Nicolas Lamaison
email: nicolas.lamaison@cea.fr

David Chèze

Cédric Paulus

MODELING AND SIMULATIONS OF SOLAR TWO-WAY SUBSTATION

Decentralized surplus feed-in of solar heat into a District Heating Network (DHN) is here addressed. The heat collected from solar panels located on rooftops of DHN connected buildings may either be used locally for domestic hot water and space heating or fed into the DHN. Two-way substations able to transfer heat from and into the network seem then to be required utilities. The present paper presents the specifications (60kW capacity, return-to-supply connection) and promising architectures of such two-way substation based on a previous analysis. A first-of-a-kind Modelica-based dynamic model of the substation together with the consumer and the solar field connected to it is then detailed. Two-day simulations considering real operating conditions of DHN were then performed. The results highlighted i) the good match between the periods of solar heat reinjection with the periods of low supply temperature and differential pressure and ii) the decisive benefit of the reinjection to increase the part of useful solar energy.

INTRODUCTION

In the "2way District Heating" course of action from the 4GDH concept [START_REF] Lund | 4th Generation District Heating (4GDH)[END_REF], decentralized feed-in of solar heat from prosumers seems to be a promising solution to increase the share of renewable energy in District Heating Networks (DHN), especially in dense urban areas with limited ground surface. However, when scattered customers roofs are used to collect and inject heat locally in a network, new problematics arise. Local consumption or total feed-in of the collected solar energy, use of storage at the building level, and management of the local differential pressure and supply temperature are the most decisive ones.

Among the various reinjection principles, Return to Supply feed-in is considered since it seems to be the most flexible option from the DHN point of view [START_REF] Beckenbauer | Validation of a District Heating System Model and Simulation-Based Investigation of Bidirectional Heat Transport by Decentralized Solar Thermal Plants[END_REF][START_REF] Lennermo | Distributed heat generation in a district heating system[END_REF][START_REF] Schäfer | Technical challenges for solar thermal plants with decentralized feed-in into district heating networks and deduced plant concept for the experimental feedin station in the SWD[END_REF]. However, R/S feed-in implies to overcome the local differential pressure between the return and supply lines, which usually exhibits significant variations due to rapid load fluctuations. Moreover, the feed-in temperature must be superior or equal to the local network supply line temperature. The latter constraints on the local differential pressure and the local supply temperature involve at the two-way substation level the use of at least a variable speed pump and a finely tuned control strategy.

Figure 1: Schematic of Return to Supply (R/S) feed-in

At the network scale, various studies investigate decentralized reinjection and its effect on the thermohydraulic behavior of the network [START_REF] Brange | Prosumers in district heating networks -A Swedish case study[END_REF][START_REF] Hassine | Control Aspects of Decentralized Solar Thermal Integration into District Heating Networks[END_REF][START_REF] Heymann | Integration of Solar Thermal Systems into District Heating -DH System Simulation[END_REF]. However, at the component scale i.e. the substation, only few studies from the open literature address the topic of the reinjection. From the simulation point of view, [START_REF] Paulus | Substations for Decentralized Solar District Heating: Design, Performance and Energy Cost[END_REF] compared nine different substation architectures, however connected in a Return to Return fashion, using TRNSYS and evaluated the influence of the return temperature, solar collectors area and type of solar collectors on thermal performances only. From the experimental point of view, [START_REF] Rosemann | A New DH Control Algorithm for a Combined Supply and Feed-In Substation and Testing Through Hardware-In-The-Loop[END_REF] addressed the challenging topic of innovative control algorithm at the substation level with Hardware-In-The-Loop testing. There conclusions were used to build various first-of-a-kind solar prosumers and decentralized feed-in substations (Rosemann et al., 2017a).

In the frame of the Horizon 2020 "THERMOSS" project, specifications, modelling and prototype testing of a twoway substation for a multi-family building is performed.

In the present paper, specifications and promising architectures are first presented in the basis of what was presented in [START_REF] Lamaison | A multicriteria analysis of bidirectional solar district heating substation architecture[END_REF]. Second, a first-of-aking Modelica-based dynamic model of the most promising two-way substation together with the consumer and the solar field connected to it is then detailed. The control strategies and operating principles associated to this substation are then discussed. Third, results of a twoday simulation considering real DHN operating conditions in terms of differential pressure and temperatures (supply and return) is then detailed in terms of temperatures, flow rates and heat power. Finally, a sensitivity analysis on the DHN operating conditions is presented.

SPECIFICATIONS AND ARCHITECTURE

Specifications

During the first year of the THERMOSS project, specifications and architecture of a two-way substation were addressed. In order to give a precise frame to study, it was decided to specifically consider the decentralized reinjection of solar heat on a DHN. Extrapolation to other local heat sources is also envisioned and will use the current development as basis.

As stated in the Introduction, R/S feed-in is considered since it is the most promising option. As highlighted schematically in Figure 2, the solar collectors are assumed to be on the rooftop of a multi-family building, equipped with a unique two-way substation. Variants relying on individual two-way stations at the apartment level have been discarded from this study due to prohibitive cost and increased complexity. Indeed, solar two-way substations seem more appropriate for multi-family buildings rather than for individual apartments (Rosemann et al., 2017b) since it simplifies the hydraulic connections at the building level while reducing the costs. It also reduces the number of reinjection points in the DHN, aggregate heat inputs and thus simplifies the operation of the network. Regarding the order of magnitudes involved for the present study, it is considered that the building consists in 6 apartments of 70m² organized in 3 floors of 2 apartments, leading to a building footprint of 7.5m of height, 10m of width and 14m of length. Firstly, assuming a rather poorly insulated envelope, the required Space Heating (SH) power is approximately 42kW (i.e. around 100W/m²). Secondly, using the daily draw-offs from COSTIC (2016), i.e. about 150 liters for an apartment of 3 people, and the "DHW-calc" calculator [START_REF] Jordan | DHWcalc: Program to generate domestic hot water profiles with statistical means for user defined conditions[END_REF] to obtain a distributed daily profile, the maximum 10 minutes average is about 16kW/apartment. The latter leads to 60kW of Domestic Hot Water (DHW) power consumption for the entire building when accounting for a simultaneity coefficient of 0.62. Finally, accounting for the building geometry, it is calculated that the maximum solar collector area is about 80m² (which covers one side of the rooftop with a 30° of inclination angle). The building solar production would reach 56kW with an assumption of 700W/m² of production based on IEA SHC recommendations (IEA SHC, 2004).

Substation architectures chosen

The present work is a continuation of the study presented in [START_REF] Lamaison | A multicriteria analysis of bidirectional solar district heating substation architecture[END_REF] that discussed the possible architectures of such two-way substation based on a set of features and selected the most promising ones based on a set of criteria.

Features such as the location of the hydraulic separation between the network and the building, local consumption of the heat or total feed-in and control strategies were combined to build an exhaustive list of possible configurations. Promising setups were chosen from that list based on a multi-criteria analysis (cost, operation, ownership, etc.). These setups are presented in Figure 3, Figure 4 and Figure 5. The two first architectures exhibit complete reinjection of the solar heat on the network without direct local consumption of this heat while the last one promotes a local usage for DHW preheating while reinjecting the excess heat into the network.

On the three Figures, various control strategies are highlighted (S0, S1 and S2). The goal of these control strategies is to obtain a feed-in temperature level above the local supply temperature in the DHN, while also addressing the following constraints: i) Minimize the temperature in the solar field to reach high efficiencies, ii) Overcome the strongly varying local flow resistance, i.e. differential pressure drop, iii) Adjust the feed-in flow rate so that the feed-in rate matches the strongly varying heat rate produced by the solar field. S0 consists in a pump and a valve in series, S1 consists in a pump and a bypass valve, and S2 consists in a hydraulic separator and two pumps. It should be noted that for the present study, only Architecture 2 (C2U0) with control strategy S0 is considered.

MODELING

The present section introduces first the modelling framework. Second, it presents in sequence the models for the consumer (SH and DHW), the solar field and the substation. Third, the network boundary conditions used in the present study are highlighted. Finally, the control associated to the operation of the substation and the basic operating principles are discussed.

Modelling Framework

The modelling framework is based on the open source modelling language Modelica used in the commercial simulation environment Dymola. Modelica is an acausal (equation-base) and object-oriented programming language with a large and fast-growing community both for industrial and academic applications [START_REF] Schweiger | District heating and cooling systems -Framework for Modelica-based simulation and dynamic optimization[END_REF]. Modelica has native multi-physical modelling capabilities (thermo-hydraulic), is structured in libraries enabling exchange of methods in the scientific community and allows for implementing new components. Moreover, the Annex 60 project from the IEA [START_REF] Wetter | IEA EBC Annex 60 Modelica library an international collaboration to develop a free open-source model library for buildings and community energy systems[END_REF] promotes the development of computational tools for building and community energy systems based on Modelica and FMI standards, motivating the choice of this modelling framework.

As mentioned in the introduction, the Modelica "Standard" Library for its common connectors and fluid ports, the "Buildings" library [START_REF] Wetter | Modelica Buildings Library[END_REF] for its general building models and the "DistrictHeating" library [START_REF] Giraud | Presentation, Validation and Application of the DistrictHeating Modelica Library[END_REF] for its DHN piping and solar collectors models will be used.

Consumer

The consumer is modelled in terms of space heating consumption using the 'Buildings' library [START_REF] Wetter | Modelica Buildings Library[END_REF] from Modelica and in terms of domestic hot water consumption using the 'DHW-calc' draw-offs profile [START_REF] Jordan | DHWcalc: Program to generate domestic hot water profiles with statistical means for user defined conditions[END_REF]. Space heating is modelled with i) a heating system and ii) a mono-zone building. Concerning the heating system, it is composed of a thermostatic valve and a radiator modelled using the "RadiatorEN442_2" model from the library "Buildings" [START_REF] Wetter | Modelica Buildings Library[END_REF]. In this model, the transferred heat is computed using a discretization along the water flow path, and heat is exchanged between each compartment and a uniform room air and radiation temperature. Concerning the mono-zone building, it is modelled using the "mixed air" model [START_REF] Wetter | Modeling of heat transfer in rooms in the Modelica[END_REF] from the same library. It considers a perfectly mixed air in the room and takes into account heat exchange through convection, conduction, infrared radiation and solar radiation. Internal heat gains due to occupation (latent heat), lighting (radiation) and home appliances (convection) are included in the model. Constant singleflow ventilation is considered with a flow-rate of about 0.4 room volume per hour. For the present study, the dimension of the building considered were listed in Section 2.1. The total glazed area for the building represents 1/6 of the building living area, shared as follows, 50% on the South wall, 15% on the West wall and 35% on the East wall. The envelope of the building (layers composition and infiltration) is set to follow the RT2000 French thermal regulations.

Concerning the equivalent radiator, it is assumed to have a nominal inlet temperature, a nominal difference and a nominal power of respectively 70°C, 15K and 42kW. For the obtained nominal mass flow rate of 0.7kg/s, a nominal pressure drop of 1bar is assumed (together with a quadratic pressure drop law). The set point for the building ambient temperature is set at 20°C. The model requires as inputs the outside temperature and the set point for the radiator inlet temperature. In the present study, the analysis of the operation of the two-way substation is studied for 2 days. Thus, Figure 6 gives this 2 days outside temperature profile calculated using the weather station of Chambéry-Aix les Bains in the software [START_REF] Meteonorm | Meteonorm: Irradiation data for every place on Earth[END_REF]. The 2 days considered are the 11 and 12 th of March. The associated set point for the thermostatic valve is also given in Figure 6. The latter follows a heating curve, i.e. 60°C as radiator inlet set point temperature for -10°C as outside temperature and 25°C for 15°C). The water draw-off system is considered without sanitary loop. As explained initially, the daily profile of draw-offs are obtained from the software DHW-calc [START_REF] Jordan | DHWcalc: Program to generate domestic hot water profiles with statistical means for user defined conditions[END_REF] from Task 26 of IEA which distributes DHW draw-offs throughout the year or the day with statistical means, according to a probability function. The mean daily DHW consumption was set to 900litres, i.e. 150l/apartment. The latter was obtained from a report of COSTIC ( 2016) based on the type of apartment and the number of people living in it. Figure 7 shows the 2 days profile considered (6 minutes time-step). The cold water temperature is considered constant equal to 10°C.

Solar Field

The solar field on the rooftop is modelled using a component developed inside the 'DistrictHeating' library specifically for the present study. It is a thermo-hydraulic model that considers collectors arranged in rows, each row being discretized in a number of element superior or equal to the number of collectors in the row. The energy balance of each discretized element follows is shown in Eq.( 1) which comes from the norm NF EN ISO 9806 (2017).

In that equation, the thermal capacity C accounts for the fluid and material capacities, Tm is the mean temperature of the fluid through the field, Ta is the ambient temperature, Afield refers to the total collector field area, η0 is the collector optical efficiency and a1 and a2 are respectively the linear and quadratic heat loss coefficients. The three latter coefficients are obtained using the Solar Keymark test results report ("Solar Keymark Database," 2018).

Finally, in Eq.( 1), GT is calculated using Eq.( 2) in which Ib and Id are the direct and diffuse solar irradiations, obtained from weather data and Kb and Kd are the incidence angle modifiers for the direct and diffuse irradiations, obtained from the Solar Keymark test results report ("Solar Keymark Database," 2018). More specifically, Kb depends on the incidence angle θ and is obtained using input table from manufacturer.

𝐶

𝑑𝑇 𝑚 𝑑𝑡 = 𝐴 𝑓𝑖𝑒𝑙𝑑 (𝜂 0 𝐺 𝑇 -𝑎 1 (𝑇 𝑚 -𝑇 𝑎 ) -𝑎 2 (𝑇 𝑚 -𝑇 𝑎 ) 2 )

+ 𝑚̇𝑠 𝑜𝑙 𝑐𝑝(𝑇 𝑖𝑛 -𝑇 𝑜𝑢𝑡 )

(1)

𝐺 𝑇 = 𝐼 𝑏 𝐾 𝑏 + 𝐼 𝑑 𝐾 𝑑 (2)
The direct irradiation Ib and incidence angle θ must be calculated on the tilted plan of the collectors while to obtain the diffuse irradiation Id on the tilted surface, the model of [START_REF] Perez | DH Networks -Concept, Construction and Measurements Results of a Decentralized Feed-In Substation[END_REF] is used. For both irradiation calculations, the horizontal solar irradiation obtained from the weather station of Chambéry-Aix les Bains in the software Meteonorm ( 2017) is used. Figure 8 presents the resulting total solar irradiation for the 2 days considered.

Hydro-dynamically speaking, the mass flow is considered perfectly distributed in the different rows for the present study. Thus, the solar field pressure drop is calculated with a quadratic law using the total solar loop flow rate 𝑚̇𝑠 𝑜𝑙 , the single collector nominal pressure drop ∆Pnom,c and flow rate 𝑚̇𝑛 𝑜𝑚,𝑐 , and the number of rows Nrows and collectors per row Ncoll_per_row, as shown by Eq. (3).

Δ𝑃 𝑠𝑜𝑙𝑎𝑟,𝑓𝑖𝑒𝑙𝑑 = 𝑁 𝑐𝑜𝑙𝑙_𝑝𝑒𝑟_𝑟𝑜𝑤 Δ𝑃 𝑛𝑜𝑚,𝑐 ( 𝑚̇𝑠 𝑜𝑙 𝑁 𝑟𝑜𝑤𝑠 𝑚̇𝑛 𝑜𝑚,𝑐 ) 2 

(3)

Finally, there are two pipes on the solar field (return and supply lines) modelled using finite volumes with heat losses (see Eq. ( 4) with 'z' being the abscissa along the pipe and UAloss the overall heat transfer coefficient). For the heat losses, the piping are considered to be in contact with the ambient air. The pressure drop in this piping also follows a quadratic law (see Eq. ( 5)).

𝐶 𝑑𝑇 𝑑𝑡 = 𝑚̇𝑐𝑝 𝑑𝑇 𝑑𝑧 -𝑈𝐴 𝑙𝑜𝑠𝑠 (𝑇 -𝑇 𝑒𝑥𝑡 ) (4)
Δ𝑃 𝑝𝑖𝑝𝑒 = Δ𝑃 𝑛𝑜𝑚,𝑝𝑖𝑝𝑒 ( 𝑚ṁ ̇𝑛𝑜𝑚,𝑝𝑖𝑝𝑒 ) 2

(5) In the present study, two rows of two double-glazing solar thermal panels ("SavoSolar SF500-15DG -Solar Keymark," 2016) with a gross unit area of 15.96m² (2.6m x 6.2m) leads to a gross area of about 64m² (5.2m x 12.4m) that fits in the estimated space in Section 2.1. The solar collector coefficients and unit capacity are respectively 0.793, 2.52 W/m²/K, 0.004W/m²/K and 12 KJ/K/m². Both the supply and return lines of the solar field are considered to be 20m long with an internal diameter of 32mm and insulated with 3cm of PUR Foam.

Substation

As shown in Figure 9, the C2U0 two-way substation is modelled with 3 Heat Exchangers, 2 valves and 2 pumps. Concerning the heat exchangers (HEXsol, HEXDHW and HEXSH), they are discretized with finite volume method on both sides. A constant overall heat transfer UA is assumed, sized for the nominal operating conditions listed in Table 1 below. The service lines between the network and the substation are modelled using the finite volume model of long pipes form the 'DistrictHeating' library [START_REF] Giraud | Presentation, Validation and Application of the DistrictHeating Modelica Library[END_REF]. The flow in these lines can switch direction. They are sized for 60kW for a temperature difference of 30°C. The latter means a flow rate of 0.47kg/s, which leads to a DN32 for a nominal pressure drop of 100Pa/m (usual sizing value for DHN). The nominal velocity is thus calculated to be about 0.6m/s below the advised limit of 2m/s. A quadratic model (similar to Eq. ( 5)) is then used to obtain the pressure drop during the simulations. A length of 50m is chosen for these service lines with an insulation of 3cm of PUR Foam.

For the consumer two-way valves (VSH and VDHW), a linear characteristic is considered assuming that the nominal flow rate of 0.47kg/s should be ensured for a differential pressure drop of 1bar. For the SH pump (PSH), it is assumed to operate at a constant differential pressure drop of 1bar in accordance with the equivalent radiator characteristics as described in Section3.2.

Figure 9: Two-Way Substation model (Similar to

Figure 4 with S0 strategy only) Finally, the feed-in pump (Pfeed) is considered to be subjected to the network pressure drop and its flow rate is controlled as explained later in Section 3.6. For the calculation of the feed-in pump consumption, an isentropic efficiency of 80% is accounted for.

Network boundary conditions

The network side inputs are the local differential pressure and supply/return temperatures. In the present model, these two variables can either be set to constants to study specific operational conditions or set to follow real DHN variations. For the latter, data were collected by Veolia Giroa in the frame of the THERMOSS project at the DHN of San Sebastian, Spain. Figure 10 below presents these data for the two consecutive days of interest (2 nd and 3 rd of March 2017) with a time step of 15 minutes. These data are here only used as typical DHN data and are not correlated to the other boundary conditions such as the outside temperature for example.

Control and Operating Principles

The control strategy of the two-way substation concerns first the valves on the consumer side, second the solar field pump speed and third, the feed-in pump speed:

 Consumers: The DHW valve (VDHW) opening is controlled using a pure proportional controller so that the secondary hot temperature on the consumer side is equal to 45°C. Similarly, a pure proportional controller is used for the opening of the SH valve (VSH) so that the secondary hot temperature is equal to the set point defined in Figure 6.  Solar field: The solar field pump starts when the irradiation in the plane of the solar field is above 100W/m² and its flow rate is controlled by a PI controller so that the outlet temperature of the solar field is constant (set at the DHN supply temperature plus a margin of 10°C). The flow rate on the field is bounded between 10 and 50 kg/hr/m² of solar panels.  Feed-In: The feed in pump (Pfeed) starts when the solar field outlet temperature is above the supply temperature of the DHN plus a margin of 5°C. The speed of the feed-in pump is controlled so that the outlet temperature of the feed-in heat exchanger is above the supply temperature of the DHN plus a margin of 5°C.

Figure 11: Operating principles of the C2U0 Substation

The operating principles of the substation are presented in Figure 11. At a given instant of time, there are three cases possible: i) In case 1, there is no heat consumption or solar production and thus there is no flow through the service lines; ii) In case 2, the heat consumption for domestic hot water and space heating is larger than the solar production. The solar energy is used for the consumer needs in addition to the heat coming from the network. The flow in the service lines is thus from the supply to the return line; iii) In case 3, the heat consumption is lower than the solar production. The solar energy is used to entirely satisfy the consumer needs and the surplus heat is reinjected to the network. The flow in the service lines is thus from the return to the supply line.

SIMULATION RESULTS

Two days operation analysis

Figure 12 presents first the results in terms of mass flow rate in the solar field, from the network, from the feed-in pump and to the consumer (DHW+SH). The 3 operating cases described in the previous Section are specifically highlighted. The peaks in the consumer flow rates are due to the DHW draw-offs. The oscillations observed for the feed-in are due to the PI controller that requires a better identification, or even a gain schedule-like identification due to the strongly varying operating conditions. The conclusion of this graph is that reinjecting the solar heat on the network when no local storage facilities are available is of prime interest since the solar production periods do not match the local consumption periods. when the temperature at the entrance of the service line is the same as the DHN supply temperature, the substation consumes heat, when it is above, it reinjects heat. It is here interesting to note that solar energy production periods are coincident with the periods where the heating demand is the lowest (due to high outside temperature and passive gains) for which in general the supply temperature from the network is also the lowest. The latter is favourable for the reinjection. A similar analysis can be performed for the network differential pressure. Figure 14 presents the results of the 2days simulation in terms of heat power. On the top chart, the difference between the solar irradiation (Qsol) and the solar field production (Qfield) is due to the solar collectors' efficiency. The difference between the solar field production (Qfield) and the collected energy (Qcoll) is due to the losses in the solar field piping mostly during the start-up phase (warming up of the volume of water that cooled down at night). On the bottom chart, all the heat power related to streams from or to the two-way substation are shown. It is worth noticing that the part of self-consumption is rather small (SH and DHW curves below the Solar Collected curve). A schematic explanation of all the heat power from Figure 14 is given in Figure 15. 4.2 Influence of DHN differential pressure, supply temperature and return temperature Using the previous simulation as a base case scenario, 3 additional simulations for which respectively the DHN differential pressure was increased by 1bar, the DHN supply temperature was increased by 10°C and the DHN return temperature was reduced by 10°C were performed. Table 2 summarizes the results in terms of energy, using the same nomenclature as Figure 15. The feed-in pump consumption is also calculated. In general, the increase of the differential pressure has only an impact on the feed-in pump consumption while an increase of either the supply or the return temperature will reduce the solar plant production (lower field efficiency and larger heat losses).

Table 3 presents 5 ratios of interest using the energies presented in Table 2. As shown before, both the efficiencies of the solar field (Esolar,field/Esolar,irr) and the plant (Esolar,coll/Esolar,irr) are reduced with an increase of either DHN lines temperature. Additionally, the ratio of the self-consumed solar energy (Esolar,coll-Eloss,SL) to the total energy consumed (ESH+EDHW) referred as ηself_vs_cons and to the collected solar energy referred as ηself_vs_solar are rather small when considering that the solar energy produced is larger than the consumed energy. The reinjection is thus primordial in such situation since it transforms the collected solar energy into useful energy (for other DHN users). Finally, the pump (Efeed,pump) to collected solar energy (Esolar,coll) referred as ηpump_vs_solar increases with a higher differential pressure and decreases with higher supply and lower return temperature.

CONCLUSIONS

The present paper presented the modelling of a specific architecture of two-way substation together with the solar field and the consumer connected to it. Real DHN operating conditions were used as boundary conditions to dynamically simulate two days of operation. The latter showed that the developed framework was appropriate to perform detailed thermo-hydraulic simulations of such solar two-way substations.

Further steps will include the generalization of these results on seasonal and yearly basis, the simulations of the other promising architectures and the evaluation of the influence of a storage.

Figure 2 :

 2 Figure 2: Schematic of a two-way substation in a multi-family building prosumer

Figure 3 :Figure

 3 Figure 3: Schematic of Architecture 1 (C0U0)

Figure 6 :

 6 Figure 6: Two days outside temperature and associated radiator inlet temperature set point profiles

Figure 7 :

 7 Figure 7: Two days DHW draw-offs profile at 10 minutes time step (obtained from DHW-calc software)

Figure 8 :

 8 Figure 8: Total (diffuse + direct) solar irradiation on the tilted plan (30°) of the solar collectors for 2 days consideredIn the present study, two rows of two double-glazing solar thermal panels ("SavoSolar SF500-15DG -Solar Keymark," 2016) with a gross unit area of 15.96m² (2.6m x 6.2m) leads to a gross area of about 64m² (5.2m x 12.4m) that fits in the estimated space in Section 2.1. The solar collector coefficients and unit capacity are respectively 0.793, 2.52 W/m²/K, 0.004W/m²/K and 12 KJ/K/m². Both the supply and return lines of the solar field are considered to be 20m long with an internal diameter of 32mm and insulated with 3cm of PUR Foam.

Figure 10 :

 10 Figure 10: Differential Pressure and Supply/Return Temperatures used as DHN inputs for the simulations

Figure 12 :

 12 Figure 12: Mass flow rates in the two-way substation and in the solar field for the 2-days simulation period Figure 13 presents the results of the 2days simulation in terms of temperature. The periods of consumption and reinjection are noticeable on the temperature chart, i.e.when the temperature at the entrance of the service line is the same as the DHN supply temperature, the substation consumes heat, when it is above, it reinjects heat. It is here interesting to note that solar energy production periods are coincident with the periods where the heating demand is the lowest (due to high outside temperature and passive gains) for which in general the supply temperature from the network is also the lowest. The latter is favourable for

Figure 13 :Figure 14 :

 1314 Figure 13: Temperatures in the supply line of the DHN, the supply service line (SL) and the solar field for the 2-days simulation period

Figure 15 :

 15 Figure 15: Schematic of the power streams used in Figure 14

Table 1 : Heat Exchanger sizing HEX Thot,in [°C] Thot,out [°C] Tcold,in [°C] Tcold,out [°C] Q [kW]

 1

	Solar	90	60	50	80	60
	DHW	80	45	10	45	60
	SH	80	50	40	70	42

Table 2 : Sensitivity on DHN operating conditions (unit: kWh) Case

 2

		Base	∆P+1bar	Ts+10°C	Tr -10°C
	Esolar,irr	760.19	760.19	760.19	760.19
	Esolar,field	361.13	361.09	342.63	372.78
	Esolar,coll	332.74	332.71	310.58	345.04
	ESH	194.28	194.42	194.34	194.28
	EDHW	74.16	74.78	74.41	74.15
	ESST,consumed	250.09	250.85	252.16	250.73
	ESST,feed-in	315.84	316.22	295.14	328.53
	Eloss,SL	89.70	89.81	92.58	85.04
	Efeed,pump	1.05	1.59	0.63	0.73

Table 3 : Energy Ratios (unit: %) Case

 3

		Base	∆P+1bar	Ts+10°C	Tr -10°C
	ηsolar,field	47.5	47.5	45.1	49.0
	ηsolar,plant	43.8	43.8	40.9	45.4
	ηself_vs_cons	6.3	6.1	5.7	6.2
	ηself_vs_solar	5.1	5.0	5.0	4.8
	ηpump_vs_solar	0.32	0.48	0.20	0.21

ACKOWLEDGEMENTS

This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No 723562. The authors also wish to thank Bosch Thermotechnik GmbH and Giroa-Veolia for their valuable inputs.


Nihat Sahin

Wanderson O Silva

Mariana R Camilo

Edson A Ticianelli

Fabio H B Lima

Julien Parmentier

Clément Comminges

Teko W Napporn

Kouakou Boniface Kokoh

CO 2 electroreduction to fuels on mesoporous carbon-embedded copper nanoparticles

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Introduction

The current global emission rate of carbon dioxide (CO2) is getting up to metric gigatonnes that brings along adversely environmental consequences causing climate changes. It is admitted that using CO2 as a sustainable carbon source in energy conversion systems has been received much attention, as it would mitigate a large amount of CO2 emission per year 1,2 . There are variety processes in which CO2 can be converted into value-added chemicals and fuels such as syngas 3 , methanol 4 , methane 5 , ethanol 6 , formic acid/formate 7 , ethylene 8 , and oxalic acid/oxalate 9 . Of the various CO2 valorisation approaches, electrochemical devices are the most promising and compatible devices due to their ability of adapting to renewable energy sources, and possible higher conversion efficiency. In past two decades, various electrode materials were prepared and studied to assess the poor selectivity, this being the major limitation to the development of industrial CO2 reduction [10][11][12] . However, the electrochemical processes have been hampered by the poor selectivity and the large overpotential in CO2 electrocatalysis, causing the decreasing energy efficiency. In addition, development of effective nanostructured copper materials for especially tuning the structure and composition is still the main drawback. Considerable efforts have therefore devoted to developing effective Cu-based catalysts with the goal of controlling structure and composition for improving reaction selectivity [13][14][15][16] . The product selectivity of CO2 reduction strongly depends on nature of the electrode 17,18 and composition of the electrolyte 19 . In particular, metal-based electrodes such as lead, mercury, bismuth, zinc, cadmium, and iridium that possess electron rich orbitals are capable of reducing the molecular CO2 at high overpotential (> 0.80 V). A variety of valuable chemicals (C 1- C3) were reported from CO2RR on copper based-metal nanoparticles 14,[20][21][22] and on copper particles derived from different Cu2O morphologies 23 at -1.10 V vs. RHE in 0.1 mol L -1 KHCO3 and/or CsHCO3 20,22 , and 0.3 mol L -1 KHCO3 23 solutions. The binding energy features of copper for CO2 and reaction intermediates make it a unique electrode material for the production of hydrocarbons. The mechanism may also include steps of readsorption of reaction intermediates (such as formic acid and CO) for hydrocarbon generation 24 . Among the wide variety of CO2 electroreduction products, short-chain hydrocarbons (methane and ethylene) are more desirable for chemical industries, mainly for using as feedstocks or for producing synthetic fuels 25 . So, this has leading studies on electrochemical production of methane and ethylene on copper-based electrodes. Recently reported studies have also been demonstrated that threedimensional porous electrode materials 18,26 that favour easy diffusion of electrolyte, fast kinetics of electrochemical reactions, and steps of readsorption of reaction intermediates are highly desirable for constructing high performance electrode materials in electrocatalysis. In particular, the influence of the interconnected porous channels as transport paths on electrolyte transportation and ion diffusion have been demonstrated [27][28][29] . In addition to catalyst morphology, the presence of cation species in the electrolyte is another crucial key factor in achieving desired selectivity and catalytic performance on metallic catalyst 30,31 . Similarly, the influence of the alkali cation nature on activity of an electrocatalyst has been reported 32 . It was argued in the literature that the presence of a larger cation such as K + instead of Na + promotes an increase in the local concentration of dissolved CO2, which results in improving the electrode performance 33 . Moreover, significant improvements towards the CO2RR have been achieved in the electrode performance and reaction selectivity, as investigated by on-line mass spectroscopy on CO2 electroreduction 34 . In addition to electrolyte nature, the correlation of the electrocatalyst composition and reaction product distribution using on-line differential electrochemical mass spectrometry (DEMS) were extensively investigated 35,36 . Inspired by these studies, we present a novel copper-based electrocatalyst with a mesoporous carbon network, carving the path for efficient CO2 electrocatalysis. The synthesis was based on the one pot soft-template process which has shown the lead easily to accessible nanostructured metal-based phase well dispersed in a mesoporous carbon structure. 26 We further discuss the role of the electrode potential on product selectivity of short-chain hydrocarbons determined by on-line differential electrochemical mass spectroscopy and chromatographic analysis.

Results and discussion

Physicochemical Characterization

The N2 adsorption/desorption isotherm at 77 K (Fig. 1a) and the corresponding pore size distribution calculated by the BJH method (Fig. 1b) were determined from the adsorption branches of the isotherms. According to IUPAC classifications, the Cu/C ex-Tannin material isotherm represents type IV isotherm with a capillary condensation step and a hysteresis, which is indicative of the presence of broadly distributed mesopore size. The capillary condensation step occurs at a relative pressure of about 0.6, in agreement with previously reported results 37 . In terms of textural properties, Cu/C ex-Tannin shows a BET surface area of 417 m 2 g -1 and a pore volume of 0.41 cm 3 g -1 (at P/P°=0.98) with a maximum pore diameter distribution around of 7 nm. In order to confirm the presence of copper nanoparticles, thermal gravimetric analysis was conducted. Fig. 2 displays differential thermal and thermo-gravimetric (DT/TG) analysis curves for Cu/C ex-Tannin material heat-treated in air. This Cu/C ex-Tannin shows a total weight loss (87.8 wt.%) in two distinct temperature regions. The first region from 25 to 220 °C with a low weight loss (3.52 wt.%), which may be attributed to desorption of physically adsorbed impurities and water molecules at the surface of the substrate material. Then, the second region from 220 to 450 °C shows a conspicuous weight loss of 78.9 wt.%, accompanied by an exothermic peak at about 335 °C on the DT pattern, which corresponds to the concomitant oxidation of the carbon substrate and the copper phase. The accurate metal loading for the Cu/C ex-Tannin material is found to be 12.2 wt.% after subtracting of adsorbed water content and considering CuO formation 38 during DT/TG analysis in air as confirmed by XRD (Figure S1). X-ray diffraction analysis was used to examine the structure of the Cu/C ex-Tannin material and this result is shown in Fig. 3a. All the diffraction phases were identified by referring to the Joint Committee on Powder Diffraction Data (JCPDS-ICDD) database. The diffraction lines at 43.25°, 50.44°, 74.10°, 89.91° and 95.13° correspond, respectively to the (111), ( 200), (220), and (222) planes of face-centered cubic (fcc) copper with a lattice constant of a=b=c=3.6150 Å. Diffraction patterns display sharp and narrow peaks, suggesting the high degree of crystallinity. Moreover, the additional peaks observed at around 28.40° and 36.32° correspond, respectively to the reflection planes of (110) and (111) of the cubic Cu2O phases with a lattice constant of a=b=c=4.2670 Å. The Debye-Scherrer`s relation was used to determine the average crystallite size measured in a direction perpendicular to the diffracting planes (hkl) of the material according to Eq. (1).

𝐿 𝑣 = 𝑘𝜆 𝛽 ℎ𝑘𝑙 𝑐𝑜𝑠𝜃 ⁄ Eq. 1
where Lv is the average crystallite size, β is the full width at half maximum (FWHM) of the peak corresponding to plane (hkl; in radian), and k is a shape factor. Based on the assumption of Gaussian line profiles and small cubic crystals of uniform size, k is 0.94. The calculated average crystallite size of the Cu (111) diffraction plane is determined as 190 nm in the [111] direction. In addition, the Williamson-Hall (W-H) procedure allows to explore the lattice micro-strain (db) influence lied behind the lattice mismatch in the presence of nanoparticles 39 . Fig. 3b shows the lattice micro-strain effect (db) in the copper particles for Cu (111) plane in the light of the W-H plot. This indicates the growth of the Cu nanoparticles in the porous network occurred without deformation in agreement with the formation of a perfect truncated octahedron copper crystallite, as evidenced by our previously reported study 26 .

To acquire further insights into nanoparticle distribution, cross-sectional TEM analysis was conducted, as presented in Fig. 4a. The CS-TEM image evidently displays that the C ex-Tannin support material presents mesoporous structure (Fig. S2) with highly dispersed copper nanoparticles as seen as black spots. The mean particle size of 160 nm is estimated from the histogram illustrated in Fig. 4c. XRD and TEM analyses provide characteristic information on the size of the prepared material. Basically, calculation from XRD diffraction patterns, based on the Scherrer's formula (see in Eq.1), gives an average size of the crystallite in the direction perpendicular to the considered (hkl) plane (herein, (111)). The similarity of the two values (in the ±60 uncertainty range) suggests a monocrystalline nature of the Cu nanoparticles. Further, the white spots on the CS-TEM image correspond to the holes of the copper nanoparticles which were removed from the surface of the support during the CS-TEM sample preparation. The high magnification image shown in Fig. 4b evidences the growth of copper nanoparticles and the presence of mesoporosity throughout the copper particles.

Therefore, the material can be described as a mesoporous carbon matrix embedding well dispersed mesostructured copper nanoparticles (12.2 wt.%). As shown in the electrochemical part, these particles behave as electrocatalyst and are greatly accessible to CO2 through the diffusion path of the mesoporous network. Moreover, high resolution TEM analysis shows that synthesized copper nanoparticles exhibit oxide structure using bright field (Fig. S3a) and dark field (Fig. S3b) measurements with the selected area electron diffraction (SAED) patterns at zone axis [011] (Fig. S2c). In the bright field TEM micrograph (Fig. S3a), copper nanoparticles obviously appear, while in dark one (Fig. S3b) Cu2O (111) structure appear as white patterns around the copper surface confirming by the SAED pattern. The SAED pattern in Fig. S3c corresponds to the [011] zone axis of a face centered cubic unit cell, corresponding to (111) crystal planes of Cu2O phase, which could be ascribed to a lattice spacing of 0.221, 0.254, 0.256 nm for the indexed spots. The presence of copper oxide, which has electrondonor property, tends to increase the carbon dioxide adsorption capacity due to its electron-acceptor features 40 .

Electrochemical characterization of the mesoporous carbon supported copper electrode

The voltammetric behaviour of carbon dioxide is studied on the mesoporous carbonembedded copper electrode in N2-deareated and CO2-saturated 0.4 mol L -1 KHCO3 containing 4.0 mM KCl aqueous electrolyte. The cyclic voltammograms (Fig. 5) display two distinct regions: (i) metallic copper behaviour between about -0.10 and +1.40 V, (ii) CO2 electroreduction (CO2RR) and hydrogen evolution reaction (HER) region between -0.10 and -1.0 V. As can be seen in Fig. 5, two oxidation peaks located at 0.62 V (j Ox1) and at 0.76 V (jOx2), represent the transformation of metallic copper into copper(I) oxide (Cu→Cu2O) and then copper(I) oxide into copper(II) oxide (Cu2O→CuO), respectively. Furthermore, the corresponding copper redox couples were observed at the potentials of 0.42 V (jRed1) and 0.21 V (jRed2) during the cathodic scan direction, which are respectively attributed to the formation of copper(I) oxide (CuO→Cu2O) and metallic copper (Cu2O→Cu). In addition to copper oxide species on the anodic scan direction, another characteristic anodic peak assigned to Cu2O3 (jOx3) appears in the course of oxidation of copper originating from hydrated CuO. Only a few papers have reported the investigations of the copper electrode behaviour in aqueous [41][42][43] and non-aqueous 44 electrolytes. It should be highlighted that comparing the cyclic voltammogram profiles on the cathodic scan direction, it is remarkable that the reduction of CuO (j Red1) and Cu 2 O (j Red2 ) species are recorded in the presence of N 2 , whereas only reduction to copper(I) oxide is solely recorded in the presence of CO2. In this way, hypothesis can be made that there is a strong interaction between Cu/Cu 2 O and dissolved CO 2 at the electrode surface just before the CO2 electrolysis. It is worth noting that the onset potential of CO2RR is observed at more positive potential (about 0.30 V) than hydrogen evolution reaction (see CV in Fig. 5). The broad reduction peak in the polarization curve as to CO2-catalysis located at about -0.50 V with a reasonable current density (jCO2RR=-18.5 mA cm -2 ), is assigned to the CO2RR, as endorsed by its 5.8-fold enhanced current density in the presence of CO2. This sharp rise in current density indicates the catalysing ability of Cu/C ex-Tannin under defined electrolysis conditions. In addition to CV test for Cu/C ex-Tannin, the mesoporous carbon material synthesized without copper precursor is examined. The CV measurements (shown in Fig. S4) show that mesoporous carbon material is not active towards CO2RR.

Electroreduction of CO2 on mesoporous carbon supported copper electrode as probed by chromatography analysis

Faradaic efficiency, abbreviated to F.E, is the fraction of electrons passed through the electric circuit to obtain a desired product, which is a key factor in determinin g the feasibility of converting CO2 to fuels. For evaluating the Faradaic efficiency, electrolysis experiments were carried out at various cathodic potentials ranging from -0.32 V to -1.12 V vs. RHE with a constant metal loading of 0.15 mgCu cm -2 . The comparison of the CO2 reduction activity and the current densities of the material electrode are presented in Fig S5 . A high initial current density is explicit in the first 200 s of electrolysis, which thereafter declines, reaching a stable profile. As can be seen in Fig. S5, the potentiostatic currents remain stable during electrolysis period, suggesting that the diffusion limited current density was estimated by using the simplified steady-state Nernst diffusion model 45 for the case of flat electrode inserted into laminar flow, promoting the diffusion limited current density. The details of the electrolysis results including the amount of charge passed per electrode area and the F.E values of HCOOH, CO, and H2 for each applied potential were listed in Table S1. As presented in Fig. 6, F.E of HCOOH typically increases with the applied potential from -0.32 V and reaches a 31.2% optimum value at -0.62 V/RHE, above which only H2 concentration continues increasing. These results expose that the Cu/C ex-Tannin electrocatalyst is capable of producing HCOOH at moderate overpotentials. We could hypothesize an interaction between electroactive catalytic sites on the Cu particles facilitated by the porosities of the electrocatalyst that intends to produce a weakly stabilized CO2 intermediate species 22 , resulting in a high yield of formic acid production than that of hydrocarbons. CO is also one of the gaseous products but less than 9.6% of F.E (at the potential of -0.52 V) was detected at all applied potentials, suggesting a stable adsorption of radical anion CO 2 •─ on the electrode surface at more cathodic potentials. With increasing the electrode potential towards cathodic window from -0.32 to -1.12 V/RHE, the F.E related to H2 formation being dominantly favoured from 45% (at -0.32 V/RHE) up to 77 % (at -1.12 V/RHE) listed in Table S1.

CO2RR electrochemical experiments coupled to on-line DEMS

As noticed in Fig. 6, the total balance of production formation is not achieved. This is due to the detection limit chromatographic analysis. Considering copper is capable of catalysing production of hydrocarbons, the eventual formation of CH4 and C2H4 in the CO2RR was monitored by on-line DEMS, more sensitive but not quantitative, under galvanostatic conditions on Cu/C ex-Tannin. Fig. 7 displays the constant electrochemical potentials (Fig. 7a), and ionic currents for hydrogen (H2, m/z = 2, Fig. 7b), methane (CH4, m/z = 15, Fig. 7c), and ethylene (C2H4, m/z = 26, Fig. 7d) acquired during DEMS measurements in CO2-saturated 0.4 mol L -1 KHCO3 containing 4.0 mM KCl aqueous electrolyte. Indeed, it was observed during these investigations that the presence of potassium chloride in the electrolyte also promotes the CO2RR efficiency as well as suppresses or slows down the HER may be due to the K + radius role on the H2O molecules. Chronoamperometry results presented in Fig. S6 show that the Cu/C ex-Tannin electrode (0.36 mgCu cm -2 ) does not lose its activity during the electrolysis. As under constant potential electrocatalysis the reaction activity reaches a steady state value (Fig. S6), it may be concluded that the accumulated intermediates are nonpoisoning compounds. When the electrode was polarized from -0.52 to -1.12V vs. RHE, an increase in the ionic currents for m/z=2, m/z=15, m/z=26 was observed, evi dencing the formation of hydrogen, methane and ethylene, as shown in Figs. 7b-d. From the DEMS measurements, it can be seen that, while the production of methane appears at the potential of -0.52 V (Fig. 7c), ethylene detection shows up at -0.72 V/RHE (Fig. 7d), suggesting that formation of these products is extremely sensitive to the value of the electrode potential. Although not quantitative, it is noted that the ionic mass signal of CH4 (m/z=15, Fig. 7c) is three-fold higher than that of C2H4 (m/z=26, Fig. 7d). This suggests fast kinetics of methane production if one assumes that the formation of these two molecules requires different protonating steps that affect the reaction sites, and, therefore reaction intermediates proportionally to ionic currents i n mass spectrometer measurements.

On smooth polycrystalline metallic copper (electropolished or H 2 treated Cu, for example), the dissociative adsorption (or activation) of CO2 is thermodynamically unfavourable 46 . However, at high overpotentials, the only hydrocarbon evolved is CH4 47 , indicating that this species is formed on high hydrogenated surface. Interestingly, on oxide-derived Cu surface or on electrodeposited Cu, C2H4 is also produced, but at lower overpotentials. This seems to be due to the formation of a surface with high number of grain boundaries and of low-coordinated Cu atoms, which are formed during the electroreduction of Cu-oxide (surface roughening) [48][49][50] . These undercoordinated copper atoms and those located at the grain boundaries, more reactive than the bulk ones, do promote the dissociative adsorption of CO2 46 . Additionally, active sites of these resulting structures may possess interatomic distances that favour the CO-CO coupling, which might explain the ethylene formation, as discussed before 50 . Regarding Cu/C ex-Tannin investigated in present study, one can observe that methane is produced at lower overpotential and at a much higher extent in relation to ethylene. This indirectly shows that the surface of this electrocatalyst is smooth, with low amount of undercoordinated Cu atoms (also explaining its relatively low F.E. for formate production). One can also argue that the electrochemical potential excursion to higher values followed by another one to lower values (see CV in Fig. 5) may induce undercoordinated Cu atoms (by Cu dissolution and redeposition in the mesoporous material), which are active for C2-product formation. So, in a future study, considering that this mesoporous material has a high-surface area of metallic copper for hydrogenation, we will explore the effect of potential and of thermal treatment on CH 4 and C2H4 selectivities.

Experimental Chemicals

Copper(II)acetylacetonate (Cu(C5H7O2)2, Sigma-Aldrich >99.9% trace metal basis), triblock copolymer pluronic® F127 as the structure directing agent (Sigma-Aldrich, with the formula of (PEO)106(PPO)70(PEO)106),absolute ethanol (AnalaR NORMAPUR reagent, 37 wt.%), potassium bicarbonate (Sigma-Aldrich, ACS reagent, 99.7% powder), potassium chloride (Sigma-Aldrich, ACS reagent, 99.0%, powder). Tannin mimosa was supplied by the company Silvachimica (Italy) in the form of a fine, light brown, powder, obtained by spray drying of aqueous wattle (Acacia mearnsii formerly mollissima, DeWild) bark extracts. All used ultrapure water is provided by Milli -Q Millipore source (18.2 MΩ cm, 20 °C).

Synthesis of the mesoporous carbon embedded copper nanoparticles.

Bio-sourced mesoporous carbon material containing copper nanoparticles was synthesized by an endo-templating route using evaporation-induced self-assembly process (EISA) 37 as schematized in Fig. S7. The process is based on the use of i) an amphilic surfactant triblock polymer Pluronic® F127 as a pore structuring agent and ii) Tannin mimosa (polyphenolic compounds) acting as both carbon source and reducing agent during the heat-treatment step. This process allows for preparation of highly uniform mesostructured carbon materials (Fig. S7d) with tubular pores locally organized in a honey-comb structure. Fig. S7 illustrates the stages of the self-assembly formation of mesoporous carbon (Fig. S7a-d) and nucleation and growth of copper nanoparticles by means of highly uniform thin films (Fig. 7a-i). For preparation of pure mesoporous carbon, firstly, non-ionic surfactant (Pluronic® F127) and carbon precursor (Tannin mimosa) are dissolved in the mixture of absolute ethanol, water and HCl, giving a weight composition of 1.10 g Pluronic® F127; 1.10 g Tannin mimosa; 5.7 g CH3OH; 3.0 g H2O; 1.10 g HCl (solution A). After continuous stirring at 500 rpm for 15 min, the mixture is poured into Petri dishes of 9.5 cm diameter, to evaporate the solvent at room temperature overnight. In this dry state, the mesophase obtain ed is composed of composite micelles with presumably tannin being located preferentially in the hydrophilic shell of the micelles 51 (Fig. S7a). It is proposed, as shown with phenolic resins, that the hydroxyl groups in the Tannin structure selectively int eract with the polyethylene oxide (PEO) chains of the triblock copolymer via hydrogen bonds which intend to be the driving force of the organic-organic self-assembly 52 (Fig. S7b).

Thereafter, the so-called mesophase undergoes condensation reactions around the surfactant phase (Fig. S7c1) during the heat-treatment (120 °C for 24 h and further).

The obtained polymerized thin films are scarped from the Petri dishes and thermally treated at the temperature of 400 °C for 18 min by heating rate of 5 °C min -1 in argon atmosphere with a flow rate of 10L h -1 , leading to elimination of the surfactant (opening of the mesoporosity) and to a partial carbonization of the polymer. For preparation of Cu/C ex-Tannin materials, another homogeneous solution (solution B) is prepared by dispersing of copper precursor Cu(acac)2 with acacH = acetylacetone in absolute ethanol via ultrasonication (Bandalin Sonorex RK106) with a maximum power output of 100 W for 5 min. To obtain a homogeneous material, solution A and solution B are mixed and stirred for 2 h. The mixture is then poured into four Petri dishes (5 mL in each Petri dish) so that obtaining thin film where the organized mesophase takes place. This mesophase transformation involves different steps: solvent evaporation with me sostructuration, thermal condensation phase, carbonization of the organic part and reduction of copper salt. Owing to the complexing ability of tannin, it is proposed that copper (II) is partly stabilized by the phenolic groups present in tannin. Tannin m imosa and its thermal decomposition products acts as reducing agents for Cu 2+ to Cu 0 clusters (Fig. S7e). Coalescence of the copper cluster (Fig. S7f) leads to formation of copper nanoparticles (Fig. S7g) through a nucleation-growth mechanism. As presented in Fig. S7i, the mesoporous carbon structure, with open pores, that contains the copper nanoparticles is obtained just after the removal of the surfactant. Herein, one may assume that the Cu crystallites have a perfect truncated octahedral shape (Fig. S7h) based on the Van Hardeveld and Hartog method 53 which describes the surface structure (corners, edges, and faces) for non-supported perfect fcc crystallites.

Physicochemical Characterization BET surface area and BJH pore size analysis

The nitrogen adsorption and desorption isotherms were conducted on a Micromeritics TriStar 3000 apparatus at 77 K. Prior to analysis, the samples were previously degassed at 363 K and then 523 K for at least 6h under vacuum. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method and the pore size distribution curve was calculated by the Barret-Joyner-Halenda (BJH) method using adsorption branch of the isotherms. Differential Thermal and Thermo-Gravimetric (DT/TG) analysis Differential thermal and thermo-gravimetric (DT/TG) analysis was achieved by TA Instruments SDT-Q600 apparatus equipped with TA Universal analysis software. Two alumina crucibles, one containing about 10 mg of the sample and another empty as a reference were thermally heated under air flow of 100 mL min -1 from 25 to 850 °C with a temperature rate of 5 °C min -1 .

X-ray diffraction analysis

X-ray diffraction data in the 2θ range of 15-100° with steps of 0.05° and fixed acquisition time of 2 min step -1 were collected using an ENPYREAN (PANalytical) diffractometer in Bragg-Brentano (θ-θ) configuration with a copper tube powdered at 45 kV and 40 mA (Cukα1=1.54060 Å and Cukα2=1.54443 Å).

Cross-section transmission electron microscopy

Cross-section transmission electron microscopy (CS-TEM) observations were conducted on a TEM/STEM JEOL 2100 UHR (200 kV) microscope working at 200 kV accelerating voltage. The point and linear resolutions were 0.19 nm and 0.14 nm, respectively. Before cutting the sample by using diamond knifes with ultra-microtome (Leica UCR RT) in the section thickness between 50 and 70 nm, the sample is embedded in a polymeric resin (Epon) at 60 °C for 48 h. The nanoparticle size distribution and mean particle size were evaluated by ImageJ® free software.

Electrochemical Characterization and CO2 Electrolysis

The electrochemical behaviour of the Cu/C ex-Tannin electrode was evaluated from cyclic voltammetry (CV) on the glassy carbon electrode (5 mm diameter) at a scan rate of 50 mV s -1 in a standard three-electrode electrochemical cell, equipped with a glassy carbon as the counter, and an Ag│AgCl (sat. KCl) as the reference electrode. Prior to measurement, 0.4 mol L -1 KHCO3 with 4.0 mmol L -1 KCl was completely deoxygenated by N2 (Linde Gas) for 30 min and then saturated by CO2 (Linde Gas) for another 30 min with the pH value chancing from 8.3 to 7.2, respectively. All the potentials applied were converted to reversible hydrogen electrode (RHE) reference scale. The catalytic ink was prepared by ultrasonically dispersing the prepared catalyst powder in a mixture of isopropyl alcohol (Sigma-Aldrich, 99.5 wt.%) and 10% Nafion® suspension (Sigma-Aldrich 5 wt.%) at the ratio of 9:1 (v/v). Then, 10 µL ink is drop casted on the glassy carbon electrode, yielding a 62.2 µg of Cu per cm 2 electrode. CO2 electrolysis measurements were conducted using a bi-potentiostat (Autolab-PGSTAT 302N) in a H-type cell separated by an ion-exchange Nafion®-117 membrane which is treated in 0.5 mol L -1 H2SO4 solution at 90 °C for 8 h. The cell was designed to have a large electrolyte volume of 100 mL in each compartment, along with a gas headspace of approximately 65 mL above the electrolyte on each side of membrane. It should be noted that the electrochemical cell was chemically cleaned with potassium permanganate acidic solution to remove the adsorbed impurities before electrochemical experiments. The counter and reference electrode are Pt/IrO 2 grid and Ag│AgCl, respectively. In the H-type electrolysis cell, a Ag wire covered with AgCl was used as a pseudo reference electrode. To better control the electrode potential, a small amount of chloride ions (4 mmol L -1 KCl) was added in the working compartment in order to stabilize the Nernst equilibrium. The working electrode is consisting of Toray carbon paper (with a total 6.3 cm 2 geometric area for both sides) with gold wire connection. The electrode loading uses the same catalytic ink, however the volume of the ink deposited on to the wet proofed Toray carbon paper yielding a 0.15 mgCucm -2 of metal loading. Electrolyses were conducted by polarization of electrode surface at various cathodic potentials ranging from -0.32 to -1.12 V/RHE in the CO2-saturated catholyte at room temperature. Prior to electrolysis, 0.4 mol L -1 KHCO3 containing 4.0 mM KCl aqueous catholyte and anolyte was de-aerated by N2 for 30 min, and then the catholyte compartment saturated by CO2 another 30 min while keeping the anolyte under N2 purging. Corresponding chronoamperograms along with electric charge and electrolysis duration presented in supporting information (Fig. S5 and Table S1).

Chromatographic analysis

Gas products were quantified with on-line gas chromatography (Agilent Technologies, 490 GC), The residual reactant and/or product gases from the outlet of cathodic compartment were vented directly into the gas-sampling loop of the GC, equipped with a thermal conductivity detector (TCD) that possesses three columns. The first one was equipped with a M5 A H BF column at 80 °C for H2 detection. The second channel contained a packed PPQ H column at 60 °C for hydrocarbon (CxHy) detection, and a third one was equipped with a packed COX HBF column at 120 °C for CO2 and CO detection. Argon (Airgas, 99.999%) and helium (Airgas, 99.999%) were used as the carrier gases to quantify hydrogen and CO concentrations, respectively. Liquid phase samples were analysed by high performance liquid chromatography (HPLC) which was equipped with Aminex®HPX-87H Ion Exclusion Column, Dionex Ultimate 3000 Pump, ASI-100 Automated Sample Injector and UV-detector. A 3.0 mM sulfuric acid solution was used as the mobile phase at 25 °C with a 0.60 mL min -1 flow rate.

On-line Differential Electrochemical Mass Spectrometry (DEMS) Measurements

The gaseous species generated during CO2RR were also monitored by using on-line DEMS. Here, qualitative analyses were performed under potentiostati c conditions 54,55 .

The DEMS equipment was constructed with a Pfeiffer Vacuum QMA 200 quadrupole mass spectrometer, and two turbomolecular pumps. The electrochemical cell was designed specifically for the coupling with mass spectrometer. The working electrodes for DEMS measurements were prepared on carbon cloth (PWB-3, Stackpole, with a 0.9498 cm 2 geometric area) pressed to a Gore-Tex PTFE membrane (pore size 0.02 µm), which served as interface layer between electrochemical cell and the high vacuum of the mass spectrometer, following a configuration similar to those utilized in a previous published study 54,56 Carbon dioxide was bubbled into the electrolyte (herein KHCO 3) to maintain saturation, and the electrolyte was constantly stirred using a magnetic b ar. A high area platinum electrode and an Ag│AgCl (sat. KCl) served as counter and reference electrodes, respectively. Typically, the electrochemical potential was recorded simultaneously with the ionic current (mass signal intensity) for selected values of mass/charge (m/z) ionic signals, being m/z = 2 (H2), m/z = 15 (CH4), and m/z = 26 (C2H4). The electrochemical cell was controlled by an Autolab (PGSTAT128N) Potentiostat/Galvanostat, and the DEMS equipment was piloted by the Quadera® analysis software (the output signal of the potential ramp from the potentiostat was registered by the DEMS equipment electronics in order to synchronized it with the mass signals). More details about DEMS electrode setup are presented in previous studies 55, 57 .

Conclusions

We have demonstrated a facile and cost-effective route on synthesis of mesoporous carbon-embedded nanostructured copper electrocatalyst from a one-pot evaporationinduced self-assembly process. Physicochemical characterizations combined with electrochemical examinations revealed that the high catalytic activity originates from the nanostructured copper particles and also porous network that provides a favourable local environment for CO2 electroreduction. Notably, the electrocatalytic activity of the Cu/C ex-Tannin material was evaluated by cyclic voltammetry measurements, showing catalytic ability of prepared electrode surface. Moreover, we have shown that Cu/C ex-Tannin catalysts preferably catalyse the CO2 reduction reaction into formic acid with a remarkable F.E of 31.2% at the potential of -0.62 V/RHE. This allows conceptualization and understanding of reaction selectivity in bench-scale fundamental studies. In addition to CO2 electrocatalysis measurements, the on-line DEMS examinations indicated that low-chain hydrocarbons (CH4 and C2H4) are selectively acquired from CO2 electroreduction on the Cu/C ex-Tannin electrocatalyst. To sum up, these results highlight the significant role of one pot soft-templating process on the preparation of the three-dimensional porous electrodes embedding nanostructured copper particles with interesting CO2 electroreduction properties regarding efficiency and selectivity. This study confers valuable insights towards engineering and designing more effective materials that overcome the reaction limitations for scalable CO2 catalysis.

Conflicts of interest

There are no conflicts to declare.

Fig. 1 a

 1 Fig. 1 a) N2 physisorption isotherms and b) corresponding pore size distributions from the BJH analysis of adsorption branches of the prepared copper-free mesoporous carbon (■) and Cu/C ex-Tannin (▼) materials.

Fig. 2

 2 Fig. 2 Differential thermal and thermogravimetric analysis (DTA/TGA) of the prepared Cu/C ex-Tannin material, recorded at the temperature rate of 5 °C min -1 under air (flow rate of 100 mL min -1 ).

Fig. 3 a

 3 Fig. 3 a) X-ray diffraction patterns and b) Williamson-Hall plot of the Cu/C ex-Tannin material.

Fig. 5

 5 Fig. 5 Cyclic voltammograms of the Cu/C ex-Tannin electrode recorded at 50 mV s -1 scan rate in N2deareated (pH 8.3) and CO2-saturated (pH 7.2) 0.4 mol L -1 KHCO3 containing 4.0 mM KCl aqueous electrolyte.

Fig. 6

 6 Fig. 6 Faradaic efficiency of Cu/C ex-Tannin material at various electrode potentials in CO2-saturated 0.4 mol L -1 KHCO3 electrolyte.

Fig. 7 a

 7 Fig. 7 a) The electrochemical potentials at -0.52 V (green), -0.72V (blue), -0.92 V (red), and -1.12 V (black) versus RHE, and corresponding ionic currents for b) hydrogen (H2, m/z=2), c) methane (CH4, m/z=15) d) ethylene (C2H4, m/z=26) obtained during on-line DEMS measurements of chronoamperometry for the Cu/C ex-Tannin electrode with a constant metal loading of 0.36 mg of Cu per cm 2 in CO2-saturated 0.4 mol L -1 KHCO3 electrolyte.

Acknowledgements

This work was financially supported by the European Union (ERDF) and "Région Nouvelle-Aquitaine". N. S. (IC2MP) is grateful for Post-doctoral research grant acknowledged by Centre National Scientific Research, France (CNRS, ANR E-CLOCK Project). F. B. H. L. and M. R. C. (IQSC) acknowledge financial support from FAPESP grants 2016/13323-0, 2013/16930-7, and 2014/26699-3, and CNPq grants 309465/2019-2 (F. B. H. L.) and 164293/2017-5 (W. O. S.).

Electronic Supporting Information


Nolwenn Mayet

Karine Servat

K Boniface Kokoh

Teko W Napporn
email: teko.napporn@univ-poitiers.fr

Electrochemical oxidation of carbon monoxide on unsupported gold nanospheres in alkaline medium

Keywords: carbon monoxide, electrocatalysis, gold nanoparticles, cyclic voltammetry, underpotential deposition

) facets. These unsupported NPs were used to study the effect of the upper potential limit and gold oxide formation on CO oxidation in alkaline medium. The results demonstrate that hydroxide species are crucial to oxidize CO, whereas the gold oxides species do not play a critical role.

Introduction

The electrochemical oxidation of organic molecules leads often to the formation of carbon monoxide which is considered as a poisoning specie for various noble metal electrodes like platinum [1]. Therefore, the oxidation of carbon monoxide (CO) becomes one of the most investigated reaction in electrocatalysis in the aim of finding the efficient way for overcoming the electrode deactivation due to its strong adsorption. On Pt electrode, CO electro-oxidation is extensively studied both in acid and alkaline media for understanding the reaction mechanism and to fabricate tolerant and active electrode materials [2]. For this purpose, the use of disseminated noble metal nanoparticles on high specific surface area materials as the carbon substrate is a way to avoid strong adsorption of CO and promote its oxidation. Conversely to platinum which strongly interacts with CO, bulk gold is considered as the most stable and less active metal [3]. Therefore, under standard conditions, the poisoning effect by CO usually evidenced electrochemically on Pt by CO stripping is not commonly experienced on Au.

Indeed, the electrochemical interaction between the surface of bulk gold electrode and CO was earlier studied in 1960s' by Roberts et al. [4] in acid and alkaline media. The important role of hydroxides in the oxidation process was pointed out. In heterogeneous catalysis, the oxidation of CO is widely studied on supported gold nanoparticles. Indeed, a pioneer work by Haruta et al. [5,6] showed that gold nanoparticles (AuNPs) deposited on transition metal oxides (Fe, Co, Ni, Ti) can oxidize CO at a temperature as low as -70 °C. These investigations have confirmed the outstanding properties of gold at nanoscale in heterogeneous catalysis. Carbon monoxide oxidation on the bulk gold surface was studied electrochemically [7][8][9][10][11][12][13]. Particularly, the studies of CO electrochemical oxidation reaction in alkaline and acid media were reported on different low-index gold single-crystals surfaces and on supported gold nanoparticles [7,8,10,[14][15][16][17]. This reaction strongly depends on the electrolyte pH and the structure of the surface [10]. Contrary to platinum or palladium, the molecule of CO weakly interacts with the gold surface [18]. It was shown that CO is irreversibly adsorbed on the (111) plane [15]. The studies concerning the CO oxidation on gold AuNPs reported mostly the effect of the presence of a metallic oxide support such as TiO2, Fe2O3 or Co3O4 [11,16,19]. Moreover, gold nanoparticles are considered as inactive in gas phase towards this reaction when they are unsupported and when the size is higher than 10 nm [11]. Studies in gas phase demonstrated the interaction between hydroxyl species and carbon monoxide towards water gas shift reaction [20]. In electrocatalysis, investigations on unsupported nanoparticles appear as one of the suitable approaches to understand their intrinsic properties towards a reaction. To our knowledge, only two works concern the behavior of unsupported AuNPs towards CO electrochemical oxidation, particularly the effect of the particle coverage of an ITO electrode and the size of the NPs [21,22]. It is also reported that a synergetic effect was observed for this reaction by combining gold with others metals like Ag (Au-Ag) or Pd (Au-Pd) [23][24][25].

In alkaline medium, the presence of CO promotes the adsorption of hydroxide ions (HO -) on the gold surface [7,10,26]. Indeed, the weakly adsorbed CO leads to a shift of the HO - adsorption toward lower potentials. At higher potentials, CO molecules react with co-adsorbed HO -species to form carbohydroxyl species.

In this work, the electrochemical interaction between CO and unsupported spherical gold nanoparticles synthesized by citrate reduction method was investigated in alkaline medium using a rotating disk electrode. Contrary to the already reported works on CO oxidation at gold nanoparticles, the present investigation concerns unsupported gold nanoparticles deposited on a rotating glassy carbon disk electrode. Extensively, the surface of AuNPs was electrochemically evaluated by lead underpotential deposition (Pbupd) as well as the effect of upper limit potential on the oxidation of CO.

Experimental

Synthesis

The AuNPs were synthesized from the method developed by Turkevich [27][28][29]. A solution (19 mL) containing 5×10 -6 mol of HAuCl4 was heated in a flask with an oil bath until 80 °C with vigorous stirring. Then, 1 mL of a 0.5% trisodium citrate solution was added to the flask.

The solution was kept stirred at 80 °C for 30 minutes. Finally, the colloidal solution was cooled down at ambient temperature (20 °C) before its centrifugation. 2 mL of this solution was centrifuged at 10 000 rpm during 10 min. The bottom fraction was washed with ultrapure water and centrifuged at 8000 rpm. The final bottom fraction was used directly as the catalytic ink.

Therefore, 5 µL of this fraction was dropped on the glassy carbon disk. This volume represents 30 µg cm -2 of gold, which covers the surface. After drying with nitrogen, the as-prepared AuNPs electrode is ready for electrochemical characterization.

Physical Characterizations

Transmission electron microscopy (TEM and HR-TEM for high resolution) analysis was performed in a JEOL 2100 UHR microscope operating at an acceleration voltage of 200 kV.

UV-vis spectra was recorded with a spectrophotometer Helios Omega UV-vis/NIR (Thermo Fisher) with quartz cell (optical path = 1 cm). The crystalline structure of samples was studied by X-ray diffraction (XRD) on a PANAnalytical "Empyrean" diffractometer in the 2θ range 20-120° using Co Kα X-ray irradiation source (λ = 0.1789 nm). The colloidal solution of AuNPs was dropped on a single crystal Si (which has a very low background) and dried under nitrogen flow. Indexation of the different phases was determined using HighScore software.

Electrochemical characterizations

The glassware was cleaned with an acidic potassium permanganate solution and then an acidic hydrogen peroxide solution. Afterwards, the glassware was rinsed with hot water and finally rinsed with ultrapure water (18.2 MΩ cm at 20 °C). A three-electrode Pyrex glass cell was employed with a reversible mercury oxide reference electrode then converted as hydrogen reference electrode (the potential was converted as 0.926 V vs. RHE); a glassy carbon slab electrically connected with a gold wire served as counter electrode, while a glassy carbon disk (GC) of 0.07 cm 2 was used as electrode support of the working catalysts. Before each experiment, the GC electrode was polished with alumina 0.5 µm and cleaned in ultrapure water under sonication. A voltammogram of the GC electrode was recorded to verify its cleanliness.

The supporting electrolyte was a 0.1 mol L -1 NaOH solution, deaerated by bubbling nitrogen to remove any dissolved oxygen before each experiment. A nitrogen stream was maintained over the electrolytic solution during the measurement in order to obtain an electrolyte free from oxygen. The electrochemically active surface area (EASA) of each AuNPs thin layer was estimated from electric charge of the reduction of Au oxides monolayer (482 µC cm -2 ) [30,31] in cyclic voltammetry at 50 mV s -1 and in 0.1 mol L -1 NaOH from 0.05 V to 1.6 V vs. RHE. Underpotential deposition of lead (upd of lead) was carried out in 0.1 mol L -1 NaOH + 1 mmol L -1 Pb(NO3)2 from 0.85 V to 0.25 V vs. RHE [32]. The growth of thick oxide layer were carried in 0.1 mol L -1 NaOH. First, linear polarizations were recorded from 0.05 V to Esup (Esup = 1.1 V, 1.2 V, 1.3 V, 1.4 V, 1,5 V and 1.6 V vs. RHE) at 50 mV s -1 . Carbon monoxide oxidation was carried out after bubbling CO during 20 minutes to saturate the electrolyte, and at different electrode rotation speeds (400, 900, 1600 and 2500 rpm) and different scan rates (10, 20 and 50 mV s -1 ). Then, chronoamperometry measurements were performed at Esup during 7200 seconds. Electrochemical experiments were carried out with a potentiostat (PGSTAT-302) interfaced with Nova 1.8 software.

Results and discussion

Physical characterizations

Transmission electron microscopy

The pictures from the Transmission Electron Microscopy (TEM) characterizations of the synthesized AuNPs were shown in Figure 1. The as-prepared AuNPs have a quasi-spherical shape and display twinned structures as shown in Figure 1b. From the TEM image (Figure 1a), about 300 isolated AuNPs were counted to estimate the size distribution (Figure 2) which is in the range from 5 to 35 nm; their mean size is evaluated to 23.04 ± 3.37 nm. First papers on this preparation method have reported size distributions of 1000 nanoparticles that varied between 14.50 ± 1.30 nm and 24.00 ± 2.90 nm according to the reaction parameters [27]. These values are close to the mean size obtained in this work.

UV-visible measurement

Figure 3 shows the UV-visible spectrum of the colloidal solution containing AuNPs. The spectrum displays a single plasmon peak centered at 520 nm. When the size of nanoparticles increases, the peak associated with surface plasmon resonance (SPR) is shifted towards higher wavelengths (red shift) [33]. This single peak shape is typically observed for spherical gold nanoparticles [34,35]. The position of the peak corresponds to a nanoparticles size of about 20 nm, which is confirmed by electron microscopy [36]. The width of the peak indicates the dispersion of the nanoparticles in the colloidal solution. Narrower the width of the SPR peak is, narrower is the size distribution [37]. In the present case, the width of the SPR peak is in agreement with the size distribution on Figure 2. peaks is 0.20. By comparison, the value for bulk polycrystalline gold is 0.53 [38]. This lower value suggests that the (111) plane was the predominant orientation of these prepared AuNPs with revisited Turkevich method. The ratio between the intensities of (220) and (111) planes is 0.08, which is much lower than the value of 0.32 obtained on polycrystalline gold [39]. These values suggest that AuNPs are composed mostly of (111) planes with a low presence of (100) and (110) ones. The CV recorded with Esup = 1.6 V vs. RHE shows the fingerprint of gold with the double-layer region from 0.05 V to 0.9 V vs. RHE, the oxidation surface from 1.1 V vs. RHE and its reduction at a single peak at 1.05 V vs. RHE. When the upper potential limit increases, the charge corresponding to the formation of the oxide increases. This is followed by the increase of the oxide reduction peak during the negative going scan. During the positive scan and until 0.8 V vs. RHE, the surface is not oxidized, and no reconstruction occurs, as mentioned in the literature [40,41]. The adsorption of hydroxide species occurs from 0.3 V to 1.1 V vs. RHE and involves a partial electron transfer [42]. The oxide formation/reduction is a complex and irreversible process. The origin of this irreversibility is attributed to the nature of the oxide film. Indeed, during the oxide growth process it is assumed that dipolar species as Au δ+ -HO δ-are formed, which repel each other, creating a place-exchange reaction [43]. The repulsion raises the energy required to create new dipoles. Hence, an increase in potential leads to an increase of the coverage. This effect explains the fact that the oxide region is a wide plate rather than a sharp peak. However, since the different crystallographic planes have different surface energies, the oxide region strongly depends on it, as the fingerprint of each single crystal is different [40,41]. No electrostatic repulsion occurs during the negative potential sweep, a single sharp reduction peak is observed. The fingerprint of the oxide region is similar to polycrystalline gold. The oxidation peak at 1.25 V vs. RHE is observed on (111) facets and the small peak at around 1.40 V vs. RHE is attributed to (100) facets [44]. Noble metal surfaces can be characterized by electrochemical methods such as underpotential deposition (upd) of monolayer of foreign metals [45]. Upd of lead on gold surfaces is a very sensitive electrochemical method to identify and quantify crystallographic orientations, particularly the low index of Miller facets [32,[46][47][48][49][50]. Figure 6 exhibits the CV of AuNPs in 0.1 mol L -1 NaOH in the presence of 1 mmol L -1 Pb(NO3)2, at a scan rate of 20 mV s -1 . During the negative potential sweep from 0.85 to 0.25 V vs. RHE, a monolayer of lead is deposited at different electrode potentials, on different crystallographic orientations of the surface. During the positive potential sweep, lead desorption occurs reversibly from the electrode surface. The CV obtained is similar to the CVs shown in the literature for upd of lead on spherical gold nanoparticles [46,51,52]. During the deposition step, a wide peak centered at 0.52 V vs. RHE corresponds to (110) facets and defect sites [53]. A second peak that appears at 0.37 V vs. RHE is assigned to (111) facets. Desorption of lead occurs at 0.41 V vs. RHE from (111) facets, at 0.46 V vs. RHE from (100) facets, and at 0.55 V vs. RHE from (110) facets. The desorption peak associated with (110) facets is split, suggesting the presence of different size domains. It is important to mention that (110) facet is itself a stepped surface represented as 2(111)- (111) in TKL (terrace kink ledge) notation [54]. As the above obtained XRD results demonstrated that AuNPs contain mostly (111) planes, the presence of deposition/desorption peaks from (110) can be attributed to the reconstruction of (111) planes. Moreover, weak dissolution peaks under 0.4 V vs. RHE assigned to steps or kinks can be distinguished [53].

X-ray diffraction

Underpotential deposition (upd) of lead on the as-synthesized AuNPs

Underpotential deposition of lead: effect of a thick oxide growth

In order to study the contribution of crystallographic orientation at each oxide formation peaks, upd of lead was carried out after growing oxides at different potentials (1.1 V, 1.2 V, 1.3 V, 1.4 V, 1.5 V and 1.6 V vs. RHE). Deposition of lead is supposed to not occur on oxide species.

The behavior of thick oxide film on gold in alkaline medium is much more complex than in acid medium, which has been largely studied [55][56][57][58][59]. Figure 7 shows the linear polarization recorded prior to oxide growth (polarization at Egrowth during 7200 s). The charge associated with the oxide formation during the growth process were calculated by integrating the chronoamperometric curves (no shown here). The values shown in Table 1 clearly indicate that several monolayers of oxides are formed (1 ML = 482 µC cm -2 ). The values of the number of monolayers increase when the value of the upper potential limit increases, especially at 1.6 V vs. RHE. E growth : Upd of lead was carried out after this oxide growth step, as shown in Figure 8. The oxide region of gold appears as a fingerprint of the crystallographic orientation, like the hydrogen region for platinum [40,41,[60][61][62]. In the literature, cyclic voltammograms of single crystals clearly demonstrate that the surface structure has a strong influence on this region [40,41]. However, a given structure does not give a single peak. All the CVs show the same profile that the CV recorded without oxide growth (reproduced as dashed line), with a lower current intensity and shifted peaks. The effect of a thick oxide is not obvious. However, the peak associated with the desorption from the (100) facet, at around 0.45 V vs. RHE, is not observed. This indicates that (100) planes were probably covered by oxides.

1.6 V 1.5 V 1.4 V 1.3 V 1.2 V I / mA E / V vs. RHE
Moreover, the intensities of the peaks are not the same, indicating the process of oxide growth affected the upd of lead. During the desorption step, the peaks are shifted toward higher potential values, suggesting that the desorption requires more energy than without the growth of oxide layers. For Egrowth of 1.4 V to 1.6 V vs. RHE, the deposition peak attributed to (111) facets has a lower intensity than that attributed to (110) facets. The potential of their desorption peak is shifted of 20 mV toward lower potentials in comparison to the peaks of electrodes polarized at 1.2 V and 1.3 V vs. RHE. This shift is certainly caused by a different size of the (111) domains [46,52,63]. It can be assumed that the oxidized surface has a different surface energy. This can explain the different observed desorption potentials. The facets (111) are more covered by oxides from 1.4 V vs. RHE, contrary to (110) facets. The process of oxide growth may also consist in a reconstruction of the surface.

Carbon monoxide oxidation

Effect of the upper limit potential

Carbon monoxide oxidation was carried out by cyclic voltammetry at different upper potential limits in order to determine the effect of the oxides present at the AuNPs surface. The electrode was first cycled from 0.05 V vs. RHE to Esup (Esup = 0.8 V, 1.0 V, 1.2 V, 1.4 V and 1.6 V vs.

RHE) (see Figure 5). Then, CO was introduced in the electrolyte and the open-circuit potential was recorded (Figure 9). Indeed, the open-circuit potential drastically decreased when CO is introduced and the steady-state starts from 300 seconds. This change evidences that CO has an interaction with the gold surface. In addition, the presence of CO at the gold electrode surface promotes the adsorption of HO -at low potentials [1,26]. Table 2 shows the values of opencircuit potentials recorded before and after 20 minutes of bubbling, according to the upper potential limit applied to cycle in supporting electrolyte. Under inert gas (N2), the open-circuit potential is between 0.81 V and 0.88 V vs. RHE, while under CO, the open-circuit potential shifts in the range 0.17 -0.23 V vs. RHE. The upper potential limit used during cyclic voltammetry has not a significant influence on the open-circuit potential in the presence of CO. The same value has been observed on gold single crystal electrodes for the oxidation of carbon monoxide in alkaline medium. Compared to the results obtained with spherical AuNPs (from 2 to 60 nm) deposited onto ITO electrodes [21], CO oxidation occurs at lower potential at the surface of the present AuNPs. The onset potential of the oxidation of CO in alkaline medium depends on the crystallographic orientation and the adsorption of hydroxide species [26].

Indeed, calculations revealed the stability of hydroxide species following the order: (110) > (100) > ( 111), while the onset potential is lower through the reverse order: (111) > (100) > (110) [10,64]. These spherical nanoparticles mainly enclose (111) facets, which display lower onset potential than (100) and (110) facets. As pointed out above, a large diffusion plateau is observed from 0.6 V to 1.3 V vs. RHE. The current density then decreases from 1.3 V vs. RHE.

During the backward scan, a diffusion plateau starting at 1.25 V vs. RHE is also observed. Two mechanistic approaches are suggested in the literature for the CO electrochemical oxidation on gold in alkaline media. The first one from the group of Koper is described as follows [10,26]:

CO + * = CO ads (1)
CO ads + HO -+ * = CO ads + HO ads -

where * is an active site and rds the rate determining step. The formed carbon dioxide reacts quickly to form carbonate in alkaline medium. In addition, DFT studies suggest that HO and CO enhance each other's adsorption when bonded to nearest-neighbor binding sites on gold surface in alkaline medium [10,26].

The second approach from Roberts et al. [4] considers that interaction between CO and OH -is very fast, as given:

CO + OH - HOCOads + e - (6)
HOCOads + OH - OCO - ads + H2O

OCO - ads + 2OH - CO3 2-+ H2O + e -

The overall equation from this second approach is the following:

CO + 4OH - CO3 2-+ 2H2O + 2e - (9)
This last mechanism never involves the production of a proton compared to Eq. 4. It suggests several hydroxide species around CO for its oxidation on gold surface.

Figure 11 shows the CVs (first and second cycle) of CO oxidation for different upper potential limits (0.8 V, 1.0 V, 1.2 V, 1.4 V and 1.6 V vs. RHE). Before each experiment under CO, the electrode was cycled from 0.05 V vs. RHE to Esup. Below the potentials of gold surface oxidation (below 1.0 V vs. RHE), CO is weakly adsorbed with a fractional coverage θCO < 0.1 [14]. It is known that the adsorption of hydroxide species occurs between 0.3 V and 1.1 V vs. RHE [42]. When the upper potential limit is 0.8 V vs. RHE (Figure 11a), the first cycle is the same than the second one, indicating no change in the surface structure. Furthermore, the first cycle at Esup = 0.8 V vs. RHE displays a lower onset potential than the other voltammograms. CO oxidation occurs even at low CO coverage. When Esup = 1.0 V vs. RHE (Figure 11b), the second cycle of CO oxidation is shifted toward lower potential, indicating the beginning of the surface reconstruction. At higher upper potential limits, when the oxides layer starts to be formed, the changes between the first and second cycles are more obvious. Indeed, more molecules of CO are adsorbed at potentials higher than 1.0 V vs. RHE and the amount of hydroxide species decreases. For Esup ≥ 1.4 V vs. RHE (Figures 11d and11e), the second cycle shifts more than 100 mV toward lower potential and displays a higher current density. More active sites are available for the reaction after the first cycle, which probably enhances the formation of hydroxide species. The current decreases after the diffusion plateau as the formation of oxides occurs. The diffusion plateau occurring at the backward scan has an onset potential coinciding with the reduction of oxides species. This phenomenon indicates the major and crucial role of hydroxide species in the CO oxidation reaction. In addition, the process of oxidation/reduction of gold surface improves the reaction by creating more active sites.

Conclusion

Unsupported spherical AuNPs were successfully synthesized by a revisited Turkevich method.

Particularly, the role of the oxides was investigated. Characterizations such as TEM and XRD

shows that AuNPs are quasi-spherical with diameters of 23.04 ± 3.37 nm and with the predominance of (111) facets. Electrochemical characterizations undertaken with cyclic voltammetry and upd of lead, were employed to reveal the surface state of these AuNPs. For this purpose, electrochemical CO oxidation was investigated in alkaline medium. The role of hydroxide species towards CO oxidation was highlighted. CO oxidation does not occur when the surface is oxidized. However, the reconstruction of the surface and the process of formation/reduction of the gold oxides enhance the CO oxidation in terms of current density and onset potential. Moreover, the onset potential of 0.2 V vs. RHE observed for the CO oxidation with the synthesized AuNPs is close to the values reported for gold single crystals.

This value is much lower than those reported for gold nanoparticles.

Figure 1 :

 1 Figure 1: a) TEM image of the gold nanoparticles, b) HR-TEM of an isolated nanoparticle.

Figure 2 :

 2 Figure 2: Distribution of the nanoparticles diameters (for about 300 nanoparticles).

Figure 3 :

 3 Figure 3: UV-visible spectrum of the colloidal solution containing gold nanoparticles.

Figure 4

 4 Figure 4 shows the diffractogram of the AuNPs. AuNPs are crystallized as face-centered cubic (fcc) gold (JCPDS, file number 04-0784). The ratio of the intensity between (200) and (111)

Figure 4 :Figure 5 :

 45 Figure 4: XRD patterns of AuNPs prepared from the revisited Turkevich method

Figure 5

 5 Figure 5 presents the CVs of AuNPs in alkaline medium recorded at 50 mV s -1 . The CVs were performed at various upper potential limits (Esup = 0.8 V, 1.0 V, 1.2 V, 1.4 V and 1.6 V vs. RHE) to probe the oxidation states of the Au particles surface as function of the electrode potential.

Figure 6 :

 6 Figure 6: Voltammetric Pbupd profile of AuNPs in 0.1 mol L -1 NaOH + 1 mmol L -1 Pb(NO3)2 recorded at 20 mV s -1 at a temperature of 20 °C.

Figure 7 :

 7 Figure 7: Linear polarization on AuNPs from 0.05 V vs. RHE to Egrowth in 0.1 mol L -1 NaOH recorded at 50 mV s -1 and at a temperature of 20 °C.

Figure 8 :

 8 Figure 8: Voltammetric upd profiles of AuNPs in 0.1 mol L -1 NaOH + 1 mmol L -1 Pb(NO3)2 recorded at 20 mV s -1 and at a temperature of 20 °C after polarization at Egrowth. Dashed line represents upd profile without the oxide growth step.

Figure 9 :

 9 Figure 9: Potentiometric curves in 0.1 mol L -1 NaOH recorded before and after the introduction of CO in the cell, corresponding to the evolution of the open-circuit potential.

Figure

  Figure10ashows the cyclic voltammogram for CO oxidation obtained on AuNPs at 50 mV s -1 , from 0.05 V to 1.6 V vs. RHE and Figure10billustrates the positive scan at different rotation

Figure 10 :

 10 Figure 10: (a) Cyclic voltammogram of AuNPs in CO-saturated 0.1 mol L -1 NaOH recorded at 50 mV s -1 , at a temperature of 20 °C and at 1600 rpm; (b) Linear positive scans of AuNPs at different rotation speeds.

Figure 11 :

 11 Figure 11: First cyclic voltammograms of AuNPs towards CO oxidation in 0.1 mol L -1 NaOH recorded at 50 mV s -1 at a temperature of 20 °C and at 400 rpm according to the upper potential limit (Esup): a) 0.8 V, b) 1.0 V, c) 1.2 V, d) 1.4 V, e) 1.6 V vs. RHE. Solid line is corresponding

Table 1 :

 1 Values of charges involved during the process of oxide growth (7200 s) and the corresponding number of the formed oxide monolayers.

	Egrowth / V vs. RHE	Charge / µC cm -2	Number of monolayers
	1.2	3151	6.5
	1.3	3496	7.3
	1.4	4130	8.6
	1.5	5248	10.9
	1.6	18215	37.8

Table 2 :

 2 Open-circuit potential values obtained in N2 and after the introduction of CO, for different values of upper potential limit.

	Esup

/ V vs. RHE OCP under N2 / V vs. RHE OCP under CO / V vs. RHE

	0.8	0.86	0.22
	1.0	0.82	0.21
	1.2	0.81	0.23
	1.4	0.88	0.20
	1.6	0.86	0.17

Acknowledgements

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