Combining impedance and hydrodynamic methods in electrocatalysis. Characterization of Pt(pc), Pt5Gd, and nanostructured Pd for the hydrogen evolution reaction

Electrochemical hydrodynamic techniques typically involve electrodes that move relative to the solution. Historically, approaches involving rotating disc electrode (RDE) configurations have become very popular, as one can easily control the electroactive species’ mass transport in those cases. The combination of cyclic voltammetry and RDE is nowadays one of the standard characterization protocols in electrocatalysis. On the other hand, impedance spectroscopy is one of the most informative electrochemistry techniques, enabling the acquisition of information on the processes taking place simultaneously at the electrode/electrolyte interface. In this work, we investigated the hydrogen evolution reaction (HER) catalyzed by polycrystalline Pt (Pt(pc)) and Pt5Gd disc electrodes and characterized them using RDE and electrochemical impedance spectroscopy techniques simultaneously. Pt5Gd shows higher HER activities than Pt in acidic and alkaline media due to strain and ligand effects. The mechanistic study of the reaction showed that the rotation rates in acidic media do not affect the contribution of the Volmer–Heyrovsky and Volmer–Tafel pathways. However, the Volmer–Heyrovsky pathway dominates at lower rotation rates in alkaline media. Besides, the HER in acidic solutions depends more strongly on mass diffusion than in alkaline media. In addition to simple and clearly defined systems, the combined method of both techniques is applicable for systems with greater complexity, such as Pd/C nanostructured catalysts. Applying the above-presented approach, we found that the Volmer–Tafel pathway is the dominating mechanism of the HER for this catalytic system.


Introduction
The concept of combining several electrochemical or non-electrochemical techniques in one dedicated experiment is relatively common [1][2][3]. However, in the case of rotating disc electrode (RDE) measurements, its combination with electrochemical impedance spectroscopy (EIS) needs more exploration (see, e.g. [4][5][6][7][8][9]) to provide a more informative methodology for electrocatalysis. A specific power of EIS is that it helps to elucidate the mechanism of electrocatalytic reactions and estimate physicochemical parameters related to the interfacial processes taking place simultaneously. EIS has been combined with various techniques under different conditions demonstrating a great increase in the informative power of experiments [10][11][12][13]. However, despite the frequent positive synergistic effect while using such different combinations of techniques, their application methodology needs to be sufficiently elaborated. This is especially true in the case of electrocatalysis.
The particular combination of EIS measurements performed under hydrodynamic conditions should be especially beneficial. The RDE configuration is standard in electrocatalysis: it facilitates the control over the mass transport and helps to benchmark the electrode activity [14][15][16]. On the other hand, one can envisage that this option will also be helpful in the identification, testing, and verification of the physical impedance models. Therefore, accurate activity information, the reaction mechanism, and physicochemical parameters like the double layer capacitance or the uncompensated resistance should be available in one experiment and valid for the same conditions.
In this work, polycrystalline Pt (Pt(pc)), as a reliable and well-studied system in electrocatalysis (see, e.g. [17][18][19][20]), is used to establish the methodology for the combination of hydrodynamic RDE techniques and EIS measurements. The hydrogen evolution reaction (HER) was investigated due to its importance and key role in a sustainable energy transition [21]. While RDE measurements typically deliver only a change in current densities, i.e. activities, upon changing hydrodynamic conditions, such as rotation speeds, they lack further information about the reaction mechanisms. However, additional EIS measurements at different rotation speeds can give further insights into the change of parameters, such as charge transfer resistance or Warburg diffusion behavior for the RDE setup, which help to elucidate the different reaction mechanisms of the HER in acidic and alkaline media. Furthermore, Pt 5 Gd, as a novel electrocatalyst material with high activity toward the oxygen reduction reaction (ORR), was chosen to test the methodology developed above and identify the connection between induced ligand and strain effects and increased electrocatalytic activity toward the HER in acidic and alkaline media compared to pure Pt (see, e.g. [22][23][24][25][26]). Finally, we used Pd nanoparticles supported on high surface area carbon (Pd/C) as a promising hydrogen evolution electrocatalyst to demonstrate the informative power of the EIS-RDE approach for a more complex system [27,28]. To harvest results with profound physical meaning, a suitable equivalent electric circuit (EEC) model for the EIS data is used in this work [19]. Subsequently, the EIS parameters were elucidated and verified in the RDE configuration on Pt(pc), Pt 5 Gd, and Pd/C. As a result, the EIS-RDE approach can address the role of the Volmer-Heyrovsky and Volmer-Tafel pathways in these different systems, motivating the combined study as a new and valid approach for the complete characterization of electrocatalyst materials.
For the electrochemical measurements, a three-electrode set-up was used with a mercury − mercurous sulfate electrode (MMS) (SI Analytics, Germany) as the reference electrode (RE) and a platinum wire (99.9% purity, MaTecK, Germany) as the counter electrode. EIS and RDE techniques were performed using a VSP-300 potentiostat (Bio-Logic, France). Since the EIS measurements were performed at high frequencies, measurement artifacts caused by the impedance of the RE were suppressed by a shunt capacitor placed between the RE and a Pt wire, which was immersed into the electrolyte close to the Luggin capillary [27,33]. This is also how to make the iR compensation for the HER/HOR polarization curves. Prior to the measurements, the cells and all glassware were cleaned with a 3:1 mixture of H 2 SO 4 (96% H 2 SO 4 , p.a., ISO, Carl Roth, Germany) and H 2 O 2 , and then rinsed several times alternately with ultrapure water.
As the acidic and alkaline electrolytes, perchloric acid (70% HClO 4 , extra pure, Acros, Germany) and lithium hydroxide (LiOH, anhydrous, 99.995% (metals basis), Sigma-Aldrich, USA) solutions, respectively, were chosen and diluted to 0.1 M concentration with ultrapure water. The alkaline medium was chosen as 0.1 M LiOH due to the better HER stability and activity values compared to other alkaline solutions reported previously [20,34].
For the electrochemical measurements on the Pt(pc) and Pt 5 Gd disc electrodes, the acidic or alkaline solution was first purged with H 2 (5.0, Westfalen, Germany) for 30 min. Subsequently, cyclic voltammograms (CVs) were recorded within a potential range of about −0.13 V to 0.32 V and −0.05 V to 0.39 V vs reversible hydrogen electrode (RHE) for acidic and alkaline electrolyte solutions, respectively, at a scan rate of 10 mV s −1 until reaching steady current densities. This step also ensured the calibration versus the RHE [35] avoiding the potential shift in the Nernst equation due to a liquid junction potential between the inner electrolyte of the MMS electrode and the operating electrolyte [36]. The measurements of the Pd/C catalyst were only performed in 0.1 M HClO 4 . To activate the Pd/C catalyst, CVs at a scan rate of 50 mV s −1 were performed in an Ar-saturated environment (5.0, Westfalen, Germany) in the potential range of 0.06 to 1.2 V vs RHE, until establishing a relatively stable state. Similar to the Pt-based disc electrodes, the electrolyte was purged with H 2 for 30 min, and HER/ hydrogen oxidation reaction (HOR) polarization curves were subsequently recorded in a potential range from −0.13 to 0.82 V at 1600 rpm. All potentials in this work are referenced to the RHE scale. The measured current densities are normalized to the geometrical surface area of the disc electrodes (0.196 cm 2 ).
In the case of the Pd/C catalyst, posterior evaluations of the iR-corrected HER/HOR polarization curves at −5 mV vs RHE determines the mass and specific activity. The former depends on the mass loading, the latter on the electrochemically active surface area (ECSA) of the Pd/C catalyst. Due to the carbon-based support material, thermogravimetric analysis quantifies the Pd/C mass loading reliably using a Mettler Toledo instrument. Consequently, the relative Pd loading to total mass of the catalyst corresponds to ∼17 wt%.
Arguably one of the essential components for the characterization of nanostructured catalysts corresponds to the specific surface area (SSA). For its estimation, the ECSA needs to be detected and normalized to the Pd mass, which can be calculated considering the Pd/C mass loading. Next to the hydrogen underpotential deposition (H upd ) technique, CO stripping belongs to the most well-known techniques for ECSA determination [16,37]. The first technique introduces a non-neglectable error in the ECSA for Pd-based systems, presumably owing to hydride formation. With these considerations in mind, CO stripping was used in the following work by assuming entire adsorbed CO monolayers on the Pd nanoparticles and a CO oxidation charge per unit area of 420 µC cm −2 . For the experiment, the 0.1 M HClO 4 electrolyte was initially purged with CO (1000 ppm CO in Ar, 4.7/5.0, Westfalen, Germany) for 15 min. Subsequent polarization of the WE at 0.1 V vs RHE for 50 min under persistent rotation of 400 rpm ensures that an entire monolayer of adsorbed CO forms on the catalytic surface. Then, the electrolyte is purged with Ar for 15 min, and afterward, two CV cycles are executed with a scan rate of 10 mV s −1 within the potential range of 0.06 to 1.2 V vs RHE.
It should be noticed that the HER activity in alkaline media was not stable and decreased gradually, which is already a known phenomenon in the literature [38]. Therefore, the disc electrodes were electrochemically cleaned and activated before each EIS measurement was performed. The EIS measurements were performed within the AC-probing frequency range from 100 kHz to 0.1 Hz at different rotation rates. For Pt(pc) and Pt 5 Gd, −10 mV vs RHE potential was applied during the EIS measurements with a 10 mV perturbation amplitude. For the nanostructured Pd/C catalyst, a lower potential of −5 mV vs RHE was selected to avoid excessive H 2 formation on the electrode surface. The impedance data were analyzed via the EIS Spectrum Analyzer 1.3 software [39,40] and checked by Kramers-Kronig Relations.
For material characterization, scanning electron microscopy (SEM) techniques visualized the structure of the elucidated Pd/C catalyst with nanometric resolution. The used ZEISS Gemini Nvision 40 accelerates electrons with a 5 kV voltage for the performed measurements. For a deeper fundamental understanding of the nanostructured system, x-ray diffraction (XRD) elucidates crystallographic and structural properties. The diffractometer of the used Rigaku MiniFlex 600-C produces monochromatic x-ray beams with a Cu-Kα source and a Ni-based filter. The angle of 2Θ in the diffraction patterns changed with 5 • min −1 step velocity in the range from 5 • to 90 • . Subsequently, the Scherrer equation can be used to estimate the average crystallite sizes of the Pd/C nanoparticles [41]. In the case of Pt(pc) and Pt 5 Gd, the XRD patterns were measured by X'Pert Pro PANanalytical instrument with Cu-Kα source and a Ni-based filter. The scanning 2Θ angle varied from 5 • to 90 • with ∼4 and ∼0.78 • min −1 step velocity for Pt(pc) and Pt 5 Gd, respectively.

Results and discussion
Before electrochemical measurements, the polycrystalline structure of the Pt(pc) and Pt 5 Gd disc electrodes in figure S1 are shown by XRD measurements. In order to fully characterize the HER on the chosen model systems, Pt(pc), Pt 5 Gd, and Pd/C, we combine the classical methodology of RDE with EIS. The RDE experiment determines the HER activity and controls the mass diffusion in a standardized way. The EIS method quantitatively provides the relevant physicochemical parameters to describe the electrochemical interface and to distinguish the relevant reaction pathways. As an overview, the overall and elementary pathways of the HER in acidic and alkaline media are illustrated in table 1. In acidic media, the hydrogen ions (H + ) are adsorbed at the electrode surface (H ads ) and produce H 2 in the overall reaction; however, in alkaline media, water molecules are reduced and form H 2 and hydroxides (OH − ). The elemental reaction processes of the HER mechanisms can be separated into the Volmer−Heyrovsky and the Volmer− Tafel  Table 1. Overall and elementary steps of the HER in acidic and alkaline media [44].

Acidic media
Alkaline media  pathway. In a previous study on a Pt(pc) microelectrode [19], an EEC model was developed to evaluate the impedance data corresponding to the hydrogen reactions and to extract the involved physicochemical properties. As shown in figure 1, the EEC model includes the uncompensated resistance (R u ), which accounts for the resistance of the electrolyte and electronic resistances. In series to the uncompensated resistance, there is a parallel connection of the double layer capacitance (C dl ), the Volmer-Heyrovsky, and Volmer-Tafel pathways. The non-Faradaic branch corresponds to the electrochemical double layer. The Volmer-Heyrovsky pathway comprises a charge transfer resistance (R ct,1 ), an adsorption capacitance (C a ), and an adsorption resistance (R a ). The Volmer-Tafel pathway is described by a charge transfer resistance (R ct,2 ) and a Warburg short element (Ws) due to the finite length transmissive diffusion with the RDE setup. Ws consists of two parameters, namely the Warburg coefficient Ws r and the time constant parameter Ws c , which is related to the Nernst diffusion layer thickness [42,43]. Further information is provided in the SI. The Pt(pc) and Pt 5 Gd systems are probed and compared in the acidic media. Initially, RDE experiments are applied to classically measure the HER activity at 1600 rpm in H 2 -saturated 0.1 M HClO 4 with a scan rate of 10 mV s −1 . As shown in figure 2(A), the HER activity of Pt 5 Gd is higher than for Pt(pc). This can be associated with the ligand and strain effects on the Pt 5 Gd surface, weakening the binding energy of the reaction intermediates to reach optimal binding conditions. The promising activity improvement on the Pt-based alloys can also be observed for the ORR activities in acidic media [23,24,45].
Additionally, the polarization curves of the HER/HOR at different rotation rates on the Pt(pc) electrode in 0.1 M HClO 4 are shown in figure 2(B). The HOR limiting current density increases with increasing rotation rates from 400 to 2500 rpm, which agrees with the linear behavior of the Koutecky-Levich equation at 0.1 V vs RHE, as shown in the inset of figure 2(B), mainly because of the limited mass transport. Similarly, the measured current densities of the HER show the same trend as the HOR current densities. This indicates that the HER current density is associated with the charge transfer reactions and limiting diffusion [46], which will be further investigated in the EIS data discussion. A comparable trend was obtained for the Pt 5 Gd electrode and is given in figure S2. In addition, EIS spectra were measured at different rotation rates at −10 mV vs RHE on the Pt(pc) and Pt 5 Gd electrodes to quantitatively analyze the relationship between the rotation rate and the HER performance. The results are displayed in figures 2(C) and (D), along with the fits according to the EEC in figure 1. At different rotation speeds, the slight deviations of the arcs in the high-frequency regime can be associated with the changes in charge transfer reactions. More importantly, the profound differences in the low-frequency region correspond primarily to the change in mass diffusion. The Bode plots provide additional information on the phase shift and the total impedance as a function of frequency in figure S11. Further, the relative contributions of the Volmer-Heyrovsky and the Volmer-Tafel pathways on the Pt(pc) and Pt 5 Gd electrodes in 0.1 M HClO 4 , R ct,1 and R ct,2 are shown as a function of the square root of the rotation rate in figures 3(A) and (B), respectively. For the Pt(pc) electrode, R ct,1 and R ct,2 slightly decrease with increasing rotation rate, as displayed in figure 3(A). Correspondingly, the charge transfer reaction on the Pt 5 Gd electrode indicates a similar trend as for the Pt(pc) electrode with the change in the rotation rate, as shown in figure 3(B). Compared to the Pt(pc) electrode, R ct,2 is considerably higher than R ct,1 at different rotation rates on the Pt 5 Gd electrode, indicating that the Volmer-Heyrovsky pathway is the preferred mechanism. The increase of the charge transfer resistance can be associated with a change in the concentration of the redox species at the electrode surface due to rotation rate variations and a time shift [47] in the EIS measurements. Moreover, figures 3(C) and (D) show the Ws c , which corresponds to the controlled finite length diffusion with transmissive boundary by the RDE setup versus the inverse of the square root of the rotation rate. In the case of Pt(pc), Ws c linearly decreases with increasing rotation rate, while the Warburg coefficient (Ws r ) increases, as shown in figures 3(C) and S3, respectively. Similar results were obtained for the Pt 5 Gd electrode, as presented in figures 3(D) and S4. Therefore, the observations indicate that Ws c is inversely proportional to the square root of the rotation rate. The thickness of the diffusion layer depends on the rotation rate and can be quantitatively calculated. Other essential parameters obtained by the fit of the EIS data, shown in the EEC, as a function of the square root of the rotation rate on Pt(pc) and Pt 5 Gd electrodes in 0.1 M HClO 4 are shown in figures S3 and S4, respectively.
In the second step, the same set of experiments was conducted in the alkaline media for comparison and to underline the analytical strength of the approach. Here, the activities of the Pt(pc) and Pt 5 Gd electrodes toward the HER at −10 mV vs RHE at 1600 rpm are approximately six times smaller in 0.1 M LiOH than in 0.1 M HClO 4 , as shown in figures 2(A) and 4(A), respectively. The activity of Pt 5 Gd is slightly higher than that of Pt(pc) in alkaline media, which could again be ascribed to the ligand and strain effects of the alloy. The polarization curves of the HER/HOR with different rotation rates on Pt(pc) in H 2 -saturated 0.1 M LiOH at a scan rate of 10 mV s −1 are given in figure 4(B). The HOR current densities are rising with increasing rotation rate, which follows the Koutecky-Levich equation with a linear relationship at 0.3 V vs RHE, as shown in the inset of figure 4(B). However, the current densities toward the HER increase slightly with higher rotation rates in alkaline media compared to the more pronounced increase in acidic media in figure 2(B). This indicates that the HER in alkaline media is rather limited by kinetics than diffusion compared to the mainly limiting diffusion in acidic media [46]. The polarization curves of HER/HOR with different rotation rates on Pt 5 Gd in 0.1 M LiOH are shown in figure S2, illustrating a similar behavior as for Pt(pc) in figure 4(B). The detailed interpretation of the HER current densities influenced by different rotation rates can be given with the help of the impedance data shown in figures 4(C) and (D) for Pt(pc) and Pt 5 Gd electrodes, respectively. The EIS spectra presented in the Nyquist plots decrease slightly with the increase of the rotation rate for Pt(pc) and Pt 5 Gd electrodes, at −10 mV vs RHE in H 2 -saturated 0.1 M LiOH compared to the dramatic change of the EIS data in acidic media shown in figures 2(C) and (D). While the Nyquist plots consist of two small semicircles at high frequencies and one larger arc at low frequencies in acidic media, as shown in figures 2(C) and (D), the Nyquist plots in figures 4(C) and (D) in alkaline media illustrate different shapes. The EIS toward HER in alkaline media includes two larger arcs within the high-frequency region and one small arc within the lower-frequency region. The Bode plots in figure S11 display further details of the influence of rotational rate on the phase shift and total impedance within different frequencies.
In the following, the fit parameters obtained from the EIS spectra for the Pt(pc) and Pt 5 Gd electrodes in the alkaline media are discussed and interpreted. In figure 5(A), the values of R ct,1 and R ct,2 on Pt(pc) in alkaline media are more pronounced than those in acidic media, which are shown in figure 3(A). This explains the slower HER kinetics on Pt(pc) in 0.1 M LiOH compared to those in 0.1 M HClO 4 under the same experimental conditions. Furthermore, R ct,1 and R ct,2 show similar values at higher rotation rates. At lower rotation rates, R ct,2 increases dramatically, while R ct,1 remains constant, as illustrated in figure 5(A). This implies that the contribution of the Volmer-Tafel and Volmer-Heyrovsky pathways are comparable at higher rotation speeds, but at lower rotation speeds, the Volmer-Heyrovsky pathway dominates in alkaline media. A similar trend is shown in figure 5(B) for the Pt 5 Gd electrode in 0.1 M LiOH. In addition, Ws c decreases while Ws r increases with increasing rotation rates for Pt(pc), as shown in figures 5(C) and S5, respectively. The trend of the finite length transmissive diffusion with the RDE setup in alkaline media is comparable to that in acidic media; however, in alkaline media, the values of Ws c are about two times larger, and the values of Ws r are more than ten times larger than in acidic media, as shown in figures 3(C) and S3, respectively. The difference in the mass transport between the acidic and alkaline solutions can be explained due to the dissimilar reactants toward HER shown in table 1. The behavior of mass diffusion on Pt 5 Gd follows a similar trend as for Pt(pc) in 0.1 M LiOH, as presented in figures 5(D) and S6. The additional critical parameters in the EEC as a function of the square root of the rotation rate for Pt(pc) and Pt 5 Gd electrodes in 0.1 M LiOH are given in figures S5 and S6, respectively. To further investigate the dominating reaction pathway toward HER in acidic and alkaline media, the ratio of R ct,1 /R ct,2 is calculated for different rotation speeds, as shown in figure S7, respectively. The relative contributions of the Volmer-Heyrovsky and Volmer-Tafel pathways toward the overall HER in acidic media remain comparably stable at different rotation rates. The Volmer-Heyrovsky mechanism is more preferred for Pt 5 Gd than for Pt(pc), according to the results. On the other hand, both reaction pathways contributed more or less equally at higher rotation rates, while the Volmer-Heyrovsky mechanism dominates only at lower rotation rates for both Pt(pc) and Pt 5 Gd electrodes in alkaline media.
The combination of EIS and RDE is applicable not only for model systems like Pt(pc) and Pt 5 Gd disc electrodes but also for more complex nanostructured catalysts. In the case of the ∼17 wt% Pd/C catalyst, we provide a general characterization of the synthesized nanoparticles. The XRD measurement is displayed in figure S8 and its subsequent analysis reveals an average Pd crystallite size of (6.6 ± 2.3) nm. Additionally, the SEM image of the Pd/C catalyst shows homogeneously distributed Pd nanoparticles on the carbon-based support. The small nanoparticle size and their proper distribution results in a large SSA of (67.3 ± 0.2) m 2 g −1 .
With this characteristic knowledge of the Pd/C catalyst, the mass and specific HER activity can be investigated via RDE techniques. Figure S9 illustrates the HER/HOR polarization curve at 1600 rpm in 0.1 M HClO 4 . Evaluating the activity at −5 mV vs RHE determines the geometric, mass, and specific activity of (1.23 ± 0.37) mA cm geo −2 , (71.4 ± 21.5) mA mg −1 and (0.11 ± 0.03) mA cm ECSA −2 , respectively. Furthermore, the following EIS analysis provides insights into the dominating hydrogen reaction mechanism. As known from the literature [48], stability problems related to agglomeration and dissolution of the nanoparticles arise for the Pd/C system under cycling voltammetry operations in 0.1 M HClO 4 . In order to guarantee an active state and relatively stable structural conditions of the Pd/C catalyst prior to the RE calibration and EIS measurements, 68 cycles in the aforementioned potential area were performed, as shown in figure S9. Figure 6(A) displays the EIS spectra as Nyquist plots for the Pd/C catalyst with different rotation rates. In the high-frequency regime, minor deviations occur, which can be attributed to a small change in the charge transfer resistance of the Volmer-Heyrovsky and Volmer-Tafel steps. Accordingly, R ct,1 and R ct,2 vary only slightly with different rotation rates, as shown in figure 6(B). The explanation for the change is similar to the disc electrodes in acidic media, as mentioned above. Since R ct,2 is considerably smaller than R ct,1 for all selected rotation rates, the Volmer-Tafel pathway is the dominating mechanism in hydrogen reactions for the Pd/C catalyst. In contrast, in the low-frequency range of the impedance spectra, significant differences occur due to the change in diffusion via different rotation rates. This is illustrated by the change in Ws c , as shown in figure 6(C). The Bode plots present further information in figure S11. As elucidated earlier in the discussion for the disc electrodes, an inversely linear dependency with the square root of the rotation rate is expected (see figures 3(C) and 5(C) for Pt(pc) and 3(D) and 5(D) for Pt 5 Gd). For the Pd/C catalyst, this linear trend is not observed. One possible explanation can be the complex structure of the nanostructured catalyst leading to different diffusion properties compared to disc electrodes. In addition, figure S10 provides further essential parameters in the EEC as a function of the square root of the rotation rate for the Pd/C catalyst in 0.1 M HClO 4 .

Summary and conclusions
In this work, the EIS combined with the hydrodynamic RDE technique provides an essential approach to investigating the different reaction mechanisms of hydrogen evolution in acidic and alkaline media under reaction conditions. The power of the strategy is demonstrated for Pt(pc) and Pt 5 Gd model systems. In general, Pt 5 Gd shows higher HER activities than Pt(pc), which could be ascribed to strain and ligand effects in both media. For both Pt(pc) and Pt 5 Gd electrodes, the activity increases with an increasing rotation speed of the electrodes. The EIS data recorded in 0.1 M HClO 4 varies stronger with the rotation rate than in 0.1 M LiOH. These findings indicate that the HER in acidic media depends more strongly on mass diffusion than in alkaline media. From a physical model of the EIS data, we can conclude that the reaction ratio of Volmer-Heyrovsky and Volmer-Tafel mechanisms are independent of the rotation rate in acidic media. However, the Volmer-Heyrovsky pathway becomes the dominating mechanism at lower rotation rates in alkaline media. The finite diffusion in the electrolytes is properly demonstrated by the Warburg short element with the RDE setup. In addition to the well-defined model systems of the disc electrodes, the more complex system of nanostructured Pd catalyst is investigated for hydrogen reactions, revealing the Volmer-Tafel pathway as the dominating mechanism. The combined study of impedance and RDE measurements provides a powerful tool for further investigations of electrolyte effects and other critical reactions for electrocatalysis in the future.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors.