STIMULATED-HEALING OF PROTON EXCHANGE MEMBRANE
FUEL CELL CATALYST
R. Latsuzbaia1, E. Negro1, G. J. M. Koper1
1Department of Chemical Engineering, TU Delft, Julianalaan 136, NL-2628BL, Delft - e-mail:
G.J.M.Koper@tudelft.nl
Keywords: fuel cell, platinum nanoparticles, self-healing, dissolution, deposition.
ABSTRACT
Platinum nanoparticles, which are used as catalysts in Proton Exchange Membrane Fuel Cells (PEMFC), tend to degrade after long-term operation. We discriminate the following mechanisms of the degradation: poisoning, migration and coalescence, dissolution, and electrochemical Ostwald ripening. There are two ways to tackle this problem. The first option involves formulation of durable catalyst, which can resist harsh fuel cell conditions, and this is the conventional route. The second option is reactivation by dissolution and then redeposition of the catalyst nanoparticles, which is an unprecedented method for platinum catalyst regeneration/stimulated-healing and the one we shall discuss.
Dissolution of platinum can be achieved electrochemically, by potential cycling of the fuel cell electrode impregnated with platinum nanoparticles in oxygen enriched acidic electrolyte according to following reactions [1]:
Pt + H2O→PtO + 2H+ + 2e- (1)
PtO + 2H+→Pt2+ + H2O (2)
During the potential cycling, platinum oxides are formed at each positive cycle and subsequently dissolved as platinum ions in the electrolyte on the negative cycle. These cycles are alternated continuously. The partial dissolution of platinum nanoparticles results in a decrease in particles size and oxidation of the poisonous species on the platinum surface. The process of dissolution is monitored in-situ via cyclic voltammetry technique. The concentration of dissolved platinum is measured with Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES).
The next step of the regeneration procedure is redeposition of the dissolved platinum back onto the carbon support of the fuel cell electrode. This can be realized by means of electrodeposition. A negative potential is applied to an electrode from where the platinum was dissolved and this results in a reduction of the dissolved platinum ions. Regenerated nanoparticles are characterized by AFM, TEM and XRD. The activity of the catalyst will be checked via voltammetric techniques.
1. INTRODUCTION
Price and durability remain major barriers for full commercialization of Proton Exchange Membrane (PEM) fuel cell technology. The main cause of these obstacles is the degradation of the fuel cell electrodes, and particularly the fuel cell catalyst. Therefore, a stimulated-healing strategy would offer a possibility to prolong the lifetime time of PEM fuel cells and to reduce their operation price.
2. MATERIALS
A fuel cell electrode was employed as a working electrode (WE) for most dissolution studies. The working electrode (20x10 mm) was prepared by spraying catalyst ink (44% Pt, 33% Carbon Black, 23% Nafion®) on carbon paper (Toray TGP-H-060)
reaching platinum loading of 0.5 mg cm-2; then the electrode was dried at 110 0C for one hour. Commercial JM Hispec 9100 was used to prepare the catalyst ink. Carbon paper of the same dimensions (without catalyst layer) was used as a counter electrode. The carbon paper was chosen as a counter electrode to ensure the dissolution of platinum only from the working electrode. A carbon electrode can be used as a counter electrode instead of platinum because hydrogen evolution is also possible on a carbon surface, however rates are much lower than on platinum [2, 3], for which reason the surface area of the carbon CE during the dissolution experiment should be several times larger than active surface area of the WE. A reversible Hydrogen Electrode (RHE) was used as a reference electrode. For all dissolution experiments 0.1 M HClO4 and x M HCl (x= 0, 0.1, 03, 0.5M) solutions were used as an electrolyte.
The electrodeposition experiments were carried out on Highly Oriented Pyrolitic Graphite (HOPG) in 0.1 M HClO4 with 1 mM H2PtCl6 at room temperature (21-22 0C). 3. METHODS
Platinum dissolution experiments were performed in a three-electrode electrochemical cell using the potential cycling technique. The potential was cycled between 0.45 and 1.1 V vs. RHE with a scan rate of 50 mV s-1. The influences of the chloride-ion concentration and of the electrolyte temperature on the dissolution rate of platinum were investigated. ICP-OES was used to determine concentration of dissolved platinum. After 800-1000 cycles the experiments were terminated.
The electrodeposition technique was employed for the redeposition procedure [4]. Platinum deposition was performed in a three-electrode cell on a HOPG working electrode. A platinum wire was used as counter electrode, and a RHE as reference electrode. The particle size and dispersion over the HOPG surface were studied with AFM.
4. RESULTS
The investigations on a possible strategy for a stimulated-healing of fuel cell electrodes have been performed ex-situ in an electrochemical cell using voltammetric techniques. The main aim of the study is optimization of platinum catalyst dissolution and redeposition for the present purpose. Therefore, the project is divided in two parts: dissolution and redeposition studies.
It was reported before that traces of chloride [5], the pH[6], the presence of oxygen [1] and high temperature [6] in the electrolyte accelerate platinum dissolution. However, most of the work has been done under simulated fuel cell conditions with the aim of studying catalyst degradation mechanisms, whereas in our investigations we tried to reach the opposite with the highest possible level of harsh conditions applicable in a fuel cell in order to achieve optimal catalyst dissolution. We studied dissolution of platinum at high chloride concentrations (0.1, 0.25 and 0.5 M HCl in 0.1M HClO4), at different temperatures. Cyclic voltammetry was used to observe the decay of the electrochemical surface area (ECSA) or hydrogen desorption peak
current of the platinum catalyst, see Figure 1, where both indicate the dissolution of the catalyst.
Figure 1: Decay of the hydrogen desorption cathodic peak current.
In our first investigations the fast initial current decay was increasing with increase of chloride concentration up to 0.5 M as well as with increase of temperature up to 55 0C.
During the dissolution experiments in the 0.1M HCl + 0.1M HClO4 at room temperature about 7.5 % of the platinum catalyst was dissolved, see Figure 2. Further investigations are required to measure the amount of platinum dissolved in harsher conditions.
Figure 2: ICP-OES analysis of the platinum dissolution in an electrolyte solution of 0.1M HCl + 0.1M HClO4, at 22 0C, 0.45-1.1 V potential cycling at 50 mV s-1. Number
of cycles: 0, 100, 400, and 700.
Electrodeposition was considered as one of the options for the deposition of dissolved platinum. The HOPG was used as a model support to study the morphology of the electrodeposited particles by AFM. Monodisperse nanoparticles of 30 nm diameter and 8 nm height were deposited and well dispersed on the support, see Figure 3, from the solution of 1mM H2PtCl6 and 0.1M HClO4. Reduction potential of 0.08 V was applied for 0.2 s to deposit the platinum from the electrolyte solution. The procedure still needs to be optimized to get smaller nanoparticles.
Figure 3: AFM image of platinum nanoparticles electrodeposited 1mM H2PtCl6 and 0.1M HClO4 mixture, cathodic pulse of 0.08V for 0.2 s.
5. CONCLUSIONS
Our first investigations on catalyst dissolution demonstrate that platinum can be dissolved electrochemically at a reasonable rate and that increasing the Cl -concentration or temperature can increase dissolutions rates. The optimum conditions still need to be identified. The dissolved platinum can be redeposited electrochemically resulting in small and monodisperse nanoparticles. As a result, the optimized conditions for both processes could be applied in a real fuel cell setup thus achieving the stimulated-healing of PEMFC catalyst.
ACKNOWLEDGEMENTS
Financial support from IOP Self-Healing Materials (Agentschap NL, Ministry of Economic Affairs) for this study (Project No: SHM1050) is gratefully acknowledged.
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