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Synthesis of the carbon-supported electrocatalysts by the polyol method
The studied carbon-supported electrocatalysts were prepared using a modified polyol method. In a typical procedure, 100 mg of 10 or 20 wt. % metal loaded electrocatalysts was prepared. The calculated amount of metal precursors, H2PtCl6, RhCl3.xH2O and SnCl2 (Aldrich), was dissolved in a 20 mL solution containing milli-Q water and ethylene glycol (EG) (volume ratio 1:1), prior to the addition of carbon black particles (Vulcan XC-72R, Cabot) dispersed in 20 mL of the same mixture (10 mL EG, 10 mL water) by sonication. 20 mL EG was then added to the solution in order to get in the end a 2:1 EG:water ratio. The pH of the whole solution was subsequently adjusted to pH = 12 using a 0.5 M NaOH solution (diluted in EG + water (1:1)) and let under vigorous stirring for one hour at ambient temperature under argon atmosphere. Thereafter the solution was heated up to 160°C (still under argon atmosphere), maintained at this temperature for three hours and cooled down overnight in air. The pH of the solution was then fixed to pH = 3 using a 0.5 M H2SO4 aqueous solution and stirred for 24 h. Finally, the electrocatalyst powder was filtered, washed copiously with milli-Q water and dried overnight in an oven at 80°C.
A considerable advantage of this colloidal method is its high simplicity and the ability to control the size of the nanoparticles.
The polyol synthesis requires the presence of only one chemical, ethylene glycol, which plays the role of the solvent and of the reducing agent. In our procedure, milli-Q water was added to the process as it was found that additional water was helping CB dispersion and the dissolution of the metal salts. No additional stabilizing agent is needed in the synthesis solution to prevent from the possible nanoparticles agglomeration, as it can be found for other colloidal syntheses such as the Bönnemann method [127–132] or methods using borohydride as reducing agents [111,133–136]. Although this method is well employed [123,137–155], very few studies focused on the reactions occurring during the synthesis [148,153–155].
Heating up the synthesis solution to 130°C (ethylene glycol boiling point) engage the solvent oxidation into glycolic acid which remains stable in the alkaline solution as glycolate anions. The reaction frees one electron per oxidized ethylene glycol molecule which is further used in the reduction of the metal salts in zerovalent metal atoms. The stabilization of the metal colloids is believed to be carried out by the glycolate anions [143,148].
Diverse temperatures from 130°C to 180°C are reported in the literature [123,139,143,152] for the synthesis. No accurate control of the temperature is usually operated as this factor is believed to be neutral. A home study investigated the effects of the temperature during the synthesis on the electrocatalysts state and concluded that this parameter does indeed not impact the nanoparticle size or the metal loading [156]. Nonetheless, a study from Fievet et al. [155] shows that the particles size decreases with the temperature increase. The effects of the temperature were however investigated above 160°C. Besides, it must be precised that the particles size in this study was comprised between 0.1 and 1 µm i.e. well above the diameters reached here. No similar study on the temperature effect on the nanoparticles size was found in the literature.
In colloidal synthesis methods in general, the nucleation phase initiates after the metal atoms concentration reaches a “supersaturation concentration” during the metal salts reduction and should take place in a very short time interval (almost instantaneously) to ensure the particles size homogeneity [157]. After the formation of the nuclei, the metal atoms concentration drops below a concentration threshold. From this moment, the number of nuclei remains constant while they start growing until the consumption of all the zerovalent atoms.
The polyol method is no exception and follows the same principle: The first step (the reduction of the metal salts resulting in the formation of zerovalent metal atoms) occurs after an induction time after the temperature is raised to 160°C (temperature chosen in the protocol). Once the metal atoms supersaturation is reached, the nucleation step initiates and stops before the nuclei starts growing until consumption of all the metal atoms in the solution. These three steps (metal salts reduction, nucleation and nuclei growth) all occur at 160°C. The three hours fixed in the protocol (see above) correspond to a sufficient duration for the reduction of the metal salts and the growth of the nuclei, and can be found in many studies [138,143,144,146]. However, alike the temperature, this parameter varies in the literature from two to five hours [142,145].
The adsorption of the formed nanoparticles on the carbon support takes place afterward during the overnight cooling step. After the cooling procedure, another step was added to the polyol method, which consists of lowering the pH to pH = 3, likewise to the protocol in [144] in order to improve the nanoparticles adsorption. Oh et al. investigated the effects of the pH on the zeta potential of the carbon support and Pt nanoparticles [144]. They found out that, after the synthesis at pH = 12, the zeta potential of the carbon support and the metal nanoparticles is negative. However, after the pH adjustment down to pH = 3, the zeta potential of the carbon support changes and becomes positive while the zeta potential of the nanoparticles remains negative. The authors assumed that the glycolate anions are more strongly adsorbed on the nanoparticles than on the carbon support, which would hinder the modification of the nanoparticles zeta potential. As a consequence, the authors believed that it could help the electrostatic attraction of the nanoparticles with the carbon support and thus render their adsorption more homogeneous on the carbon surface. Other publications added similar steps in their synthesis protocol but without explaining the reason [123,146].
The gas environment of the synthesis solution is also an influent parameter which impacts significantly the yield of the total deposited nanoparticles mass on the carbon support. Oh et al. [143] discovered that when the complete synthesis (at 160°C and after at room temperature) is carried out in an inert gas atmosphere, the control of the nanoparticles size is good but the metal loading on the carbon support poor. On the opposite, an entire process in open air results in a good metal loading but in the presence of agglomerates on the carbon support. Finally, the optimum result (good particle size control + good metal loading) was obtained in inert gas during the three-hour step at 160°C and in open air during the next steps at room temperature. No conclusions were drawn related to the impact of the gas environment on the reactions occurring during the synthesis.
A significant advantage of the polyol process is the good control on the metal nanoparticle size by pH adjustment. The decrease of the nanoparticles size by increasing NaOH concentration (and a fortiori the pH) was already reviewed in the literature [143,148,154,158]. According to Fievet et al. [154], an increase of the pH by NaOH addition in the synthesis solution leads to faster kinetics of the metal salt reduction, which would result in the formation of a larger number of nuclei. As a consequence, the nuclei growth would be hindered. pH = 12 was chosen for the experiments in the present study in order to hinder the formation of agglomerates (and the decrease of the electrocatalyst specific area) which could occur at low pH and the formation of too small nanoparticles (and their increased stability on the carbon support) at too high pH (for the sake of concision, the impact of the nanoparticles size on electrochemical reactions is introduced in section III).
This synthesis method was finally chosen among others due principally to its simple operability, the use of only one chemical (EG) as reducing and stabilizing agent and the good control of the nanoparticle size.
Physical characterisations
ICP-AES
Inductively coupled plasma atomic emission spectroscopy (ICP-AES) enables the quantitative determination of elemental chemical compounds present in a solution. Two distinct elements constitute the ICP-AES: the plasma and the photodetector.
The ignition of an argon gas constrained in a strong electromagnetic field results in inelastic collisions between argon neutral atoms and ions, which in return gives rise to a stable plasma, the temperature of which is in the order of 7000 K.
A solution containing the sample is pumped into the ICP device where it is evaporated by a nebulizer before being introduced inside the plasma. There, the sample molecules/atoms/ions enter in collision with the ions and electrons constituting the plasma and break into exited ions which further stabilize after photon emission. The emitted radiation, unique for each element, is then analyzed and quantified by a photodetector.
The ICP-AES analysis was used to determine the metal-carbon and metal-metal ratio in the bi- and tri-metallic electrocatalysts. The device employed for the measurements was a iCAP 6300 Thermo.
Thermogravimetric analysis
The thermogravimetric analysis (TGA) allows recording continuous material mass changes as a function of the temperature during a thermal treatment (the temperature usually varies linearly against the time at ca. 5-10 K min-1). The pressure and the atmosphere (air, inert gas…) in the analysis chamber are also important parameters which impact the profile of the material mass evolution.
The mass variations of the sample, placed on a micro-balance, are recorded by measuring the voltage required to maintain the micro-balance in its initial position. The variation of the balance position is determined by a photo-sensor which permits to adjust the voltage response of the induction coils located at two extremities of the micro-balance. The voltage is then converted to mass during the experiments.
Concretely, the metal loading on the carbon support of the synthesized electrocatalysts was measured by TGA. Due to the high melting point of the investigated metals (Pt and Rh melting point are over 1500°C), only the carbon mass loss can be measured and thus the carbon-metal ratio estimated (the maximal temperature reached by the device is ca. 1200°). For tin oxide containing electrocatalysts (Pt-SnO2/C, Pt-Rh-SnO2/C), this ratio could not be estimated due to tin low melting point (T ≈ 200°C). Thus, the use of ICP-AES was primordial for the knowledge of the metal-metal and carbon-metal ratio. The TGA analyses were carried out with a Q 5000 from TA Instruments apparatus.
Transmission Electron Miscroscopy
The principle of the transmission electron miscroscopy (TEM) relies in electrons-atoms interactions. Depending on the TEM mode, information related to the studied sample either concerns its topography or its chemical contrast.
Primary electrons are generated by a thermo-electronic source, typically a tungsten or a LaB6 filament, and expulsed from the anode with an energy up to ca. 200 kV. The beam is then directed through a column where its focus is ensured magnetically by the presence of one or several condensers, before being finally directed on the sample by the objective lenses. The primary electrons pass trough the sample owing to its small thickness (< 100 nm). The imaging contrast is ensured by the the thickness of the sample, its chemical composition (in this case, between the Pt nanoparticles and the carbon support) and by the sample cristallinity (deflection of the inciding electrons on the sample diffraction planes).
Other analyses such as the energy dispersive X-ray spectroscopy (X-EDS or EDX) are enabled by TEM. The collision of primary electrons with highly energetic electrons of the sample atoms can engender the ionization of the latter. The hole in their valence band will be replaced by an electron of an upper layer which will in return emit energy in the form of a photon. This energy in the X-range will subsequently be detected by a sensor. Information on the chemical composition of the sample is thus made possible since each atom has its own X-ray spectrum.
The preparation of the samples is trouble-free and only consists of dipping a properly-cleaned copper grid baring a thin carbone membrane inside the studied electrocatalyst powder. A Jeol 2010 TEM was employed for this characterization (1.9 Å point-to-point resolution at 200 kV).
Fig. 5.TEM Scheme [159].
Determination of the particle size distribution (PSD) was performed from TEM imaging. This evaluation was performed measuring manually the diameter of ca. 400-500 nanoparticles visualized on ca. 20 different TEM images randomly taken on the sample, which usually provides data with sufficient statistics. Those images were photographed in four distinct areas of the studied sample (five images per sample area), which themselves were shot at × 200 000 magnification.
For each sample, the number-averaged (dN), the surface-averaged (ds) and the volume-averaged diameter (dV) were calculated as follows: with ni the number of nanoparticles and di their respective diameter.
The knowledge of these three parameters is essential as it enables verifying that the surface- (dS) and volume-averaged diameter (dV) match with dElec and dXRD respectively (introduced hereafter).
X-Ray Diffraction (XRD)
X-Ray Diffraction (XRD) permits identifying the crystalline structure of the studied sample. The XRD analysis was also used to evaluate the volume-average cristallite size and the lattice parameter of the corresponding electrocatalysts.
The X-radiation originates from the violent collision of electrons, bombarded from a tungsten filament, with a copper anode emitting heat and X-rays in the process. More precisely, these X-rays correspond to the photons emitted by the electrons from the L shell replacing the vacancies let by the electrons from the K shell ejected from their Cu atoms as a result of the W filament electrons collision with the copper anode. The electrons from the other electron shells (M, N…) emit too-low-energy photons to be functional (they are rapidly absorbed in the air). Only the Kα and Kβ X-photons can be detectable at the exit of the X-ray tube. A monochromator is placed between the source and the sample in order to select the photons with the highest energy (Kα: Kα1 and Kα2). The incident X-photons issued from the source collide the atoms of the sample and are diffracted following three conditions:
– The 1st Snell-Descartes law should be respected: incident and diffracted rays and the line perpendicular to the crystallographic planes should be contained in the same plane.
– The angle (θ) between the incident rays with the crystallographic planes and between the diffracted rays with the crystallographic planes (θ’) should be equal (θ’ = θ).
– The diffraction angle (θ) should verify Bragg’s law (Eq.II.4): Eq.II.4 with λ the wavelength of the incident ray, n the diffraction order and dhkl the distance between the crystallographic planes with (hkl) orientation (h, k and l are also known as the Miller indices) of the crystalline lattice (see Fig. 6).
Finally, the diffracted X-photons are collected by a detector which also forms a θ angle with the crystallographic planes. During the acquisition, the sample support is rotated on itself so that the different sample diffraction planes can be detected.
A Bruker AXS D8 diffractometer was used to analyze the studied electrocatalysts from 2θ = 15° to 90° with a scan rate of ca. 0.74° min-1.
Fig. 6. X-ray diffraction scheme on the crystallographic (hkl) planes.
The average cristallite size was estimated using Scherrer’s equation: Eq.II.5 with λKα the wavenumber of the incident ray and θmax the angular position and B the width at half-maximum of the (111) and of the (220) peaks.
The (111) and (220) diffraction peaks were used because of their high intensity. Inaccuracies can be found in the determination of B due to the possible overlap of the carbon support peak and the Pt (111) diffraction peak. In case of a sample with a large nanoparticle size distribution, the average size value can be biased because of the presence of the bigger nanoparticles, which impacts the diffraction peaks width. Accurate determination of the average size may also render difficult or even impossible in case of exclusive presence of very small nanoparticles, which will broaden the diffraction peaks.
Discrepancies between dV with dXRD can regularly be witnessed and often reflect the formation of heterogeneous nanoparticles (in shape, size…), which can only be observed by TEM. Due notably to the weak reflection by small nanoparticles of the incident X-ray beam, XRD spectra are more sensitive to the presence of small fractions of larger nanoparticles in the sample, which results in the formation of sharper diffraction peaks.
Electrochemical characterization
Three electrode assembly
A three electrode assembly was used to carry out electrochemical experiments. The system is constituted with a working electrode (WE), a counter electrode (CE) and a reference electrode (RE). The electrochemical reactions of interest generally occur at the working electrode. The cell voltage is generally measured as the difference between the WE potential and the RE potential. The latter should not be polarized so as to provide a reliable comparison to the WE and should be located as close as possible from the latter in order to minimize the ohmic drop engendered by the electrolyte resistance. Its position should however not interfere with the ions/molecules mass-transport. The CE serves only as a current sink. In other words, it helps the current flow through the cell. The reactions occurring at the counter electrode are usually of no concern (in aqueous electrolyte, it is usually predominantly H2 or O2 evolution).
Table of contents :
Chapter I. Introduction on the Direct Ethanol Fuel Cell
I.1. Context
I.2. Brief history of fuel cells
I.3. Overview of a fuel cell
I.3.1. Overall operation
I.3.2. Fuel cell components
I.4. Electrochemical reactions: DEFC
I.4.1. Anode side
I.4.2. Cathode side
I.4.3. Global reaction
I.5. Energy efficiency
I.5.1. Thermodynamic efficiency
I.5.2. Potential efficiency
I.5.3. Faraday yield
I.5.4. Overall energy
I.6. Ethanol oxidation reaction
I.6.1. Problematic
I.6.2. Mechanism of the EOR in acidic medium on platinum
I.6.3. Ethanol adsorption modes
I.6.4. Ethanol dehydrogenation
I.6.5. C-C bond splitting
I.6.6. CO-stripping
I.6.7. Acetaldehyde oxidation reaction
I.6.8. Acetic acid or ethyl acetate
I.6.9. Pt-M alloys
I.6.9.1. Pt-SnO2/C
I.6.9.2. Pt-Rh/C
I.6.9.3. Pt-Rh-SnO2/C
I.7. Overview of the investigations carried out in this thesis
Chapter II. Experimental section
II.1. Synthesis of the carbon-supported electrocatalysts by the polyol method
II.2. Physical characterisations
II.2.1. ICP-AES
II.2.2. Thermogravimetric analysis
II.2.3. Transmission Electron Miscroscopy
II.2.4. X-Ray Diffraction (XRD)
II.3. Electrochemical characterization
II.3.1. Three electrode assembly
II.3.2. Cyclic voltammetry
II.3.3. Chronoamperometry
II.3.4. Rotating Disc Electrode
II.3.5. Evaluation of the electrochemical active surface area
II.3.6. Normalization of the current
II.4. Coupled physical and electrochemical techniques
II.4.1. In situ Fourier Transform InfraRed spectroscopy
II.4.2. Differential Electrochemical Mass Spectrometry (DEMS)
II.4.2.1. Basics of operation
II.4.2.2. MS calibration
II.5. Conclusions
Chapter III. Ethanol Oxidation Reaction (EOR) investigation on Pt/C, Rh/C, and Pt-based bi- and tri-metallic electrocatalysts: a DEMS and FTIR study
III.1. Influence of the Pt/C electrode structure on the EOR
III.1.1. Introduction
III.1.2. Physical characterization
III.1.3. Effect of the mass-transport
III.1.4. Influence of the electrocatalyst thickness
III.1.4.1. Thickness determination
III.1.4.2. Effect of the thickness on the EOR
III.1.4.2.1. DEMS
III.1.4.2.2. RDE
III.1.5. Effect of the scan rate
III.1.5.1. CO-stripping CVs
III.1.5.2. EOR CVs
III.1.6. Discussions
III.1.7. Conclusions
III.2. Effect of Rh- and Sn- addition on the Pt-based electrocatalyst on the EOR
III.2.1. Physical characterization
III.2.2. CV in base electrolyte
III.2.3. CO stripping CV
III.2.3.1. On-line DEMS
III.2.3.2. In situ FTIR
III.2.4. Comparison of the in situ FTIR and on-line DEMS measurements
III.2.5. Ethanol electrooxidation
III.2.5.1. EOR on Pt/C studied by in situ FTIR
III.2.5.2. Comparison with on-line DEMS measurements
III.2.5.3. In situ FTIR – EOR on Pt- and Rh-based electrocatalysts
III.2.5.4. Comparison in situ FTIR and on-line DEMS measurements
III.2.6. Conclusions on the addition of transition metals to platinum
Chapter IV. Influence of H- and OH-adsorbates on the ethanol oxidation reaction – A DEMS Study
IV.1. Influence of adsorbates on the oxidation of organic molecules
IV.2. Hydrogen and hydroxide adsorption procedure
IV.3. Potentiodynamic ethanol oxidation reaction
IV.3.1. On Pt/C
IV.3.2. On Rh/C
IV.3.3. On Pt based bi- and tri-metallic electrocatalysts
IV.4. EOR comparative study between the electrocatalysts
IV.5. CO2 current efficiency
IV.6. Zoom on the CA at Ead = 0.05 V vs. RHE
IV.6.1. On Pt/C and Rh/C
IV.6.2. On Pt-based multi-metallic electrocatalysts
IV.7. Potentiodynamic acetaldehyde oxidation reaction
IV.7.1. On Pt/C
IV.7.2. On Rh/C
IV.7.3. On Pt-based electrocatalysts
IV.8. Acetaldehyde potentiostatic adsorption at Ead = 0.05 V vs. RHE
IV.9. Conclusions
Chapter V. Mass spectrometric investigation of ethanol and acetaldehyde adsorbates electrooxidation on Pt- and Rh-based electrocatalysts
V.1. Ethanol and acetaldehyde adsorbates electrooxidation
V.2. Ethanol and acetaldehyde adsorbates formation
V.3. On Pt/C
V.3.1. Ethanol adsorbates stripping
V.3.2. Acetaldehyde adsorbates stripping
V.3.3. Discussion
V.4. On Rh/C
V.5. On Pt-based electrocatalysts
V.5.1. Ethanol adsorbates stripping
V.5.2. Acetaldehyde adsorbates stripping
V.6. Conclusions
Chapter VI. Influence of the temperature for the ethanol oxidation reaction (EOR) on Pt/C, Pt-Rh/C and Pt-Rh-SnO2/C
VI.1. Influence of the temperature on the EOR
VI.2. Physical characterization
VI.3. CVs in supporting electrolyte
VI.4. COad monolayer electrooxidation
VI.5. Potentiodynamic EOR
VI.6. Tafel plot
VI.7. Apparent activation energy
VI.8. Conclusions
Discussion, conclusions and prospects
Appendix.
A1. CV in supporting electrolyte
A2. CO stripping
A3. Potentiodynamic ethanol oxidation
A4. Potentiostatic ethanol oxidation
A5. Conclusions
Literature references
