Investigation of the Performance of Aucore-Pdshell / C as the Anode Catalyst of Direct Borohydride-Hydrogen Peroxide Fuel

The carbon-supported bimetallic Au-Pd catalyst with core-shell structure is prepared by successive reduction method. The coreshell structure, surface morphology, and electrochemical performances of the catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet-visible absorption spectrometry, linear sweep voltammetry, and chronopotentiometry. The results show that the Au-Pd/C catalyst with core-shell structure exhibits much higher catalytic activity for the direct oxidation of NaBH4 than pure Au/C catalyst. A direct borohydride-hydrogen peroxide fuel cell, in which the AuPd/C with core-shell structure is used as the anode catalyst and the Au/C as the cathode catalyst, shows as high as 68.215 mW cm−2 power density.


Introduction
Fuel cells constitute an attractive class of renewable and sustainable energy sources alternative to conventional energy sources such as petroleum that has finite reserves.Among all types of fuel cells, the direct borohydride-hydrogen peroxide fuel cell (DBHFC) has recently attracted increasing attentions because of some excellent features such as high open circuit potential, low operation temperature, and high power density [1][2][3][4][5][6][7][8].The DBHFC is comprised of BH 4 − oxidation at the anode and H 2 O 2 reduction at the cathode; thus, it can be used as a promising power source for space and underwater applications.Generally, the key of development of DBHFC is anode electrocatalyst Cao et al. [9] have prepared Au/Ni-foam electrode by spontaneous deposition of Au nanoparticles on nickel foam substrates and achieved an open-circuit voltage of about 1.07 V and a peak power density of 100 mW cm −2 at 170 mA cm −2 and 0.6 V at 60 • C. Chatenet et al. [10] reported that electrooxidation of BH 4 − can exchange about 7.5 electrons on Au and Ag electrodes in contrast to about 4 electrons on Pt electrode.The high electron utilization efficiencies of Au and Ag towards BH 4 − electrooxidation are attributed to their low activity towards BH 4 − hydrolysis.However, the catalyst, which causes the low activity towards BH 4 − hydrolysis, simultaneously has a low catalytic activity towards BH 4 − electrochemical oxidation reaction.Thus, it is very important for DBHFC application to develop high-activity electrocatalysts for the borohydride oxidation reaction (BOR).
In our previous works, high-activity catalysts such as carbon-supported hollow gold nanoparticles (HAu/C) [11,12] and AuCo/C [13] have been studied.Usually, the core-shell structure is an effective morphology to enhance catalytic activity.The improvement of catalytic activity of core-shell bimetallic nanoparticles is attributed to the unique structure in electrocatalysis, which has gained much attention in recent years [14].Besides, the activity of core-shell structure is related with the underlying interface between the core and shell metals due to the bimetallic mechanism [15].It is reported that the core-shell nanoparticle catalysts such as Ni@Pd/MWCNTs [16] and Pt shell -Au core /C [17] have effectively enhanced the catalytic activity for the electrooxidation of methanol.Palladium is an appropriate choice as the shell of the catalyst since it has the similar catalytic activity as platinum, which is more expensive than the former.In this paper, carbon-supported bimetallic Au-Pd catalyst

Experimental
2.1.Preparation of Carbon-Supported Core-Shell Au-Pd Nanoparticles.The preparation of Au seed was reported elsewhere [18,19].Concisely, the citrate-stabilized Au nanoparticles were prepared by NaBH 4 reduction of 0.5 mM HAuCl 4 aqueous solution in the presence of trisodium citrate in 100 mL ice ultrapure (18.2 kΩ) water with vigorous stirring [20].Stirring was continued for 12 h to decompose the unreacted NaBH 4 .Then 0.25 mL 0.1 M H 2 PdCl 4 aqueous solution and freshly prepared 1.25 mL 0.1 M ascorbic acid aqueous solution were added dropwise to the Au hydrosol.The solution was continuously stirred for 2 h and then added 50 mg Vulcan XC-72R carbon slurry (metal loading was 20 wt.%).After stirring 5 h, the suspension was filtrated to recover solid product, which was fully washed with water and alcohol.The recovered solid product was dried in vacuum at 80 • C overnight.Comparatively, carbon-supported monometallic Pd and Au samples with the same overall metal loading were prepared by similar procedures.

Characterization of Anode Catalysts.
The samples for transmission electron microscopy (TEM) were prepared by putting one drop of the sample slurry on a copper grid followed by drying in a desiccator.Electron micrographs were taken with a FEI Tecnai G2 microscope at 200 kV.A diffractometer (D/MAX-3C) was employed using Cu Kα radiation (λ = 1.54056Å) and a graphite monochromator at 50 kV, 100 mA, to obtain XRD spectra of the samples.Continuous X-ray scans were carried out from 10 • to 90 • of 2θ.The UV-Vis spectra of the hydrosols were measured by a Lambda 25 spectrometer equipped with quartz cells.

Electrochemical Performance of Anode Catalysts.
A conventional three-electrode cell was used to perform the electrochemical tests of the anode catalysts at 25 • C with a CHI660A Electrochemistry Workstation.The Au-Pd/C was used as working electrode, an Ni foam mesh with 3 × 5 cm 2 as counter electrode and an Ag/AgCl, KCl std as the reference electrode.The electrolyte was 0.1 M NaBH 4 + 3.0 M NaOH.The working electrode was prepared as follows: 5 mg of Au-Pd/C powder was dispersed by ultrasonic for 2 h in 0.5 mL blend solution of 0.125 mL 5 wt.% Nafion solution and 0.375 mL deionized water.Then 5 μL of slurry was pasted on the surface of the glassy carbon (GC) electrode (3 mm in diameter) which was polished to mirror by 0.5 μm alumina and sonicated 5 min prior to use.The dispersed catalyst on the GC surface was dried for 5 h at ambient temperature.The fresh anolyte and catholyte were continuously supplied and withdrawn from the cell at a rate of 0.7 mL min −1 during the testing process.The load was applied in steps of 5 mA within the range of 0-150 mA.Each step lasted 2 min, and the current was continuously applied from one value to the next without disconnecting the cell.

Fuel Cell Test. The performance of the direct NaBH
The catalyst ink was made by mixing isopropyl Nafion solution and carbon-supported catalyst and coated onto stainless steel gauze.Then it was dried in vacuum at 80 • C for 12 h and pressed at 16 MPa for 1 min.Finally, the loading mass of catalysts was about 4.5 mg cm −2 .all three samples are attributed to Vulcan XC-72R carbon (002) crystal face.In Au and core-shell Au-Pd catalysts, there are other five main characteristic peaks, which could be attributed to the face-centered cubic (fcc) structure of crystalline Au (111), ( 200), ( 220), (311), and (222).These indicated the absence of both isolated large-size Pd particles and the bulk Au-Pd alloy phases in the core-shell Au-Pd/C catalyst.The morphologies and particle distributions of the carbon-supported Au/C, core-shell Au-Pd/C, and Pd/C samples were showed in Figure 3.The corresponding histograms on size distribution of electrocatalysts were also simultaneously given in Figures 3(d), 3(e), and 3(f).It could be seen from Figure 3 that the spheroidal nanoparticles were homogeneously dispersed on the surface of the carbon supporter.The average diameter of Au, core-shell Au-Pd, and Pd particles in samples from counting more than 300 International Journal of Electrochemistry particles were about 5.955 nm, 6.705 nm, and 3.613 nm, respectively.Usually, the size of the particle with core-shell structure, D, can be calculated by [21]

Results and Discussion
Here, D core was the measured Au particles size; V Pd and V Au were the academic Pd and Au mole volume, respectively; [Pd] and [Au] were the overall concentrations of the two metals involved, respectively.It had been found from the above formula that the calculated size of core-shell Au-Pd particle was 6.719 nm, which is close to actually measured particle size (6.705nm).Besides, the calculated Pd shell thickness was about 0.375 nm.
The absorption spectra of Au hydrosol, core-shell Au-Pd hydrosol, and mixed Au-Pd hydrosol were shown on Figure 4.The absorption peak positioned at 516 nm, which was ascribed to the surface plasmon resonance (SPR) of Au, could be clearly observed for Au hydrosol.For the mixed Au-Pd hydrosol, the Au plasmon absorption peak declined slightly because of the decrease of the Au hydrosol concentration.However, for the core-shell Au-Pd hydrosol, the remaining Au plasmon absorption peak indicated that the Pd shell was not too thick to overwhelm the SPR response of the Au nanoparticles [22].at −0.2 V versus Ag/AgCl, respectively.The peak current density of Au-Pd/C (34.65 mA cm −2 ) was nearly close to Pd/C (35.44 mA cm −2 ), which was about 1.7 times higher than that of Au/C (20.80 mA cm −2 ).Besides, the area enclosed by LSV on core-shell Au-Pd/C electrode was apparently bigger than that on Au/C and Pd/C electrodes.Obviously, the above results indicated that the core-shell Au-Pd/C catalyst had much better catalytic activity than pure Au and Pd catalysts.Moreover, the core-shell Au-Pd/C (4.25 wt.% Pd loading) was more economical than Pd/C (20 wt.% Pd loading).The reasons why the core-shell Au-Pd/C enhanced the catalytic activity of the borohydride oxidation reaction could correlate with the long-range electromagnetic enhancement between the Au core and the Pd shell [23].

Electrochemical Performance of Anode Catalysts
Chronopotentiometry was a usefully qualitative screening method for catalyst since it simulated the constant current operation of a fuel cell.Figure 6 showed the chronopotentiometry curves of BH 4 − oxidation on the Au/C, coreshell Au-Pd/C, and Pd/C at a current density of 8.5 mA cm −2 in a solution of 0.1 M NaBH 4 + 3 M NaOH, respectively.The open-circuit potentials (OCPs) were −0.953V versus Ag/AgCl for Au/C, −1.203 V versus Ag/AgCl for core-shell Au-Pd/C, and −1.188 V versus Ag/AgCl for Pd/C.After 120 s, the operating potential for Au/C, core-shell Au-Pd/C, and Pd/C was −0.653 V, −0.920 V, and −0.800 V, respectively.Therefore, it can be expected that the core-shell Au-Pd/C catalyst had higher direct electrooxidation catalytic activity and power output than those of Au/C and Pd/C under the same experimental condition, which also suggested the possibility of potentially obtaining a higher power output.
In chronoamperometry experiments (Figure 7(a)), the potential was stepped from −1.2 to −0.2 V versus Ag/AgCl in 0.1 M NaBH 4 + 3 M NaOH solution.At the beginning of the experiment, the initial current densities of three carbon-supported catalysts corresponded mainly to doublelayer charging.The core-shell Au-Pd/C and Pd/C showed a gradual current density decay with the increase of the time before a steady current density was reached.However, the current density slowly decayed on Au/C, which would be related to the formation of borohydride oxidation products [10].The current in the steady-state region represented the current of the electrochemical reaction, so it could reflect partly the electrocatalytic activity of the electrodes.As being shown in Figure 7(a), the core-shell Au-Pd/C catalyst maintained a higher borohydride oxidation current density than those of the other two catalysts.In order to analyze well the electrocatalytic activity of BH 4 − on the different catalysts, the chronoamperometry results were summarized as a Cottrell plot in Figure 7(b).The approximate constancy of the It 1/2 versus t on Au/C in the steady process could be attributed to electrode deactivation.On the other hand, the increase of It 1/2 value with the increase of the time on coreshell Au-Pd/C and Pd/C indicated that neither the intracatalyst layer diffusion nor the external mass transfer of BH 4 became rate limiting under the employed conditions [24].And core-shell Au-Pd/C had larger It 1/2 value than Pd/C, which indicates the higher activity of oxidation of BH 4 − .Moreover, the increasing value suggested the surfaces of Au-Pd/C and Pd/C are not poisoned during the experiment process.

Fuel Cell
Performance.The cell polarization and power density curves for DBHFC using 1 M NaBH 4 + 3 M NaOH solution as fuel and 2 M H 2 O 2 + 0.5 M H 2 SO 4 solution as oxidant were presented in Figure 8.In the experimental cell, the Au/C was used as the cathode catalyst and Au/C, coreshell Au-Pd/C, and Pd/C as the anode catalyst, respectively.The cell voltage had an initially abrupt reduction at the range of 0-10 mA cm −2 current density on Au/C, which showed a kinetic process of electrochemistry polarization.For the Pd/C and core-shell Au-Pd/C electrodes, the cell

Conclusions
The carbon-supported bimetallic Au-Pd catalyst with the core-shell structure was successfully prepared by successive reduction method.The spheroidal Au-Pd nanoparticles were homogeneously dispersed on the surface of carbon supporter.Compared with carbon-supported monometallic Au and Pd catalysts, the core-shell Au-Pd catalyst had an enhanced electrocatalytic activity for the BH 4 − electrooxidation.The fuel cell using core-shell Au-Pd as anode catalyst presented a maximum power density of 68.215 mW cm −2 and a cell voltage of 0.757 V in 1 M NaBH 4 + 3 M NaOH solution at 25 • C. Therefore, the carbon-supported bimetallic Au-Pd catalyst with the core-shell structure would be a promising anode electrocatalyst for the application of DBHFC.
4 -H 2 O 2 fuel cell (DBHFC) was evaluated by a battery testing system at 25 • C in standard atmospheric pressure.The schematic of direct NaBH 4 -H 2 O 2 fuel cell was shown in

Figure 1 ,
Figure1, and it consisted of Au/C cathode, core-shell Au-Pd/C anode, activated Nafion 117 membrane separator, anolyte composed of 1 M NaBH 4 + 3 M NaOH, and catholyte composed of 2 M H 2 O 2 + 0.5 M H 2 SO 4 .The fresh anolyte and catholyte were continuously supplied and withdrawn from the cell at a rate of 0.7 mL min −1 during the testing process.The load was applied in steps of 5 mA within the range of 0-150 mA.Each step lasted 2 min, and the current was continuously applied from one value to the next without disconnecting the cell.The catalyst ink was made by mixing isopropyl Nafion solution and carbon-supported catalyst and coated onto stainless steel gauze.Then it was dried in vacuum at 80 • C for 12 h and pressed at 16 MPa for 1 min.Finally, the loading mass of catalysts was about 4.5 mg cm −2 .

3. 1 .
Physical Characterization.Figure 2 showed the XRD patterns of Au/C, core-shell Au-Pd/C, and Pd/C.As shown in Figure 2, the rather wide diffraction peaks at 2θ ≈ 25 • in 50 nm

Figure 3 :
Figure 3: TEM images of Au/C (a), core-shell Au-Pd/C (b), and Pd/C (c) and their corresponding histograms on size distribution.

Figure 4 :
Figure 4: Absorption spectra of Au, core-shell Au-Pd, and mixed Au-Pd hydrosols before deposition on carbon.