Carbon Nanotube-Supported Bimetallic Core–Shell (M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni)) Cathode Catalysts for H2O2 Fuel Cells

M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni) core–shell and Pd/CNT nanoparticles were prepared by sodium borohydride reduction and explored as cathode catalysts for the hydrogen peroxide reduction reaction. Electrochemical and physical characterization techniques are applied to explore the characteristics of the produced electrocatalysts. The cyclic voltammetry (CV) experiments show that Zn@Pd/CNT-modified electrodes have a current density of 273.2 mA cm–2, which is 3.95 times higher than that of Pd/CNT. According to the chronoamperometric curves, Zn@Pd/CNT has the highest steady-state current density for the H2O2 electro-reduction process among the synthesized electrocatalysts. Moreover, electrochemical impedance spectroscopy (EIS) spectra confirmed the previous electrochemical results due to the lowest charge transfer resistance (35 Ω) with respect to other electrocatalysts.


INTRODUCTION
Fuel cells have gained much attention as effective and clean energy devices.They are emerging as prime energy transformation ways for several areas in fixed and mobile applications. 1However, price is a big barrier interrupting fuel cell distribution because of the requirement of a high loading of noble metals such as platinum. 2The cost can be reduced meaningfully by using alternative noble metals. 3,4enchmarked with liquid fuels, hydrogen peroxide (H 2 O 2 ) is a promising liquid fuel consisting of hydrogen and oxygen. 5 2 O 2 fuel cells have been receiving growing consideration due to their high energy density and cell potential.Hydrogen peroxide fuel cells have the benefits of no side products and, therefore, no poisoning of the modified electrodes. 6In addition, the H 2 O 2 reduction reaction with two electrons has better kinetics than O 2 reduction to H 2 O 2 with four electrons. 7,8H 2 O 2 can be used in a H 2 O 2 fuel cell system in two ways, i.e., as a fuel or oxidant, and it can be constructed according to its usage. 9he superiority of H 2 O 2 reduction has an outstanding influence on the execution of H 2 O 2 fuel cell behaviors.Moreover, liquid H 2 O 2 has the benefits of suitable carriage and storage for both space crafts and submarines.Concurrently, electro-reduction of H 2 O 2 takes place in the two-phase boundary, which is more achievable than the reduction of O 2 electro-reduction taking place in a three-phase zone. 10As a consequence of the above properties, H 2 O 2 is an impressive substitute for O 2 in fuel cells. 11The reduction of H 2 O 2 is executed with the subsequent reaction (eq 1) in an alkaline medium Over the past few years, crucial attempts have been made to improve the electrode modification to improve the electron transfer effectiveness, which is extremely related to fuel cells' performances.Among these above-mentioned options for the applications of fuel cells, electrode modification has a great effect on electron transfer. 12−15 Among these, Pd as the illustrative noble metal is usually taken into consideration to improve catalyst activity, since it has the same features as Pt, but it is more abundant and cheaper.Considering both metals present the same conformation, 9,16 Pd can be used instead of Pt due to its high electron utilization rate and adsorption capacity. 17Moreover, Pd could encourage the breaking of O− O bonds in H 2 O 2 . 11However, the main drawback in the design of fuel cells is the cost of the precious metals used for catalyst manufacturing.This problem will be solved by developing high-performance and low-price electrode catalysts.Therefore, researchers have concentrated on adding less expensive metals into Pd catalysts to increase the catalytic activity and reduce the cost of Pd metal by exploiting the structure, electronic, and synergistic effects among different materials.Literature studies 7,15,16,18 have confirmed that owing to synergistic catalytic outcomes, bimetallic catalysts express higher catalytic activity than any single one. 19According to research by Sun et al., 20 Cu in the PdCu/C electrode enters the Pd lattice and modifies its electronic structure, giving the hydrogen peroxide reduction reaction better performance than commercial Pd/C.For instance, because of the strong electronegativity of Au and the synergistic interaction between Pd and Au, the catalytic performance of the electro-reduction of H 2 O 2 might be enhanced by doping the Pd electrode with a second metal of Au. 21ypically, the core−shell form has an efficient morphology for increasing catalytic activity.These nanoparticles are commonly preferred to alloy nanoparticles due to the improvement in the usage level of a noble metal at the external surface. 22Otherwise, the core and shell structure decreases the load of the metal and increases its impact. 23,24his structure can exhibit the effects of synergistic between the core and the shell that improves the electrochemical reaction activity. 25Besides, the activity of the core−shell structure is related to the underlying interface between the core and shell metals due to the bimetallic mechanism.
As noble metals employed in catalyst synthesis are expensive, support materials are used to moderate the cost.Catalysts are dispersed over the support materials rather than being used as one piece, reducing the load of the catalyst used and increasing the catalyst surface area. 26Besides that, the support materials used together with the catalyst can enhance the catalytic activity. 27Carbon-supported materials such as graphene, graphene oxides, and carbon nanotubes have higher activity and stability than unsupported metal catalysts. 28With their benefits of a high specific surface area, good chemical stability, low resistance, thermal stability, great flexibility, and superior electrical properties, carbon nanotubes (CNTs) are a common choice for electrode materials. 29n this paper, the NaBH 4 reduction method was used to create M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni) and Pd/ CNT.The structural, morphological, and electrochemical features of the prepared materials were characterized.Using a three-electrode setup, the behavior of the catalysts regarding the hydrogen peroxide reduction reaction was studied with electrochemical procedures.Also, the impact of experimental conditions on the catalytic activity of electrocatalysts was evaluated.

Apparatus.
A CHI660E potentiostat device was used for electrochemical measurements.The electrode cleaning and dispersion processes by adding Nafion to catalyst powders were carried out in a Branson 1510-MTH ultrasonic bath.A Scilogex MS−H-S was used as a magnetic stirrer.

Catalyst Preparation.
Bimetallic core−shell catalysts with 10% Pd content were obtained using the NaBH 4 reduction technique.In order to synthesize M/CNT, an appropriate quantity of ZnCl 2 , MnCl 2 , AgNO 3 , CoCl 2 , VCl 3 , and NiSO 4 solutions was prepared in water first.The solution was then agitated for 2 h while the CNT support material was added under sonication.A certain quantity of NaBH 4 was mixed to obtain M/CNT nanoparticles.Then, it was filtered, cleaned, and vacuum-dried at 90 °C for 14 h.A suitable quantity of M/CNT nanoparticles and K 2 PdCl 4 was added to water to synthesize bimetallic M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni) catalysts.
2.4.Catalyst Characterization.M@Pd/CNT and Pd/ CNT nanoparticles were characterized by using X-ray diffraction (XRD) investigations.The crystallographic struc- ture of these catalysts was examined with XRD.The particle crystallite size was obtained from Scherrer equality.The morphology and particle dimensions were examined by transmission electron microscopy (TEM) (Zeiss Sigma 300) operating at 200 kV.The Specs-Flex gadget used X-ray photoelectron spectroscopy (XPS) analysis to detect the oxidation status of the Zn@Pd/CNT catalyst.
2.5.Electrode Modification.The prepared solid catalyst was spread in Nafion solution.Catalyst inks were obtained by an ultrasonic bath for 30 min.3 μL was taken from the catalyst ink with the help of a micropipette, then applied on the working electrode surface, and dried.
The TEM analysis of the Zn@Pd/CNT catalyst at different scales and the graphs of particle size distribution are presented in Figure 2a,2b.The inset of Figure 2c shows that Zn nanoparticles have a black core and Pd nanoparticles have a bright shell, representing the formation of core−shell nanostructures.As seen in the figure, metal nanoparticles show a regular distribution on the CNTs.From the ImageJ program, the average particle size of Zn@Pd/CNT was found to be 7.7 nm.
Following that, the oxidation states of the Zn@Pd/CNT catalyst were determined by XPS.Zn@Pd/CNT total spectra revealed C 1s, O 1s, Pd 3d, and Zn 2p peaks (Figure 3a). Figure 3b shows the binding energy of C 1s.The spectra of Pd 3d in Figure 3c could be ascribed to Pd 0 (at 334.8 and 341.6 eV), PdO (at 342.2 eV), Pd(OH) x (at 336.4 and 340.9 eV), and PdO 2 (at 337.6 eV).Furthermore, the spectra of Zn 2p at 1022.1 and 1045.2 eV demonstrated that Zn was mostly present as Zn 2+ (Figure 3d).In addition, the uncurved peak at 1023.1 eV is considered to be Zn(OH) 2 .The Zn@Pd core− shell had a greater Pd concentration, suggesting that Pd was mostly dispersed in the outer layer shell.
The cyclic voltammograms of M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni) and Pd/CNT-modified GCE were obtained in the potentials varying from −0.1 to −0.7 V. Without H 2 O 2 , no recognizable change in current was detected over the full potential range, indicating that no distinguishable activity occurred on the electrode surface.However, the cathodic scan at around −0.3 V, which represents the H 2 O 2 reduction that follows a two-proton and two-electron reaction, showed a substantial rise in the response after the H 2 O 2 addition to the solution (eq 3). 34

+ +
As shown in Figure 5, the current density of hydrogen peroxide reduction on Zn@Pd/CNT (273.2 mA cm −2 ) is 3.95, 2.68, 2.27, 2.18, 1.93, and 1.41 times higher than those on Pd/ CNT (69.2 mA cm −2 ), Ni@Pd/CNT (102.1 mA cm −2 ), V@ Pd/CNT (120.3 mA cm −2 ), Co@Pd/CNT (125.6 mA cm −2 ),  Ag@Pd/CNT (141.5 mA cm −2 ), and Mn@Pd/CNT (193.4 mA cm −2 ), respectively.Core metals (Zn, Mn, Ag, Co, V, Ni) coupling with palladium atoms can change the outer electronic structure of palladium, which affects the energy involved in the adsorption/desorption of palladium to H 2 O 2 .In other words, the lattice strain and synergistic effects in the core−shell region can increase the utilization of metal atoms and result in changes in the electronic structure, which can significantly improve the catalyst activity and cause differences in the current densities.Among the catalysts prepared, the Zn@Pd/ CNT catalyst showed the best activity in the H 2 O 2 electroreduction reaction with the highest peak current density.The consequences confirmed that the modification of GCE with highly conductive Zn@Pd/CNT modification to the GCE increased the electrode conduction and significantly improved the conductivity of the electrode.Subsequent experiments were carried out with the Zn@Pd/CNT catalyst.
The electro-reduction of H 2 O 2 on the Zn@Pd/CNTmodified GCE has been investigated for different experimental conditions.First, the effect of the NaOH concentration was studied by cyclic voltammetry for various concentrations of NaOH and a 0.25 M H 2 O 2 solution.Clearly, the NaOH concentration effect for H 2 O 2 electro-reduction on the Zn@ Pd/CNT-modified GCE is much more significant, as shown in Figure 6.The H 2 O 2 electro-reduction current increases as the NaOH concentration increases up to 3.0 M. 3,35 The current density then decreases with a further increase in NaOH molarity until 5.0 M. 36 The increase in the NaOH concentration has no advantage for the improvement of hydrogen peroxide reduction; therefore, it is useless, and as a result, the optimum NaOH value is 3.0 M.  The effect of H 2 O 2 concentration on H 2 O 2 electro-reduction was investigated with Zn@Pd/CNT-modified GCE.The cyclic voltammetric results of different concentrations of H 2 O 2 are shown in Figure 7. Similar to NaOH, the electro-reduction current density on Zn@Pd/CNT-modified GCE also shows a remarkable increase with the change in H 2 O 2 concentration.The peak current does not significantly increase when the H 2 O 2 concentration is increased further, though.Given that H 2 O 2 is known to be unstable in strong alkaline electrolytes, this tendency is likely caused by severe H 2 O 2 chemical breakdown. 3,30n@Pd/CNT is an excellent catalyst for the reduction of hydrogen peroxide by comparison with other catalysts synthesized for the reduction of H 2 O 2 in the literature. 8,14,30,37,38By the evaluation of H 2 O 2 electro-reduction on different modified electrodes in Table 1, the prepared Zn@ Pd/CNT cathode catalyst displays a lower reduction potential and a higher current density than other reported cathode catalysts for H 2 O 2 reduction.
Chronoamperometric studies were performed at various applied potentials to further demonstrate the performance of the Zn@Pd/CNT catalyst for H 2 O 2 electro-reduction.The response of Zn@Pd/CNT-modified GCE at each potential increased significantly and reached a stable value in a short time after the test began as the potential went negative (Figure 8).The CA curves exhibit a gentle fluctuation when the potential is switched to a more negative value, which can be attributed to the disruption of the O 2 bubbles brought on by H 2 O 2 breakdown.
EIS is a commonly used technique for exploring the charge transfer characteristics of the electrodes.At this point of the study, EIS was applied to explore the charge transfer resistance of electrodes modified with different catalysts.The typical Nyquist diagrams of M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni)-modified GCEs obtained at −0.3 V are given in Figure 9.A single semicircle that is associated with the charge transfer resistance (Rct) at the solid−electrolyte interface can be seen in the high-frequency region.The Rct values of M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni) and Pd/CNT catalysts are about 35, 73, 105, 157 and 200, 350 Ω, respectively.It appears that Zn@Pd/CNT has the smallest resistance at the catalyst− electrolyte interface, indicating that it would be better suited to serve as the cathode catalyst for a H 2 O 2 fuel cell.

CONCLUSIONS
Palladium-based catalysts were prepared on CNT by the sodium borahydrate reduction method.M@Pd/CNT (M: Zn, Mn, Ag, Co, V, Ni) electrocatalysts' activities toward H 2 O 2 reduction in alkaline solution were evaluated using different electrochemical methods.The Zn@Pd/CNT catalyst has a current density of 273.2 mA cm −2 , which is 3.95 times higher than that of Pd/CNT catalyst-modified electrodes with respect to the CV results.The findings of CA curves further demonstrated that Zn@Pd/CNT has the highest steady-state current density for the H 2 O 2 electro-reduction process among the synthesized electrocatalysts.Additionally, because Zn@Pd/ CNT showed the lowest charge transfer resistance (35 Ω) compared to the other electrocatalysts, the findings of the EIS spectra confirmed the results of CV and CA.The results showed that Zn@Pd/CNT-modified GCE for the H 2 O 2 electro-reduction reaction had excellent catalytic activity and stability, and it could be used as a catalyst.

Table 1 .
Comparison of Different Catalysts for Electrochemical Reduction of H 2 O 2