A novel electrochemical cell for operando X-ray absorption measurements at low energies: Probing electrochemically induced electronic changes in palladium
Graphical abstract
Introduction
Operando spectroscopies have provided unique insights into the structural properties of catalytic materials under working conditions [1]. In particular, X-ray absorption spectroscopy (XAS) has become an important tool in the study of materials for many important electrochemical processes. The distinctive properties of Pt for fuel cell applications have meant that research on materials for low-temperature fuel cells has been focused mainly on carbon-supported Pt and Pt-based electrocatalysts [2]. Fuel cell reactions, such as the reduction of oxygen and the oxidation of hydrogen, methanol and ethanol, involve reaction steps in which adsorption of the reactant molecules and/or reaction intermediates takes place. Because the outermost orbitals are the ones relevant to chemisorption processes, changes in their electronic occupancy, which can be probed by XAS measurements at the L3 edge (involving excitation of 2p electrons to unfilled d orbitals), might have considerable impact on the strength of adsorption, ultimately affecting the catalytic activity and reaction mechanisms. A range of electrochemical cells have been developed for in situ experiments using hard X rays [[3], [4], [5]]. The high energy of the Pt L3 edge (11,564 eV) has allowed X-ray absorption spectroscopy experiments to be performed under electrochemical conditions for carbon-supported Pt [4,6,7]. The Pt 5d band electronic occupancy can be measured directly at different applied potentials from the near edge region of the absorption spectra (X-ray absorption near edge structure, XANES) [8]. The analysis of spectra beyond the absorption edge (Extended X-ray Absorption Fine Structure, EXAFS) has allowed bond distances and coordination numbers to be assessed, providing valuable results for the characterization of carbon-supported Pt-based alloys [9].
In recent years, rapid progress in the development of membranes for alkaline fuel cells has led to consideration of Pd and Pd-based catalysts as possible alternatives to expensive Pt catalysts [10]. Most X-ray absorption studies of Pd materials have been carried out around the Pd K edge (24,350.3 eV), for which transitions occur only to the final states of p symmetry. In contrast, despite the disadvantages of smaller fluorescence yields than for K shells, the possibility of more visible multielectron transitions and the fact that for some metals the proximity of L edges might prevent structural analysis by EXAFS, measurements at the L3 edge (excitation p → d), which for transition metals show strong absorption peaks, allow the electronic occupancy of the outermost orbitals to be probed. In this case, direct assessment of the electronic occupancy of the Pd 4d band under electrochemical conditions is constrained by the low Pd L3 edge energy (3173 eV), which lies in the tender X-ray region.
Despite the fact that measurements around the Pd L3 edge are rather scarce for carbon-supported Pd and Pd-based nanoparticles, analysis of the XANES region of spectra acquired under high-vacuum conditions has shed some light on the role of the electronic occupation of the Pd 4d band in electrocatalytic reactions [11,12] and electrochemical surface processes [13].
Although measurements using low-energy X-rays under electrochemical conditions can be quite challenging, a range of experimental setups have been previously reported for operando XAS studies in the soft X-ray region. Different types of electrochemical cells have been designed for use inside a vacuum chamber, such as coin-type cells [14], cells involving liquid flow [15,16], and small tightly sealed cells [[17], [18], [19]]. The main types of electrochemical cells developed for use under vacuum conditions and the results obtained with them have been discussed in review articles [20,21]. With such experimental arrangements, however, changing the samples is not an easy task as this involves the time-consuming processes of venting/pumping the vacuum chamber.
In contrast, conventional electrochemical cells filled with a liquid electrolyte under ambient pressure allow experiments to be performed under conditions that are closer to the real environment of fuel cells and are therefore more suitable for studying materials for fuel cell applications. To the best of our knowledge, operando X-ray absorption studies around the Pd L3 edge under conventional electrochemical conditions have never been reported for supported Pd nanoparticles. Yet, studies of this kind are highly desirable as information on the changes occurring in the electronic properties at different applied potentials and in different electrolytic solutions might offer valuable insights into their roles in electrocatalytic reactions and mechanisms.
In this work, we describe a new electrochemical cell in an out-of-chamber setup that makes it possible to measure X-ray absorption spectra around the Pd L3 edge and to probe, for the first time, the changes in the electronic properties of carbon-supported Pd nanoparticles induced by electrochemical oxidation of the Pd surface.
Section snippets
Electrochemical cell and out-of-chamber system
The body of the electrochemical cell was made of PEEK® and the cell cap with openings for reference and auxiliary electrodes was made of Teflon®. Fig. 1 shows the main parts of the electrochemical cell. For the working electrode, a window-catalyst assembly was mounted on a removable aluminum holder with a 5 mm diameter orifice at the center for the X-ray beam to pass through. An Ultralene® film (4 μm) was fixed on the removable aluminum holder and a 50 nm film of titanium was deposited on it by
Results and discussion
The spectra of the Pd/C catalyst were acquired in sequence at constant applied potential, after potential steps from 0.4 V as shown in Fig. 3. The first spectrum was recorded with the sample kept at a potential in the region where the Pd surface is not yet oxidized (0.6 V vs. RHE) and the subsequent spectra were collected with the Pd/C catalyst polarized at 0.8, 0.9 and 1.0 V vs RHE, i.e., within the region where oxidation of the Pd surface takes place.
A comparison of the normalized X-ray
Conclusions
We have developed a new electrochemical cell and a simple out-of-chamber setup that provides an easy way for operando measurements in the soft and tender X-ray region under electrochemical conditions. We probed the changes in the electronic occupancy of the Pd 4d band resulting from the application of different potentials and observed, for the first time, that electrochemical oxidation of the Pd surface led to an increase in the band vacancy, indicating that charge is transferred from the metal
Acknowledgments
This work was supported by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) (2014/12255-6), and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), (407143/2013-0). FBO thanks CAPES and GMA thanks FAPESP (2016/05041-5) for the scholarships granted.
Conflicts of interest
The authors declare no competing interests.
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