Size-Dependent Hydrogen Oxidation and Evolution Activities on Supported Palladium Nanoparticles in Acid and Base

The study of particle size effect provides the fundamental understanding of the active sites that is necessary for guiding the design and development of better catalysts. Here we report a systematic investigation of particle size effect of hydrogen oxidation and evolution reaction (HOR/HER) on carbon supported Pd nanoparticles with sizes ranging from 3 to 42 nm in both acidic and alkaline electrolytes using rotating disk electrode (RDE) method. Similar particle size effect was obtained in both acid and base: the HOR/HER activity in terms of specific exchange current density increases as Pd particle size increases from 3 to 19 nm, and then reaches a plateau with activity similar to that of bulk Pd. The enhanced activity with rising particle size could be attributed to the increased ratio of the sites with weaker hydrogen binding energy revealed in cyclic voltammograms (CVs). Pd catalysts with different Pd particle sizes all showed much higher HOR/HER activity in acid than in base, which is largely attributed to their smaller hydrogen binding energy in acid evidenced by the lower potential of the underpotential deposited hydrogen (Hupd) peak in CVs as well as the smaller activation energy in acid. © The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0661606jes] All rights reserved.

Hydrogen oxidation and evolution reactions (HOR/HER) are of substantial practical importance for their application in H 2 -fueled fuel cells and water electrolyzers. The high HOR/HER activity on Pt in acid (exchange current density of about 216 mA/cm 2 Pt at 313 K 1 ) enables the low loading of Pt (e.g. ≤0.05 mg Pt /cm 2 ) at the anode of a proton exchange membrane fuel cell (PEMFC). The cathode of a PEMFC however still requires a high loading of Pt (e.g. 0.2 mg Pt /cm 2 ) because the oxygen reduction reaction (ORR) in acid is sluggish. By contrast, hydroxide exchange membrane fuel cells (HEMFCs) offer the possibility to replace precious metals (e.g. Pt) required for both the ORR on the cathode and HOR on the anode with non-precious metals or metal-free catalysts owing to their alkaline nature, which could reduce the cost of fuel cells substantially. While much progress has been made in the search for non-precious metal or metal-free catalysts for ORR in both acid [2][3][4] and base, 5,6 non-precious metalbased catalysts with sufficient activity and stability for HOR remain a challenge. 7 Additionally, the HOR/HER activity on Pt in base is about two-orders of magnitude lower than that in acid 1,8 and thus a much higher Pt loading is needed on the anode of an HEMFC in order to achieve the same anode performance as a PEMFC. Therefore, the development of affordable and active HOR catalysts is highly desirable.
Pd, a member of the Pt-group-metals, is not only 50% cheaper than Pt ($724/oz for Pd vs. $1449/oz for Pt for 2010 -2015 averaged price) but also about 50 times more abundant in the known natural reserves. Pd and Pd-based catalysts have been studied as HOR catalysts in fuel cells, 9,10 and lower HOR/HER activities were observed on Pd/C than bulk Pd, 11-13 suggesting a particle size effect. However, a fundamental understanding of HOR/HER kinetics on Pd and a systematic study of the particle size effect are largely missing.
Particle size effect on Pd/C has been investigated for various reactions including ORR in both acid and base 14,15 and formic acid oxidation reaction in acid. 16 For HOR/HER, studies are mainly focused on Pt. Antoine et al. reported an increase of specific and mass HOR activity for smallest Pt particles; 17 whereas Sheng et al. showed no particle size effect for HOR/HER on Pt in alkaline media as they obtained similar exchange current densities on both 2 nm Pt nanoparticle supported on carbon and bulk polycrystalline Pt. 8 Sun et al. reported increased activity with increased particle size for HOR/HER on 2-7 nm Pt nanoparticles in acid, 18 and Ohyama et al. reported the same trend for HOR/HER on 2-4 nm Pt nanoparticles in base. 19 However, the rotating disk electrode (RDE) method cannot be used to measure HOR/HER activity on Pt in acid due to the overlapping polarization and concentration overpotential curves, 8,20 and thus the results from Sun et al. 18 via RDE measurements might not be reliable. Using the H 2 -pump method in a proton exchange membrane fuel cell configuration, Durst et al. characterized the exchange current densities of HOR/HER on 2-9 nm Pt/C and reported no particle size effect. 21 Clearly, the discrepancy still exists for HOR/HER on Pt with respect to particle size effects. Ohyama et al. explored the particle size effect of HOR/HER on Ru nanoparticles in 0.1 M NaOH, and their results indicated an optimum exchange current density at particle size of 3 nm due to the optimum ratio of amorphous-like Ru on the surface at 3 nm. 19 Zheng et al. reported that the specific HOR/HER exchange current density on Ir/C in 0.1 M KOH increases as particle size increases from 3 to 12 nm. 22 In this study, we investigate the particle size effect for HOR/HER via RDE measurements in both acidic and alkaline electrolytes on Pd/C catalysts with the Pd particle size ranging from 3 to 42 nm. The HOR/HER activity in terms of specific exchange current density (i 0 ) increases with rising Pd particle size in both acid and base.

Preparation and physical characterization of palladium nanoparticles with different sizes.-Carbon-supported
Pd nanoparticles with varying particle sizes were prepared by annealing the commercial 20 wt% Pd on Vulcan XC-72 (Pd/C, Premetek Co.) at different temperatures in an Ar/H 2 atmosphere in a tube furnace: the quartz tube was first purged with Ar for 30 min at room temperature to remove air. Furnace temperature was then raised to 100 • C at a rate of 10 • C/min in Ar and maintained at 100 • C for 30 min to remove adsorbed H 2 O on the catalyst. Finally, H 2 was introduced into Ar (5 vol. % H 2 ) and the temperature was increased to 300 • C, 400 • C, 500 • C and 600 • C at a rate of 10 • C/min and maintained for 2 h. 14 The obtained catalysts are denoted as Pd/C-300C, Pd/C-400C, Pd/C-500C and Pd/C-600C, respectively. Diameters of Pd nanoparticles were measured from transmission electron microscopy (TEM) images obtained on a JEOL JEM-2010F TEM. X-ray diffraction (XRD) patterns of Pd samples were measured using a Philips X'Pert X-ray diffractometer with Cu Kα radiation.
Electrochemical measurements.-The electrochemical measurements were conducted in a three-electrode glass cell with a doublejunction silver/silver chloride (Ag/AgCl) electrode as the reference electrode, a Pt wire as the counter electrode and a glassy carbon (5 mm diameter, PINE Ins.) as the working electrode. The ink Transmission electron microscopy (TEM) images of (a) Pd/C, (b) Pd/C-300C, (c) Pd/C-400C, (d) Pd/C-500C, (e) Pd/C-600C and (f) number averaged particle size determined from TEM images for all Pd/C samples. Insets of (a-e) are the histograms of the particle size for each sample counted over 300 particles. dispersions of Pd/C samples were prepared by dispersing Pd/C (20 wt%) in 0.05 wt% Nafion isopropanol solution to a final concentration of 2 mg Pd/C /mL with ultrasonication for 1 h. The thin-film electrodes were prepared by pipetting 2.5 μL of the ink (2 mg Pd/C /mL) once, four times or six times to achieve a final loading of 5, 20 or 30 μg Pd /cm 2 disk . The Pd loading on the electrodes were 20 μg Pd /cm 2 disk for all the five Pd samples measured in 0.1 M KOH prepared from KOH tablet (85 wt%, 99.99% metal trace, Sigma Aldrich). For the samples measured in 0.1 M HClO 4 prepared by diluting 70 wt% HClO 4 (EMD) with DI water, the final loadings for Pd/C, Pd/C-300C, Pd/C-400C were lowered to 5 μg Pd /cm 2 disk so that the HOR/HER polarization curves will not overlap with the concentration overpotential curve, and those for Pd/C-500C and Pd/C-600C were 20 μg Pd /cm 2 disk and 30 μg Pd /cm 2 disk to maintain roughness factors greater than 1 due to their low electrochemical active surface areas (ECSAs). Cyclic voltammetry experiments were performed in both Arsaturated 0.1 M KOH and 0.1 M HClO 4 electrolytes at a scanning rate of 50 mV/s in the potential range of 0.07 to 1.25 V vs. reversible hydrogen electrode (RHE). All the potentials reported were converted to the RHE scale. The electrochemical surface areas of all the Pd/C samples were determined from PdO reduction peak from cyclic voltammograms (CVs) based on a charge density of 424 μC/cm 2 Pd . 23,24 Activity measurements for hydrogen oxidation and evolution reactions (HOR/HER) on Pd catalysts were carried out in H 2 -saturated 0.1 M KOH or 0.1 M HClO 4 at a scanning rate of 50 mV/s and a rotating speed of 1600 rpm by cycling the potential several times until a stable polarization curve was obtained. The scanning rate was then switched to 1 mV/ to minimize the contribution of capacitance current, and the positive scan of the first cycle at 1 mV/s was reported as the HOR/HER polarization curve. The internal resistance was determined by electrochemical impedance spectroscopy (EIS) measured from 300 kHz to 100 mHz at open circuit voltage right after the HOR/HER measurement, which was then used to correct the measured potential to iR-free potential based on the following equation, where E is the measured potential, i is the corresponding current, R is the internal resistance, and E i R− f ree is the internal-resistance free potential. The internal resistances in 0.1 M KOH and 0.1 M HClO 4 were determined to be about 40 and 20 for, respectively.

Results and Discussion
TEM images.-The Pd nanoparticle size increases as the annealing temperature rises: the number-averaged particle diameter (d n TEM ) calculated by d T E M n = n i=1 d i /n grows from 3.2 nm for Pd/C to 33.6 nm for Pd/C-600C as shown by the TEM images and the corresponding histograms (Figure 1). The increase of standard deviation of d n TEM as particle size becomes larger ( Figure 1f and Table I) is caused by the thermal annealing method, which is consistent with literature. 22,25 The volume/area averaged particle diameters (d v/a TEM ) calculated by are larger than d n TEM (Table I). The d v/a TEM is considered a better measure of average particle diameter than d n TEM for calculating the specific surface area (S v/a TEM ) 26 (Table I),  according to where ρ is the density of the metal (12.02 g/cm 3 for Pd), d is the particle diameter in nm, S v/a TEM is in unit of m 2 /g. , respectively. The width of diffraction peaks width becomes narrower as the annealing temperature increases, suggesting an increase in the particle size, which is consistent with the TEM observations. The particle diameter determined from XRD, denoted as the volume-averaged particle diameter (d v XRD ), 26 can be calculated using the Scherrer equation, where λ is the wavelength of the X-ray wavelength (1.54 Å), (2θ) is the full width of the half maximum (FWHM) of the diffraction peak and θ is the Bragg angle. Pd(111) peak was used in the particle size calculations. The d v/a TEM and d v XRD agree well for Pd/C, Pd/C-300C, Pd/C-400C and Pd/C-500C. However, the average particle diameter determined by TEM is significantly larger than that determined by XRD for Pd/C-600C (Table I), which can be caused by: 1) the width of XRD patterns reflects the primary crystalline size, while particles in TEM images could be polycrystalline, i.e., agglomerates of several primary crystals. The larger the particles are, the more likely the particles are polycrystalline. 2) The particle size distribution of Pd/C-600C is broad (Figure 1f), which could introduce errors in calculating the average particle size with by counting a finite number of particles. Specific surface areas of different Pd samples calculated from d v XRD using Eq. 2, denoted as S v XRD , are comparable with S v/a TEM (Table I).
Electrochemical surface area (ECSA) measurement from cyclic voltammograms.-Specific surface areas of the supported Pd catalysts can also be measured from cyclic voltammograms ( Figure 3). The lower limit was chosen to be 0.07 V to avoid the diffusion of H atom into the Pd lattice. 11 In 0.1 M KOH, the underpotential deposited hydrogen (H upd ) adsorption and desorption peaks in the potential region of 0.07-0.45 V are very asymmetric and the double layer regions are not well defined (Figure 3a), making it difficult to determine sur-   (Figure 3b), both of which are at a lower potential than the desorption peak in base (0.35 V), and could represent different facets of Pd. When the current is normalized to the peak current after the double layer current deduction, the height of the H upd desorption peak at about 0.26 V is fixed while that of the shoulder at about 0.19 V increases with increases of particle size (Figure 3c). The change of line shape of the H upd desorption peak suggests a redistribution of facets in different Pd samples, which is consistent with a previous report on Pd catalysts. 15 The PdO reduction peak shifts to a more positive value as Pd particle size increases (from Pd/C to Pd/C-600C sample) in both KOH and HClO 4 (Figures 3a and 3b), indicating a weaker Pd-O binding as the Pd particle size increases. In this study, we use integrated area of the PdO reduction peak to determine the electrochemical surface area (ECSA) of supported Pd catalysts. ECSAs calculated from CV conducted in KOH (S Base CV ) and HClO 4 (S Acid CV ) are comparable (Table I), in agreement with Henning et al.'s earlier observations. 13 Consistent with the surface area determined by TEM and XRD, ECSA decreases with increasing particle size (Table I), however, ECSAs are roughly a factor of two smaller than S v/a TEM and S v XRD (Table I). One potential cause for this discrepancy is that a fraction of every particle is in contact with the support and thus is not electrochemically accessible. Alternative methods such as H upd adsorption/desorption peaks and CO-stripping can also be used to determine surface area, all of which have advantages and drawbacks. 13 Although the ECSAs derived from PdO reduction might not be the most accurate in absolute terms, the general trend is consistent with values obtained with other methods. Thus, it is still meaningful to compare activities normalized by ECSAs obtained with PdO reduction.
HOR/HER kinetics.-The HOR/HER polarization curve on Pd/C measured in 0.1 M KOH before (black line) and after (red line) iR correction deviate significantly from the concentration overpotential curve (grey-dash line) and reach the HOR limiting current at overpotentials above 0.3 V (Figure 4a). The concentration overpotential curve can be calculated using the following equation, where I d is the diffusion limited current, I l is the maximum HOR limiting current, F is the Faraday constant, η is overpotential, R is the gas constant, and T is temperature in Kelvin. The HOR/HER polarization  (Figures 5a and 5b). The HOR limiting current densities for all the polarization curves in base are about 2.9 mA/cm 2 disk (Figure 5a) which agrees with the theoretical values within experimental errors. Similar HOR limiting currents of about 2.9 mA/cm 2 disk were observed in acid as in base (Figure 5b). Atomic hydrogen is known to be able to diffuse into the Pd lattice (H bulk ), therefore the HOR/HER process might be accompanied by hydrogen absorption/desorption reaction (Eq. 5), which could appear as an extra oxidation peak in the polarization curve. 13,27 Pd − H bulk ↔ Pd − H upd [5] Smaller or no extra peaks associated with oxidation of absorbed hydrogen were observed in the polarization curves in 0.1 M KOH To determine the exchange current densities of HOR/HER, the kinetic currents (I k ) were first calculated according to the Koutecky-Levich (Eq. 6), 20,22 [6] where I is the measured current and I d is the diffusion limited current defined in Eq. 4. The kinetic currents are then normalized by their corresponding Pd surface area (which are referred to as specific kinetic current densities, i k ) and plotted as a function of overpotential. A linear relationship was observed between i k and η in the micropolarization regions (Figures 5c and 5d) where i k and η can be fitted into the linearized Butler-Volmer equation with the assumption that the summation of anodic and cathodic transfer coefficient equals to 1 (α a + α c = 1) (Eq. 7) to obtain the exchange current density (i 0 ).
Alternatively, the kinetic currents can also be fitted into the Butler-Volmer equation (Eq. 8) to get the exchange current densities and transfer coefficients (Figures 5e and 5f).
The specific exchange current density determined using linear fitting for Pd/C in 0.1 M KOH at 293 K is 0.052 ± 0.002 mA/cm 2 Pd (Table II), which is higher than the value obtained on Pd/C reported (0.06 ± 0.02 mA/cm 2 Pd at 313 K corresponding to 0.02 mA/cm 2 Pd at 293 K). 1 The Butler-Volmer fitting with α a + α c = 1 yields a very similar exchange current density value of 0.051 ± 0.002 mA/cm 2 Pd and a α a value of 0.54 ± 0.02 (Table III) which corresponds to an anodic Tafel slope about 108 mV/dec. This suggests that the Volmerstep (H ad ↔ H + + e) is the rate-determining step, in agreement with  1 Without constraining α a + α c = 1, the best fitting into the Butler-Volmer equation generates α a value of 0.45 ± 0.01 and α c value of 0.38 ± 0.02 (Table III), and a slightly higher exchange current density of 0.060 ± 0.003 mA/cm 2 Pd . The HOR/HER activity on Pd/C is much higher in 0.1 M HClO 4 than in 0.1 M KOH -the specific exchange current densities determined from linear fitting and Butler-Volmer fitting with α a + α c = 1 on Pd/C are 2.56 ± 0.12 and 2.70 ± 0.04 mA/cm 2 Pd respectively, which represent a 50-fold increase of that in 0.1 M KOH. The exchange current density determined using a H 2 -pump in a proton exchange membrane fuel cell configuration (which resembles that in electrolyte with pH = 0) is 5.2 ± 1.2 mA/cm 2 Pd at 313 K, 1 which can be converted to 2.3 ± 0.5 mA/cm 2 Pd at 293 K according to Arrhenius equation with an activation energy of 31 kJ/mol. 27 The excellent agreement in the exchange current densities measured using RDE in this work and H 2pump indicates the validity of RDE method to characterize HOR/HER activity on Pd/C in acid.

Particle size effect for HOR/HER.-
The specific exchange current density of HOR/HER increases as the Pd particle size increases from 3 to 19 nm and then levels off afterward in both acid and base (Table II and Figure 6a). The specific exchange current density in base (from linear fitting) reaches about 0.12 mA/cm 2 Pd as the particle size exceeds 20 nm. Sheng et al. reported an exchange current density value of 0.13 mA/cm 2 Pd on polycrystalline bulk Pd disk electrode in 0.1 M KOH by extrapolating from the Tafel plot in the HER region to equilibrium potential (0 V). 11 Alia et al. obtained an exchange current density of 0.18 mA/cm 2 Pd by fitting the kinetic current from HOR/HER polarization curve into the Butler-Volmer equation. 12 The specific exchange current density on Pd nanoparticles larger than 20 nm approaches that of a bulk Pd electrode. Specific exchange current densities determined from the following three methods show good agreement: 1) linear fitting (black squares in Figure 6a); 2) Butler-Volmer fitting with α a + α c = 1 (red circles in Figure 6a); and 3) Butler-Volmer fitting with unconstrained α a + α c (blue triangles in Figure 6a). The mass exchange current density shows a general decreasing trend with the growing particle size (Figure 6b), due to the decrease of ECSA.
Transfer coefficient (α) and Tafel slope (TS) are related according to TS = 2.303 RT/αF, which are important parameters to reveal the reaction mechanism. For HOR/HER on different-sized Pd particles in 0.1 M KOH, the α a obtained from the Butler-Volmer fitting decreases from 0.54 on Pd/C (3 nm) to 0.45 on Pd/C-600C (42 nm) when constraining α a + α c = 1 (Table III), which corresponds to a change of anodic TS from 108 to 129 mV/dec. However, the R 2 value for the regression is relatively low (0.991). By contrast, a much better Butler-Volmer fitting can be achieved by removing the constraining of α a + α c = 1 (R 2 = 0.999). Still though the α a decreases with increase of Pd particle size (from 0.45 to 0.32) which translates to a TS change from 129 to 182 mV/dec, while the α c 's are about 0.38 (TS = 153 mV/dec) and barely change with particle size (Table III). In 0.1 M HClO 4 , the α a value ranges from 0.57 to 0.71 when the fitting was conducted with α a + α c = 1 (Table III). Only currents in the narrow potential regions (-20 mV to 20 mV) can be used for data analysis in acid, thus introducing a large degree of uncertainty in the curve fitting. As a result, we did not calculate α a and α c without constraining α a + α c = 1. mV/dec on Pd/Au(111) at 293 K. 28 The particle size effect of HOR/HER activity on Pd/C could be attributed to the redistribution of surface facets among different-sized Pd nanoparticles (known as geometric effect), as revealed by the change in peak ratios in the H upd desorption peak profiles (Figure 3c), together with the possible structure sensitivity of the reaction activity on different facets. Studies on single crystalline Pt showed that the exchange current density of HOR/HER increases in the order of Pt(111) < Pt(100) < Pt(110) in both acidic 29 and alkaline 30 electrolytes. HOR/HER activities on single crystalline Pd surfaces have not been reported, which is likely also due to diffusion of hydrogen atoms into the bulk of single crystal samples. It is reasonable to conclude that HOR/HER on Pd is structure sensitive based on the particle size dependence of HOR/HER activities in this study. The particle size effect of HOR/HER on carbon supported Ir nanoparticle in  alkaline electrolyte also suggests the possibility of structure sensitivity of HOR/HER on Ir. 22 Additionally, the H upd desorption peak potential is related to hydrogen binding energy (HBE) as in Eq. 9, which has been proposed to be a dominating descriptor for HOR/HER activities. 22 where E peak is the H upd desorption peak potential, and S 0 H 2 is the standard entropy of H 2 . Since Pd belongs to the strongly-binding branch in the "volcano plot" (HOR/HER exchange current densities vs. HBE), 11,31 sites with weaker HBE will possess higher HOR/HER activity. Zhou et al. deconvoluted the H upd desorption in the cyclic voltammogram of Pd/C obtained in 0.1 M HClO 4 , and attributed the peaks to low-index Pd facets (111), (100) and (110). 15 In the current study, the sites associated with the H upd desorption peak centered at around 0.19 V on Pd samples in 0.1 M HClO 4 bind to hydrogen more weakly than those associated with peak centered at around 0.24 V, and thus should have higher HOR/HER activity. The increased ratio of the sites with lower HBE with rising particle size is responsible for the improved HOR/HER activity from untreated Pd/C (about 3 nm) to Pd/C-500C (about 19 nm). Similar result has been observed on Ir/C where the exchange current density and the fraction of sites with lowest HBE increase with particle size and a good correlation between exchange current density and the fraction of sites with lowest HBE is obtained. 22 Meanwhile, it is generally accepted that larger particles contain less defect sites compared with smaller particles. The lower ratio of the defect sites with low coordination number and a weaker binding strength to adsorbates (Pd-H binding in this case) in larger nanoparticles could lead to a higher HOR/HER activity. Therefore, Pd nanostructures with extended surfaces and less defect sites such as Pd nanowires (PdNWs) or Pd nanotubes (PdNTs) will have higher activity toward HOR/HER than Pd nanoparticles. Indeed, a much higher exchange current density of 0.96 mA/cm 2 Pd was obtained on PdNTs synthesized via galvanic displacement reaction using copper nanowires (CuNWs) as templates, which is even about 5 times of bulk Pd possibly attributed to the small amount of Cu left in Pd lattice. 12  (Table II and Figure 6a). Durst et al. reported that the exchange current density on Pd/C (about 3 nm) measured using H 2 pump in a PEMFC configuration (equivalent to pH = 0) is about 90 times of that obtained in 0.1 M NaOH using RDE method at 313 K, 1 which is consistent with our results. Consensus has not been reached regarding the root cause for the higher HOR/HER activity in acid than in base on Pt or Pt group metals so far: whether HBE is the sole and unique descriptor for HOR/HER 1,22,32,33 or there is a change in reaction mechanism from acid to base and adsorption OH will facilitate HOR/HER in base. 34,35 The H upd adsorption/desorption peaks on Pd samples in CVs measured in 0.1 M HClO 4 are located at more negative potentials compared with those measured in 0.1 M KOH (Figures 3a and 3b), suggesting a weaker Pd-H binding energy in acid than in base. Therefore, HOR/HER should have a smaller activation energy in acid according to the Brønsted-Evans-Polanyi (BEP) principle. In this regard, we measured HOR/HER polarization curves on Pd/C in both 0.1 M HClO 4 and 0.1 M KOH at temperatures from 275 to 313 K, and determined the exchange current densities at each temperature to generate the Arrhenius plots (Figure 7). The apparent activation energy of HOR/HER on Pd/C in 0.1 M HClO 4 is 32.3 ± 0.7 kJ/mol, which is similar to that reported when the exchange current densities were determined using H 2 -pump (31 ± 2 kJ/mol). 27 A higher activation energy is obtained in 0.1 M KOH (38.9 ± 3.0 kJ/mol) than in 0.1 M HClO 4 , which confirms our prediction. In addition, the activation energies of HOR/HER on Pt/C, Pd/C and Ir/C in 0.1 M KOH are in the sequence of Pt/C (29.5 ± 4.0 kJ/mol) 8 < Ir/C (32.9 ± 1.5 kJ/mol) 22 < Pd/C (38.9 ± 3.0 kJ/mol, this work), which correlates well to the sequence of HOR/HER activity in base in the order of Pt/C (0.57 ± 0.07 mA/cm 2 Pt ) 8 > Ir/C (0.21 ± 0.02 mA/cm 2 Ir ) 22 > Pd/C (0.052 ± 0.002 mA/cm 2 Pd , this work). Similar correlation between the activation energy and the exchange current density on Pt/C, Pd/C and Ir/C is also valid in acid. 27 Therefore, the higher Pd-H binding energy in base is likely responsible for the lower HOR/HER activity.

Conclusions
In summary, we studied the kinetics of HOR/HER on Pd catalysts with different particle sizes ranging from 3 to 42 nm in both acidic and alkaline electrolytes using RDE measurement. Similar particle size effects were observed on Pd in acid and base: the HOR/HER specific exchange current density increases as Pd particle size increases and then levels off at about 19 nm while the mass exchange current density increases slightly initially, and then decreases with the increase of particle size in both acidic and alkaline electrolytes. This particle size effect suggests that HOR/HER activity on Pd is structure sensitive with the sites of lower HBE (possibility low-index facets) being more active. The HOR/HER activities on Pd in acid are about 50 times of those in base, which is likely due to the lower Pd-H binding energy in acid revealed from the negative shift of H upd peak from base to acid as well as the smaller activation energy obtained in acid (32.3 ± 0.7 kJ/mol) than in base (38.9 ± 3.0 kJ/mol).