The Nafion®-Pt/C electrocatalysts prepared as described in the previous chapter were characterised electrochemically in a half-cell rotating (ring-) disc electrode (RDE/RRDE) configuration using liquid electrolytes to determine their electrochemical surface area (ECSA), investigate their durability and measure their catalytic activity and selectivity towards the ORR.
5.1 Aims and objectives
The aims and objectives for the electrochemical characterisation of the Nafion®-Pt/C catalysts produced in this work are as follows:
• To develop a reproducible method for the preparation of thin-film catalysed working
electrodes for RDE studies, from a range of catalyst materials both commercial and in-
house prepared.
◦ Catalyst films should be homogeneous and cover the entire area of the disc.
◦ For a given catalyst, variations in ECSA, Jlim and specific activity should be <10%
between identically-prepared electrodes.
• To develop reliable, reproducible methods for the electrochemical characterisation of electrocatalyst materials:
• The ECSA and ORR activities measured for bulk polycrystalline Pt and commercially-
available Pt/C catalyst should agree quantitatively with those reported in references [109]
and [108].
• To apply these methods for the characterisation of Nafion®-Pt/C catalysts, including: • Measurement of ECSA, O2 mass transport parameters (diffusion coefficient), ORR
specific and mass activities and reaction mechanisms
• To assess the viability of determining optimal Nafion® content for maximum catalyst utilisation using the RDE technique with high catalyst loadings on the working electrode.
• To conduct measurements at elevated temperature (60-70 ºC), to simulate PEMFC operating conditions and allow comparison of ORR activities between RDE and in-situ tests.
5.2.1 Hardware
The experimental setup employed for the electrochemical characterisation of electrocatalysts in this work is shown in Figure 5.1. Electrochemical measurements were carried out using an Autolab PG302N potentiostat with an FRA2 impedance module, a SCAN250 analogue scan generator and a BA bipotentiostat/array. The electrochemical cell was enclosed in a grounded Faraday cage.
5.2.2 Selection and preparation of materials
A jacketed glass electrochemical cell was cleaned by soaking in concentrated nitric acid for several hours. The cell was then rinsed thoroughly and boiled for 1 hour in UHQ water.
A 0.1 M HClO4 electrolyte solution (prepared from 70% TraceSelect, Sigma and UHQ water, 18.2 MΩ.cm Millipore) was de-oxygenated by purging with ultra-pure nitrogen (BOC) for 20 minutes. For this work, a reversible hydrogen electrode (RHE) was prepared in-house. The detailed procedures for the preparation and maintenance of these electrodes are described in the appendix, but briefly the electrode consisted of a platinised Pt gauze (300 mesh) sealed in a glass pipette, which was then filled with 0.1M HClO4. A suitable volume of hydrogen was generated on the Pt gauze by electrolysis. Unless otherwise indicated, all potentials in this work are referred against the RHE. Counter electrodes for this study were made from platinised Pt gauze (300 mesh, Alfa Aesar) with surface areas of >10 cm2,which were flame-annealed prior to experiments to remove impurities.
Figure 5.1: Experimental setup employed for the ex-situ electrochemical characterisation of electrocatalysts in this work.
5 Electrochemical characterisation of Nafion®-Pt/C catalysts
5.2.3 Preparation of working electrodes
Glassy carbon RDE tips (5mm diameter, geometric surface area A=0.196 cm2) enclosed in PTFE or PEEK (for high-temperature experiments 70ºC) were purchased from Pine Instruments (USA). The electrodes were polished for 5 mins on Microcloth™ (Buehler) using 1 μm, 0.3 μm and 0.05 μm alumina slurries in sequence. The polished electrodes were rinsed with UHQ water, then sonicated (40 kHz bath) for 3 minutes in UHQ water to remove any residual alumina.
A known volume of catalyst ink was deposited on the glassy carbon (GC) disc to produce a Pt loading between 20 – 100 µgPt.cm-2 as required, and dried at room temperature either in a vacuum oven, or under ambient conditions whilst rotating at 200 - 500 RPM on an inverted RDE rotator, depending on which method produced the most homogeneous catalyst film for a particular ink.
5.2.4 Electrochemical methods
The catalysed working electrode was mounted in an RDE assembly (ASR, Pine Instruments, USA) and immersed in the electrolyte, taking care to remove any air bubbles from the electrode su rface by rotating the electrode briefly at 1000 RPM. The cell was thermostated at 25ºC or 70ºC using a circulation heater.
The electrolyte was purged with nitrogen (Grade 5, ultra-high purity, BOC) for at least 30 minutes prior to electrochemical conditioning of the electrode by cycling between +0.05 – +1.0 V at 250 mV.s-1 for up to 200 cycles, until stable cyclic voltammograms were observed.
For the measurement of ECSA, cyclic voltammograms were recorded using analogue scan cyclic voltammetry (using the SCANGEN module) at 25 mV.s-1 between +0.05 and +1.1 V.
Background linear sweep voltammograms (LSVs) were recorded whilst still under N2 purge, from +0.3 – +1.1 V at 25 mV.s-1 at various rotation rates 400, 505, 660, 896, 1286 and 2000 RPM. The electrolyte was then purged with oxygen (ultra-high purity, BOC) for at least 30 mins, or until the open circuit potential stabilised.
LSV scans were then repeated in the O2 – purged electrolyte at the same rotation rates.
The Ohmic resistance of the electrolyte was measured using electrochemical impedance spectroscopy at 10 kHz, using an AC perturbation of 5 mV (see section 7.2.3 for a full description of this technique).
Finally, to ensure that the catalyst had not detached from the electrode, the electrolyte was purged once more with N2 before repeating ECSA measurements using cyclic voltammetry as above.
5.3.1 Method validation: Pt disc
Prior to conducting experiments with supported catalysts, the RDE methods were validated using a polycrystalline Pt disc. A 2mm diameter Pt disc electrode (geometric surface area 0.0314 cm2) was polished to a 0.5 μm mirror finish and mounted immediately in the RDE assembly.
ECSA was measured from the charge due to Hupd desorption, found by integrating the shaded area of the cyclic voltammogram shown in Figure 5.2 (a) and using equation (3.8) (page 35). For the 2mm Pt disc, the ECSA was measured as 0.035 ± 0.004 cm2, giving an electrode roughness factor (rf = 1.1 ± 0.1) close to unity as expected for a highly-polished surface.
O2 diffusion coefficient The Levich plot in Figure 5.2 (c) shows a linear relationship (R2 value