Characterization of Nanomaterials in Electrochemistry

Abstract

Establishing new protocol for nanomaterial characterization of functional materials is an important step in our knowledge for understanding the correlation between atomic changes and electrochemical performances. We propose a combination of different state-of-the-art techniques as a robust approach for nanomaterial characterization, which is suitable in structural refinements of nanocrystalline active systems. This technique of studying microscopic properties of nanomaterials includes XAS (X-ray absorption spectroscopy) ex situ and in situ, XRD (X-ray diffraction), high-resolution TEM (transmission electron microscopy), and XRF (X-ray fluorescence). In particular, we are using the site-selective XAS (performed at international synchrotron radiation facilities) that is sensible to the local structure (up to 5–10 Å around photoabsorbing sites) for characterization of the nanomaterials with singular accuracy. An investigation of the local structure and chemical disorder dynamics of a commercial Pt-Co alloy nanocatalyst, used as electrode material in proton exchange membrane fuel cells (PEMFC), will be presented and discussed.

Appendix

Design and Performances of the XAS PEM Fuel Cell

This section describes the experimental setup used to perform in cell measurements. For optimizing in situ XAS measurements on the catalyst during original electrochemical activity, a standard pemmican has been modified ad hoc [40]. In situ X-ray investigations of active materials first need to reduce the background absorption due to the various components of the cells. The thickness of the electrodes and membrane cell components has to be severely restricted if we would like to probe the valence and structure of metals participating in the catalytic process.

Due to this restriction, standard PEM fuel cells must be modified; the body of the cell was modified designing suitable windows for X-ray investigations. For obtaining XAS measurements in transmission mode with low noise, an EFC-05-02 (Electrochem) fuel cell was modified to achieve high transmission rates for photon energies in energy range about 515 keV, where the materials used as catalyst have most of the core levels.

The commercial fuel cell is made by two isotropic graphite separator plates, a 5 cm² active area (serpentine flow pattern) and two gold-plated current collectors. Eight screws keep together the plates of graphite; in this way, a high enough compression on the membrane electrode assembly (MEA) is induced providing a good electrical contact. In order to obtain maximum X-ray transmittance, two thin graphite windows (light and gas tight) are hollowed in the EFC-05-02 cells. The total graphite thickness obtained up to 0.25 mm, over a flat 1 mm × 7 mm area, providing a double window for XAS in transmission mode and possible X-ray fluorescence and X-ray diffraction measurements owing to the wide angular acceptance (~100° on the beam plane, ~5 % covered solid angle), as shown in Fig. 9. The X-ray window is positioned to be parallel (y) and in correspondence (z) with the serpentine channels (width 0.8 mm) in order to minimize absorption.

Fig. 9

Schematic view of the modified fuel cell optimized for in situ X-ray absorption measurements (front and side views, dimensions given in mm). The front view shows the drilling of the electrode of graphite in order to reduce the X-ray absorption saving space for the fuel channels (hydrogen and oxygen). Each graphite plate has a minimal thickness about 0.25 mm, thus minimizing the absorption along the X-ray path (x, see inset). The side view shows the wide angular aperture allowing for possible X-ray diffraction and X-ray fluorescence measurements; the cell MEA positioning is also shown (From reference [40])

This modified cell allows standard condition XAS measurements at EXAFS beamlines with a typical beam size of ~ 0.4 mm × 5 mm. The cell position can be easily modified during the experiment in respect of beam, allowing the best geometry for the experimental technique in use. More details are listed in [40].

Sample Preparation

Electrochemical and structural measurements were performed on electrodes. The catalytic layers were prepared using E-TEK 30 % Pt₃Co supported on Vulcan XC-72 powder (with total metal loading of about 1.0 mg/cm²).

The metal loading was counted from the weight. Membrane electrode assemblies (MEAs) used to perform cathode catalyst degradation were composed of Nafion® N-112R as a proton conductive membrane and Pd (30%Pd/Vulcan XC-72 powder, Pd loading 1.0 mg cm⁻²) as an anode catalyst.

In the X-ray beam window region, a Pd counter electrode [41] was used in order to prevent perturbations of the electrical field on the catalyst under consideration [42, 43]. Before XAS measurements, each new MEA was subjected to conditioning process at a voltage of ~ 0.5 V for 15 min, and then Rapid Check cell performances by electrochemical cycles were done.

In Situ Experimental Setups

A typical setup for in situ XAS experiments in transmission mode at the BM29 (ESRF) or XAFS 11.1 (ELETTRA) beamlines is shown in Fig. 10. In those beamlines, the XAS fuel cell described before has been tested.

Fig. 10

Typical setup for XAS experiments on fuel cells under operating conditions (From reference [40])

In a standard transmission configuration, the fuel cell has been positioned and aligned along the beam with motorized translation stages. The reference sample has been included for a more precise energy calibration. The cell channels for oxygen and hydrogen gas were connected to their lines working at 1.2 bar (the gas flow was set to ~100 ml min⁻¹). During XAS measurements, the voltage output was remotely controlled by a computer-driven potentiostat/galvanostat and continuously stored.

Figure 11 shows the setup, which allows measurements both in transmission and in fluorescence modes. Looking at Fig. 11, the fuel cell has been installed between the ionization chambers I₀ and I₁ at the X-ray spectrometer BM29 (ESRF) close to a 13-channel Ge X-ray detector (I _f ). Figure 10 shows the setup schematic view, and it is possible to identify the fuel cell, the gas lines (humidified O₂ and H₂), the electrical connections necessary to operate the cell, and the X-ray fluorescence detector located on the left side. The pressure of the gases has been set to 1 atm; the gas flux was about 100 ml min⁻¹. The fuel cell has been located with an angle of 45° with respect to the X-ray beam direction. In order to maximize the recovered solid angle, the multichannel detector has been positioned close to the cell at 90° to the incident beam in the synchrotron (horizontal) plane.

Fig. 11

(a) Picture taken at the BM29 beamline (ESRF) that shows the modified fuel cell positioned close to the 13-channel Ge X-ray detector (on the left) on the beam direction. (b) The schematic experimental setup top view θ = ϕ = 45°. The absorption coefficient μ(E) of selected atomic species embedded inside the electrode can be measured both in transmission mode (μ(E) = ln(I₀/I₁)) and in fluorescence mode (μ(E) ∝ I _f  /I₀). The absorption coefficient of a reference sample is measured (μ(E) = ln(I₁/I₂)) at the same time

Taking advantage of this geometry and the twofold wide aperture machined on both sides of the cell, simultaneous XAS measurements in transmission and energy-dispersive fluorescence (EDXRF) mode can be carried out (see Fig. 11) without further alignment.

So make the most of this geometry simultaneous XAS measurements in transmission and energy-dispersive fluorescence (EDXRF) mode can be carried out (see Fig. 11) without further alignment. The quality of the carried-out signals is really good as shown in Fig. 12 where extracted EXAFS signals of Pt-Co nanocatalyst at different potentials are presented for both ages. It has also to be underlined that the Co quantity in the electrode is very few (0.1 mg/cm²) in spite of the quality of the signal being more than satisfactory

Fig. 12

30 % Pt-Co/C electrode in situ EXAFS signals at different potentials. Starting from the top of the figures: electrode conditioned, at OCV, 0.75, 0.70, and 0.60 V. Dotted lines represent the experimental signals and the solid lines the theoretical ones