H-Implanted Pd Icosahedra for Oxygen Reduction Catalysis: From Calculation to Practice

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Aug 2021H-Implanted Pd Icosahedra for Oxygen Reduction Catalysis: From Calculation to Practice Lingzheng Bu†, Xiaorong Zhu†, Yiming Zhu†, Chen Cheng, Yafei Li, Qi Shao, Liang Zhang and Xiaoqing Huang Lingzheng Bu† College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123 State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 , Xiaorong Zhu† Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, Jiangsu Key Laboratory of New Power Batteries, School of Chemistry and Materials Science, Nanjing Normal University, Jiangsu 210023 , Yiming Zhu† College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123 , Chen Cheng Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Jiangsu 215123 , Yafei Li Jiangsu Collaborative Innovation Centre of Biomedical Functional Materials, Jiangsu Key Laboratory of New Power Batteries, School of Chemistry and Materials Science, Nanjing Normal University, Jiangsu 210023 , Qi Shao College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123 , Liang Zhang Institute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Jiangsu 215123 and Xiaoqing Huang *Corresponding author: E-mail Address: [email protected] College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, Jiangsu 215123 State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005 https://doi.org/10.31635/ccschem.020.202000319 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail Although palladium (Pd) has gradually emerged as the most likely candidate to replace platinum (Pt) in the oxygen reduction reaction (ORR), the specific electronic structure of conventional Pd results in too strong Pd–O binding strength and unsatisfactory ORR performance. Herein, guided by density functional theory, we have explored a strategy to expand lattice spacing by implanting hydrogen (H) atoms in Pd nanocrystals (NCs), which realizes decreased electron density to boost ORR. We found that all H-implanted Pd NCs show universally promoted ORR activities compared with their corresponding Pd NCs counterparts. Significantly, the H-implanted Pd icosahedra/C with twinned {111} facet not only exhibits the highest ORR activity among all studied catalysts, but also possesses good durability with limited changes after 30,000 cycles and excellent chemical stability with preservation up to 5 months. X-ray absorption of the fine structure demonstrates that enhanced ORR performances of H-implanted Pd NCs can be attributed to the reduced coordination number and valence band, as a result of the incorporation of H atoms. Furthermore, the twinned {111} facet in H-implanted Pd icosahedra is the critical factor in further minimizing the ORR barrier and stabilizing key reaction intermediates, resulting in the highest properties among all investigated catalysts. Download figure Download PowerPoint Introduction To solve the increasingly serious energy and environmental crises, direct methanol fuel cells (DMFCs) have attracted considerable attention as energy sources due to their high efficiency and low consumption.1–3 However, the kinetically sluggish oxygen reduction reaction (ORR) in the cathode is a major bottleneck for the widespread application and commercialization of DMFCs.4–7 To bridge the gap, tremendous efforts have been devoted to exploring promising electrocatalysts for ORR, among which platinum (Pt)-based nanomaterials with high activity are regarded as the most ideal candidates.8–12 Nevertheless, the scarcity and high price of Pt remain large obstacles for their practical applications.6,13,14 Therefore, exploring cost-effective non-Pt nanocatalysts with competitive ORR performances becomes increasingly important. Palladium (Pd) has gradually emerged as a promising alternative to replace Pt because it is more abundantly available and has the potential to outperform Pt in ORR.15–18 However, conventional Pd catalysts usually exhibit unsatisfied ORR performances, which is mainly due to the slow electron-transfer rate and strong Pd–O binding strength derived from the intrinsic electron structure of Pd.19–21 Therefore, seeking effective strategies that can tune the electronic properties of Pd for improving ORR kinetics is essential. Recently, light elements with small atomic radii, which can easily permeate into metals to expand lattice spacing, have been paid tremendous attentions. Essentially, the changed lattice parameters of metals can result in different behaviors of charge transfer between host and guest, generating available electron structures for catalysis. Among various light atoms, hydrogen (H) atoms are supposed to be the most appropriate participants to implant into metal due to their smallest atomic radius and favorable affinity.22–26 In the view of this point, a potential method was presented by incorporating H atoms into the space lattice of Pd, which may provide a fundamental pathway for addressing the relatively poor ORR performances of Pd catalysts. Herein, we initially performed density functional theory (DFT) calculations on H-implanted Pd nanocrystals (NCs) to investigate their electronic behaviors in ORR. Corresponding results theoretically verify that H-implanted Pd NCs with decreased electronic densities process alleviated Pd–O binding strength, benefitting reaction rate acceleration and promotion of catalytic activity. With the guidance of DFT, a class of H-implanted Pd NCs with optimized electronic structures were successfully synthesized as targeted nanomaterials via a wet-chemical approach. As expected, all shaped H-implanted Pd NCs exhibit generally enhanced ORR performances compared with Pd/C and Pt/C, as well as the corresponding Pd NCs counterparts. Particularly, H-implanted Pd icosahedra/C featured with twinned {111} facets not only displays the highest activity among all studied catalysts, but also demonstrates excellent lifetime and stability with limited changes even after 5 months run time or 30,000 potential cycles. X-ray absorption fine structure (XAFS) results further confirm that the incorporated H atoms can effectively reduce the coordination number and bandwidth of valence band for Pd catalysts, boosting the catalytic activities of H-implanted Pd NCs. DFT calculations also reveal that the twinned {111} facet in the H-implanted Pd icosahedron plays an important role in adjusting the binding strength with key intermediates and minimizing the ORR barrier, resulting in the highest ORR performance among all investigated catalysts. This work displays that the theory-guided H-implanted Pd icosahedron can be developed as an advanced non-Pt nanocatalyst with high performance for fuel cells and beyond. Experimental Methods Preparation of various Pd NCs For the synthesis of Pd icosahedra, 7.5 mg sodium tetrachloropalladate (II) (Na2PdCl4), 100 mg polyvinylpyrrolidone (PVP), 35.6 mg L-ascorbic acid (AA), and 10 mL N,N-dimethylformamide (DMF) were added into a vial with volume of 35 mL. After the vial had been capped, the mixture was ultrasonicated for 30 min. The resulting homogeneous mixture was then heated from room temperature to 140 °C in 20 min and maintained at 140 °C for 5 h in an oil bath, before it was cooled to room temperature. The resulting colloidal products were collected by centrifugation and washed with an ethanol/acetone (v/v = 1/9) mixture. The synthesis conditions for the Pd tetrahedra are similar to those of the Pd icosahedra except by using palladium (II) acetylacetonate [Pd(acac)2, 7.5 mg] as the Pd precursor and replacing AA with hexacarbonylmolybdenum [Mo(CO)6, 5 mg] and formic acid (100 μL). The reaction temperature was changed to 120 °C. The synthesis conditions for Pd nanocubes are similar to those of Pd icosahedra except for replacing AA with formic acid (300 μL) and adding sodium iodide (NaI, 10 mg) in the synthesis. The reaction temperature was changed to 120 °C. Preparation of various Pd–H NCs/C The Pd NCs dispersed in 5 mL ethanol and 5 mg Vulcan XC-72C dispersed in 5 mL ethanol were mixed and sonicated for 1 h to make Pd NCs/C. The Pd NCs/C was collected by centrifugation and washed with ethanol twice. About 4 mg Pd NCs/C and 15 mL DMF were added into a vial with volume of 35 mL. After the vial had been capped, the mixture was ultrasonicated for 30 min. The resulting homogeneous mixture was then heated from room temperature to 160 °C in 30 min and maintained at 160 °C for 28 h in an oil bath, before it was cooled to room temperature. The resulting products were collected by centrifugation and washed with ethanol. The products were then dried under ambient conditions. For the preparation of commercial Pd–H/C, 7 mg commercial Pd/C was directly used in the transformation. DFT models and calculations In this work, we implement the VASP code to perform the first principle DFT calculations with exchange–correlation interactions modeled by GGA-PBE functional to determine the electronic energies of O-containing intermediates along the oxygen reduction dissociation path on Pd–H surfaces.27,28 We use Pd (111) and Pd (200) as benchmarks to evaluate the improved oxygen reduction activity when introducing H elements to the systems. We constructed a 3 × 3 supercell with nine exposed Pd atoms on the Pd (111), Pd (200) surfaces and nine Pd atoms, 9H atoms on Pd–H (111), Pd–H (200), Pd–H (111)-twin surfaces. These slabs were constructed with six atomic layers and a vacuum space of 15 Å along the z-direction, of which the top two layers were allowed to relax. Specifically, the twin-crystal Pd–H (111) was obtained by combining the (111) and (11-1) slab. Plane-wave cutoff energy of 420 eV with Fermi-level smearing of 0.05 eV for slabs and 0.01 eV for gas-phase species was used in all calculations. The k-space samplings were set as 3 × 3 × 1 for the geometric optimization. The convergence thresholds of energy and forces were set as 1 × 10−5 eV and 0.02 eV Å−1, respectively. The solvent effect toward the intermediate adsorption was achieved using the Poisson–Boltzmann implicit solvation model with a dielectric constant of 80.29 Results and Discussion Calculation study on different Pd/Pd–H surfaces for ORR In general, Pd-based nanocatalysts exhibit poor ORR properties, severely restricting their broad application in fuel cell technology. To address this issue, we first used DFT calculations to analyze the intrinsic origins from the perspective of facets for Pd-based nanocatalysts. We performed DFT calculations based on the computational hydrogen electrode (CHE) model.30 In the ORR procedure, the reduction is initialized with O2 first being adsorbed on the catalyst surface. Due to the strong adsorption ability of Pd-based systems, the reactant O2 can be effectively activated and then spontaneously dissociated into two adsorbed *O species. Therefore, we mainly focus on the dissociative mechanism, as it appears to be the most likely reduction pathway. As a result, the energy released via breaking the O–O bond splitting on Pd and Pd–H surfaces follows a decreasing trend of Pd (111), Pd (200), Pd–H (111)-twin, Pd–H (111), Pd–H (200), and so does the relative stability of two adsorbed *O species (Figures 1a and 1b). We then take the Pd–H (111)-twin as an example to analyze the geometric structure change along the splitting process. The results indicate that the O–O bond of O2 can be extended from 1.32 Å in its initial state to 3.01 Å in its final state, and the cracked *O favors anchoring at the Pd–H (111)-twin hollow sites (Figure 1c). Furthermore, the coadsorbed *O structure was 1.05 eV more stable than the O2 adsorption ones, which means the O2 cracking is beneficial on the Pd–H (111)-twin surface from both thermodynamic and kinetic perspectives. Figure 1 | DFT calculations for oxygen splitting energy and ORR reaction barrier on different Pd and Pd–H surfaces. The energy barrier for the oxygen splitting pathway on (a) Pd (200)/Pd (111) surfaces and (b) Pd–H (200)/Pd–H (111)/Pd–H (111)-twin surfaces. (c) The optimized geometric structures along the O2 splitting pathway. The blue, white, and red balls represent Pd, H, and O atoms, respectively. (d and e) The minimum energy profile of ORR on (d) Pd (200)/Pd (111) surfaces and (e) Pd–H (200)/Pd–H (111)/Pd–H (111)-twin surfaces. The onset potentials of ORR shown in (d and e) are 0.96 V, 1.44 V, 0.60 V, 0.48 V, and 0.43 V for Pd (200), Pd (111), Pd–H (200), Pd–H (111), and Pd–H (111)-twin surfaces, respectively. Download figure Download PowerPoint To further analyze the catalytic properties of different Pd and Pd–H facets, we calculated the minimum energy profile of ORR on Pd (111), Pd (200), Pd–H (111)-twin, Pd–H (111), and Pd–H (200) surfaces (Figures 1d and 1e). On Pd (111) and Pd (200) surfaces, the cracked *O species can be strongly attached to the surfaces, causing poisoning of active sites and high energy demand for the desorption of the first H2O on Pd (111) and the second H2O on Pd (200). The overpotential was calculated to be 1.44 and 0.96 V on Pd (111) and Pd (200) surfaces, respectively (Figure 1d). When introducing H atoms into the Pd lattice, the interaction between intermediates and catalyst surfaces can be greatly weakened, especially on the Pd–H (200) surface. Due to the smallest driven force for O2 splitting (Figure 1b), the limiting step throughout the ORR process on Pd–H (200) surface was predicted to be the O2 activation, and the overpotential for this step was 0.6 V (Figure 1e). While for Pd–H (111) surface, the combination effects between close-packed structure (strengthening binding strength) and introduced H (weakening binding strength) can effectively tune the adsorption behavior and reduce the overpotential of Pd–H (111) to 0.48 V (Figure 1e). The overpotential can be further reduced to 0.43 V when constructing the twin-crystal Pd–H (111) surface (Figure 1e). The limiting step under this circumstance became the desorption of the second H2O. Such low overpotential indicates that the twin-crystal Pd–H (111) surface has ultrahigh catalytic ability for ORR, which even outperforms the commercialized Pt (111) surface. These calculations demonstrate that the Pd–H (111)-twin facet should be a promising active surface for ORR. Practical construction of a unique Pd–H (111)-twin surface Encouraged by the DFT calculations, we began to prepare Pd–H NCs via a wet-chemical strategy. In the first step, uniform Pd icosahedra were synthesized as the starting NCs (details in the "Experimental Methods" section), as revealed by the transmission electron microscopy (TEM) image ( Supporting Information Figure S1a). We then loaded those Pd icosahedra on commercial carbon powder (C, Vulcan XC-72R) to prepare Pd icosahedra/C ( Supporting Information Figure S1b). In the next step, we transformed the obtained Pd icosahedra/C into Pd–H icosahedra/C ( Supporting Information Figure S1c) via a wet-chemical approach. After the addition of H atoms, the lattice distance was expanded from 0.22 nm of Pd icosahedra/C to 0.23 nm of Pd–H icosahedra/C, as revealed by the high-resolution TEM (HRTEM) images (Figure 2a and 2b). The powder X-ray diffraction (PXRD) pattern of Pd icosahedra/C exhibits a typical facet-centered cubic (fcc) structure (JCPDS No. 05-0681), which is consistent with the high-resolution transmission electron microscopy (HRTEM) results (Figure 2c). Interestingly, all the diffraction peaks of Pd–H icosahedra/C shift to lower degrees in comparison with the Pd icosahedra/C. The PXRD pattern of Pd–H icosahedra/C is in accordance with the standard PdH0.64 phase (JCPDS No. 84-0300), clearly demonstrating the successful creation of twinned {111} faceted Pd–H icosahedra/C (Figure 2d). During the in situ transformation strategy from Pd NCs/C to Pd–H NCs/C, Pd icosahedra/C catalyzed the decomposition of DMF. The H atoms originating from the decomposition of DMF were readily absorbed into the lattice of Pd icosahedra/C due to the strong interaction between Pd and H, leading to the expanded lattice distance of Pd icosahedra/C and the facile synthesis of Pd–H icosahedra/C, as illustrated in Figures 2e and 2f. Figure 2 | Construction of twinned (111) surface and corresponding structure analysis. HRTEM images of (a) Pd icosahedra/C and (b) Pd–H icosahedra/C. The insets in (a) and (b) are their corresponding 3D models. (c and d) PXRD patterns and corresponding crystal structures of (c) Pd icosahedra/C and (d) Pd–H icosahedra/C. (e) Schematic of the facile preparation of Pd–H nanostructures in DMF. (f) Atom models of lattice spacing changes from Pd to Pd–H. Download figure Download PowerPoint ORR performance of constructed Pd–H icosahedra/C We then explored the ORR performance of Pd–H icosahedra/C and benchmarked it against Pd icosahedra/C, commercial Pd/C ( Supporting Information Figures S2a and b), and Pt/C ( Supporting Information Figures S3a and b). Figure 3a shows the ORR polarization curves of Pd–H icosahedra/C, Pd icosahedra/C, Pd/C, and Pt/C carried out in O2-saturated 0.1 M KOH solution after considering the mass-transport correction. With the help of the Koutecký–Levich equation, the mass activity (MA) of each catalyst at 0.90 V versus reversible hydrogen electrode (RHE) can be calculated. We can see that Pd–H icosahedra/C exhibits the most positive half-wave potential of 0.909 V versus RHE among these four catalysts, revealing its highest ORR activity (Figure 3a). The MA Tafel plots for different catalysts also demonstrate the highest ORR activity of Pd–H icosahedra/C (Figure 3b). To be specific, the MA of Pd–H icosahedra/C at 0.90 V versus RHE was measured to be 0.550 A mg−1Pd, 4.1, 5.9, and 5.3 times larger than those of Pd icosahedra/C (0.134 A mg−1Pd), Pd/C (0.094 A mg−1Pd), and Pt/C (0.104 A mg−1Pd) (Figure 3c and Supporting Information Table S1), clearly showing that Pd–H icosahedra/C is highly active for ORR. Moreover, we also compared the ORR activity of Pd–H icosahedra/C with those of reported Pd- and Pt-based catalysts in alkaline electrolytes. As shown in Supporting Information Table S2, the ORR activity of Pd–H icosahedra/C is among the best reported activities of state-of-the-art Pd-based nanocatalysts to date, even higher than those of many Pt-based catalysts, clearly demonstrating the high electroactivity of Pd–H icosahedra/C for ORR. Figure 3 | ORR performances of Pd–H icosahedra/C and its counterparts. (a) ORR polarization curves and (b) ORR MA Tafel plots of different catalysts. (c) MA comparison of different catalysts. (d) ORR polarization curves of Pd–H icosahedra/C before and after 30,000 potential cycles between 0.6 and 1.1 V versus RHE. (e) The changes in normalized mass activities of different catalysts before and after 30,000 potential cycles between 0.6 and 1.1 V versus RHE. (f) ORR polarization curves and (g) the changes in normalized mass activities of Pd–H icosahedra/C tested after 5 months before and after 30,000 potential cycles between 0.6 and 1.1 V versus RHE. (h) HRTEM image of Pd–H icosahedra/C after storage for 5 months. Download figure Download PowerPoint To evaluate the electrochemical durability of Pd–H icosahedra/C, the accelerated durability tests (ADTs) of different catalysts for ORR were performed. Figure 3d and Supporting Information Figure S4 show the polarization curves of Pd–H icosahedra/C, Pd/C, and Pt/C before and after 30,000 potential cycles. After 30,000 sweeping cycles of ADTs, we found that the normalized MA loss for Pd–H icosahedra/C was calculated to be only 7.6%, showing high ORR stability (Figure 3e). By sharp contrast, the commercial Pd/C and Pt/C exhibited as large as 46.8% and 57.7% MA losses, respectively (Figure 3e). Moreover, Pd–H icosahedra/C, Pd/C, and Pt/C after 30,000 CV cycles of ADTs were studied by TEM and HRTEM ( Supporting Information Figures S2c and d, S3c and d, and S5a–c). Obviously, the size, shape, lattice spacing, and crystal phase of Pd–H icosahedra/C can be largely maintained after long-term ORR stability measurement ( Supporting Information Figure S5a–c), while the commercial Pd/C and Pt/C showed large size change and serious aggregation after 30,000 potential cycles of ADTs ( Supporting Information Figures S2c and d, S3c and d). All these results reveal that Pd–H icosahedra/C is electrochemically durable for ORR. Expect for catalytic activity and durability, many other evaluation criteria, including the lifetime and chemical stability of nanocatalysts, are also very important for their practical applications.31,32 To evaluate the lifetime and chemical stability for Pd–H NCs/C, we measured the ORR performance and characterized the structure of Pd–H icosahedra/C stored for 5 months under ambient conditions. As shown in Figure 3f, the Pd–H icosahedra/C after storage for 5 months exhibits the MA of 0.540 A mg−1Pd, very close to that of the fresh Pd–H icosahedra/C (0.550 A mg−1Pd). We also found that it can still maintain 91.5% of its initial MA after 30,000 more voltage cycles of ADTs, which demonstrates the long-term ORR stability of Pd–H icosahedra/C even after storage for 5 months (Figure 3g). Figure 3h shows the HRTEM image of Pd–H icosahedra/C after storage for 5 months, in which the lattice spacing of (111) plane is measured to be 0.23 nm, along with the well-maintained Pd–H phase ( Supporting Information Figure S5d), the excellent lifetime and chemical stability of Pd–H icosahedra/C under ambient conditions. enhanced for ORR Significantly, the ORR strategy is which can be readily extended to other Pd–H NCs/C with different including Pd–H Pd–H and even commercial In when the structure of Pd NCs is the corresponding Pd–H NCs can be which is facile for the preparation of Pd–H NCs. For when Pd Pd and commercial Pd/C were used as the starting NCs ( Supporting Information Figures S2a and and corresponding Pd–H NCs/C can be readily obtained (details in the "Experimental Methods" and PXRD results demonstrate the successful preparation of Pd–H Pd–H and commercial (Figure Supporting Information Figures and we found that the Pd–H NCs/C can exhibit generally enhanced ORR performances than those of corresponding Pd NCs/C ( Supporting Information Figure as well as commercial Pd/C and Pt/C ( Supporting Information Table To be specific, compared with corresponding Pd NCs/C, the MA factor for Pd–H Pd–H and can up to and respectively (Figure and times MA for ORR can be by these Pd–H NCs/C when compared with the commercial Pd/C and Pt/C (Figure and Supporting Information Table S1), showing promising performance for Pd-based ORR nanocatalysts. Figure 4 | strategy of various Pd–H NCs for ORR. HRTEM images of (a) Pd–H (b) Pd–H and (c) commercial The insets in are their corresponding 3D models. (d) ORR activity of different Pd NCs/C and Pd–H NCs/C. (e) MA of each Pd–H NCs/C versus commercial Download figure Download PowerPoint DFT and origins of performance To the of the enhanced ORR activities for various Pd–H NCs/C, X-ray and DFT were As revealed in Figure in Figure and Supporting Information Figure the Pd X-ray absorption structures show that the of Pd for various Pd–H NCs/C and Pd NCs/C is the as that of Pd but different from that of the we compared with the transformation of extended for various Pd–H NCs/C, Pd NCs/C, Pd and (Figure and Supporting Information Figure To be specific, different Pd NCs/C and Pd exhibit the typical distance of bond in Pd-based catalysts. By contrast, those Pd–H NCs/C similar positive on the distance of the bond in comparison with the Pd and different Pd NCs/C, but largely from the profile of Due to the of H atoms, the of Pd–H icosahedra/C, Pd–H Pd–H and expand from Å from Pd to and respectively (Figure and Supporting Information Table which is consistent with other results (Figures 2 and Supporting Information Figure As by the results of the coordination number of the bond was to be for Pd–H icosahedra/C, lower than that of Pd icosahedra/C ( Supporting Information Table which is beneficial to the of ORR performance for Pd-based catalysts. with other Pd NCs/C and Pd–H NCs/C, the of Pd–H icosahedra/C can be to the unique crystal structure of Pd–H for the twinned from was performed to further confirm the bond for Pd–H icosahedra/C, Pd icosahedra/C, and Pd As shown in Supporting Information Figure the of Pd icosahedra/C and Pd reveal only at Å−1, which can be to the By contrast, due to the of H atoms, a shift of the to can be for Pd–H icosahedra/C, which is in accordance with the In was used to analyze the valence band structures of Pd–H icosahedra/C and Pd icosahedra/C. As shown in Supporting Information Figure the bandwidth of Pd–H icosahedra/C was measured to be 0.23 eV lower than that of Pd icosahedra/C directly demonstrating the decreased electron density of Pd for Pd–H icosahedra/C. These results confirm that low and decreased electron density of Pd the twinned {111} faceted Pd–H icosahedra/C with ORR performance. Figure 5 | atomic and electronic structures of Pd–H icosahedra/C and their counterparts. (a) Pd of different catalysts. The in (a) is the structures of different catalysts. (b) Pd in space for different catalysts. The indicate the and the represent the (c) The optimized geometric structures of key intermediates in dissociative on Pd–H (111)-twin surface. (d) of the dissociative by the calculated versus limiting (e) density of state of different Pd and Pd–H surfaces. Download figure Download PowerPoint We further the of ORR performance by DFT Figure shows the optimized geometric structures of key intermediates in

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