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Nano Research 2016, 9(6): 1590–1599 DOI 10.1007/s12274-016-1053-6

Pd-Ag alloy hollow nanostructures with interatomic charge polarization for enhanced electrocatalytic formic acid oxidation Dong Liu1,§, Maolin Xie1,§, Chengming Wang1 (), Lingwen Liao2, Lu Qiu1, Jun Ma1, Hao Huang1, Ran Long1, Jun Jiang1 (), and Yujie Xiong1 () 1

Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), Hefei Science Center (CAS), and School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China 2 Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230021, China § These authors contributed equally to this work.

Received: 13 December 2015

ABSTRACT

Revised: 2 February 2016

Formic acid oxidation is an important electrocatalytic reaction in protonexchange membrane (PEM) fuel cells, in which both active sites and species adsorption/activation play key roles. In this study, we have developed hollow Pd-Ag alloy nanostructures with high active surface areas for application to electrocatalytic formic acid oxidation. When a certain amount of Ag is incorporated into a Pd lattice, which is already a highly active material for formic acid oxidation, the electrocatalytic activity can be significantly boosted. As indicated by theoretical simulations, coupling between Pd and Ag induces polarization charges on Pd catalytic sites, which can enhance the adsorption of HCOO* species. As a result, the designed electrocatalysts can achieve reduced Pd usage and enhanced catalytic properties at the same time. This study represents an approach that simultaneously fabricates hollow structures to increase the number of active sites and utilizes interatomic interactions to tune species adsorption/ activation towards improved electrocatalytic performance.

Accepted: 17 February 2016 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2016

KEYWORDS palladium, silver, electrocatalysis, formic acid oxidation, hollow nanostructures

1

Introduction

In the past decade, research on catalysis has focused on establishing structure-property relationships, which allow the function of each material to be appreciated and provide guidance for the design of

multi-component nanomaterials for high-performance catalysis. To address environmental and energy issues via a catalytic approach, one has to first develop methods for the design and synthesis of low-cost, highly efficient catalysts according to the understanding gained from fundamental research. Proton-exchange

Address correspondence to Yujie Xiong, [email protected]; Jun Jiang, [email protected]; Chengming Wang, [email protected]

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membrane (PEM) fuel cells provide a potential method of achieving highly efficient energy conversion whilst producing minimal environmental pollution and allowing low working temperatures for renewable energy systems [1–6]. As one of the more effective classes of electrochemical catalysts [6–11], Pd nanomaterials have been widely utilized in the formic acid oxidation (FAO) reaction—an important reaction in PEM fuel cells. In the FAO process, formic acid can be adsorbed onto a Pd surface to form a HCOO* intermediate, followed by a 2-electron oxidation step that directly converts the absorbed intermediate into CO2 [12, 13]. As a result, CO production can be suppressed to avoid potential CO poisoning, allowing Pd nanomaterials to exhibit FAO performances exceeding even that of Pt. Nevertheless, palladium is not abundant enough for large-scale application in the future. Typical methods of fully utilizing Pd atoms in reactions are maximizing the number of active sites by increasing the exposed surface area, or improving the activity of each reaction site by designing unique shapes and surface structures [7–11, 14]. In a different approach, Pd-based bimetallic or alloy nanostructures offer a valuable method of reducing Pd usage whilst retaining or enhancing catalytic performance [15]. To this end, Pd-based bimetallic or alloy nanomaterials incorporating other metals such as Co, Ni, Cu, or Ag have been developed for use in FAO catalysis [12, 13, 15–22]. Most recently, we have recognized that the interfacial charge polarization between Pd and Pt leads to accumulation of negative charges on the Pt surface, promoting an electrocatalytic hydrogen evolution reaction driven by their work function difference [23]. In this work, we propose that the incorporation of Ag, with its lower work function, into the Pd lattice may induce a similar interatomic charge polarization and thus tune the electron density of the Pd reaction sites for the FAO reaction. The tunable electron density of the Pd sites would provide the opportunity for enhancement of the HCOO* adsorption step and thus the FAO activity. To maximize the surface area of the catalysts, we employ a galvanic replacement between Ag nanocubes and Pd precursor to fabricate Pd-Ag alloy hollow nanostructures with well-defined

[100] exposed crystallographic facets. The electrocatalytic activity of these hollow nanostructures shows an interesting volcano-shaped relationship with the Pd-to-Ag atomic ratio, reflecting the importance of Ag in promoting FAO activity of the Pd sites. This work thus presents a new strategy for reducing Pd usage while at the same time enhancing electrocatalytic properties.

2 2.1

Experimental Synthesis of Ag nanocubes

In a standard procedure for Ag nanocubes [24, 25], ethylene glycol (EG, 50 mL, Sigma-Aldrich, 324558) was added into a 250-mL round-bottom flask and heated while stirring in an oil bath at 150 °C. NaHS (0.6 mL, 3 mM in EG, Sigma-Aldrich, 02326AH) was quickly injected into the heated solution, and after 2 min, an HCl solution (5 mL, 3 mM in EG) was added, followed by poly(vinyl pyrrolidone) (PVP, 12.5 mL, 20 mg·mL–1 in EG, MW = 55,000, Sigma-Aldrich, 856568). After another 2 min, silver trifluoroacetate (4 mL, 282 mM in EG, Aladdin, S109509) was added. The reaction solution was heated at 150 °C for 1 h. After this, the reaction was cooled using an ice bath, then the reaction solution was washed with acetone and water by centrifugation, then re-dispersed in water for further use. 2.2

Synthesis of Pd-Ag alloy hollow nanostructures

Pd-Ag alloy hollow nanostructures were synthesized via galvanic replacement using Ag nanocubes [26]. Water (5 mL) was stirred and heated to 90 °C in a 50 mL round-bottom flask fitted with a reflux condenser, then 0.1 mL of 1 mg·mL–1 Ag nanocubes was added. After 10 min, K2PdCl4 (0.5 mM in water, 0.1 mL·min–1, Aladdin, P123385) was added using an injection pump. The different molar ratios of the Pd-Ag hollow nanostructures depended on the volume of Pd solution injected. After the solution had been heated for a further 10 min, the reaction mixture was cooled with an ice bath. Solid KCl was added into the reaction mixture while stirring until the saturation of Cl– ions had removed AgCl from the sample, helping the

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reaction to yield well-defined nanoboxes or nanocages [25, 26]. The supernatant was then washed 5 times with water by centrifugation. Further etching of Ag from these Pd-Ag alloy hollow nanostructures was carried out by adding different doses of H2O2 to aqueous suspensions of the Pd-Ag hollow nanostructures [27]. After 30 min in a sample shaker, a small amount of HCl was added to react with the possible products (such as Ag2O or AgOH), and then solid KCl was added to saturate the solution with Cl– ions to produce well-defined Pd-Ag alloy nanocages [25, 26]. Finally, the supernatant was washed 10 times as above. 2.3

Electrochemical measurements

A glassy carbon (GC) rotating disk electrode (RDE, PINE, PA, USA, geometric surface area = 0.196 cm2) with catalysts dispersed on it was used as the working electrode. To prepare the working electrode, 50 μL of normalized aqueous suspension containing 0.25 mg·mL–1 of nanoparticles was transferred to the electrode and then dried under ambient conditions. Before electrochemical measurements, the working electrodes were cleaned with RF plasma (Plasma Cleaner pdc-002, Harrick, NY, USA) at a medium level for 1 min to remove residual organic matter, and then covered with Nafion dispersed in water (10 μL, 0.025%, NR50, 309389). Pt foil and a reversible hydrogen electrode acted as the counter and reference electrodes, respectively. The electrochemical data were collected by a CHI 760E electrochemical work station (Shanghai Chenhua, China). Perchloric acid (HClO4, 0.1 M) aqueous solution deoxygenated by a N2 stream was used as the electrolyte for cyclic voltammetry (CV) measurements in the potential region from 0.05 to 1.05 V at a scan rate of 50 mV·s–1. A mixed solution of HCOOH (0.1 M) and HClO4 (0.1 M) was employed as the electrolyte for the measurement of electrocatalytic formic acid oxidation. Chronoamperometry measurements were conducted at a practical operating voltage of 0.3 V vs. a RHE over 4,000 s in a mixed solution of 0.1 M HCOOH and 0.1 M HClO4. CO stripping tests were carried out by first adsorbing CO at 0.2 V for 900 s from a CO-saturated 0.1 M HClO4 solution, then collecting CO stripping CVs at a scan rate of 10 mV·s–1 in 0.1 M HClO4 solution after bubbling with Ar for 30 min.

2.4

First-principles simulations

The Vienna ab initio simulation package (VASP) was employed to simulate the geometric, electronic, and catalytic properties of model systems of Ag, Pd, and Pd-Ag based on spin-polarized density functional theory [28]. Projector augmented wave (PAW) potentials were used for electron ion interactions, and the generalized gradient approximation (GGA) and Perdew-Burke-Ernzerhof (PBE) functions with a dipole correction were used to describe the exchangecorrelation functions. When simulating the electronic structures and molecular interactions in the different model systems, the surface was modeled by a five-layer (2 2  2 2) surface unit cell with a 15 Å vacuum slab. For all the alloy model systems, Pd and Ag atoms were distributed evenly in each layer with different Pd-Ag ratios. An energy cutoff of 450 eV was used for the plane-wave expansion of the electronic wave function. The force and energy convergence criteria were set to 0.01 eV·Å–1 and 10–5 eV, respectively. The Brillouin zone was integrated using Monkhorst-Pack generated sets of k-points. A 7 × 7 × 7 k-point mesh was chosen for bulk states when examining the effect of polarization charge, while 5 × 5 × 1 k-point tests were done to simulate molecular adsorption on the surfaces. In the surface optimization calculations, the top two layers of adsorbates were relaxed and three bottom layers were fixed to reflect bulk positions. The HCOO* was mainly absorbed on the top sites. The adsorption energy was calculated with the equation Ea = Emolecule/substrate – Emolecule – Esubstrate.

3

Results and discussion

The molar ratios of Pd-to-Ag in the hollow alloy nanostructures could be controllably tailored by altering the amount of K2PdCl4 precursor used in the galvanic replacement step. As shown by transmission electron microscopy (TEM) images (Figs. 1(a)–1(e)), all the Pd-Ag nanocrystals inherited the well-defined cubic profile of the Ag nanocube precursor; however, their structures gradually become hollow as the content of Pd increased. Given the cubic profile and hollow nature, this set of samples can be referred to as

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Figure 1 TEM images of the samples: (a) Ag nanocubes, and (b)–(e) Pd-Ag alloy hollow nanostructures with molar contents of Pd of 0.072, 0.172, 0.317, and 0.423, respectively. (f) XRD patterns of as-obtained Pd-Ag alloy hollow nanostructures with Pd molar contents of 0.172, 0.317, and 0.423, respectively.

nanoboxes. The high-resolution TEM (HRTEM) image of a typical sample (Fig. S1 in the Electronic Supplementary Material (ESM)) indicates that the nanoboxes were enclosed by [100] crystallographic facets and were single crystals. As determined by inductively coupled plasma mass spectrometry (ICP-MS), the molar contents of Pd in the nanoboxes could be controlled between 0.072 and 0.470. These nanobox samples were further characterized by X-ray powder diffraction (XRD) (Fig. 1(f)). When compared with the standard diffraction patterns of face-centered cubic (fcc) Ag (JCPDS No.04-0783) and Pd (JCPDS No.46-1043), two sets of XRD peaks can be identified: the ones at smaller angles indexed to Ag, and the others assigned to Pd-Ag alloys. This suggests that the galvanic replacement allowed formation of Pd-Ag alloys in the shells while a certain amount of unreacted Ag remained in the center. As the galvanic replacement proceeded, the XRD peaks

relating to Ag become weaker, and those of the alloys gradually shifted towards the standard Pd peaks. This indicates that the Ag cores were removed from the samples and that the Ag content in the alloys was reduced, both due to the replacement reaction. Such changes substantially increased the content of Pd in the nanoboxes. This conclusion is further supported by the energydispersive spectroscopy (EDS) line scan profiles and scanning TEM (STEM) images shown in Fig. 2. As the Pd content increased, the nanoboxes become hollow, indicating that the material in the center had been gradually removed. The EDS profiles confirm that the material removed was Ag, and that the Pd concentrations in the shells had increased. A rapidly increase in the specific surface areas was also demonstrated by nitrogen adsorption–desorption measurements for the two typical Pd-Ag nanoboxes with molar contents of Pd of 0.172 and 0.423 (Fig. S2 in the ESM). The enlarged specific surface areas would facilitate the application of these materials to the FAO reaction. H2O2 etching was used to remove additional Ag from the center of the nanoboxes, and the technique could be used to remove Ag not only from the core but also from the Pd-Ag alloy shells. As a result, the molar contents of Pd could be increased from 0.470 to

Figure 2 STEM images and EDS line scan profiles of the samples: (a)–(d) Pd-Ag alloy hollow nanostructures with Pd molar contents of 0.072, 0.172, 0.317, and 0.423, respectively. Red and cyan curves correspond to the Pd and Ag signals, respectively.

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0.670. The TEM image in Fig. S3(a) (in the ESM) shows that the Pd-Ag nanostructures become hollower inside and that the walls became more porous. These samples are therefore referred to as nanocages. To better assess the FAO performance of hollow Pd-Ag alloy nanostructures with different Pd-Ag molar ratios, we also prepared Pd nanocubes and Pd-Ag bimetallic nanoparticles (or Pd-Ag bi-NPs) as reference samples (Figs. S3(b) and S3(c) in the ESM). To study the FAO catalytic activity of these samples, a blank CV was first measured for the samples on GC electrodes at a scan rate of 50 mV·s–1 in 0.1 M HClO4 solution deoxygenated by N2. The loading weight of Ag and Pd in the samples was 12.5 μg, regardless of the Pd-Ag molar ratio. The blank CV curves of the Pd-Ag alloy nanoboxes and nanocages with different Pd molar contents are displayed in Fig. 3 and Fig. S4 (in the ESM), compared with those of bare Ag nanocubes and Pd nanocubes. In Fig. 3(a), the positive sweep of the Ag nanocubes can be seen to exhibit an extensive double-layer charging region from 0.05 to about 0.6 V [29]. A main peak associated with Ag oxidation transition appears at about 0.68 V, and a rising slope is observed toward the end due to the formation of a layer of adsorbed hydroxyl species. The maximum currents in the CV curves of the Pd-Ag hollow nanostructures increased with increasing Pd content (Fig. 3), demonstrating the importance of Pd in FAO. The CV curves of the Pd-Ag alloy nanostructures and Pd nanocubes displayed two distinct regions, one corresponding to the oxidative desorption of underpotentially deposited hydrogen on the surface at potentials between 0.05 and 0.4 V, and the other

beyond 0.6 V caused by the formation of a layer of adsorbed hydroxyl species [30, 31]. The gradually enlarged peak area between 0.05 and 0.4 V indicates that the electrochemical active area was promoted by increasing the Pd content (i.e., through the formation of hollow nanostructures by galvanic replacement). Upon revealing the structures of the samples using electrochemical measurements, we investigated their suitability for electrocatalysis of the FAO reaction. Figure 4 and Fig. S5 (in the ESM) show CVs of the samples used for formic acid oxidation, normalized by the mass of the metal nanocatalysts (12.5 μg in total). Ag nanocubes alone did not show formic acid oxidation (Fig. 4(a)), meaning that Ag alone could not directly catalyze the FAO reaction. The Pd-Ag hollow nanostructures with minimal Pd content (0.072) showed very low FAO activity, but the reduction and oxidation peaks began to appear. As the Pd content increased, the FAO activity was dramatically altered, following a volcano-shaped trend (Figs. 4(a) and 4(b)). This trend is directly reflected by the dependence of the FAO currents both at their peak and at 0.3 V on the molar content of Pd (Fig. 4(c)). The Pd-Ag alloy nanoboxes with Pd contents of 0.317 and 0.423 showed the highest activity. These two alloy nanobox samples showed superior performance to Pd-Ag bimetallic nanoparticles (Fig. S6 in the ESM) as well as to Pd nanocubes both in blank CVs and FAO curves, suggesting that the hollow nanostructures and interatomic effects may both make contributions to the improved performance. Onset potential and oxidation peak potential are two key parameters for FAO. From Fig. 4 and Fig. S5 in the ESM, one can visually recognize the onset potential

Figure 3 CV curves of electrodes composed of hollow Pd-Ag alloy nanostructures with molar contents of Pd ranging from 0.072 to 0.670 in comparison with bare Ag and Pd nanocubes, in 0.1 M HClO4 solution from 0.05 to 1.05 V at a scan rate of 50 mV·s–1. | www.editorialmanager.com/nare/default.asp

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Figure 4 (a) and (b) CV curves for formic acid oxidation on electrodes composed of hollow Pd-Ag alloy nanostructures with molar contents of Pd ranging from 0.072 to 0.670 in comparison with bare Ag and Pd nanocubes. Electrolyte: 0.1 M HCOOH + 0.1 M HClO4; scan rate: 50 mV·s–1. Dependence of (c) peak current and current at 0.3 V, (d) onset potential, and (e) peak potential for formic acid oxidation (obtained from the data in (a), (b) and Fig. S5(c) in the ESM) on molar contents of Pd. (f) FAO performance stability shown by the decay of CV peak currents.

and peak potential. By adding more Pd to the catalyst lattice, the onset potential and peak potential were initially reduced and then increased (see the summary in Figs. 4(d) and 4(e)). Remarkably, the Pd-Ag alloy nanoboxes with molar contents of Pd of 0.317 and 0.423 showed the lowest onset potential and the lowest oxidation peak potential, respectively, and as a result were the best FAO electrocatalysts of the samples

studied. Note that the hollow Pd-Ag alloy nanostructures presented lower onset potentials, indicating higher tolerance towards CO poisoning [32, 33]. The results above clearly show that the content of Ag incorporated in alloy catalysts is critical to the FAO activity. From this dependence, one can easily recognize that hollow Pd-Ag alloy nanostructures with Pd contents of 0.3–0.4 should be the best candidates

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for FAO electrocatalysis when considering the cost of materials. Catalyst durability is another significant factor when considering future applications. Figure 4(f) and Fig. S7(a) in the ESM show the FAO performance stability of the Pd-Ag alloy nanostructures. The hollow Pd-Ag alloy nanostructures with Pd contents of 0.3–0.4 were among the top performing materials, displaying greater stability than both Pd nanocubes and Pd-Ag bi-NPs. In order to further assess the long-term stability of the materials, chronoamperometry measurements were performed for three representative samples, two Pd-Ag alloy hollow nanostructures with molar contents of Pd of 0.172 and 0.423, as well as a Pd-Ag bi-NP, in terms of the two key factors—Pd content and surface area. Figure S7(b) in the ESM reveals that the hollow Pd-Ag alloy nanostructure with molar contents of Pd of 0.423 maintained the highest FAO current density with little decay in activity, especially over the first 500 s, in sharp contrast to the other two samples. In addition, CO stripping experiments were also performed directly after the stability tests to further investigate the effect of CO adsorption and poisoning on catalyst longevity (Fig. S7(c) in the ESM). The Pd-Ag with molar contents of Pd of 0.423 nanobox catalyst displayed the lowest peak CO oxidation potential at 0.81 V, demonstrating facile removal of CO from the catalyst surface. This feature could improve the longterm stability of the catalyst for the FAO reaction [32–35]. The next question was why the hollow Pd-Ag alloy nanostructures exhibited high FAO performances. As the galvanic replacement proceeded, the catalysts contained more Pd atoms and possessed hollower structures. As a result, the number of Pd active sites would be expected to increase; however, the FAO performance starts to decay as the Pd content increases beyond 0.5. This observation indicates that the presence of Ag improved the FAO activity of Pd. To uncover the mechanism, we performed theoretical simulations of model Pd-Ag alloy structures in reference to bare Pd and Ag [100] (Fig. S8 in the ESM). The ratios of Pd to Ag in the three simulated Pd-Ag [100] alloy model systems were 1:3, 1:1, and 3:1, respectively, roughly reflecting the variation in the alloy compositions. The simulations of the electronic structures revealed that

the work function of Pd (5.0 eV) was higher than that of Ag (4.3 eV), as shown in Fig. S9 (in the ESM). As a result, the work function difference would induce polarization charges on Pd (Table 1), tuning the electron density of the Pd atoms. As the catalytic reaction takes place on the surface, we paid special attention to the Bader charges on surface Pd atoms, which turned out to have a volcano-shaped relationship with the Pdto-Ag ratio. The Bader charge transformation is an intuitive expression of the surface charge polarization effect, which normally favors the adsorption of polar molecules. The tunable charge density is anticipated to have an impact on species adsorption. As Pd has a propensity to break only the O–H bond of the HCOOH molecule over the entire potential region, FAO on Pd surfaces proceeds exclusively through the dehydrogenation reaction step [36, 37]. Therefore, HCOO* is the key intermediate in the FAO process. In order to illustrate the importance of the surface charge polarization in the FAO reaction, the adsorption energies of HCOO* on the three model systems were calculated (Fig. 5). We started with three types of adsorption sites, Pd– Pd, Ag–Ag, and Pd–Ag bridges, and all geometry optimizations indicated molecular adsorptions would occur on the Pd-Ag bridge. As shown by the adsorption energy values in Fig. 5, the Ag surface had the least ability to adsorb HCOO*. With increasing Pd content, the absorption energies of HCOO* first increased and then decreased, with the adsorption energy for the 1:1 Pd-to-Ag ratio being the largest of the model alloy systems. This trend was nearly consistent with the results of the electrochemical measurements. Note that the Pd-Ag nanoboxes with Pd contents between 0.072 and 0.470 contained higher amounts of Ag in their centers, so the Pd-to-Ag ratios in their alloy shells should be higher than the values indicated by ICPMS. This explains why the Pd-Ag alloy nanoboxes Table 1 The Bader charge of total and surface Pd atoms in PdAg3, PdAg, and Pd3Ag [100] alloy structures, obtained from firstprinciples simulations. The lattice structures are shown in Fig. S8 (in the ESM)  bader charge (e–)

PdAg3

PdAg

Pd3Ag

Total Pd atoms

1.12

1.34

0.91

Surface Pd atoms

0.38

0.57

0.45

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of electrocatalytic active sites, and more importantly, the interatomic charge polarization effect on the Pd-Ag surface enhanced the adsorption of HCOO*, leading to high electrocatalytic performance in the FAO process. This work represents an approach to the design of low-cost, high-performance electrocatalysts by tailoring electronic and surface structures.

Acknowledgements This work was financially supported by the National Basic Research Program of China (No. 2014CB848900), National Natural Science Foundation of China (Nos. 21471141, 21573212, and 21473166), Anhui Provincial Natural Science Foundation (No. 1508085MB24), Recruitment Program of Global Experts, CAS Hundred Talent Program, Hefei Science Center (CAS) Funds for Users with Potential, and Fundamental Research Funds for the Central Universities (Nos. WK2060190025, WK2310000035, and WK2090050027). Figure 5 Optimized theoretical structures for the adsorption of HCOO* on (a) Pd[100], (b) Ag[100], (c) PdAg3[100], (d) PdAg[100], and (e) Pd3Ag[100], together with their adsorption energies (Ead), obtained from first-principles simulations. Note that the models have been simplified to illustrate the interface between the molecule and metal, while simulations were actually performed on metallic crystal surfaces.

Electronic Supplementary Material: Supplementary material (TEM, BET, XRD and CV data, and simulation models) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-016-1053-6.

References with Pd contents of about 0.4 showed the highest degree of FAO of the samples. Since the adsorption of the reactant to the catalyst is the first dominant process in surface reactions, heterogeneous catalytic activity often increases with enhanced molecular adsorption [38, 39]. In addition to the molecular adsorption, a strong negative–positive charge dipole enhancing the electrostatic attraction to polar molecules can impact on other reaction processes, for example by lowering reaction barriers [37].

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In summary, hollow Pd-Ag alloy nanostructures have been developed for electrocatalytic formic acid oxidation, aiming to reduce usage of expensive Pd and enhance catalytic performance. The hollow structures contributed to an increase in the number

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