Electrochemical Dealloying-assisted Surface Engineered Pd-based

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Electrochemical Dealloying-assisted Surface Engineered Pd-based Bifunctional Electrocatalyst for Formic Acid Oxidation and Oxygen Reduction Siniya Mondal, and C. Retna Raj ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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ACS Applied Materials & Interfaces

Electrochemical Dealloying-assisted Surface Engineered Pd-based Bifunctional Electrocatalyst for Formic Acid Oxidation and Oxygen Reduction Siniya Mondal, and C. Retna Raj* Functional Materials and Electrochemistry Lab, Department of Chemistry Indian Institute of Technology, Kharagpur Kharagpur 721302, India E-mail: [email protected] KEYWORDS: Galvanic displacement, electrochemical dealloying, surface engineering, lattice strain, formic acid oxidation, oxygen reduction reaction Abstract Synthesis of non-Pt bifunctional electrocatalyst for the anodic oxidation of liquid fuel and cathodic reduction of oxygen is of great interest in the development of energy conversion devices. We demonstrate a facile room temperature synthesis of surface engineered trimetallic alloy nanoelectrocatalyst based on Co, Cu and Pd by thermodynamically favorable transmetallation reaction and electrochemical dealloying. The quasi-spherical CoxCuyPdz trimetallic catalysts were synthesized by the thermodynamically favorable reaction of K2PdCl4 with sheet-like ComCun bimetallic alloy nanostructure. The surface engineering of CoxCuyPdz was achieved by electrochemical dealloying. The surface engineered alloy electrocatalyst exhibits excellent bifunctional activity toward formic acid oxidation reaction (FAOR) and oxygen reduction reaction (ORR) at same pH. The elemental composition and lattice strain control the electrocatalytic performance. The elemental composition-dependent compressive strain weakens the adsorption of oxygen containing species and favors the facile electron transfer for FAOR and ORR. The engineered alloy electrocatalyst of Co0.02Cu13.8Pd86.18 composition is 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

highly durable and delivers high mass-specific activity for ORR and FAOR. It delivers the mass specific activity of 1.50 and 0.202 A/mgPd for FAOR and ORR, respectively in acidic pH. The overall performance is superior to that of as-synthesized Pd and dealloyed bimetallic Co2.7Pd97.3 and Cu5.61Pd94.39 nanoelectrocatalysts.

1. Introduction The depletion of traditional energy sources such as fossil fuels and the environmental consequences of the extensive use of traditional fuels motivate intensive research on development of efficient energy conversion and storage technologies for future energy requirements. Several efforts are being taken in the past few decades to develop alternative and eco-friendly energy technologies to address the energy demand and adverse environmental issues. The fuel cell technology is emerging as one of the promising technologies for the conversion of chemical energy into electrical energy as it is environmentally benign and can be used for portable devices and to meet the energy requirement for automobiles.1 The development of direct alcohol fuel cell such as direct methanol fuel cells (DMFC) received significant interest owing to its high energy density.2 Although alcohol (methanol, ethanol) based fuel cell technology is well investigated in the past, the high inflammability, volatility (boiling point: 64 78 ºC) and fuel cross-over are some of the major concerns for the widespread commercialization.2 Formic acid as an anode fuel received considerable attention in the recent years owing to its low toxicity and environmentally benign nature. High boiling point of formic acid (100.8 ºC) compared to methanol and ethanol offers its utilization in wide range of temperature.3 Interestingly, the theoretical open circuit potential of direct formic acid fuel cell (DFAFC) is higher (1.48 V) than that of direct methanol fuel cell (1.21 V). Although DFAFC has low energy density (2104 Wh/L) compared to methanol (4900 Wh/L),2-4 it has relatively high 2 ACS Paragon Plus Environment

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power density at room temperature.5 Moreover, the formic acid crossover flux through Nafion membrane is two-order magnitude lower than methanol and it compensates the disadvantage of having low energy density.4 Formic acid undergoes electrochemical oxidation in two pathways on traditional electrocatalysts: (i) direct oxidation of HCOOH to CO2 via dehydrogenation pathway and (ii) dehydration pathway via the formation of CO intermediate. The dehydration pathway with strongly adsorbed intermediate (COad) lowers the efficiency of DFAFC. It has been suggested that the Pd-based electrocatalysts are promising candidates as they can provide high power density with remarkable tolerance toward in-situ generated poisoning intermediate CO.6-9 The Pd-based catalysts favor the direct dehydrogenation pathway. However, it is a challenging task for the development of highly efficient and durable Pd-based electrocatalyst. One of the main concerns with the Pd-based electrocatalyst is the lack of durability; the dissolution of Pd in acidic solution during fuel cell operation significantly limits the catalytic activity. Moreover, the upward shift of d-band center of Pd leads to a strong bonding between metal surface and adsorbate.10,11 The ideal electrocatalyst should have the ability to weaken the bond between Pd and adsorbed oxygenated species, i.e. the catalyst has to be significantly less oxophilic. One effective approach to impart high activity and durability is to engineer the Pd catalyst in terms of lattice contraction, d-band structure, etc. Such engineered catalyst can be obtained by synthesizing bimetallic alloy or core-shell nanostructure with high abundant first row transition metals.11,12 Alloying with transition metal lowers the d-band energy and weakens the bonding between Pd and surface adsorbed oxygenated species.10-16 In DFAFC, the anodic oxidation of formic acid is counterbalanced by cathodic oxygen reduction reaction (ORR). In acidic medium, O2 reduces to H2O either directly (4e‾ process) or 3 ACS Paragon Plus Environment


via the formation of H2O2 involving 2e‾. The 4-electron direct pathway for ORR is highly preferred for fuel cell applications. Sluggish kinetics at cathode persuades to search for efficient inexpensive electrocatalysts. The Pt-based catalysts are well known for the cathodic reduction of oxygen.17,18 However, these catalysts may not be suitable for DFAFC as the Pt catalyst is prone to undergo poisoning due to the adsorption of in situ generated CO in the anode compartment. Furthermore, the scarcity of Pt and the lack of durability are some of the other major concerns with Pt-based catalysts. Synthesis of non-Pt electrocatalyst is of recent interest and much effort have been devoted for the development of non-Pt metallic alloy based and metal-free electrocatalyst.19-24 Although U.S. Department of Energy set the 2017 target ORR mass activity of 0.44 A/mgPt, the non-Pt electrocatalysts could not deliver the target value.25 For instance, the non-Pt catalysts such as Pd-Rh, Au-Rh, Rh-Sn, etc. could deliver a mass specific activity of ~0.20 A/mg which is far below the target value.26-28 During the course of the present investigation, Li et al demonstrated the ORR and FAOR activity of Pd-based trimetallic alloy catalyst in two different pH (alkaline and acidic).29 Zuo et al reported the ORR and ethanol oxidation activity of Pd-based alloy electrocatalyst in acidic and alkaline pH, respectively.30 Although these catalysts have bifunctional activity, they could not catalyze both cathodic and anodic reaction at same pH. The bifunctional catalyst that catalyze both anode and cathode reaction at the same pH is highly desirable for the development of DFAFC. Rational surface engineering of Pd is critically required to achieve the high mass specific activity and durability for FAOR as well as ORR at same pH. In an effort to engineer the Pd-based catalyst for the electrocatalytic FAOR and ORR, we have rationally designed the synthesis of a trimetallic alloy nanoelectrocatalyst based on Co, Cu and Pd at room temperature by thermodynamically favorable galvanic displacement and further tailored by electrochemical dealloying. 4 ACS Paragon Plus Environment

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Interestingly, the engineered catalyst could effectively catalyze both FAOR and ORR at same pH without compromising its overall performance. Our catalyst is highly durable and delivers high mass specific activity for both reactions.

2. Experimental Section 2.1 Materials Potassium tetrachloropalladate (K2PdCl4) and Nafion® were purchased from sigma Aldrich. Cobalt chloride hexahydrate (CoCl2∙6H2O), copper chloride dihydrate (CuCl2∙2H2O), sodium borohydride (NaBH4), sodium citrate (C6H5Na3O7∙2H2O), dimethylformamide (DMF) and formic acid were obtained from Merck and other chemicals were of analytical grade. All the solutions used in this investigation were prepared with Millipore water (Milli-Q system). 2.2 Instrumentation The

absorption

spectra

were

acquired

using

CARY

5000

UV-visible-NIR

spectrophotometer. X-ray Diffraction (XRD) analysis was performed with a BRUKER D8 Advance XRD unit using Cu-Kα (λ=1.54 Å) radiation. The XRD profiles were obtained using powder samples at a scanning rate of 3º /min in 2θ angular range between 30−90º. The morphology and composition was characterized by field emission scanning electron microscope (FESEM) (FEI Nova NanoSEM 450) using in-lens secondary electron detector at an acceleration voltage of 5.00 kV. Transmission electron microscopic (TEM) images of the nanoparticles were obtained with FEI-TECNAI G2 20S-TWIN electron microscope operating at 200 kV Carboncoated copper grids (400 mesh) were used to prepare the TEM sample. For the preparation of TEM sample, the catalyst was dispersed in DMF using bath-sonicator and drop casted on the grid. The modified grid was kept in vacuum for overnight for the removal of DMF. X-ray 5 ACS Paragon Plus Environment


photoelectron spectroscopic (XPS) analysis was carried out using XPS−AES module with Ar ion as well as C60 sputter gun spectrometer (PHI 5000 Versa Prob II, FEI Inc.). Powder sample was used in the XPS analysis. The elemental analysis was performed using inductively coupled plasma-optical emission spectroscope (ICP-OES) (Perkin Elmer, Optima 8300). Sample of measured weight was digested in aqua regia and used in the ICP-OES analysis. The respective standard solution was prepared for the standardization of each element. The electrochemical experiments were performed with Autolab potentiostat-galvanostat (302N) using computer controlled GPES software (NOVA 1.10). Glassy carbon (GC), Pt wire and Ag/AgCl (3 M NaCl) were used as working, counter and reference electrode, respectively. The ORR activity was evaluated using rotating disk electrode (RDE) (0.197 cm2), and the FAOR activity was evaluated using GCE (0.07 cm2). All the potentials are referred against the reversible hydrogen electrode (RHE). 2.3 Synthesis of Co nanoparticles The Co nanoparticles were synthesized by the borohydride reduction method according to the literature procedure.31 Typically, an aqueous CoCl2 solution (30 ml of 0.08 M) was taken in a two-neck round bottom flask and was deaerated by purging Ar for 1 h and the flask was closed well with rubber septum. Sodium citrate solution (15 ml of 0.03 M) was then injected to the solution under Ar purging. Inert atmosphere is critically required in order to avoid the formation of cobalt oxide. Then freshly prepared NaBH4 solution (5 ml, 2 M) was injected to the reaction mixture under constant stirring in ice-cold condition. Ice-cold condition is essential to control the rate of the reaction and to avoid the unwanted aggregation of the particles. Evolution of hydrogen was observed immediately after addition of borohydride and the pink colored aqueous Co2+ solution turned to black suspension. The reaction was allowed to continue until the 6 ACS Paragon Plus Environment

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gas evolution ceased. The ice-bath was removed and the stirring was continued for another 5 min in aerobic condition. The black colored precipitate of Co nanoparticles was collected at room temperature by centrifugation and dried under vacuum. Cu nanoparticles were also synthesized in the similar procedure using CuCl2 instead of CoCl2. 2.4 Synthesis of CoxCuyPdz nanoelectrocatalyst ComCun bimetallic alloy nanoparticle was synthesized by galvanic displacement reaction between the as-synthesized Co nanoparticle and Cu precursor. Briefly, required concentration (0.2, 0.6 and 1.0 M) of aqueous solution of CuCl2 (5 ml) was added dropwise in three different reaction vessels containing 5 ml aqueous dispersion of as-synthesized Co nanoparticle (59 mg) under argon atmosphere. The reaction mixture was stirred constantly under inert atmosphere for 15 min. The black colored product ComCun of different compositions was centrifuged and subjected to further analysis (m and n represent the corresponding atomic percentage of respective elements). The supernatant was spectrally analyzed to know the extent of galvanic displacement reaction. CoxCuyPdz with different compositional ratios was obtained by the further galvanic displacement reaction of as-synthesized ComCun with Pd precursor. In a typical procedure, the as-synthesized ComCun (50 mg) was dispersed in 10 ml of H2O and K2PdCl4 (5 ml) of required concentration (0.2−0.6 M) was added under constant stirring for 15 min in inert atmosphere. CoxCuyPdz trimetallic alloy catalyst was collected by centrifugation and dried in vacuum (x, y and z are the atomic percentage of respective elements). The supernatant was spectrally analyzed to know the extent of galvanic displacement. 2.5 Synthesis of CoPd and CuPd nanoelectrocatalysts In order to understand the contribution of Co and Cu in the electrocatalytic activity of CoxCuyPdz nanoelectrocatalysts, CoPd and CuPd bimetallic electrocatalysts were synthesized. In 7 ACS Paragon Plus Environment


a typical procedure, aqueous solution of K2PdCl4 (5 ml of 0.6 M) was added dropwise to an aqueous dispersion of Co nanoparticles (5 ml) and stirred for 15 min in inert condition. The CoPd alloy nanoparticle was collected by centrifuging the mixture after 15 min. The CuPd bimetallic alloy was also synthesized following the same procedure using Cu nanoparticle instead of Co nanoparticle. The as-synthesized bimetallic nanoparticles have the elemental composition of Co8.5Pd91.5 and Cu10.82Pd89.18. The Pd nanoparticle was synthesized by the borohydride reduction of aqueous K2PdCl4. Required amount of K2PdCl4 (1.63 mg) was dissolved in 10 ml of H2O and kept in an ice-bath. Freshly prepared aqueous NaBH4 solution (0.1 M, 2 ml) was then added drop wise to the aqueous solution of K2PdCl4 under constant stirring in ice-cold condition for 15 min. The black colored product of Pd nanoparticle was collected by centrifugation and subjected to the further analysis. 2.6 Electrode modification The working electrodes were polished well with alumina slurry (0.3 followed by 0.05 m) on a polishing cloth and sonicated in Millipore water for 10 min. The electrode was then thoroughly rinsed with Millipore water and dried under vacuum. The catalyst ink was prepared by sonicating 0.5 mg of as-prepared catalyst with the mixture of 20 µL of Nafion® and 180 L of ethanol-water-DMF (3:1:2 v/v ratio) solution for 30 min. RDE and GCE were modified with 410 µL of this prepared catalyst ink, and dried at ambient condition. 2.7 Electrochemical dealloying The as-synthesized trimetallic and bimetallic catalysts were electrochemically dealloyed by 100 consecutive cycling in the potential range of 0 to 1.4 V in deaerated 0.5 M H2SO4 at a scan rate of 100 mV s−1. The electrode after dealloying was subjected to further electrochemical

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analysis in fresh electrolyte. For the characterization of the dealloyed catalysts, the electrode material was collected after dealloying and was washed with ethanol followed by water to remove the binder Nafion®. The washing process was repeated two times and finally dealloyed catalyst was washed with ethanol and collected by centrifugation followed by drying at vacuum. The dried dealloyed catalysts were subjected to XRD, ICP-OES, TEM characterization.

3. Results and discussion 3.1 Synthesis and characterization of CoxCuyPdz Scheme 1 illustrates the synthesis of CoxCuyPdz trimetallic alloy nanoelectrocatalyst. The synthetic approach involves the following three steps: (i) synthesis of Co nanoparticle by borohydride reduction, (ii) galvanic displacement/transmetallation of Co with Cu2+ to obtain ComCun and (iii) further galvanic displacement/tansmetallation reaction of ComCun with PdCl42− to obtain the final product CoxCuyPdz. The reduction of transition metals using NaBH4 as a reducing agent is a common procedure. Though the H2/H‾ redox potential is ‒2.25 V, the negative reduction potential of Co2+/Co (‒0.28 V) makes it less favorable for reduction. In inert

Scheme 1. Scheme illustrating the synthesis of CoxCuyPdz alloy nanoelectrocatalyst.



atmosphere the primary product of the reaction between Co2+ and NaBH4 is Co2B, which further converted to metallic Co in open air according to the following equations 1 and 2.32 4Co2+ + 8BH4‾ + 18H2O → 2Co2B + 25H2 + 6B(OH)3

(1)

4Co2B + 3O2 + 6H2O → 8Co + 4B(OH)3

(2)

The second step involves the galvanic displacement/transmetallation of Co with Cu2+. The 0 0 2+ difference between the 𝐸 0 of Co2+/Co (𝐸𝐶𝑜 2+ /𝐶𝑜 = −0.28 V) and Cu /Cu (𝐸𝐶𝑢2+ /𝐶𝑢 = 0.337 V)

redox couples is adequate enough for the thermodynamically favorable spontaneous displacement reaction. In the third step, the ComCun obtained in the second step was subjected to 0 further transmetallation with Pd. The high positive reduction potential of PdCl42−/Pd (𝐸𝑃𝑑𝐶𝑙 2− /𝑃𝑑 4

= 0.599 V) makes the galvanic displacement reaction of ComCun with PdCl42− favorable. As the 0 𝐸 0 of Co2+/Co and Cu2+/Cu is less than 𝐸𝑃𝑑𝐶𝑙 2− /𝑃𝑑 , both Co and Cu can simultaneously undergo 4

redox displacement reaction with PdCl42−, though displacement of Co is highly favorable. The redox reaction between (i) Co and Cu2+ and (ii) ComCun and PdCl42− was spectrally

0.9

2-

(A)

(111)

Co78Cu22+ PdCl4 2-

(B) Co0.25Cu8.25Pd91.5 (200) (220)

(311)

Co51Cu49+ PdCl4

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6

2-

Co7Cu93+ PdCl4

0.3

Co0.05Cu26Pd73.95

Cu(II) Co29Cu13Pd58

Co(II) Pd # 46-1043

0.0 300

600 900 Wavelength (nm)

1200

40

50

60

2

70

80

90

Figure 1. (A) Absorption spectral profile of the supernatant solution obtained after the galvanic displacement reaction between ComCun and PdCl42−. (B) XRD profiles of assynthesized CoxCuyPdz. 10 ACS Paragon Plus Environment

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followed by monitoring the absorbance at ~510 (Co2+) and ~810 nm (Cu2+) (Figure 1A and Figure S1). Figure 1A represents the spectral profile obtained for the supernatant solution after galvanic displacement reaction between ComCun and PdCl42−. The peak observed at 810 nm corresponds to Cu2+ whereas the hump at 510 nm is ascribed to Co2+ species. In the case of Co7Cu93, the spectral signature for Co2+ is not clearly seen, as the concentration of cobalt is very small. The elemental composition of CoxCuyPdz was obtained from ICP-OES analysis; the assynthesized nanoelectrocatalysts have the composition of Co29Cu13Pd58, Co0.05Cu26Pd73.95 and Co0.25Cu8.25Cu91.5. The XRD analysis shows that the as-synthesized Co nanoparticles are amorphous in nature (Figure S2A) whereas the ComCun bimetallic alloy nanoparticles are crystalline (Figure S2B). The XRD profile of CoxCuyPdz shows the signature for cubic palladium (Figure 1B). The close inspection of the diffraction profiles of CoxCuyPdz alloys shows a gradual positive shift in 2 value with respect to Pd, as the Pd content was decreased. No additional peak was observed, confirming the absence of Co/Cu oxides or impurity. The calculated lattice parameter (a) as well as d-spacing of (111) plane of Pd in CoxCuyPdz increases while decreasing the total Co/Cu content. The lattice parameter of Co29Cu13Pd58, Co0.05Cu26Pd73.95 and Co0.25Cu8.25Pd91.5 was calculated to be 0.382, 0.385 and 0.388 nm, respectively. Broadening of diffraction while increasing the Co and Cu content is visibly seen in the XRD profile. Such peak broadening can be ascribed to the lattice strain.33 The crystallite size was calculated according to Debye-Scherrer

rule.34

The

Co29Cu13Pd58,

Co0.05Cu26Pd73.95

and

Co0.25Cu8.25Pd91.5

nanoelectrocatalysts have the crystallite size of 4.82, 4.5 and 4.21 nm, respectively. The lattice strain () was calculated using Williamson-Hall analysis (Table 1) using equation 3.35  cos  = K/D + 4 sin 

............. (3)



Here,  is the full width at half maxima (FWHM) of the peak and  is the Bragg’s angle. K is the shape factor (0.3080 Å), D is the crystallite size and  is the X-ray wavelength. The lattice strain in Co29Cu13Pd58, Co0.05Cu26Pd73.95 and Co0.25Cu8.25Pd91.5 nanoelectrocatalyst was calculated to be 1.125, 1.47 and 0.74%, respectively. The presence of both Co and Cu results in larger strain variation compared to the introduction of single transition metal in Pd-based materials.29,36 It is generally known that such increase in the lattice strain would enhance the electrocatalytic performance of the catalyst. The shape and structural morphology of Co, ComCun and CoxCuyPdz

Figure 2. FESEM images of (A) Co78Cu22, (B) Co51Cu49, (C) Co7Cu93, (D) Co29Cu13Pd58, (E) Co0.05Cu26Pd73.95 and (F) Co0.25Cu8.25Pd91.5.


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were analyzed by electron microscope. The electron microscopic analysis shows that the assynthesized Co nanoparticles are quasi-spherical in shape with an average size of 90 nm (Figure S3) whereas the ComCun nanoparticles have largely different shape. It is observed that the concentration of Cu2+ in the galvanic displacement reaction controls the shape and surface morphology of ComCun. The Co7Cu93 obtained at high concentration of Cu2+ has crumbled sheetlike nanostructures, whereas Co78Cu22 and Co49Cu51 obtained at low concentration of Cu2+ has mixture of quasi-spherical and crumbled sheet-like nanostructures (Figure 2A-C). Interestingly, the redox displacement of ComCun with PdCl42− yields CoxCuyPdz trimetallic alloy nanoelectrocatalyst with small spherical and quasi-spherical shape (Figure 2D-F). The TEM images are in well agreement with those of FESEM images (Figure 3, S3, S4). The fringe spacing of ComCun nanoparticles correspond to the (100) plane of cubic Cu (Figure S4). The dspacing of ComCun gradually increases with increasing amount of Cu in ComCun bimetallic alloy (Figure S4). It should be noted here that the as-synthesized cobalt nanoparticles are amorphous

60 40

average particle size: 11.57 nm

20

60 40

average particle size:

(B)

20 0

(C)

9.2 nm probability of particles

(A)

Probability of particles

in nature and we did not observe any characteristic signature for Co in XRD as well as High

probability of particles

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

7 8 9 10 11 Particle size (nm)

0

60

average particle size: 5.35 nm

40 20 0

9 10 11 12 13 14 15 particle size (nm)

3

4 5 6 7 particle size (nm)

20 nm

(E) (D) 0.220 nm (111)

(111) (200)

(F) (G) 0.222 nm (111)

(H) (111) (200)

0.224 nm (111)

(I) (111) (200)

Figure 3. (A-C) TEM images and corresponding (D, F, H) inverse fast Fourier transform (FFT) and (E, G, I) SAED patterns of as-synthesized (A, D, E) Co29Cu13Pd58, (B, F, G) Co0.05Cu26Pd73.95 and (C, H, I) Co0.25Cu8.25Pd91.5. 13 ACS Paragon Plus Environment


resolution

TEM

(HRTEM)

analysis.

The

HRTEM

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images

of

CoxCuyPdz

alloy

nanoelectrocatalyst shows fringe spacing of 0.220−0.224 nm corresponding to the (111) plane of cubic Pd. The d-spacing increases while increasing the content of Pd in CoxCuyPdz (Figure 3D, F, H). The selected area diffraction pattern (SAED) confirms the crystallinity of CoxCuyPdz. The average particle size of as-synthesized Co29Cu13Pd58, Co0.05Cu26Pd73.95 and Co0.25Cu8.25Pd91.5 is 11.57, 9.2 and 5.35 nm, respectively (Figure 3). The XPS surface scan and high resolution profile of as-synthesized Co nanoparticle show characteristic signature for cobalt (Figure S5). The absence of XPS signature for boron confirms that the cobalt nanoparticle does not have the intermediate Co2B. The surface scan profile of CoxCuyPdz alloy nanoelectrocatalyst shows characteristic signature of Co, Cu and Pd (Figure S6). The high resolution Pd 3d profile of CoxCuyPdz shows two peaks corresponding to 3d5/2 and 3d3/2 (Figure 4). Pd 3d binding energy of CoxCuyPdz is downshifted with respect to Pd nanoparticles. The monometallic Pd is known to show the 3d5/2 and 3d3/2 peaks at the binding energy of 335.0 and 340.3 eV, respectively. The shift in the band position can be ascribed to the change in electronic structure of Pd. The shift is more significant in the case of Co29Cu13Pd58. This chemical shift is dictated by the electronegativity of Pd (2.20), Cu (1.9) and Co (1.88) Table 1. Chemical composition and characteristics of CoxCuyPdz alloy nanoelectrocatalysts. Lattice strain  (%)#

Catalyst composition* As-synthesized

Particle size, nm**

Dealloyed

As-synthesized

Dealloyed

As-synthesized

Dealloyed

Co29Cu13Pd58

Co0.55Cu24.4Pd75.05

1.12

2.3

11.57

10.2

Co0.05Cu26Pd73.95

Co0.02Cu13.8Pd86.18

1.47

3.1

9.2

5.93

0.74

1.9

5.35

3

Co0.25Cu8.25Pd91.5 Co0.2Cu4.6Pd95.2

* Obtained from ICP-OES; # Calculated using Williamson-Hall analysis; ** Obtained from TEM analysis


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where the increase of valence electron charge (higher electronegativity) decreases the binding energy.37 Due to higher electronegativity, Pd withdraws electrons from the other two transition metals in CoxCuyPdz, inducing the downshift of Pd 3d peak.

345

Co29Cu13Pd58

3d5/2

3d3/2

342

339

336

Binding energy (eV)

333

Intensity

raw fitted background 334.25 eV 339.37 eV

(C)

(B)

330

raw fitted background 334.5 eV 339.58 eV

Co0.05Cu26Pd73.95

3d3/2

342

3d5/2

339

336

Binding energy (eV)

333

Intensity

(A)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


330

Co0.25Cu8.25Pd91.5 raw fitted background 334.9 eV 340.32 eV

3d5/2

3d3/2

345

342

339

336

Binding energy (eV)

333

Figure 4. High resolution Pd 3d XPS profiles of as-synthesized (A) Co29Cu13Pd58, (B) Co0.05Cu26Pd73.95 and (C) Co0.25Cu8.25Pd91.5. 3.2 Electrochemical characterization The electrochemical dealloying is one of the promising approaches to introduce micro/nanoporosity in the catalytic materials. The selective electrochemical etching of one or more component of the homogeneous alloy can introduce porosity and lattice strain.38-40 Such surface engineering of alloy catalyst can enhance the electrocatalytic performance.39,40 The assynthesized CoxCuyPdz catalysts were electrochemically dealloyed prior to explore the electrocatalytic activity. The dissolution of more reactive components (Co and Cu) of CoxCuyPdz can enhance the surface active sites of Pd. Dealloying was achieved by 100 consecutive cycling in the potential range of 0 to 1.4 V in deaerated 0.5 M H2SO4. The dissolution of Co and Cu from CoxCuyPdz nanoparticle leads to the formation of rough and highly porous structure with surface active sites. The careful observation of the voltammetric profiles for electrochemical dealloying shows gradual disappearance of the shoulder peak in the potential range of 0.55 to 0.7 V indicating the removal of Co/Cu from CoxCuyPdz nanoparticles (Figure S7). The ICP-OES and 15 ACS Paragon Plus Environment


XRD analysis of the catalyst after electrochemical dealloying evidences significant change in the composition and lattice strain (Table 1) (Details are summarized in supporting information). Significant decrease in the atomic percentage of Co and Cu was observed (Table 1). The diffraction peaks of dealloyed CoxCuyPdz are broadened compared to the as-synthesized catalysts, indicating the further increase in lattice strain (Figure S8). For instance, ˃50% increase in the lattice strain was noticed after the electrochemical dealloying. Moreover, the dealloying significantly influences the shape and size of the alloy nanoelectrocatalyst. The shape and size of the dealloyed CoxCuyPdz particles are different from the as-synthesized catalysts (Figure S9, S10). The as-synthesized Co29Cu13Pd58 nanoparticles have the mixture of sheet-like and quasispherical structures, whereas Co0.05Cu26Pd73.95 and Co0.25Cu8.25Pd91.5 have quasi-spherical shape. The sheet-like and quasi-spherical nanostructures transforms into aggregated particles of irregular shape upon electrochemical dealloying. The particle sizes of the dealloyed catalysts are smaller compared to the as-synthesized catalysts (Table 1). The HRTEM images of the dealloyed CoxCuyPdz nanoelectrocatalysts shows increase in the d-spacing corresponding to the Pd(111)

0.5

(A)

(B)

E@Pd-O

0.5

Co0.55Cu24.4Pd75.05

0.70 V

Co0.02Cu13.8Pd86.18

0.74 V

Co0.2Cu4.6Pd95.2

0.69 V

0.0

E@CO Co0.55Cu24.4Pd75.05 Co0.02Cu13.8Pd86.18 Co0.2Cu4.6Pd95.2

0.4

I (mA)

1.0

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.02 V 0.979 V 1.05 V

0.3 0.2 0.1

-0.5

0.0 0.0

0.3

0.6 0.9 E/V vs RHE

1.2

0.4

0.6

0.8 E/V vs RHE

1.0

1.2

Figure 5. (A) Characteristic cyclic voltammetric and (B) CO stripping profiles of Co0.55Cu24.4Pd75.05, Co0.02Cu13.8Pd86.18 and Co0.2Cu4.6Pd95.2 alloy nanoelectrocatalysts in 0.5 M H2SO4. Scan rate: 50 mV s−1. Stripping analysis was performed in CO saturated electrolyte.


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(Figure S10) due to the removal of highly active Co and Cu. Spotty SAED pattern suggests that CoxCuyPdz trimetallic alloy nanoelectrocatalysts retains the crystallinity after dealloying. The voltammetric profile of the dealloyed catalysts in 0.5 M H2SO4 shows the characteristic signature for Pd (Figure 5A, S11A). The close inspection of the voltammetric profile reveals that (i) the potential corresponding to surface oxide reduction (E@Pd-O) on the Co0.02Cu13.8Pd86.18 surface is more positive than the other two catalysts, (ii) Co0.55Cu24.4Pd75.05 and Co0.2Cu4.6Pd95.2 catalysts show two anodic peaks in the low potential region and it can be assigned for the removal of adsorbed hydrogen on (111) and (100) plane of Pd active site,41,42 whereas Co0.02Cu13.8Pd86.18 shows a single peak corresponding to the hydrogen desorption from Pd(100) and (iii) two anodic peaks were observed at high positive potential for the oxidation of Pd (100) and Pd (111) surface planes at 0.95 and 1.1 V, respectively. This positive shift in E@Pd-O for Co0.02Cu13.8Pd86.18 catalyst suggests the decreased oxophilicity as well as weakly bonded oxygenated species (OHad, OOHad, COad) to Pd surface.43,44 Such positive shift can be accounted for the crystal facets, oxophilicity and associated geometric and electronic effects. The electrochemically accessible surface area (ECSA) of the dealloyed catalysts was estimated by the CO-stripping method (Figure 5B, S11B). The charge associated with the stripping peak of adsorbed CO is proportional to the ECSA of the catalysts (Table 2).45 The CO stripping is highly facile on Co0.02Cu13.8Pd86.18 compared to the other two catalysts, suggesting that it can favor facile oxidation of formic acid. 3.3 Electrocatalytic performance The oxidation of formic acid on Pd proceed either through direct pathway involving adsorbed HCOOHad or via formate pathway.46-48 Direct pathway involves equation 4. Pd + HCOOHad → Pd + CO2 + 2H+ + 2e‾

(4)

On the other hand, the formate pathway involves the following steps (equations 5−7): 17 ACS Paragon Plus Environment


Pd + HCOOHad → HCOOad + H+ + e‾

(5)

Pd + H2O → Pd−OH + H+ + e‾

(6)

Pd-HCOOad + Pd-OH → 2Pd + CO2 + H2O

(7)

The electrocatalytic performance of the dealloyed catalysts toward FAOR was examined by cyclic voltammetric measurements in 0.5 M H2SO4. Figure 6A displays the electrocatalytic FAOR activity of dealloyed CoxCuyPdz catalysts. At the potential lower than 0.3 V, the peaks corresponding to hydrogen adsorption/desorption is still observed in presence of HCOOH. Welldefined anodic peak at potential ˃0.3 V is attributed to the oxidation of formic acid by the alloy catalysts. In the anodic sweep, the main peak denoted as (∗) is ascribed to the oxidation of formic acid via formate pathway. The hump denoted as (#) is attributed to the direct oxidation of formic acid involving HCOOHad. The small hump (**) at more positive potential is due to the surface oxidation of Pd.49 In the high potential region (˃1.0 V), oxidized Pd surface suppress the FAOR activity. During negative sweep a sharp increase in current density is due to the oxidation of Co0.55Cu24.4Pd75.05

(A)

Mass specific activity (A/mgPd)

100 (B)

Co0.02Cu13.8Pd86.18

2

Co0.2Cu4.6Pd95.2 * * #

1

#

*

#

**

0.3

96

Co0.55Cu24.4Pd75.05 Co0.02Cu13.8Pd86.18 Co0.2Cu4.6Pd95.2

92

**

88

0 0.0

retention of current (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 0.9 E/V vs RHE

84

1.2

Figure 6. (A) Cyclic voltammograms illustrating the electrocatalytic activity of Co0.55Cu24.4Pd75.05, Co0.02Cu13.8Pd86.18 and Co0.2Cu4.6Pd95.2 alloy nanoelectrocatalysts in 0.5 M H2SO4 in the presence 0.5 M formic acid. Scan rate: 50 mV s-1. (B) Plot illustrating the durability of the dealloyed catalysts in terms of the retention of voltammetric current in the forward sweep after amperometric durability test. 18 ACS Paragon Plus Environment

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HCOOH on fresh regenerated bare Pd surface. In the case of Co0.02Cu13.8Pd86.18, the peaks in the anodic as well as in the cathodic sweep appear at less positive potential (120−180 mV) compared to the other two catalysts. The negative shift in the peak potential during the forward and reverse sweeps implies the highly favorable oxidation of formic acid and it can be ascribed to the geometric and electronic effects. The mass specific activity was obtained by normalizing the current with the mass of Pd loaded on the electrode surface. The mass specific activity of Co0.02Cu13.8Pd86.18 is higher than the other two catalysts (Table 2), implying enhanced catalytic performance. For instance, at the potential of 590 mV the Co0.02Cu13.8Pd86.18 catalyst could afford 1.50 A/mgPd, whereas Co0.55Cu24.4Pd75.05 and Co0.2Cu4.6Pd95.2 could afford 1.24 and 0.86 A/mgPd, respectively (Figure 6A). The ratio of the peak current obtained in the forward and backward scan (if/ib) can be considered as a measure of tolerance of the electrocatalyst toward carbonaceous species (i.e. COad). The ratio is less than unity, suggesting the poisoning species (i.e. COad) generated during positive sweep, if any, gets oxidized at higher potential and makes the catalyst more active during negative sweep.50 It is well known that the Pd based catalysts follow dehydrogenation pathway for the oxidation of formic acid and the further reduction of dehydrogenated product (i.e. CO2) generates CO.51,52 According to Cai et al, COad on Pd surface can be spectrally detected only at the potentials where Had exists and not in the high positive potential range.51 During positive sweep, the COad on the Pd surface, if any, should be oxidized at higher potential range (˃ 0.95 V). In our case, the absence of any additional peak for COad oxidation at potential higher than 0.95 V (Figure 6A) implies the generation of either negligible amount of COad or absence of COad in the potential range examined. The careful analysis of the CO-stripping profile provides further insights into the electrocatalytic FAOR activity. The catalyst that weakly binds 19 ACS Paragon Plus Environment


in situ generated CO is highly preferred for the development of DFAFC as the strongly bound CO blocks the active site of Pd and limits the overall performance. Among the dealloyed CoxCuyPdz catalysts, Co0.02Cu13.8Pd86.18 is superior to that of the other two catalysts. The peak corresponding to the stripping of surface adsorbed CO on the Co0.02Cu13.8Pd86.18 is significantly less positive (40-70 mV) than the other two catalysts (Figure 5B), suggesting the facile removal of COad and enhanced CO-tolerance. The main concern of the Pd-based electrocatalyst is the durability, as the catalytic activity decreases due to the dissolution of Pd in acidic solution during fuel cell operation.53 The durability of the catalysts was examined by chronoamperometric analysis by holding the potential at 0.5 V for 3600 s. Stable steady-state response was obtained for long time, suggesting the durability of the catalyst (Figure S12A). The voltammogram recorded with Co0.02Cu13.8Pd86.18 catalyst after amperometric durability test shows only 7% decrease in current density. However, >9% decrease was observed for the other two catalysts (Figure 6B, S12). Such high stability suggests that the electrode surface does not undergo deactivation during the oxidation process. To understand the contribution of Co and Cu in the electrocatalytic activity, control experiment was performed with CoPd and CuPd based bimetallic catalysts (Figure S13). The mass specific activity of Co2.7Pd97.3, Co5.61Pd94.39 and Pd electrocatalysts is only 0.98, 1.03 and 0.34 A/mgPd, respectively (Table 2). It suggests that the enhanced performance of trimetallic alloy nanoelectrocatalyst originates from the synergistic effect. Figure 7A illustrates the ORR activity of CoxCuyPdz alloy nanoelectrocatalyst. Welldefined polarization curves were obtained for all the three catalysts, suggesting a facilitated electron transfer. The Co0.02Cu13.8Pd86.18 nanoparticle shows the onset potential for the reduction of oxygen at 0.95 V, which is 20-30 mV more positive than that of Co0.55Cu24.4Pd75.05 and 20 ACS Paragon Plus Environment

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Co0.2Cu4.6Pd95.2. The limiting current density on Co0.02Cu13.8Pd86.18 is higher (5.12 mA/cm2) than that of Co0.55Cu24.4Pd75.05 and Co0.2Cu4.6Pd95.2. The kinetics of ORR was further examined by analyzing the hydrodynamic voltammograms obtained at different rotation rate using Koutecky−Levich (K−L) equations 8 and 9 (Figure 7B).54 1

=

i

1 i

=

1 iD

+

1

. … … … … . . . . . (8)

iK

1 2 1 − 0.69 nFAC0 D𝑂2 3 ν 6

1 ω2

+

1 nFAkC0

. … … … … … . . . (9)

Here, iK is kinetic current and iD is the diffusion limited current. ‘n’ is the number of electrons transferred, ‘F’ is Faraday’s constant, ‘A’ is the surface area of the electrode, ‘C0’ is the concentration of oxygen, ‘𝐷𝑂2 ’ is the diffusion coefficient of oxygen, ‘’ is the kinematic viscosity, ‘’ is the angular velocity, and ‘k’ is the heterogeneous rate constant. The ECSA specific activity of the Co0.55Cu24.4Pd75.05, Co0.02Cu13.8Pd86.18 and Co0.2Cu4.6Pd95.2 are 0.37, 1.53 and 0.29 mA/cm2, respectively. Mass specific activity of Co0.02Cu13.8Pd86.18 is ~3 to 5 times

8

-3

6 Co0.02Cu13.8Pd86.18

51 mV/dec -1.6

Co0.55Cu24.4Pd75.05

-1.2

log jk

-0.8

-0.4

Co0.02Cu13.8Pd86.18

5 4

Co0.2Pd4.6Pd95.2

3

-6

0.6 0.8 E/V vs RHE

0.5

MA= 0.202 n= 3.8

n= 3.9 MA= 0.064

1.0

0.4 0.3

Co0.2Cu4.6Pd95.2

2

0.4

1.0

0.10

0.15 0.20 -1/2 (rad/s)-1/2

n= 3.8 MA= 0.038

0.2 0.1 0.0

Mass specific activity (MA) (A/mgPd)

0

0.80

Co0.55Cu24.4Pd75.05

mA-1 cm2)

0.88

7

Co0.55Cu24.4Pd75.05 Co0.02Cu13.8Pd86.18 Co0.2Cu4.6Pd95.2

(

3

48 mV/dec

1.5

-1

0.96

0.5 (B)

57 mV/dec

j

E/V vs RHE

1.04

no. of electron transfer (n)

6 (A)

j (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (A) Polarization curves for ORR in O2-saturated 0.5 M H2SO4. Scan rate: 5 mV s-1. Corresponding Tafel plot is shown in the inset. (B) Plot illustrating the mass specific activity and no. of electrons involved in ORR. Inset in (B) is the K-L plot. 21 ACS Paragon Plus Environment


higher than the other two catalysts (Figure 7B, Table 2). To understand the beneficial role of presence of both Co and Cu in the CoxCuyPdz catalysts, the ORR catalytic activity was compared with bimetallic Co2.7Pd98.3, Cu5.61Pd94.39 and Pd catalyst (Figure S14). As summarized in Table 2, the ORR activity of Co2.7Pd97.3, bimetallic and Pd catalyst is inferior to that of the Co0.02Cu13.8Pd86.18 alloy electrocatalyst. The high activity of dealloyed CoxCuyPdz nanoparticles over Pd nanoparticle Co2.7Pd98.3, and Cu5.61Pd94.39 proves the beneficial role of the presence of both Co and Cu (Table 2). The kinetics of electrocatalytic reduction of oxygen was further examined by the Tafel analysis (Figure 7A). The mass transport-corrected Tafel plot was made using equation 10.17 𝐽𝑘 = (𝐽𝑙𝑖𝑚 × 𝐽𝑑𝑖𝑓𝑓 )/(𝐽𝑙𝑖𝑚 − 𝐽𝑑𝑖𝑓𝑓 )

.…………..... (10)

Tafel slope in the low current density region on Co0.02Cu13.8Pd86.18 is 57 mV/dec and it is closely similar to the conventional Pt-based electrocatalysts. The Co2.7Pd97.3, Cu5.61Pd94.39 and Pd catalyst show relatively high slope compared to the trimetallic catalysts (Figure S14A). Durability of the catalyst during ORR is very important and highly durable catalysts are required for fuel cell applications. The durability of the catalysts was examined by recording the steady-state current at 0.85 V for 5 h. Only a 14% decrease in the initial current was observed with Co0.02Cu13.8Pd86.18. The durability of Co0.55Cu24.4Pd75.05, Co0.2Cu4.6Pd95.2 catalysts is rather low; 17−20% decrease in the initial current was obtained during the durability test (Figure S15A). In the case of Co2.7Pd97.3 and Cu5.61Pd94.39, initial current loss was 35−32% whereas Pd loses 55%. To further evaluate the durability, polarization curves for ORR obtained with the electrode used in amperometric durability test. Interestingly, the Co0.02Cu13.8Pd86.18 catalyst does not show any changes in the onset potential, though 5% decrease in the limiting current density was noticed. The other two trimetallic catalysts show small negative shift in the onset potential 22 ACS Paragon Plus Environment

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(1−3 mV) and 4−9% decrease in the current density (Figure S15B-C). The Co0.02Cu13.8Pd86.18 catalyst is highly durable and it retains its initial catalytic activity even after holding the potential for 5 h. Table 2. Electrocatalytic performance of dealloyed CoxCuyPdz, Co2.7Pd97.3, Cu5.61Pd94.39 and Pd catalysts towards FAOR and ORR. FAOR

ORR

Catalysts

ECSA (cm2)

Co0.55Cu24.4Pd75.05

1.35

0.71

1.38

0.93

0.064

Co0.02Cu13.8Pd86.18

1.30

0.59

1.50

0.95

0.202

Co0.2Cu4.6Pd95.2

1.40

0.77

1.13

0.92

0.038

Co2.7Pd97.3

1.5

0.53

0.98

0.9

0.01

Cu5.61Pd94.39

1.1

0.59

1.03

0.92

0.003

Pd

0.873

0.59

0.34

0.87

0.016

Peak Mass activity Onset Mass activity potential (V) (A/mgPd) potential (V) (A/mgPd)

The enhanced electrocatalytic activity of the trimetallic nanoelectrocatalyst is attributed to (i) chemical shift in the binding energy of the catalyst, (ii) compressive lattice strain, (iii) particle size, and (iv) the synergistic effect between Co, Cu and Pd. The d-band center of Pd is higher than Pt and it is downshifted due to alloying with Co and Cu and favorably tunes the surface adsorption/desorption capacity towards oxygen containing species.10,11,55,56 However, the removal of Co/Cu by electrochemical dealloying would increase the energy of d-band center.57 In such case what is the need for dealloying of the alloyed catalyst? Although the d-band energy of the dealloyed catalyst is not known, our experimental results show that the dealloying might have provided an optimum d-band energy required for the electrocatalytic reduction of oxygen and oxidation of formic acid. The dealloying significantly increases the lattice strain (Table 1) 23 ACS Paragon Plus Environment


and it makes the catalyst highly porous and increase the electrochemically accessible surface area. The dealloying-induced lattice strain weakens the binding of in situ generated oxygenated intermediate species such as COad, OHad, OOHad to the Pd active sites.29,39 It enhances the antipoisoning capacity of the catalyst towards FAOR and makes the catalyst highly durable. Moreover, it has been demonstrated that the particles size has considerable effect on the electrocatalytic activity.58,59 Pd nanoparticles of 5−7 nm has high activity toward FAOR.58 The average particle size of Co0.02Cu13.8Pd86.18 is 5.93 nm further supporting its superior activity. The enhanced performance of Co0.02Cu13.8Pd86.18 catalyst is ascribed to the combined electronic and geometric effects arising from the electrochemical dealloying.

4. Conclusions Surface engineered Pd-based trimetallic alloy nanoelectrocatalysts of different composition (CoxCuyPdz) have been synthesized by galvanic displacement reaction and subsequent electrochemical dealloying in aqueous solution at room temperature for the electrocatalytic FAOR and ORR. Among the CoxCuyPdz nanoelectrocatalysts, Co0.02Cu13.8Pd86.18 has excellent FAOR and ORR activity. The mass activity of Co0.02Cu13.8Pd86.18 towards ORR is >12 times higher than the as-synthesized Pd nanoparticles and 20−65 times higher than the bimetallic Co1.7Pd97.3, Cu5.61Pd94.39 catalysts. The FAOR activity of Co0.02Cu13.8Pd86.18 is >2.5 times higher than the as-synthesized Pd nanoparticle and 1.5 times higher than the bimetallic nanoelectrocatalyst. Our catalyst is durable and it does not undergo deactivation or sintering during electrocatalysis. It could retain 85−90% of its initial activity towards FAOR and ORR after extensive durability test. The elemental composition and dealloying-induced lattice strain and the change in the electronic structure due to the downshift in d-band center of Pd controls the overall performance of the alloy electrocatalysts. It is interesting to highlight here that our 24 ACS Paragon Plus Environment

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catalyst has bifunctional activity towards FAOR and ORR at the same pH and it can be a promising catalyst for the development of direct formic acid fuel cell. Our studies further demonstrate that the galvanic displacement reaction and the electrochemical dealloying can be used as powerful tool to engineer the surface structure of the catalysts for electrocatalytic reaction.

Associated Content Supporting Information. Supporting information includes figure S1-S15 and table S1. Detailed characterization and electrocatalytic performance of bimetallic catalysts and electrochemically dealloyed catalysts are includes.

Author Information Corresponding Author *E-mail: [email protected]. ORCID C. Retna Raj: 0000-0002-7956-0507 Notes The authors declare no competing financial interest.

Acknowledgement This work was financially supported by the Science and Engineering Research Board (SERB) (Grant No. EMR/2016/002271), New Delhi and IIT Kharagpur. We acknowledge Dr. Bikash Kumar Jena (CSIR-IMMT, Bhubaneswar) for ICP-OES analysis and Central Research Facility (IIT Kharagpur) for TEM analysis.



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Electrochemical Dealloying-assisted Surface Engineered Pd-based

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