied in the range of 1-4.5 mW.cm -2 . Fig. 5 shows the influence of the light intensity on the α during acrylonitrile degradation. The initial concentration and velocity were fixed at 2 ppm and 1 m.s -1 respectively. From the graph, it can be seen that the α increases from 0.005 to 0.12 when the light intensity increases from 1 to 4.5 mW.cm -2 . of ppm. For this reason, the initial concentrations of acrylonitrile were therefore studied from 0.5 to 10 ppm. This would provide some knowledge on the performance of PCO in degrading these compounds under possible OR concentrations. The air velocity v and light intensity I were kept constant at 1 m.s -1 and 4.5 mW.cm -2 respectively. Fig. 6 shows the effect of the initial concentration on α.

It can be observed that the single-pass removal efficiencies decreased from 0.032 to 0.0066 when the concentration is increased. Due to the limited number of active sites on the media, an increase in concentration leads to competition between molecules for chance to be adsorbed onto the active sites and degraded. As the number of active sites is fixed, more molecules leave the media without undergoing degradation. This is in accordance with what Jafarikojour et al. [START_REF] Jafarikojour | Evaluation and optimization of a novel immobilized photoreactor for the degradation of gaseous toluene[END_REF] observed for toluene degradation at concentration range of 20 to 100 ppm and with Kricheevskaya et al. [START_REF] Krichevskaya | Gas-phase photocatalytic oxidation of acrylonitrile[END_REF] who also noticed a decline in the removal efficiencies during acrylonitrile degradation from 10 to 100 ppm.

Mineralization rates obtained during the degradation of acrylonitrile

In order to completely characterize the performance of PCO it is necessary to also determine the mineralization rates. This is done by measuring the amounts of CO 2 produced and comparing this value to the amount of CO 2 which is expected to be produced when there is complete mineralization. The mineralization rates were studied by looking at the influence at different initial concentrations. The conversion rates were calculated as:

= - C C C Conversion rate t 0 0 (4)
where C 0 is the initial concentration and C t is concentration at time t.

The mineralization rate (%) was determined by comparing the concentration of CO 2 produced to the theoretical one (if mineralization is complete: 1 mol of acrylonitrile should lead to the formation of 3 mol of CO 2 ), using the following equation:

= CO theoretical CO x Mineralization rate (%) [ ] [ ] 100 2 2 (5)
where

= x measured inlet acrylonitrile theoretical CO [ 2] 3 [ ] (6)
Figs. 789show the evolution of the mineralization rate and the conversion rate for different acrylonitrile concentrations.

For all three studied concentrations: (i) CO 2 was formed from the beginning of the degradation and it increased until a stable value (ii) 100% mineralization was achieved. All the eventual organic intermediates are adsorbed and oxidized into CO 2 .

In the case of initial concentration of 0.5 ppm and 2 ppm, 100% mineralization was achieved almost instantaneously to 100% conversion. For 0.5 ppm, 100% conversion was achieved at around 18 min whilst 100% was achieved at around 23 min. For 2 ppm 100% conversion is achieved in about 60 min whilst complete degradation is achieved in about 70 min. However, for 10 ppm, at 100% conversion of acrylonitrile about 70% of the organic compounds had been transformed to CO 2 . Then about 120 min after total removal of acrylonitrile from the gas phase, 100% mineralization was achieved. This delay could possibly be attributed to the fact that at higher concentration of 10 ppm, higher concentrations of intermediates were formed which would require more time to be mineralized. A two step mineralization rate was also observed (Fig. 10), with the first step involving the fast transformation of acrylonitrile into organic intermediate compounds which were also quickly transformed into CO 2 (0.008 ppm min -1 ) followed by a slower mineralization (0.003 ppm min -1 ) of more stable intermediates into CO 2 .

To study the promptness of conversion of a compound and its reaction intermediates to CO 2 , Debono et al. [START_REF] Debono | VOC ternary mixture effect on ppb level photocatalytic oxidation: removal kinetic, reaction intermediates and mineralization[END_REF] suggested that the mineralization rates be plotted as a function of the conversion rates. The results are acrylonitrile conversion rate vs mineralization rates for the three concentrations studied are shown in Fig. 10.

The dashed line represents the instantaneous conversion of the target compound into CO 2 . Consequently, if acrylonitrile was instantaneously converted in CO 2 the plotted data would fit the dotted line. For all three concentrations, the results show a gap between the dotted line and the plotted data. It can also be observed that the mineralization rate at the beginning is low compared to the conversion rate but it increases until the total conversion of acrylonitrile. This gap corresponds to the fact that photocatalytic degradation of acrylonitrile Fig. 6. Effect of initial concentration on the single pass removal efficiency determined during acrylonitrile degradation (v = 1 m.s -1 ; I =4.5 mW.cm -2 ; RH = 50%). Fig. 7. Evolution of pollutant conversion and CO 2 mineralization rate during the photocatalytic oxidation of acrylonitrile (C 0 = 0.5 ppm; v = 1 m.s -1 ; I =4.5 mW.cm -2 ; RH = 50%). Conclusively, even though 100% mineralization is reached at around 100% removal for 0.5 ppm and 2 ppm, acrylonitrile is not immediately converted to CO 2 but first into intermediate compounds.

In this study, no intermediate compound was detected during the degradation of acrylonitrile. However, when Krichevskaya et al. [START_REF] Krichevskaya | Gas-phase photocatalytic oxidation of acrylonitrile[END_REF] studied the degradation of acrylonitrile inlet concentrations of 40 ppm in annular reactor, they identified carbon dioxide, water and nitrogen dioxide as the main products. They also noticed the formation of hydrogen cyanide, a compound classified by the US EPA as an extremely toxic compound. The inability to detect intermediates including hydrogen cyanide in this study could be explained by the fact that the analytical methods were probably not adapted for this or that intermediates were at very low concentrations.

Conclusion

In this work, acrylonitrile which is a pollutant found in surgical smoke is degraded in a closed loop laboratory reactor to evaluate the efficiency of degradation by PCO. The use of a model developed by Dumont and Héquet enables the calculation of the single pass removal efficiency, the qualitative descriptor that allows the study of the Fig. 9. Evolution of conversion and CO 2 mineralization rate during the photocatalytic oxidation of acrylonitrile (C 0 =10 ppm; v = 1 m.s -1 ; I =4.5 mW.cm -2 ; RH = 50%). Fig. 10. Evolution of mineralization rates as a function of conversion rates during the photocatalytic oxidation of acrylonitrile at initial concentrations of (a) 0.5 ppm (b) 2 ppm and (c) 10 ppm (v = 1 m.s -1 ; I =4.5 mW.cm -2 ; RH = 50%). influence of the air velocity, light intensity and initial concentration on the degradation process.

The results showed that PCO is able to degrade acrylonitrile. Decreasing the air velocity resulted in improved single-pass removal efficiencies due to higher contact time between the molecules and the photocatalytic media. Higher single-pass removal efficiencies were achieved at higher average light intensities. The relationship between light intensity and removal efficiency was seen to follow a half-order regime. This meant that for the range of light intensities studied the electron-hole recombination was dominant. In this case, further increment in the light intensity would not significantly improve the removal efficiencies but would only impose unnecessary energy costs. Higher initial concentrations of acrylonitrile led to a decrease in their degradation efficiencies attributed to limited active sites. However, due to the fact that the concentrations of pollutants vary over time, the use of multiple photocatalytic banks would help to maximize the removal efficiencies of pollutants. This would also be beneficial in reducing concentrations of intermediates and keeping them at levels that would not pose health risks.

In this work, the absence of intermediates such as hydrogen cyanide that was observed by Krichevskaya et al. [START_REF] Krichevskaya | Gas-phase photocatalytic oxidation of acrylonitrile[END_REF] does not rule out their existence. Indeed by plotting the mineralization rates as a function of the conversion rates it was shown that during the course of degradation, acrylonitrile was first converted into organic intermediate compounds which were then converted into CO 2 . Further analysis using different and possibly more sensitive methods are recommended as these could help to identify anticipated and additional intermediates.

Fig. 1 .

 1 Fig. 1. Schematic representation of the closed-loop photocatalytic reactor.

⎜

Fig. 3 .

 3 Fig. 3. Degradation curves of acrylonitrile velocity experiments fitted to model to obtain α (C 0 = 2 ppm; I =4.5 mW.cm -2 ; RH = 50%).

Fig. 5 .

 5 Fig. 5. Effect of light intensity the single pass removal efficiency determined during acrylonitrile degradation under experimental conditions of C 0 = 2 ppm and v = 1 m.s -1 ; RH = 50%.

Fig. 8 .

 8 Fig.8. Evolution of conversion and CO 2 mineralization rate during the photocatalytic oxidation of acrylonitrile (C 0 = 2 ppm; v = 1 m.s -1 ; I =4.5 mW.cm - 2 ; RH = 50%).

Acknowledgements

The authors would like to express their gratitude to ATA Medical and IMT Atlantique for their financial support and also to Eric Chevrel, Patrick Brion, Francois-Xavier Blanchet, Yvan Gouriou and Katell Chaillou for their invaluable technical assistance and Eric Dumont for his help with the modeling.


Nidhal Ganfoud

Assane Sene

Matthieu Haefele

Abel Marin-Laflèche

Barbara Daffos

Pierre-Louis Taberna

Mathieu Salanne

Patrice Simon

Benjamin Rotenberg
email: benjamin.rotenberg@sorbonne-universite.fr

Effect of the carbon microporous structure on the capacitance of aqueous supercapacitors

Using a combination of cyclic voltammetry experiments and molecular dynamics simulations, we study the effect of microporous carbon structure on the performance of aqueous supercapacitors using carbide derived carbon (CDC) electrodes. The structures investigated by molecular simulations are compatible with the experimental results for CDC synthesized at 800 • C, but not with the other two materials (CDC-1100 and YP-50F), which are more graphitic. In fact, the specific capacitance obtained for the latter two are in good agreement with molecular simulations of graphite electrodes, assuming that all the charge is localized in the first plane in contact with the electrode (a very good approximation). Our molecular simulations further allow to examine the solvation of ions inside the electrodes. Unlike what was observed for large organic ions dissolved in acetonitrile, we find that most Na + cations remain fully solvated. Overall, microporous carbons such as CDCs are good candidates for applications involving aqueous supercapacitors, in particular the harvesting of blue energy or desalination, but their performance remains to be optimized by tailoring their microstructure.

I. INTRODUCTION

Nowadays, with the the increase of the world population, the demand for energy is growing rapidly and has become a major societal and geopolitical issue. In the last 50 years, the energy consumption has increased six times and studies predict a further doubling of energy consumption by 2050. On the one hand, fossil fuels (oil, natural gas, coal), which account for about 85 % of the total energy supply, have a negative impact on the environment and generate significant emissions of greenhouse gases which lead to global warming and climate changes. On the other hand, the use of fossil fuels will be limited in time due to the depletion of available resources, and resort to alternative energy sources is urgent. Among the various possibilities of renewable energies, "blue" or osmotic energy, which aims at recovering the entropy from mixing of fresh and salty water (e.g. in estuaries), have the advantage of being non-intermittent and could provide a significant contribution to the mix of electricity production sources on the global scale [1][2][3][4]. Nevertheless, the efficient harvesting of this energy remains a great practical challenge.

In this context, electrochemical devices are likely to play a crucial role, either to store the electric current produced by diffusio-omosis with new generations of membranes [5,6], or at the core of alternative approaches such as Capacitive Mixing [7][8][9][10]. This process involves the charge/discharge of electrodes, successively using solutions with different salt concentration as electrolytes. In such Electric Double Layer Capacitors, or supercapacitors, the charge is stored by adsorbing the ions of the electrolyte on the surface of the electrodes. In order to maximize this adsorption, carbide-derived carbons (CDCs) are a promising family of electrode materials. Their main advantage is that they are mostly microporous, with a very well-controled pore size distribution depending on the synthesis temperature and carbide precursor. One of the key factors governing the capacitance is the carbon local structure, which plays an important role in the process of the charge and the discharge of the electrodes.

Hence, a fundamental understanding of the effect of carbon structure is essential to predict the capacitance and to optimize the energy storage device.

A second essential feature influencing the capacitance value is the electrolyte local structure, since the charge compensation at the surface of the electrode is governed by the adsorption properties of the ionic species. In principle, increasing the contact surface area between the liquid and the electrode material should generally lead to better performances. However, it was shown recently for ionic liquid electrolytes that the degree of confinement of the various adsorption sites plays a major role [11], which leads to situations where materials with an optimal accessible surface area do not show a large enhancement of the capacitance [12].

The importance of the degree of confinement was recently confirmed in the case of aqueous solutions by Prehal et al., who combined modelling with in situ X-ray scattering inside nanoporous carbon supercapacitors [13].

Unlike the case of ionic liquids, only a few studies on the structure and the capacitance of concentrated aqueous electrolytes inside nanoporous carbon electrodes have been reported.

We have recently shown [10] for a single CDC structure that conventional theories should be applied with some caution under conditions relevant to the context of blue energy, due to i) the large ionic concentration and ii) the strong confinement effect. Molecular dynamics simulations then appear as an efficient tool to quantitatively predict the capacitance and the ion adsorption capacity, and to get access to the microscopic processes occuring inside the electrode. It can further be used to parametrize more conventional theories to extrapolate to other conditions (e.g. at low salt concentration).

In this work, we investigate the effect of the nanoporous carbon structure on the capacitive properties of supercapacitors using NaCl solutions as electrolytes, at concentrations which are representative of sea water (0.5 and 1 mol L -1 ). We compare the experimental values obtained for two different CDC materials by cyclic voltammetry to the simulation values obtained for two CDC structures as well as a graphite electrode. The simulations involve a constant-potential method to simulate realistically the electrochemical conditions.

Experiments show that the CDC carbon with both the lower average pore size and the smaller graphitic character has a larger capacitance. The simulations made with the CDC yield very similar capacitance despite the different porosities, pointing towards a proeminent role of the graphitization, a result which is further confirmed by the results obtained with the pure graphite electrode.

II. SYSTEMS AND METHODS

A. Electrochemistry experiments

Microporous Carbide Derived Carbons (CDC) powders (Carbon-Ukrain) were prepared by chlorination of TiC powder at 800 • C or 1100 • C as reported elsewhere [14,15]. CDC powders were further annealed for 2h at 600 • C under H 2 to remove traces of chlorine and other surface groups. The resulting materials were denoted as CDC-800 and CDC-1100, respectively. CDC carbons show 100% of the porous volume in the microporous range (< 2 nm) [14,15]. A commercial actived carbon YP-50F (coconut shell-derived carbon from Kuraray Chemical Co., Japan) was also used for comparison purpose [16]. The specific surface area is 1,730 m 2 g -1 and the total porous volume is 0.75 g cm -3 . YP-50F shows an average pore size of 0.9 nm with 92% of pores being smaller than 2 nm and 8% of mesopores, while the other two are exclusively microporous.

Electrochemical tests are performed using two-eletrode Swagelok cells. Active films are made by mixing 95 wt% CDC with 5 wt% polytetrafluoroethylene (PTFE from Dupont TM ) binder. Once calendered, 11 mm diameter electrodes are cut. The active film thickness is around 300 µm, with a weight loading of 15 mg cm -2 . Platinum disks are used as current collectors and two layers of 25 µm-thick porous cellulose (from Nippon Kodoschi Corporation, NKK) as a separator. Cyclic voltammetry experiments are carried out with a multi-channel potentiostat (VMP3, Biologic), for two NaCl concentrations (0.5 and 1 M) at a scan rate of 1 mV s -1 . Two series of measurements per system for two ranges, between 0.0 and 0.6 V and between 0.0 and 0.7 V, are performed, leading for each of them to four estimates of the capacitance. The values and uncertainties reported here are the corresponding averages and standard deviations. Such voltages are sufficiently low to avoid faradic (redox) processes linked with water decomposition on the high surface area carbon electrodes.

B. Molecular Dynamics Simulations

The simulation cell consists of two CDC-based electrodes and an aqueous electrolyte composed of sodium chloride salt dissolved in water. We have used similar supercapacitor cells to the ones described in our previous work [10]. Two different CDC electrodes, denoted CDC-1 and CDC-2 in the following, as well as graphite electrodes, were studied with the same electrolyte (aqueous NaCl) at two different salt concentrations 1.0 and 0.5 mol L -1 .

The CDC electrodes were obtained by quenched molecular dynamics [17] and consist of 3821 and 3649 atoms for CDC-1 and CDC-2, respectively. Their structure will be described in more detail below. In the case of CDCs, repulsive walls are placed on both extreme sides of the non periodic dimension of the simulation cell to prevent the mobile molecules from exiting. As discussed below, both CDC-1 and CDC-2 serve here as models for the experimental CDC-800, while the experimental performance of CDC-1100 is rationalized using insights from the planar electrodes.

The system corresponding to CDC-1 with an electrolyte salt concentration of 1.0 M (resp. Following the approach detailed in ref [18,19], the electrode is modeled as a perfect conductor, i.e. a fixed potential difference of ∆Ψ = 1V is imposed between the two electrodes and the charge of each electrode atom is recomputed at each step of the molecular simulation, leading to the fluctuation of the (local and total) charge of the electrode in response to the instantaneous microscopic configuration. This method was previously shown to be suitable for the simulation of ionic liquids, organic electrolytes as well as aqueous electrolytes in nanoporous carbon electrodes [10,11,[20][21][22][23][24]. We used an in-house code dedicated to such classical MD simulations of metallic electrodes. The potential difference of 1.0 V is chosen to be as high as possible in order to maximize the effects and hence improve the sampling of the properties, while staying within the electrochemical window of water (1.23 V).

The simulations are conducted in the NVT ensemble with a timestep of 1 fs. The temperature of the electrolyte is maintained at room temperature (298 K) by applying a Nosé-Hoover thermostat with a time constant of 1 ps while the electrode atoms are fixed. Two dimensional periodic boundary conditions are used, so that there is no periodicity in the direction perpendicular to the electrodes (z direction). Short-range interactions are computed using a cut-off radius equal to half of the shortest box length, while the computation of electrostatic interactions involves an Ewald summation in 2D (x and y directions only) for the combination of point charges (for the electrolyte) and Gaussian charges (for the electrode atoms)

as described e.g. in Refs. 18 and 25. The force field consists of a sum of coulombic and Lennard-Jones interactions. Lorentz-Berthelot mixing rules were applied for Lennard-Jones parameters. The SPC/E model was used for water molecules [26] that are kept rigid using the Shake algorithm [27,28]. Interaction parameters for carbon atoms and for ions are taken from Refs. 29, 30. The systems are first prequilibrated by fixing the electrode charges to zero (about 600 ps for the CDCs, 2 ns for graphite), then under constant potential conditions.

Once a steady state is reached, equilibrium trajectories are sampled for about 10 ns (resp. 17 ns) for the two CDC systems (resp. graphite) to compute all the relevant properties such as capacitance, number density profiles, as well as solvation numbers.

III. RESULTS AND DISCUSSION

A. Capacitance

C cell = Q /∆Ψ (1)
It is related to the capacitance of the two electrodes as 1

C cell = 1 C + + 1 C -.
Each of them can then be obtained by assuming that the electrodes behave symmetrically (C + = C -). The experiments performed using a 3-electrode cell confirm that this assumption is reasonable since C -is only slighly larger than C + (by less than 10%). The capacitances corresponding to the CVs of Figure 1 are summarized in Table I. The values for CDC-800 are consistent with those previously reported for the same material [10].

The increase in capacitance with salt concentration is relatively limited compared to the Debye-Hückel prediction (increase as the square-root of salt concentration), which is not unexpected in this high concentration range.

Table I. Electrode specific capacitance (in F g -1 ) from cyclic voltammetry.

Carbon structure Salt concentration Capacitance (F g -1 )

CDC-800

0.5 M 113±3 1 M 123±3 CDC-1100 1 M 87±3 YP-50F 1 M 93±3
The effect of the carbon material is much larger. Since CDC-800 contains smaller pores than the other two materials, a larger capacitance is not unexpected, however the effect is much more pronounced than previously reported for aqueous electrolytes, for which alkali cations and the chloride anion generally do not show strong sieving effects for the pore size range we investigate [31]. This suggests that the carbon structure plays a role beyond the average pore size. In particular, the extent of graphitization may contribute to the observed differences between the microporous carbons. Large planar surfaces should indeed be detrimental since they do not promote enough the confinement of the adsorbed ions [11,13].

B. Effect of the microporous structure

In order to investigate the effect of the carbon structure, we compare the simulation results for the capacitance obtained for the two carbon structures shown in Figure 2a. The pore size distribution in both materials, computed using the software Zeo++ [32,33] with a probe of radius of 1. Based on the pore size distribution analysis, the CDC-1 was shown by Palmer et al.

to be consistent with an experimental structure of a CDC synthesized at 800 • C. While the average pore size of CDC-2 corresponds approximately to the average pore size of the experimental CDC-1200 (synthesized at 1200 • C), it contains much less graphitized areas than the experimental CDC-1200 and CDC-1100 [17]. In fact, recent data obtained from X-ray diffraction (XRD), suggests that the local structure inside the CDC-2 resembles that of CDC synthesized from silicon carbide (SiC), rather than titanium carbide (TiC) as in the present experiments, as illustrated in Figure 2c [34]. Therefore, both CDC-1 and CDC-2 serve here as models for the experimental CDC-800, while the experimental performance of CDC-1100 is rationalized later using insights from the planar electrodes.

Table II. Electrode specific capacitance (in F g -1 ) from molecular simulations for the two carbon structures CDC-1 and CDC-2.

Salt concentration

Capacitance (F g -1 )

CDC-1 CDC-2

0.5 M 103 ± 2 124 ± 3 1.0 M 107 ± 3 135 ± 3
The specific capacitances (per unit mass of the electrode) from molecular simulations of the two electrode materials, with the same aqueous NaCl electolyte at two different concentration (0.5 and 1.0 M), are summarized in Table II. Both of them yield capacitances that are in good agreement with the experimental results of Table I for CDC-800, with a difference of ≈10 % in both cases (with CDC-1 underestimating it while CDC-2 overestimates it). The relative increase with concentration is also similar, even though not quantitative. This is consistent with the structural analysis of the carbons, since CDC-1 shows a pore size distribution typical of CDC-800 while CDC-2 has a slightly larger average pore size but a very similar local structure as shown in the structure factors comparison.The larger capacitance obtained with CDC-2 compared to CDC1 confirms that even if small pore sizes are necessary to confine the ions, knowing the average pore size is not sufficient to predict the capacitance, and that it is necessary to have a correct representation of the pore shape as well. Note that a larger specific capacitance was already obtained in our previous simulations of CDC-2 (denoted CDC-1200 in Ref. 23) in contact with ionic liquids as electrolytes.

The larger specific capacitance of CDC-2 also correlates with its lower density (larger porosity), which results into more ions inside the pores per unit mass of carbon, and fewer small pores, so that the access of the ions from the bulk electrolyte into the pore is less hindered. The composition of the liquid inside the electrodes can be determined from the density profiles shown in Figure 3, which clearly indicates that the main charging mechanism is ion exchange, i.e. the replacement of anions (resp. cations) by cations (resp. anions) in the negative (resp. positive) electrode. The capacitance of the system, i.e. the amount of electronic charge stored inside the electrode material, is then determined by the extent to which the electrode can accommodate such a net charge in the confined electrolyte (which compensates that of the electrode).

Table III. Average number of cations and anions numbers inside both electrodes and in the bulk electrolyte, for both systems under a voltage of 1 V.

System Negative electrode

Bulk Positive electrode

N + N - N + N - N + N - CDC-1 27 ± 2 6 ± 1 108 ± 2 110 ± 2 4 ± 0.7 22 ± 1 CDC-2 35 ± 1 7 ± 1 119 ± 2 120 ± 2 7 ± 1 33 ± 2
Table III summarizes the number of cations and ions inside the electrodes (and the region corresponding to the bulk electrolytes), for the two simulated systems under a voltage of 1 V. It shows that not only the total number of ions inside the pores, but also the charge imbalance is larger in CDC-2 than CDC-1. Since the size of the electrode is comparable in both simulated systems, this means that the larger specific capacitance (charge per unit mass of electrode) is not only due to the smaller mass density of CDC-2. Together with the fact that the surface area per unit volume (SSA divided by mass density) is slightly larger in CDC-2 than CDC-1, this underlines the importance of the interface between the carbon and the electrolyte -as expected for supercapacitors.

C. Planar electrodes

Going back to the cases of CDC-1100 and YP-50, our simulations with the CDC-1 and CDC-2 electrodes cannot be used to interpret the experimental results because these carbons have a local structure which has a stronger graphitic character. We have therefore performed additional simulations of the NaCl-concentrated electrolytes in contact with a pure graphite electrode to gain further insight. As discussed in Ref. [START_REF] Méndez-Morales | Energy Storage Materials[END_REF], for planar electrodes the best metric is the capacitance per unit area, since the mass of the electrode depends on the number of graphite planes, while most of the charge is stored in the first plane in contact with the electrolyte. Nevertheless, for comparison purposes it is useful to estimate the specific capacitance by assuming that all the charge is located in the graphite plane in contact with the electrolyte (which is a very good approximation, of the order of 3%).

Table IV. Electrode surfacic capacitance (in µF cm -2 ) of graphite electrodes, with a 0.5 M and 1 M aqueous NaCl solutions as electrolytes, obtained by molecular simulations.

Electrolyte concentration (mol L -1 ) 0.5 1.0 Surfacic capacitance (µF cm -2 ) 6.9 ± 0.7 6.9 ± 0.7

The capacitance per unit area of graphite electrode, using aqueous NaCl solutions as electrolyte, is reported in Table IV for two concentrations. We first note that within the accuracy of our simulations, the capacitance is independent of concentration in this high concentration regime. Converting the surfacic capacitance to specific capacitance, within the above approximation, leads to an estimate of ∼ 90 F g -1 , which is comparable to that of CDC-1100 and YP-50F (see Table I). Even though in these materials not all the interface is purely graphitic and not all the carbon atoms are in contact with the electrolyte, this confirms the importance of the degree of graphitization in the performance of the electrode material.

D. Ion solvation

Finally, molecular simulations also gives access to information that is difficult to obtain experimentally than the electrode capacitance, in particular pertaining to ion solvation inside the electrodes. Conventional electrochemical techniques such as CV are not sufficient and it is for example necessary to use electrochemical quartz crystal microbalance [START_REF] Levi | [END_REF] or sophisticated in situ diffraction techniques [13]. In contrast, the simulation, provide a high level of details on the local structure. For example, the solvation number of an ion is easily defined as the number of water molecules located in its first solvation shell, the latter being defined by a cutoff radius which is determined beforehand from the position of the first minimum observed in the radial distribution functions between the water and each ion.

The corresponding cutoff radius are the following: 3.9 Å and 3.3 Å for Cl -and Na + . Here we investigate how the adsorption of ions inside the microporous structure (for CDC-1 and CDC-2) or within the first layer of ions at the surface of graphite (see below) modifies their solvation shell with respect to the bulk electrolyte. As shown on Figure 4a, the desolvation effect is rather scarce for Na + cations, with small differences in solvation numbers for the sodium ions adsorbed at the surface of the electrodes and in the bulk: In contrast to organic ions dissolved in acetonitrile [11], which can lose several solvent molecules from their solvation shell, most of the sodium ions remain fully solvated. However, a fraction of them (around 10 %) lose one water molecule in the case of the microporous carbon, while almost no effect is seen in the case of graphite. For the latter, the density profiles (Figure 4b) show that the sodium ions adsorbed at the surface of the electrode lie between two molecular layers of water, so that they can keep their solvation shell intact. Figure 4c finally shows that the desolvation of Cl -anion is more pronounced that of Na + cations for all electrodes, with a typical loss of 1-2 molecules, or even more in the case of the CDCs. In the case of graphite (Figure 4b), the position of the first layer of chloride anions coincides with that of the water molecules, which indeed requires the partial desolvation of the anion to allow direct contact with the carbon plane. In terms of performance, the overall moderate decrease in the ion solvation number in these systems suggests that this effect does not play a significant role in their capacitive properties.

IV. CONCLUSION AND PERSPECTIVES

Using a combination of cyclic voltammetry experiments and molecular dynamics simulations, we have studied the effect microporous carbon structure on the performance of aqueous supercapacitors using carbide derived carbon (CDC) electrodes. The structures investigated by molecular simulations are compatible with the experimental results for CDC synthesized at 800 • C, but not with the other two materials (CDC-1100 and YP-50F), which are more graphitic. In fact, the specific capacitance obtained for the latter two are in good agreement with molecular simulations of graphite electrodes, assuming that all the charge is localized in the first plane in contact with the electrode (a very good approximation). Our molecular simulations further allowed to examine the solvation of ions inside the electrodes.

Unlike what was observed for large organic ions dissolved in acetonitrile, we found that most Na + cations remain fully solvated. Overall, microporous carbons such as CDCs are good candidates for applications involving aqueous supercapacitors, in particular the harvesting of blue energy or desalination, but their performance remains to be optimized by tailoring their microstructure. Beyond these applications, which involve aqueous electrolytes with composition imposed by the sources (sea and river water), it would be particularly interest-ing to investigate the effect of the nature of the electrolyte (ion specific effects). Such a study would shed more light on the relation between ion solvation thermodynamics, which also depends on the choice of the force field [37], and the capacitive properties of such systems.

Work in this direction is in progress.

0. 5 M

 5 ) contains 7615 (resp. 7700) water molecules and 139 (resp. 70) NaCl ion pairs, while for CDC-2, the concentration of 1.0 M (resp. 0.5 M) corresponds to 8800 water molecules and 160 (resp. 80) NaCl ion pairs. The box dimensions for CDC-1 (resp. CDC-2) are 43.3×43.3×183.0 Å 3 (resp. 43.7×43.7×186.4 Å 3 ). These dimensions, combined with the above-mentioned number of atoms lead to the correct density of water in the bulk region between the two electrodes. For the graphite case, the system consists of three layers of graphene in each electrode containing 1440 carbon atoms per electrode and 2200 water molecules with 40 NaCl pairs, leading to a concentration of 1.0 M. The box dimensions are 34.1×36.9×67.9 Å 3 .

Figure 1

 1 Figure1displays the cyclic voltammograms (CV) for i) two different salt concentrations and ii) three different microporous carbons, for potentials varying between 0.0 and 0.6 V (to stay within the electrochemical stability potential window of water). The rectangular shape of the CV is typical of a capacitive electrochemical response in the considered range of concentrations and voltages. The experimental capacitance is then determined by differentiating the electrode charge with respect to the potential. The latter is obtained by integrating the electric current during the discharge of the electrochemical cell. In molecular simulations, the integral cell capacitance is computed from the average charge Q of the electrodes:

Figure 1 .

 1 Figure 1. Cyclic voltammograms of electrochemical cells based on microporous carbons and aqueous solutions of sodium chloride as electrolytes: (a) for CDC-800 and two concentrations (0.5 and 1 M); (b) for 3 carbons (CDC-800, CDC-1100 and YP-50F) at an electrolyte concentration of 1 M. The potential scan rate is 1 mV s -1 in all cases.

  3 Å (comparable to water molecule size), are shown in Figure 2b. The pore sizes for CDC-1 (resp. CDC-2) are in the range 3-14Å (resp. 3-17Å) with an average of 7.5Å (resp. 9Å). Geometrical analysis of the nanoporous structure results for CDC-1 (resp. CDC-2) in a porosity of Φ = 23.3% (resp. Φ = 35.0%) and a specific surface area ofSSA = 1934 ± 2 m 2 g -1 (resp. 1857 ± 2 m 2 g -1). The two carbons also have different mass densities, of ρ solid = 0.939 g cm -3 and 0.872 g cm -3 , respectively.

Figure 2 .

 2 Figure 2. (a) Snaphots of the two carbon structures CDC-1 and CDC-2 simulated in the present work. (b) Pore size distribution for CDC-1 and CDC-2 electrodes calculated using a probe with radius 1.3 Å, corresponding to the size of a water molecule. (c) Structure factor computed from the atomic positions for CDC-1 and CDC-2, compared with X-ray diffraction experiments on CDC synthesized from silicon carbide (SiC) of Ref. 34.

Figure 3 .

 3 Figure 3. Density profiles along the z axis perpendicular to the electrodes for CDC-1 (top) and CDC-2 (bottom) with the same salt concentration of 1.0M for both systems. The negative (resp. positive) electrode is on the left (resp. right) side.

Figure 4 .

 4 Figure 4. (a) Change in the probability distribution of solvation number of Na + between the bulk electrolyte and at the negative electrode, for the three electrode types considered in this work. In the case of graphite, only ions in the first adsorbed layer are considered. (b) Density profiles in the case of graphite electrodes. (c) Same as (a) for Cl -in the positive electrode.

ACKNOWLEDGMENTS

The authors acknowledge financial support from the Ville de Paris (Emergences, project Blue Energy), the French National Research Agency (Labex STORE-EX, Grant No. ANR-10-LABX-0076), as well as HPC resources granted by PRACE (Grant No. 2015122929), GENCI (resources of TGCC, Grant No. t2016087725 and Cines, Grant No. a0050907725) and Sorbonne Université (MeSu). We acknowledge support from EoCoE, a project funded by the European Union Contract No. H2020-EINFRA-2015-1-676629.