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a) 3D model of the electrochemical cell used during the FEXRAV measurements with the position of the working electrode (W), reference electrode (R) and counter electrode (C); b) experimental setup for fluorescence (If) and transmission (I1) measurements. I0 is the incoming monochromatic X-ray radiation.
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![Figure 2. Pourbaix diagrams of the Pd-O-H system a) [Pd] = 1 · 10 −3...](publication/319080439/figure/fig2/AS:526523740454912@1502544450325/Pourbaix-diagrams-of-the-Pd-O-H-system-a-Pd-1-10-3-mol-kgw-b-Pd-1-10-6_Q320.jpg)
In this study, the first coupled FEXRAV and chemical speciation modelling study of the Pd deactivation is presented. Due to the high brilliance of synchrotron light, FEXRAV can investigate deeply buried surfaces. More specifically, we directly analyzed the evolution of the Pd/C catalytic layer during a voltammetric cycle, through a specifically des...
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Context 1
... the setup hereby discussed, the lower detection limit is evaluated to be approximately 10 13 atoms/cm 2 . In order to perform our experiments we used a standard three elec- trode cell, specifically designed (3D model in Figure 1a) to be mounted in the experimental XAS chamber of the LISA beamline at ESRF. 14 The schematic of the experimental setup is presented in Figure 1b. ...
Context 2
... the setup hereby discussed, the lower detection limit is evaluated to be approximately 10 13 atoms/cm 2 . In order to perform our experiments we used a standard three elec- trode cell, specifically designed (3D model in Figure 1a) to be mounted in the experimental XAS chamber of the LISA beamline at ESRF. 14 The schematic of the experimental setup is presented in Figure 1b. ...
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Citations
... The major issue with Pd electrocatalysts seems to be the stability of the materials [19]. In the latest years, it has been shown that the oxidation of the palladium surface can compete with the ethanol oxidation reactions even in the conditions that the materials experience in fuel cells [353,354]. Such oxidation can result in the partial dissolution of the nanoparticles that may lead to significant decrease of the surface area [355]. ...
In the last 20 years, direct alcohol fuel cells (DAFCs) have been the subject of tremendous research efforts for the potential application as on-demand power sources. Two leading technologies respectively based on proton exchange membranes (PEMs) and anion exchange membranes (AEMs) have emerged: the first one operating in an acidic environment and conducting protons; the second one operating in alkaline electrolytes and conducting hydroxyl ions. In this review, we present an analysis of the state-of-the-art acidic and alkaline DAFCs fed with methanol and ethanol with the purpose to support a comparative analysis of acidic and alkaline systems, which is missing in the current literature. A special focus is placed on the effect of the reaction stoichiometry in acidic and alkaline systems. Particularly, we point out that, in alkaline systems, OH ⁻ participates stoichiometrically to reactions, and that alcohol oxidation products are anions. This aspect must be considered when designing the fuel and when making an energy evaluation from a whole system perspective.
Graphical Abstract
... However, anode potential could vary widely in operative conditions in FC applications, exceeding palladium oxidation potential. Our previous experiments [46,47] at the European Synchrotron Radiation Facility (ESRF, BM-08 "LISA" CRG beamline), performed by stressing Pd/C nanoparticles in the 0 to 1.2 V vs RHE potential range using KOH 2M and KOH 2M + EtOH 2M electrolytes, highlighted loss in the electrochemical performances of the catalyst as a consequence of loss in palladium mass at the electrode. During these experiments, we used the Fixed Energy X-Ray Absorption Voltammetry technique (FEX-RAV [48]) to uncover changes in the speciation of palladium. ...
... Results Fig. 1 demonstrated that, apart from local variations inside a single voltametric cycle (the valley-peak absorption coefficient variation in phase with the variation of potentials at the electrode), a constant loss of metallic palladium nanoparticles occurs in the long run (decrease in the overall absorption signal μ) by formation of Pd 2+ hydroxides into solution, when the electrode is subject to oxidative potentials for both alkaline and alkaline + ethanol electrolytes. A comparison between our results and computational speciation models [47] highlighted the presence of two main species in our system, Pd (0) and Pd 2+ (Pd(OH) 4 2− ), and only a minor amount of PdO as transient species at the electrode, thus confirming a dissolution pathway instead of a poisoning effect. We were then able to observe dissolution in the Pd/C catalyst in a half-cell set-up under high oxidative potentials. ...
... Oppositely, a dependency can be seen considering μ 1 variation, which fastly increases for the first six cycles, and tends to smoothen up for the following ones. We observed similar trends during our previous FEXRAV half-cell experiments [46,47] performed using the same catalyst and fuel ( Fig. 11-b, top row). Here, we adopted disareated KOH 2 M + EtOH 2 M electrolyte ( Fig. 11-b) in a three electrodes set-up, reaching higher oxidative potentials as voltammetry upper vertex (1.2 V vs. RHE). ...
Palladium is one of the best anodic catalysts for Alkaline Direct Alcohol Fuel Cell (DAFC) applications. However, its adoption in Fuel Cells (FC) is still limited because of the deactivation of the catalyst, which hinders the long-term performance stability of the devices. In this article, we report the results of a Fixed Energy X-Ray Absorption Voltammetry (FEXRAV) investigation of a carbon-supported palladium electrocatalyst operating in a complete alkaline direct ethanol fuel cell (DEFC). Our results show palladium dissolution unambiguously, especially at cell polarizations corresponding to high anodic stresses. We have also investigated the behaviour of Pd electrocatalyst in a direct alkaline formate fuel cell (DFFC). Data show that when using formate as a fuel, the electrode Pd loss is negligible compared to the case of DEFC.
... The incipient HER reaction is probably related to the lower magnitude of the impedance than Pd. Indeed, the Pd electrode seems to behave as a blocking surface, which, in turn, is consistent with an OCP value falling in the immunity domain according to Pd Pourbaix diagram [39] . ...
The electrochemical hydrogenation (galvanostatic charging) and desorption (potentiostatic discharging) of Nb in an alkaline solution was investigated, using Pd as a reference model system. The phases formed during hydrogenation and those present after desorption were investigated by X-ray diffraction (XRD). The cathodic charging induced the formation of NbH0.89 and NbH2 for Nb and PdH0.6 for Pd. Nb hydrides appeared to be irreversibly formed, unlikely to the PdH0.6 that spontaneously decomposed back into Pd. The irreversible character of absorption process in Nb is likely to be related to oxide/hydroxides formation on the electrode surface in the alkaline media, which might hinder the dehydrogenation reaction. The electrochemical behavior was evaluated through potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Results were analyzed according to both thermodynamic and kinetic approaches. EIS results indicated that Pd and Nb have very similar hydrogen evolution reaction (HER) kinetic parameters. A theoretical development based on the different kinetic contributions to the overall charge-transfer resistance, including the absorption step's indirect contribution, is presented.
... Indeed, Pd 0 is known to oxidise to Pd(II) oxides/hydroxides at pH larger than 13 for potential larger than 0.5 -0.6 V vs RHE [29,30] with no formation of Pd(IV) species. However, at higher potential palladium is also dissolved in the electrolyte [29,[31][32][33], an aspect that is limited by the presence of reducing agents such as NaBH4 [23] or even milder ones such as ethanol [34,35], ethylene glycol [36][37][38] and glycerol [38][39][40]. It is also known that only metallic Pd shows activity towards ethanol electrooxidation, while Pd(II) species are not electroactive toward this reaction [20]. ...
... To perform the FEXRAV experiments, we have designed a new dedicated electrochemical cell that fits the LISA beamline hardware set up, like already done by other groups [43]. Our cell is designed to acquire XAS spectra in the Fluorescence configuration, with a 45° angle between the normal to the working electrode (WE) surface and the axis of the incoming beam [31]. The cell consists of two main sections: i) the cell head assembly and ii) the cell body ( Figure 3). ...
... Pd dissolution in bare alkali has been previously demonstrated by Pourbaix diagram calculation, XAS analysis and EQCM [20,31,45]. Liang et al. found that Pd oxidation happens with the formation of the same species even in ethanol containing electrolytes. ...
The oxidation of palladium nanoparticles causes the performance degradation of alkaline direct ethanol fuel cells. Quantifying this oxidation is a task of tremendous importance to design mitigation strategies that extend the service life of catalysts and devices. Here, we show that the Fixed Energy X-ray Absorption Voltammetry (FEXRAV) can provide this information with an in-situ approach. To do so, we have developed a quantification method that assumes the linear response at fixed energy. With this method, we have investigated the oxidation of carbon black-supported palladium electrocatalysts during cyclic voltammetry in the same solution employed as a fuel in the direct ethanol fuel cells. We have shown that up to 38% of the palladium is oxidised at 1.2 V vs RHE and that such oxidation also happens at lower potentials that the catalyst can experience in real direct ethanol fuel cells. The result of this study is a proof of concept of quantitative FEXRAV.
... Moreover, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis. Initially introduced for the study of IrOx materials, FEXRAV has been applied to operando studies of the electrochemistry of a variety of materials, from Ag nanoparticles [44], to copper oxide materials [45], to iron oxide and oxo-hydroxide materials [46], to palladium based electrocatalysts [47]. Figure 7 reports an example of a FEXRAV experiment in the case of Ag nanoparticles studied as electrocatalysts for the electrodehalogenation of trichloromethane in aqueous media. ...
... Moreover, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis. Initially introduced for the study of IrO x materials, FEXRAV has been applied to operando studies of the electrochemistry of a variety of materials, from Ag nanoparticles [44], to copper oxide materials [45], to iron oxide and oxo-hydroxide materials [46], to palladium based electrocatalysts [47]. Figure 7 reports an example of a FEXRAV experiment in the case of Ag nanoparticles studied as electrocatalysts for the electrodehalogenation of trichloromethane in aqueous media. ...
This minireview aims at providing a complete survey concerning the use of X-ray absorption spectroscopy (XAS) for time-resolved studies of electrochemical and photoelectrochemical phenomena. We will see that time resolution can range from the femto-picosecond to the second (or more) scale and that this joins the valuable throughput typical of XAS, which allows for determining the oxidation state of the investigated element, together with its local structure. We will analyze four different techniques that use different approaches to exploit the in real time capabilities of XAS. These are quick-XAS, energy dispersive XAS, pump & probe XAS and fixed-energy X-ray absorption voltammetry. In the conclusions, we will analyze possible future perspectives for these techniques.
In this article, we present an in-situ and operando time-resolved X-ray Absorption Spectroscopy (XAS) technique which exploits a combination of Grazing Incidence XAS (GIXAS) and Fixed Energy X-ray Absorption Voltammetry (FEXRAV), the Grazing Incidence FEXRAV (GI-FEXRAV). A case-study is also outlined. Palladium ultra-low loadings were deposited above Au polycrystalline iso-oriented substrates adopting three different deposition methods: surface-controlled electrochemical methods, direct electrodeposition, and physical vapour deposition (PVD). These catalytical surfaces were prepared for the investigation by GI-FEXRAV of the Pd oxidation/dissolution phenomenon that could occur when the metal is used in the anodic compartment of Direct Alcohol Fuel Cells (DAFCs) or in electrochemical reformers. Moreover, we report a robust, low cost and versatile procedure to obtain wide and flat iso-oriented gold substrates that can mimic monocrystalline gold (1 1 1) in the electrochemical response. The use of GI-FEXRAV for the operando characterization of the catalysts, in conjunction with the designed experimental cell and our flexible Au-based electrochemical substrates show an invaluable potential in the operando study of fundamental phenomena in heterogeneous electrocatalysis model systems and, due to its versatility, paves the way to further studies on a wide selection of electrochemical systems.
The development of anion-exchange membrane fuel cells (AEMFCs) has recently accelerated due to synergistic improvements yielding highly conductive membranes, stable ionomers, and enhanced alkaline electrocatalysts. However, cell durability, especially under realistic conditions, still poses a major challenge. Herein, we employ low-loadings of Pt-free Pd-based catalysts in the anode of AEMFCs and elucidate potential degradation mechanisms impacting long-term performance under conditions analogous to the real-world (high current density, H2-Air (albeit CO2-free), and intermittent operation). Our high-performing AEMFCs achieve impressive performance with power densities approaching 1 W cm-2 and current densities up to 3.5 A cm-2
. Over a 200 h period of continuous operation in H2-Air at a current density of 600 mA cm2 , our model Pd/C-CeO2 anode cell exhibits record stability (~30 µV h-1 degradation) compared to the literature and up to 6× better stability than our Pd/C and commercial Pt/C anode cells. Following an 8 h shutdown, the Pd/C-CeO2 anode cell was restarted and continued for an additional 300 h with a higher degradation rate of ~600 µV h-1 . Thorough in-situ evaluations and post-stability analyses provide insights into potential degradation mechanisms to be expected during extended operation under more realistic conditions and provide mitigation strategies to enable the widespread development of highly durable AEMFCs.
Electrochemical characterization of an electromaterial normally combines steady-state and dynamic analysis techniques to identify and parameterize key charge-transfer processes along with coupled material transformations and chemical/catalytic reactions. On the other hand, highly informative complementary characterization methods, such as in situ X-ray spectroscopy, are most commonly applied in a potentiostatic mode, probing only quasi-stabilized states of the material. While being of immense importance to the field, such approaches provide very limited information on dynamic processes and their kinetics. This limitation can be resolved by combining the steady-state analysis with Dynamic Single Energy X-ray Spectroelectrochemistry – a technique of focus herein. The manuscript outlines the basic principles of this method, identifies the scope of its applications, and provides an overview of its progressively increasing applications in in situ / operando research of electromaterials.
