Nanoporous PdCr alloys as highly active electrocatalysts for oxygen reduction reaction.

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ARTICLE Nanoporous PdCr alloys as highly active electrocatalysts for oxygen reduction reaction Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

Huimei Duan and Caixia Xu* A simple and convenient dealloying method is used to prepare nanoporous (NP) PdCr alloy with uniform ligament dimension and controllable bimetallic ratio. The structure characterizations demonstrate that NP–PdCr alloy is comprised of a nanoscaled interconnected network skeleton and hollow channels extending in all three dimensions. Electrocatalytic measurements indicated that the as–made NP–Pd75Cr25 alloy exhibits superior specific and mass activities as well higher catalytic stability toward oxygen reduciton reaction compared with NP–Pd67Cr33, NP–Pd, and commercial Pt/C catalysts. X– ray photoelectron spectroscopy and density functional theory calculations both demonstrate that the weakened Pd–O bond and improved ORR performances depend on the downshifted d–band center of Pd due to the alloying Pd with Cr (25 at.%). It is expected that the as–made NP–PdCr alloy has prospective applications as a cathode electrocatalyst in fuel–cell–related technologies with the advantages of superior overall ORR performances, unique structure stability, and easy preparation.

Introduction Proton exchange membrane fuel cells (PEMFCs) have been receiving great attention because of their broad application prospect as alternative energy sources to the environmentally detrimental use of fossil fuels, particularly in the fields of automotive systems and portable electronics.1–4 Oxygen reduction reaction (ORR) occurred at cathode is of significant importance for PEMFCs to extend the life–time of the fuel cells and achieve the desirable energy output efficiency.5,6 It is noted that the state–of–the–art Pt/C catalyst has been recognized as the most popular ORR electrocatalyst in PEMFCs. Unfortunately, the mass activity and structure stability of Pt/C catalyst is still far from satisfactory as a result of the corrosion of carbon support, de–adhesion between the Pt nanoparticles and carbon support as well as dissolution/aggregation/Ostwald ripening of Pt nanoparticles.7,8 Although great efforts have been dedicated toward the development of various Pt–based catalysts in recent years, the high cost and low reserves of Pt are still the major hurdles that hinder its broad applications as cathode electrocatalysts.9 Especially, in direct methanol fuel cells (DMFCs) methanol penetrated to cathode will also be oxidized over Pt–based catalyst, leading to a mixed potential

Address: School of Chemistry and Chemical Engineering, University of Jinan, Jinan, 250022, China. Fax: +86-531-82765969 Tel: +86-531-82767367 E-mail: [email protected] Electronic Supplementary Information (ESI) available: [EDS data of the PdCrAl precursor and the resulted NP–Pd75Cr25 sample, SEM and High–resolution EDS elemental mappings of NP–Pd75Cr25 alloy, SEM image of NP–Pd67Cr33 alloy, the pore size distribution of NP–Pd75Cr25 alloy, CV curves for all catalysts before and after 600 and 1, 000 potential cycles in the potential from 0 to 1.4 V, and the TEM image of NP–Pd75Cr25 alloy upon stability tests]. See DOI: 10.1039/x0xx00000x

as well the efficiency decay of fuel cell.10 Thus, searching alternative non–Pt electrocatalysts with high catalytic activity and durability are imperative for the practical application of fuel cells.11–13 Interestingly, Pd has been researched extensively as non–Pt ORR catalyst due to its competitive intrinsic electrocatalytic performance toward the ORR compared with Pt.14,15 More importantly, Pd is less expensive and more abundant, which has attracted greatly increasing attention recently. Meanwhile, alloying Pd with transition metals, such as Ti, Ni, Fe, and Cu, can often greatly improve the overall catalytic performance and simultaneously reduce the catalyst cost.16–21 Cr, as one of the 3d transition metals, has been demonstrated to have positive synergistic catalytic effect for the ORR activity of Pt.22–24 It is noted that the combination of Cr with Pd is also preferred to obtain high ORR activity on Pd. Kamiya et al.25 reported that PdCr alloy fabricated by a sputtering method showed a higher ORR activity than Pd. More recently, Li et al.26 prepared carbon supported Pd–Cr alloy (Pd–Cr/C) catalysts with different bimetallic ratios by chemical reduction method, and found that these catalysts showed better performances toward the electrooxidation of formic acid than Pd/C and Pt/C catalysts. However, by so far there is still a lack of systematic investigations on the ORR performances of PdCr alloy nanomaterials. It is noted that electrocatalytic reaction is an integrated display associated with the nanoarchitecture of electrocatalyst as well surface reactivity.27 Based on such understanding, it is of great meaning to maximize the ORR performances of PdCr alloy by engineering the morphology of the catalysts with simple and scaled preparation route as well to explore the origination of high ORR activity on Pd upon alloying with Cr. Recently, nanoporous metallic materials obtained by the dealloying method have been an intense research point

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because the perfect combination of the highly accessible open porosity and the highly conductive network facilitates the electron conductivity and mass transport during electrocatalysis.28–30 In addition, dealloying method has been recognized as an effective, convenient, and controllable strategy for the preparation of nanoporous alloy materials without using organic agents.31,32 In current work, we focus on alloying Pd with Cr to explore the effect of Cr in PdCr alloy on the catalytic activities of Pd toward ORR. Meanwhile, combining with special nanoporous architecture is of important significance for further improving its mass activity and structure stability along with reducing the fabrication costs. Herein, we successfully fabricated nanoporous (NP) PdCr electrocatalysts by a highly controllable dealloying manner at room temperature. Compared with commercial Pt/C and NP–Pd catalysts, the as–made NP–PdCr alloys display dramatically improved structure stability besides superior specific and mass activities. In addition, NP–Pd75 Cr25 alloy also show excellent ORR activity with enhanced methanol tolerance compared with the commercial Pt/C catalyst.

Experimental Section Sample preparation Pd20Al80, Pd15Cr5Al80, and Pd13.5Cr6.5Al80 (at.%) alloy foils were made by melting the corresponding pure metals Pd, Cr, and Al (99.99%) in an arc–furnace, respectively, and followed by melt–spinning under the protection of Ar atmosphere similar to our previous studies.33 All agents were obtained in analytical grade from Shanghai Sinopharm Chemical Reagent Ltd. Co of China. NP–PdCr and NP–Pd alloys were fabricated by dealloying the corresponding alloy foils in 0.2 M NaOH solution for 48 h at room temperature. The prepared samples were cleaned with ultra–pure water (18.2 MΩ) before drying at room temperature in air. The John–Matthey Pt/C (20 wt.%) catalyst was purchased by Alfa Aesar. Characterization Powder X–ray diffraction (XRD) data were collected using a Bruker D8 advanced X–ray diffractometer with Cu Kα radiation (λ=1.5418 Å) at a scan rate of 0.02º s–1. JEOL JSM– 6700F field emission scanning electron microscope (SEM) with an Oxford INCA X–sight energy dispersive X–ray spectrometer was used for the microstructure and chemical compositions characterization. Inductively coupled plasma (ICP) spectrometry was further employed to detect the bulky composition of Pd and Cr atoms in the two NP–PdCr alloys. Transmission electron microscopy (TEM) images were taken on a JEM–2100 high–resolution transmission electron microscope (200 kV). X–ray photoelectron spectroscopy (XPS) spectra were obtained on an ESCALab250 X–ray photoelectron spectrometer, using monochromatized Mg Ka X–ray as the excitation source. Binding energies were calibrated using the C 1s peak (284.6 eV) as the reference line. The pore size distribution was measured with a Quadrasorb

SI–MP (Quantachrome Instruments) using the Brunauer– Emmett–Teller (BET) method. Electrochemical tests All electrochemical measurements were performed using a PINE AFCBP1 electrochemical workstation. A conventional three electrode cell was employed in all experiments with Pt foil as a counter electrode and mercury sulfate electrode as the reference electrode. The ORR activity was taken on MSR rotation ring–disk electrode (RDE, Phychemi Company Limited) in 0.1 M HClO4 solution saturated with oxygen at a scan rate of 10 mV s–1. All potentials in this work are provided with respect to the reversible hydrogen electrode (RHE). The two NP–PdCr and NP–Pd catalysts were dispersed in a mixed solution of 1.0 mg of catalyst sample, 1.5 mg of carbon powder, 800 µL of ethanol, and 200 µL of Nafion (0.5 wt.%), and the mixture was ultrasonicated for one hour to obtain a well–dispersed suspension. Pt/C catalyst ink (about 1 mg mL– 1 ) was also prepared following the same procedure as described above except for further addition of carbon powder. Appropriate catalyst inks were dropped onto a pre–polished rotating disk electrode (5 mm in diameter). After drying, the catalyst film modified glassy carbon rotating disk electrode was obtained. The loading of Pt for the Pt/C catalyst was 15.7 µg cm–2, and the loadings of Pd for NP–Pd75Cr25, NP–Pd67 Cr33, and NP–Pd was determined to be 11.7, 14.9, and 17.2 µg cm– 2 , respectively. Prior to electrochemical measurements, the electrolytes were first purged with high–purity N2 for 30 min except for the ORR measurements. The electrochemical active surface areas (ECSAs) of the Pd–based catalysts were determined by measuring the charge of the Pd oxide reduction region from the cyclic voltammetry in N2–purged 0.1 M HClO4 solution in the range of 0 to 1.35 V at the scan rate of 50 mV s–1, divided by the theoretical charge for a palladium oxide monolayer reduction (420 µC cm–2).34 The ECSA of Pt was evaluated by integrating the charge correlated with H desorption in N2–purged 0.1 M HClO4 solution at the scan rate of 50 mV s–1, in which the theoretical charge for a desorption of a hydrogen monolayer is 210 µC cm–2 Pt with the double layer corrected.29 The ECSAs of NP–Pd75Cr25, NP–Pd67Cr33, NP–Pd, and Pt/C catalysts can be expressed by the two following equations, respectively: ∑

ECSAs() =

(∙

ECSAs( ) =

210 (∙

!" )× $

%% $&% %

'$ ()

∑ the charge of correlated with hydrogen desorption !" )×the total mass of catalyst sample (g)

The ECSAs of NP–Pd75Cr25, NP–Pd67Cr33, NP–Pd, and Pt/C were measured to be 64, 60, 77, and 73 m2 g–1 by using the methods stated above. Density functional theory (DFT) calculations All density functional theory (DFT) simulations were carried out with VASP program35 based on the gradient–corrected PBE exchange–correlation functional.36 The electronic wave functions were expanded in a plane wave basis set with an energy cutoff of 400 eV. For all the cases, we used a four–

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layer slab model with the forth layer fixed. Repeated slabs were separated by more than 10 Å to avoid artificial interactions between each other. The Pd3Cr (111) surface was modeled as a periodically repeated 2 × 2–unit cell in three dimensions with sixteen atoms per cell and four layers of atoms to model the case of the used electrocatalyst. The Pd3Cr (111) surface was modeled as a pure Pd outermost monolayer with 50% Pd in the second layer, and 75% Pd in the third and below. The geometries were optimized until the forces on all the unconstrained atoms were less than 0.02 eV Å–1. The Brillouin zone integration was done on a regular 5 × 5 × 1 Monkhorst–Pack k–points grid.37 The density of state calculations were evaluated at the same level except with a 13 × 13 × 1 Monkorst–Pack k–points mesh. For all the surfaces modeled in this study, face centered cubic hollow site is the most favorable adsorption site for oxygen atom. The oxygen adsorption energy (∆EO) is calculated as ∆EO = E(slab) + E(O) – E(ads/M), in which E(slab) and E(O) are the energies of the clean slab and isolate oxygen atom in vacuum, respectively, while E(ads/M) is the energy of the adsorbate–M adsorption system.

Results and Discussions It is well known that selective removal of more reactive element from an alloy frequently generates a porous structure with nanosized backbone and pores.38 Considering that Al is amphoteric and more reactive, and thus can be easily dissolved in a common alkaline electrolyte under free corrosion with almost no loss of Cr and Pd, we chose ternary PdCrAl as the source alloy. Generally, it is believed that the ternary route is more preferable for generating a nanoporous structure with higher porosity and better composition control. If taken alloy–fabrication cost into account, Al is more abundant with a relatively low price in the designed ternary PdCrAl alloy.39 NP–PdCr alloys with two different ratios were designed in order to explore the effects of Cr contents on the ORR activity of Pd. The composition of the PdCrAl source alloy with Pd:Cr around 3:1 was first identified by X–ray energy–dispersive spectroscopy (EDS) as the representative. As illustrated in Fig. S1a, the composition of the ternary alloy is around Pd15Cr5Al80 (at.%), which is identical with our initial feeding ratio during alloy melting. The bimetallic ratio between Pd and Cr in the dealloyed sample was further confirmed by EDS (around 75:25), which indicates an excellent control of the resulting nanoalloy compositions by means of the dealloying strategy. ICP was further employed to detect the bulky composition of Pd and Cr atoms in the two dealloyed PdCr alloys. Based on the ICP tests the atomic ratios of Pd and Cr in the two PdCr alloys are confirmed around 74.78:25.22 and 66.41:33.59, which is in good agreement with initial feeding. Upon dealloying a most majority of Al atoms were removed to a lower level as illustrated in Fig S1b. It is noted that the surface reactivity of the as–made electrocatalyst will not influenced by the little residual undetected Al atoms which were usually trapped inside the nanoporous ligaments.

Fig. 1. (a) Top–view and (b) cross–section SEM images, (c) TEM and (d) HRTEM images of the resulted sample by dealloying of Pd15Cr5Al80 alloy in 0.2 M NaOH solution for 48 h at room temperature.

The microstructure of the resulted sample upon dealloying Pd15Cr5Al80 was characterized by electron microscope as the representative. By selective etching Al from Pd15Cr5Al80 alloy in 0.2 M NaOH solution, an open nanosponge morphology consisting of an interconnected network ligament with a typical size of around 6 nm can be obtained as shown in Fig. 1a. The cross–section SEM image (Fig. 1b) further indicates that NaOH penetrates the whole sample with uniform porosity extending in all dimensions upon dealloying. TEM was further employed to explore the detailed microstructure. The sharp contrast between the inner bright region and interconnected dark backbone in TEM image (Fig. 1c) further confirms the formation of a bicontinuous network nanostructure. The typical high–resolution TEM (HRTEM) image (Fig. 1d) further verifies the nanoporous structure of NP–Pd75 Cr25 alloy. It is observed that the highly ordered lattice fringes were clearly resolved along several ligaments with the lattice space measured to be ~2.23 Å, corresponding to the (111) crystal plane spacing for PdCr alloy. Fig. S2 gives the corresponding high resolution EDS elemental mappings of Pd and Cr in the resulted NP–Pd75 Cr25 alloy, indicating that Pd and Cr atoms uniformly distributed on the nanoporous skeletons. Fig. S3 shows the SEM image of the resulted sample by dealloying Pd13.5Cr6.5Al80 alloy under the same conditions. As is observed, the obtained sample also has similar uniform bicontinuous interconnected nanosponge structure with the typical ligament size of ~6 nm. The pore size distribution of the NP–Pd75Cr25 alloy was measured by using the BET method as the representative. As shown in Fig. S4, the pore size of NP–Pd75Cr25 mainly distributes in the range of 5 to 7nm, which is consistent with the above electron microscope analysis.


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Fig. 2. XRD patterns of the NP–Pd75Cr25 alloy. The standard patterns of Pd (JCPDS 65–2867) and Cr (JCPDS 65–3316) are also attached for clear comparison.

To gain insight into the structure formation, XRD analysis was performed to characterize the crystal structure of the resulted NP–Pd75Cr25 alloy as the representative. It is noted that the crystal structure of PdCrAl source alloy was discussed in our previous report.39 Upon selective leaching of Al from PdCrAl alloy, it is clearly found that the resulted PdCr sample (Fig. 2) shows three broad diffraction peaks at 2θ values of 40.4, 45.5, and 68.4, which occur slight shifts to more positive positions relative to pure Pd. These peaks can be indexed to the (111), (200), and (220) planes of face–centered cubic (fcc) PdCr alloy structure as observed in our previous work,39 indicating that Cr atoms uniformly distributed into the fcc structure of Pd. On the basis of the above analysis, it can be concluded that the combination of dealloying process and alloy melting is a simple and effective method for the preparation of NP–PdCr alloys and especially suitable for the controlled production of bimetallic catalysts.

XPS was further performed to examine the electron structure of NP–Pd75Cr25 alloy as the representative. As shown in Fig. 3a, the core–level Pd 3d spectra of NP–Pd75Cr25 display a doublet signal with binding energies located at 335.4 and 340.7 eV for Pd 3d5/2 and Pd 3d3/2, respectively, showing a shift to a higher binding energy upon alloying with Cr as compared with 335.1 eV for pure Pd 3d5/2.40 The upshift of the binding energy for Pd in NP–Pd75Cr25 alloy could be ascribed to the changes in the electron density around Pd due to the formation of PdCr alloy as demonstrated by XRD. This is also observed in the previous reports,25,26 suggesting the density of states at the Fermi level decrease (or an increase of the d– vacancy) by filling the Pd d–band, i.e., the electron transfer from Cr to Pd. Another weak doublet signals around 336.7 eV and 342.1 eV can be assigned to Pd 3d5/2 and Pd 3d3/2 peaks of PdO species.41 Fig. 3b presents the XPS spectra for the Cr 2p core level region, in which the Cr 2p signals around 577.2 eV and 587.1 eV can be assigned to the presence of both metallic Cr (587.1 eV) and CrOx (577.2 eV).26 The existence of Cr oxides can be ascribed to the surface oxidation of the products during dealloying and drying. As mentioned above, NP–PdCr alloys with excellent structure integrity favor the electron conductivity and mass transport as well as the enhancement of the structure stability during electrocatalysis.42 In addition, the interconnected void run through the network skeleton is beneficial for molecular mobilization along the channels. Such an integral architecture provides a desired multifunctional platform for electrocatalysis applications. Motivated by these results, we carried out a series of explorations on the ORR electrocatalytic activities of NP–PdCr alloys.

Fig. 4. (a) CV curve of NP–Pd75Cr25 in 0.1 M N2–purged HClO4 solution. Scan rate: 50 mV s–1. (b) Polarization curves for the ORR on NP–Pd75Cr25, NP–Pd67Cr33, NP–Pd, and Pt/C catalysts in the O 2–saturated 0.1 M HClO4 solution at room temperature at 1600 rpm, Scan rate: 10 mV s–1. (c) Specific kinetic current densities for all catalysts at different potentials. (d) Specific kinetic and mass kinetic current densities at room temperature for all catalysts at 0.9 V.

Fig. 3. XPS spectra of Pd 3d (a) and Cr 2p (b) of NP–Pd 75Cr25 alloy.

The relatively stable cyclic voltammetric (CV) curve of NP– Pd75Cr25 alloy was shown in Fig. 4a, which was recorded in 0.1 M HClO4 solution at room temperature. The CV curve is composed of three characteristic potential regions, namely typical hydrogen region with sharp peak at 0–0.35 V due to its

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strong H adsorption/desorption and absorption, a double layer at 0.35–0.60 V, and the formation of metal oxides and their reduction around 0.65V. Clearly, there is no evident oxidation signal emerged associated with Cr oxides, further indicating the formation of uniform PdCr alloy as illustrated in XRD. The CV feature of PdCr alloy is very similar to that of pure Pd, revealing a surface state closing to pure Pd skin. Fig. 4b illustrates the ORR polarization curves of NP–Pd75Cr25, NP– Pd67Cr33, NP–Pd, and commercial Pt/C catalysts obtained at a rotation speed of 1600 rpm. It is evident that there are two distinguishable potential features emerged with a well–defined diffusion–limited currents region below 0.7 V followed by a mixed kinetic/diffusion–controlled region between 0.7–1.0 V.43 To further compare their intrinsic ORR activities, the specific kinetic activities of all catalysts were calculated based on ECSAs. As shown in Fig. 4c, NP–Pd75Cr25 alloy exhibited a much higher specific activity than that of NP–Pd67Cr33, NP– Pd and Pt/C catalysts over a potential from 0.84 to 0.92 V. Fig. 4d further provides the specific and mass activities for the catalysts at 0.9 V. Clearly, NP–Pd75Cr25 alloy displayed the highest specific kinetic activity with the value of ~0.24 mA cm–2 at 0.9 V, which is more than three times higher than that of NP–Pd, and also higher than that of NP–Pd67 Cr33 (0.20 mA cm–2) and Pt/C (0.15 mA cm–2) catalysts. Similar to specific kinetic activity, NP–Pd75 Cr25 also exhibited the superior mass activity among all catalysts. If taken the noble metal mass into account solely, NP–Pd75Cr25 alloy exhibited higher mass activity, which was nearly 1.2 times of that of NP–Pd67Cr33 , 1.4 times of that of Pt/C, and 3.1 times of that of NP–Pd. It is also meaningful to compare the specific kinetic activity of NP–Pd75Cr25 alloy with those of some other Pd–based alloy nanomaterials. For instance, Manthiram et al.44 reported that low cost Pd–W nanoalloy showed a specific mass activity with the value of ~0.027 A mg–1 at 0.75 V. More recently, Manthiram et al.45 reported that carbon–supported Pd–Ni nanoalloy electrocatalyst showed the mass activity at ~0.02 A mg–1 at 0.8 V with the catalytic durability remained 60% after 400 potential cycles. Neergat et al.46 also reported that the mass activities of Pd3 Co and Pd3–Co (OHT) were ~0.09 and 0.13 A mg–1 at 0.9 V. By comparison, NP–Pd75 Cr25 (0.16 A mg–1) alloy showed much higher mass activity than these similar Pd–based alloy nanomaterials as stated above. In current work, the measured activities of the commercial Pt/C catalyst are analogous to the reported in other literature.47 Taking these results into consideration demonstrates that apart from the unique nanoporous architecture, the appropriate alloying effect of Cr with Pd also contributes to the improved ORR activities, which was also identical with Kamiya et al.’ report.25 DFT calculations further verified this point as discussed below.

Fig. 5. CV curves (scan rate: 50 mV s–1) and ORR polarization curves (scan rate: 10 mV s–1) for NP–Pd75Cr25 (a and b), NP–Pd67Cr33 (c and d), NP–Pd (e and f), and Pt/C (g and h) catalysts before and after 5, 000 potential cycles in 0.1 M oxygen–containing HClO4 solution from 0.6 to 1.0 V vs RHE with the scan rate at 50 mV s–1.

Besides the required high activities, the stability is another one of the most important factors for the commercialization of the PEMFCs. Herein, detailed investigations on the long–term stability of all catalysts were conducted by applying continuous potential sweeps between 0.6 and 1.0 V in oxygen containing 0.1 M HClO4 solution for 5, 000 cycles at room temperature. After 5, 000 potential cycles, the ORR activity of Pt/C catalyst underwent a decrease of 19% in ECSA, corresponding to a 22 mV degradation in half–wave potential (Fig. 5g and h). It is interesting to find that NP–Pd75Cr25 alloy showed relatively higher long–term structure stability with a less degradation in the half–wave potential (17 mV) and ECSAs (~14%) than Pt/C catalyst despite Pd is more reactive than Pt in acid media (Fig. 5a and b). NP–Pd67 Cr33 alloy (Fig. 5c and d) showed a relatively lower long–term structure stability with more degradation in the half–wave potential (20 mV) and ECSAs (~17%) compared with NP–Pd75Cr25 alloy. As for the NP–Pd catalyst, an evident activity decay for ORR occurred with a decrease of 24% in the ECSAs and a degradation of 25 mV in half–wave potential (Fig. 5e and f). The structure stability of all catalysts were further evaluated by examining the loss of the ECSAs of all catalysts through continuous potential scans in a wider potential range from 0 V to 1.4 V in 0.1 M HClO4 solution considering the higher electrode potential in the fuel cells. The CVs of all catalysts after 600 and 1, 000 cycles were given in Fig. S5. NP–


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Pd75Cr25 and NP–Pd67Cr33 alloys underwent a decrease of 31% and 36% in ECSAs after 600 potential cycles, respectively. As for NP–Pd and Pt/C catalysts (Fig. S5c and d), a dramatic decline in ECSAs could be observed with a loss of 45 % and 39%. When scanning for 1, 000 cycles, a majority of the ECSAs of all catalysts lost due to the higher potential limit. Based on the experimental observations above, the long–term stability tests under different parameters both demonstrate that NP–Pd75Cr25 alloy has a respectable structure stability relative to NP–Pd67Cr33, NP–Pd, and Pt/C catalysts. Clearly, the combination of special structure configuration of this catalyst and the appropriate alloying effect of Cr atoms not only favors the improvement of the electrocatalytic activity of NP– Pd75Cr25 alloy but also makes for enhancing the catalytic stability despite the incorporation of more reactive Cr elements. The morphologies of NP–Pd75 Cr25 alloy after 5, 000 long–term potential scans were further investigated by TEM. As shown in Fig. S6, it is observed that NP–Pd75Cr25 alloy kept its original nanoporous structure. It is clear that the ligaments of NP–Pd75 Cr25 alloy suffer from slight coarsening compared with the initial structure upon continuous potential scans.

Fig. 6. ORR curves of Pt/C (a) and NP–Pd75Cr25 (b) in 0.1 M O 2–saturated HClO4 solution without and with 0.1 M methanol at 1600 rpm, respectively. Scan rate: 10 mV s–1.

Considering that the decay of fuel cell’s power efficiency induced by methanol permeation to the cathode, the methanol tolerance for ORR catalysts is another significant issue to be addressed in the application of direct methanol fuel cells (DMFCs).48 The results of methanol tolerance tests of NP– Pd75Cr25 and Pt/C catalysts were further provided in Fig. 6. Clearly, an obvious methanol oxidation peak was observed in Fig. 6a, while an evident decline in ORR current for Pt/C catalyst occurred with a half–wave potential degraded by ~17 mV in the presence of methanol compared with that in the absence of methanol in the potential range of 0.4 to 0.9 V. By contrast, NP–Pd75 Cr25 alloy (Fig. 6b) underwent a little decay when the potential range from 0.6 and 0.85 V in the presence of 0.1 M methanol, which may originate from the formation of the oxidized Pd surface during the methanol oxidation. However, there was almost no currents decay when the potentials higher than 0.85 V even in the presence of 0.1 M methanol. Taking these results into consideration demonstrates that NP–Pd75 Cr25 alloy possesses a higher methanol tolerance than Pt/C catalyst in the 0.1 M HClO4 and 0.1 M methanol mixed solution. With regard to Pd, there is almost no catalytic activity for methanol electrooxidation in acidic media.49 As expected, NP–Pd75Cr25 alloy has a better

selectivity for ORR in the presence of methanol, which makes NP–Pd75Cr25 alloy a preferable ORR catalyst in DMFCs.

Fig. 7. The most stable adsorbed schematic model of O atoms adsorbed on Pd3Cr(111), Pd(111), and Pt(111) slabs, and the corresponding d band center and ∆EO.

DFT calculations enable us to directly correlate the enhanced ORR performances of NP–Pd75Cr25 alloy with the changes of Pd electronic structure. Pd3 Cr (111) slabs with pure Pd surface were employed in order to be close to the experimental case. Fig. 7 shows that the ∆EO of the Pd3Cr (111) (–3.27 eV) corresponding to the most stable fcc hollow site is markedly lower than that of pure Pd (111) (–4.33 eV). Interestingly, upon incorporating with Cr the ∆EO of the Pd3Cr (111) slab was even substantially reduced relative to that of the Pt (111) slab (–4.24 eV). It is believed that strong adsorption of O atoms on Pd usually results in the formation of the oxidized unreactive Pd surface, where the ORR rate is seriously delayed by the removal of oxygenated species.27 Thus, it is considered that the decreased ∆EO of PdCr is favorable for achieving an optimum balance between the adsorption energies of oxygenated species and their surface coverage, eventually leading to more active sites for O2 adsorption and activation. Notably, the d–band center of Pd in PdCr is calculated to be –2.32 eV, which is a little lower than that of Pt (–2.72 eV). Meanwhile, an obvious negative shift occurs relative to pure Pd (–1.95 eV), which is in good agreement with the above XPS observations. It was proposed that the decrease of an electron back–donation from the Pd 4d orbital to the 2p* orbital of O usually results in the downshift of the d–band center of Pd, generating a weaker metal–O bond.50,51 The downshift of the Pd d–band center and the decreased ∆EO on Pd are proposed to be the possible origination for the higher ORR activity of NP–PdCr alloy than Pt/C and NP–Pd catalysts. Therefore, it is conclusive that alloying Pd with Cr could effectively tune the ORR reaction kinetics by appropriately modifying the electron structures of Pd, eventually contributing to the superior ORR activity of Pd. As stated above, the NP–Pd75Cr25 alloy displays superior ORR performances with enhanced specific and mass activities as well higher structure stability compared with NP–Pd67Cr33 , NP–Pd and commercial Pt/C catalysts. In addition, NP– Pd75Cr25 alloy also shows superior methanol tolerance relative to Pt/C catalyst. It is considered that the nanoporous architecture is preferable for the oxygen reduction reaction in terms of easy transport of electrons and reaction medium as well effectively suppresses the structure coarsening. Meanwhile, a nearly pure Pd skin with PdCr alloy core formed through the dissolution of surface Cr atoms also contributes to

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alleviating the dissolution and redeposition of Pd. More importantly, the appropriate incorporation of Cr with Pd could cause favorable modification of Pd electron structure to modify the reaction kinetics for ORR on NP–PdCr alloys, thus resulting in the improved ORR activities.


Conclusions The nanoporous PdCr alloys with uniform ligament dimension and controllable bimetallic ratio were synthesized by an extremely simple dealloying process, which shows enhanced specific and mass activity for ORR with high methanol tolerance and structure stability. The improved overall ORR performances of NP–Pd75Cr25 alloy could be ascribed to its excellent structure integrity and continuity as well as the appropriate changes of Pd electronic structure induced by alloying with Cr. Thus, the as–made NP–PdCr alloy holds great practical potential in green–energy technologies with the obvious advantages of excellent ORR performances and facile preparation without complicated processes and assistance of organic reagents.

Acknowledgements This work was supported by National Science Foundation of China (51001053, 21271085).

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ARTICLE

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