39 mol% SiC of ceramic pellets ZrB 2- αSiC and TiB 2- αSiC were synthesized by the reactive hot pressure RHP process at 1850°C under 40 Mpa in vacuum. The XR diffraction displays the absence of other reagents apart from ZrB 2, SiC and TiB 2 confirming the purity of the pellets. The cathodic exploitation of both of them through electrochemical study shows that TiB 2- αSiC is the most active for Hydrogen Evolution Reaction (HER) and Hydrogen Oxidation Reaction (HOR) in 0.5 M of H 2SO 4 solution at room temperature. Moreover, the kinetic exploitation shows that for both pellets the system is controlled by mass transport when they are used as HER. However, in the case of HOR, the system is controlled by the electron transfer.
Proton Exchange Membrane Fuel Cell PEMFC remains one of the competitive methods to produce a renewable energy up today. The combustible hydrogen provides a protected environmental clean energy; it is abundant and lightweight. The product from its oxidation that is water is environmentally benign [
Considering these advantages, setting PEMFC requires however several attentions. We have the hydrogen storage, the efficiency of both electrodes, the tolerability of electrolyte toward electrodes and the efficiency of ion exchange membrane used. At the cathode of the cell, and because of their high activity for the hydrogen evolution reaction (HER), platinum/palladium black and carbon supported platinum or palladium nano-particles are most of the time used [
Conductivity characteristic of ZrB2-αSiC has been developed in previous articles emphasizing its resistance in corrosion even at high temperature (2000˚C), its high conductivity depending on the amount of α-SiC in the sample [
The starting reagents were TiB2, ZrB2 and α-SiC with respectively an average size of 1.2 - 2 µm, 4.7 - 5.3 µm and 0.7 - 1.0 µm. They were used as received with high purity about 98%+. Both ZrB2-αSiC and TiB2-αSiC were prepared according to the same protocol thanks to the route described explicitly in ref.9. Nevertheless, the sintering process has been slightly modified to well improve the density of the pellets. In fact, the sintering was made following a program.
First, during 1 h, the sample is heated with a slope of 15˚C/min from home temperature until 1000˚C. The High pressure 40 MPa equivalent of 1282 kg is then applied on the sample. The temperature dwelling at 1000˚C for 15 min, the heating is then pursued for 1 h always with the slope of 15˚C/min until 1850˚C. Then, this temperature is maintained during 2 hours whence the high pressure is stopped and the cooling starts with the slope of 30˚C/min. This final operation last out 1 h.
After cooling the samples, they are cleaned out of BN that was coated on the graphite die and weighted. Density of 98.7% for ZrB2-αSiC and 99% for TiB2-αSiC were obtained.
After polishing the pellets, a D5000 diffractometer equipped with a Cu as the anticathode and a back mono- chromator was used for characterization. The wavelength used was λ = 1.5406 Å. The diffraction data were collected at a constant rate of 0.02˚ min−1 over an angle range of 2θ = 10˚ - 60˚.
Cyclic voltammetric and polarization were carried out at room temperature in a standard three-electrode cell over a model 362 scanning potentiostat. The scanning was controlled by the software “potentiostat”. The solution was 0.5 M H2SO4 aqueous solution (Aldrich and Millipore MiliQ+ water). It was deoxygenated by bubbling pure nitrogen or argon to chase all trace of oxygen molecule. The reference electrode was RHE and the
counter electrode was glassy carbon. The working electrode was the ceramic ZrB2-αSiC and TiB2-αSiC with geometry area of 18.46 mm2, mounted on a rotating disc electrode RDE (CTV 101) provided by radiometer analytical. First the time, the pellets are surrounded by non conducting sheath (polyethylene, PTFE) constructed so that the faces of the electrode and the sheath are flush and only the face of disc electrode is in contact with the electrolyte solution. The speed of RDE for HER and HOR varies from 0 to 3000 rpm. The reactions on both ceramic catalysts were performed at quasi stationary conditions with sweep rate at 2 mV/s. The hydrogen was provided in the solution with a mass flow controller “Brooks 5850TR” (15 ml/min). Before recording the polarization, voltammery of bar platinum was carried out to notice the purity of the electrolyte with the sweep rate of 50 mV/s. The platinum used was a small disk with 0.15 cm2 as geometric area.
After sintering and polishing the ceramic pellets, they were submitted to XRD diffraction.
Both pellets show that the α-SiC is present with a small amount. It is also shown that no compound apart from the reagents was detected. The patterns also confirm the resistance of reagents at the temperature higher than 1800˚C during high pressure HP sintering. This result has been confirmed by Zhao et al. [
Ceramics pellets have been exploited to perform an electrochemical test. So, the RDE (2 mv∙s−1, 0.5 M H2SO4, room temperature) measurements were performed to evaluate their electrocatalytic properties. The tests were undertaken at different rate between 0 to 3000 rounds per minute (rpm). They were also made both for hydrogen evolution reaction (HER) and for hydrogen oxidation reaction HOR.
Many authors have been interested of the HER over different types of metal used as indicative electrode. So far, a consensus has not been reached on the predominant reaction mechanism for the electrochemical formation of hydrogen molecule resumed by the following reaction:
This reaction is divided up through three electron transfer steps:
The Volmer step (Equation (2)) that is the initial adsorption of the proton is admitted to be the fastest. And the subsequent step relies on two possible routes: the Tafel reaction (Equation (4)) is the homolytic step and the Heyrovsky (Equation (3)) that corresponds to the heterolytic step [
It shows that the hydrogen is produced at high overpotential when scanning in negative side. At 0 rpm regarding the steady-state, the potential is −0.541 V/RHE and −0.413 V/RHE over ZrB2-αSiC (a) and TiB2-αSiC (b) respectively. These values go down when the disk speed is high. They reach −0.417 V and −0.334 V/RHE at the rotation speed of 3000 rpm where the curves are well linear comparing to those at low speeds. This is caused by a lack of hydrogen bubbles formed by HER on the electrode surface that is responsible for the ohmic drop.
Moreover, the comparison of both pellets shows that TiB2-αSiC is the most active since its activity requires low overpotential. Always at the speed of 0 rpm, comparison of both electrodes has been made with the linear voltammetry performed on bare platinum as displayed on
The analysis of the curves was carried out using Koutecky-Levich equation:
where
The
Assuming that n is the total number of electron transferred during the hydrogen evolution, and the kinematic viscosity of the solution admitted to be 10−2 cm2∙s−1 for aqueous solution, we can calculate the diffusion coefficient for each value of B given in
Electrode | Parameters | Potential (V/RHE) | ||||
---|---|---|---|---|---|---|
−0.60 | −0.65 | −0.70 | −0.75 | −0.80 | ||
ZrB2-αSiC | |jk| (mA∙cm−2) | 52.30 | 52.50 | 69.40 | 92.00 | 106.20 |
B | 0.45 | 1.20 | 2.56 | 3.47 | 5.26 | |
D0 (10−6 cm2∙s−1) | 0.85 | 3.70 | 12.00 | 18.00 | 34.00 | |
TiB2-αSiC | |jk| (mA∙cm−2) | 31.80 | 54.60 | 95.40 | 128.50 | |
B | 1.72 | 3.19 | 4.10 | 5.58 | ||
D0 (10−6 cm2∙s−1) | 6.40 | 16.00 | 23.00 | 37.00 |
The electrode geometric surface is 0.18 cm2 for the pellets diameter of 4.85 mm.
It is performed by bubbling and dissolving hydrogen gas through the electrolyte solution (0.5 M H2SO4). For each rate of the RDE, linear voltamogram was carried out from 0 to 3000 rpm. The limited potentials were −0.8 and 0 V/RHE with identical parameters during the HER. The potential sweep was 2 mV/s. The general equation concerning HOR in acidic solution is:
This reaction takes place at the interface through three recognized kinetic steps.
And the Volmer step is
Nevertheless, we can note the diffusion step of molecular hydrogen from bulk solution to the electrode surface
The
limited by the same current density (8.4 mA∙cm−2) from −0.508 V to 0.0 V/RHE although the rate of the rotating disk increases. This remark shows that the curves don’t depend on the RDE rotation rate. Therefore, the system is not controlled by diffusion. Thus, the concentration of the dissolved hydrogen is the same in the bulk solution and at the electrode surface, regardless the electrode reaction.
The limit current recorded for each RDE rate is 4.91 mA∙cm−2. In this case, it is impossible to determine the Levich plots. Therefore, there is no proportionality between the limit current and the speed of the RDE. Then, to determine the kinetic parameters for ceramic electrodes during HOR,
In this study, non-oxide ceramic pellets ZrB2-αSiC and TiB2-αSiC with 39 mol% SiC were synthesized by hot pressure sintering methods at 1850˚C. The DRX performed on them shows the presence of only all raw materials used as ZrB2, SiC and TiB2 confirming their non-destruction and their purity. The electrocatalytic activity of both pellets was determined as rotating disk electrode for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) for rotating speed comprised between 0 and 3000 rpm.
For HER characterization, we discover that the TiB2-αSiC electrode is more active than ZrB2-αSiC with production of hydrogen for an overpotential of −0.413 V/RHE and the current drop occurs by 0.8 V/RHE at a steady state in H2SO4 0.5 M solution. Moreover, the comparison with bare platinum with the same parameters reveals that the platinum remains the most active electrode with −0.035 V/RHE of overpotential.
Regarding HOR reaction, we found out that with both electrodes, the rotating disk is useless as the limit current observed remains the same regardless of the disk speed. It means that the determining step is not governed by diffusion.
On behalf of the resistance of both pellets in acidic middle, we need to vary the amount of αSiC in the compounds to increase the electrical conductivity. This will certainly reduce the overpotential for hydrogen evolution and make these ceramic electrodes better candidates as PEMFC electrodes than Pt electrode.
KafoumbaBamba,NahosséZiao, (2016) Cathodic Using of ZrB2-αSiC and TiB2-αSiC for PEM Electrolysis and Water Electrolysis at Low Temperature. American Journal of Analytical Chemistry,07,1-11. doi: 10.4236/ajac.2016.71001