PdPbAg alloy NPs immobilized on reduced graphene oxide/In2O3 composites as highly active electrocatalysts for direct ethylene glycol fuel cells

rGO-modified indium oxide (In2O3) anchored PdPbAg nanoalloy composites (PdPbAg@rGO/In2O3) were prepared by a facile hydrothermal, annealing and reduction method. Electrochemical tests showed that the as-prepared trimetallic catalyst exhibited excellent electrocatalytic activity and high resistance to CO poisoning compared with commercial Pd/C, mono-Pd and different bimetallic catalysts. Specifically, PdPbAg@rGO/In2O3 has the highest forward peak current density of 213.89 mA cm−2, which is 7.89 times that of Pd/C (27.07 mA cm−2). After 3600 s chronoamperometry (CA) test, the retained current density of PdPbAg@rGO/In2O3 reaches 78.15% of the initial value. Its excellent electrocatalytic oxidation performance is attributed to the support with large specific surface area and the strong synergistic effect of PdPbAg nanoalloys, which provide a large number of interfaces and achievable reactive sites. In addition, the introduction of rGO into the In2O3 matrix contributes to its excellent electron transfer and large specific surface area, which is beneficial to improving the catalytic ability of the catalyst. The study of this novel composite material provides a conceptual and applicable route for the development of advanced high electrochemical performance Pd-based electrocatalysts for direct ethylene glycol fuel cells.


Introduction
Modern society urgently needs to replace traditional fossil fuels with new clean energy sources. 1 Direct alcohol fuel cells (DAFCs) are currently considered attractive candidates for reducing future energy demand. 2 Direct ethylene glycol fuel cells (DEGFCs) have unique advantages such as safe storage, easy availability, high energy density, and convenient transportation. 3 However, the anodic electrocatalysts of DAFCs tend to adsorb the toxic intermediates generated in the anodic ethylene glycol oxidation reaction, resulting in decreased electrocatalytic activity and stability. 4 The design of anode electrocatalysts usually depends on the size, surface electronic structure, shape, and composition of the material. 5,6 It is effective to further construct porous structures with large surface area and active sites. Metal-organic frameworks (MOFs) have the advantages of high porosity, tunable pore size, and large internal specic surface area. 7,8 Reduced graphene oxide (rGO) can anchor metal nanoparticles through its inherent residual oxygen-containing functional groups and defect sites, 9 which can enhance the conductivity of catalysts. Improve the dispersion of the supported metals to enhance the electrocatalytic activity and stability of EGOR by providing more active centers and electron transport. 10,11 MOFs are oen used as ideal templates for the preparation of MOFs-derived porous metal oxides with high catalytic activity and stability to compensate for the poor electrical conductivity and acid-base corrosion resistance of MOFs. In 2 O 3 is a wide bandgap n-type semiconductor that has been widely used in the microelectronic eld and gas sensors. 12,13 Previous studies have shown that it has good catalytic activity and low resistivity. 14 However, there are few reports on the application of In 2 O 3 used as the substrate in the EGOR. Pd-based materials have abundant storage and good catalytic activity, so they are oen used as effective active components in anodic alcohol oxidation reaction (AOR). 15,16 However, many studies have shown that single Pd catalysts have poor electrocatalytic performance and stability. Therefore, the d-band center of Pd can be effectively tuned by modifying the Pd surface by doping with appropriate transition metals or alloying Pd. 16 In addition, metal particles supported on supports can promote the uniform dispersion of nanoparticles (NPs). For example, Pd-based bimetallic catalysts Pd-Cu 17 and Pd-Sn/Pd-Ni/CNT; 18 trimetallic catalysts PdCuBi/ C, 19 Pd-Co-Ni/G 20 and Pd-Ru-Bi. 21 Thereby increasing the active center of electrocatalyst, promoting the transfer of electrons, and optimizing the performance of the catalyst. 22,23 At the same time, it exhibits excellent synergistic effect and high resistance to CO poisoning, leading to improved electrocatalytic performance of Pd catalysts. 15 Taking the above considerations into account, in this study, graphene oxide-modied In 2 O 3 nanoparticles were successfully synthesize by hydrothermal method and annealing at 500 C under N 2 atmosphere. The PdPbAg NPs were then embedded into the previously obtained GO/In-MOF, and the reduction method was used to obtain the PdPbAg@rGO/In 2 O 3 trimetallic electrocatalyst, as shown in Scheme 1. Compared with Pd/C, the current density of PdPbAg@rGO/In 2 O 3 (213.89 mA cm À2 ) for EG is 7.89 times higher than that of Pd/C (27.1 mA cm À2 ). At the same time, PdPbAg@rGO/In 2 O 3 has the highest catalytic activity, the lowest E onset , the smallest resistance to EG, and better anti-toxicity. Aer 3600 s CA test, the retained current density of PdPbAg@rGO/In 2 O 3 still maintains the highest value of 64.79 mA cm À2 and 78.15% of the original current density, which is superior to that of Pd/C (4.03 mA cm À2 , 20.15%). This is attributed to the strong electronic effect between Pd-Pb-Ag nanoalloys, which can signicantly enhance the adsorption of oxygen-containing species on the Pd surface. Thereby accelerating the oxidation of intermediate carbon species on the Pd surface and the removal of adsorbed CO species. Meanwhile, the carrier rGO/In 2 O 3 enhances the specic surface area and electron transfer rate, with good electrical conductivity and unique structure, providing abundant active sites for the Pd-Pb-Ag alloy NPs.

Synthesis of GO/In 2 O 3
Graphene oxide (GO) was prepared from graphite powder via the modied Hummer's method. 24,25 1.4670 g of In(NO 3 ) 3 $xH 2 O, 0.2337 g of H 2 ATA, 12.4 mL of DMF and 0.0467 g of GO were added to the beaker and ultrasonicated for 30 min. The mixture was then transferred into a 50 mL teon-lined stainless steel autoclave and heated at 125 C for 5 h. Aer cooling to room temperature, the obtained black product was ltered, washed three times alternately with deionized water, DMF and absolute ethanol, and dried under vacuum at 40 C overnight. Finally, the obtained product was annealed at 500 C for 2 h under N 2 atmosphere to obtain the precursor GO/In 2 O 3 .

Physical characterization
The crystalline structures of prepared electrocatalysts were characterized by X-ray diffraction (XRD) using a Bruker-D8 Advance X-ray diffractometer equipped with Cu Ka radiation (l ¼ 1.5406Å). The XRD patterns were collected at 2q values ranging from 5 to 80 . The morphology and chemical component of catalyst were investigated by eld emission scanning electron microscopy (FE-SEM, Zeiss ULTRA 55) and energydispersive X-ray spectrometer (EDS). The particle size distribution and microstructure of composites were investigated using a high-resolution transmission electron microscope (HRTEM, FEI Tecnai F20). The chemical valence state of element was determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi) with a monochromatic Al Ka source of 1486.6 eV. The inductively-coupled plasma optical emission spectrometer (ICP-OES) measurements were conducted on Agilent ICP-OES 725 analyzer.

Electrochemical measurements
All the electrochemical measurements were operated in CHI660E electrochemical workstation (CH Instruments, Inc., Shanghai) with standard three-electrode system. The glassy carbon electrode (GCE, 0.1256 cm 2 ), Pt wire and Hg/HgO electrode were used as the working, counter and reference electrode, respectively. In a typical procedure, 5 mg of catalyst and 5 mL of 0.5 wt% Naon solution were mixed with 950 mL of isopropanol, followed by sonication for 30 min to produce a uniformly dispersed suspension, and then 10 mL of the suspension solution containing catalyst was dropped onto the surface of the GCE and dried at room temperature to obtain the working electrode. First, the working electrode was tested by cyclic voltammetry (CV) in the range of -0.9 V to 0.8 V at a scan rate of 50 mV s À1 in 1.0 M KOH solution to activate the prepared catalyst. The electrocatalytic oxidation activity of ethylene glycol was characterized by CV in N 2 -treated solution of 1.0 M KOH + 0.5 M EG. The scanning potential range was from -0.9 V to 0.8 V and the scan rate was 50 mV s À1 . Chronoamperometry (CA) experiments were performed at a potential of -0.1 V for 3600 s to test the durability of the catalysts. Electrochemical impedance spectroscopy (EIS) measurements were implemented on different catalysts at open circuit potentials from 0.01 to 10 6 Hz. Before testing, ultrapure N 2 was introduced into the electrolyte for 30 min to remove dissolved oxygen.

Results and discussion
Physical characterization of catalysts  Fig. 1(B)). The Pd (111) planes are shied to 38.8 , 39.1 and 39.4 from 39.6 , respectively. 29 This indicates that alloys are formed between Pd, Pb and Ag, which changes the lattice constant of Pd and increases the synergistic effects and electrocatalytic activity of Pd-Pb-Ag for EGOR. Furthermore, in bimetallic PdPb, PdAg and trimetallic PdPbAg@rGO/In 2 O 3 catalysts, due to the low Pb and Ag content on the catalyst surface, or thin and amorphous phase, no obvious Pb and Ag diffraction peaks were observed. 30,31 From the above analysis, it can be seen that the formation of metal nanoalloys is of great signicance for improve the synergistic effect and catalytic activity of ethylene glycol electrooxidation.
The surface morphologies of PdPbAg@rGO/In 2 O 3 were investigated by FE-SEM. Comparing the FE-SEM image of GO/ In 2 O 3 (Fig. 2(A)), Fig. 2(B) shows the FE-SEM image of the PdPbAg@rGO/In 2 O 3 catalyst, the nanoparticles of PdPbAg are uniformly loaded to the support. By analyzing the element mapping diagram (Fig. 2(C)), the catalyst contains Pd, Pb, Ag, In, C and O elements, conrming the uniform dispersion of Pd-Pb-Ag nanoalloys. Additionally, in Fig. 2(D), the EDS spectrum further shows the presence of In, Pd, Ag Pb, C and O. The actual atomic loadings of Pd, Pb, and Ag in the PdPbAg@rGO/In 2 O 3 catalyst are 13.23%, 5.34%, and 3.36%, respectively. The mass percentages of Pd, Pb and Ag measured by ICP-OES are 12.45%, 6.02% and 4.25%, respectively. The test results of the two are close, which are also consistent with the initial addition ratio of the reactants. It demonstrated that the Pd, Pb and Ag precursors have been completely converted into products, and the trimetallic composite was successfully synthesized. 32 The morphology and particle size distribution of PdPbAg@rGO/In 2 O 3 were further analyzed with HR-TEM (Fig. 3). Fig. 3(D) show the crystalline properties of PdPbAg@rGO/In 2 O 3 . The well-shaped lattice fringes wuth interplanar spacing of 0.228 nm, corresponding to the Pd (111) plane. Compared with the pure Pd (0.223 nm), the interplanar spacing of Pd in the catalyst is slightly larger. This indicates that the lattice of Pd expands with the addition of Pb and Ag NPs, conrming the formation of PdPbAg nanoalloys. 33 This small PdPbAg nanoalloy can easily obtain more active cites, which is benecial to the electrocatalytic oxidation of ethylene glycol. Fig. 4 shows the XPS spectrum of PdPbAg@rGO/In 2 O 3 . According to the XPS spectrum, the elemental valence states of Pd, Pb and Ag in the catalyst can be determined. The survey of    Fig. 4(B), the two typical peaks at 335.3 eV and 340.7 eV are assigned to Pd 3d 5/2 and 3d 3/2 of Pd(0). There are no obvious other valence peaks, which means that Pd ions are completely reduced to Pd(0). The binding energies (BE) of Pb 4f were depicted in Fig. 4(C), the two peaks at 137.0 eV and 141.8 eV are ascribed to Pb(0) 4f 7/2 and 4f 5/ 2 , while the other two peaks at 142.8 eV and 144.2 eV correspond to Pb(II) 4f 7/2 and Pb(II) 4f 5/2 , such as Pb-O. 34,35 The surface modication of Pb/PbO NPs can enhance the electrocatalytic performance for EG. 36,37 Fig. 4(D) shows the XPS of Ag 3d, and the peaks at BE of 368.0 eV and 378.8 eV correspond to Ag 3d 5/2 and Ag 3d 3/2 , respectively, which is the characteristic peak of Ag(0). In addition, the BE of Pd 3d, Pb 4f and Ag 3d in PdPbAg@rGO/In 2 O 3 show a slight negative displacement compared to pure Pd (Pd 3d 5/2 ¼ 335.0 eV, Pd 3d 3/2 ¼ 340.3 eV), Pb (Pb 4f 7/2 ¼ 136.9 eV, Pb 4f 5/2 ¼ 141.7 eV) and Ag (Ag 3d 5/2 ¼ 367.7 eV, Ag 3d 3/2 ¼ 374.2 eV), indicating the formation of PdPbAg nanoalloys. Fig. 4(E) is the XPS spectrum of C 1 s, two peaks can be seen in the gure, the prominent peak at 285.1 eV belongs to the C-C bond of rGO, and the other peak at 289.1 eV corresponds to the O-C]O functional group of rGO. 38 It can be found that the peaks of oxygen-containing functional groups are particularly small, indicating that GO is reduced to rGO in the electrocatalyst. Fig. 4

(F) depicts the XPS spectrum of In 3d
showing two peaks at 444.8 and 452.4 eV, which can be attributed to the characteristic spin-orbit splitting 3d 5/2 and 3d 3/2 , respectively. This proves that the valence state of indium in In 2 O 3 is mainly + 3. 39 The BE of In also shows a slight negative shi compared with pure In (In 3d 5/2 ¼ 443.8 eV, In 3d 3/2 ¼

Electrochemical study on electrooxidation of ethylene glycol
The typical cyclic voltammetry (CV) curves of commercial PdPbAg@rGO/In 2 O 3 , PdPb@rGO/In 2 O 3 , PdAg@rGO/In 2 O 3 , Pd@rGO/In 2 O 3 and Pd/C, were recorded in N 2 -saturated 1.0 M KOH at the scanning rate of 50 mV s À1 , as shown in Fig. 5(A). It can be observed that the oxidation peak of each catalyst is around 0.3 V and the reduction peak is around -0.5 V. The electrochemical active surface area (ECSA) of a catalyst is closely related to the active center of the catalyst and can be calculated by the following formula: Q in the formula is the coulombic charge observed during the reduction of Pd oxide, 40 and S is the proportionality constant during the reduction of the PdO monolayer, which is 0.405 mC     (Table 1). It is pointed out that there may be highly active reaction centers in ternary alloy catalysts. This is related to the wide extended surface area of the support In 2 O 3 and the high conductivity of rGO.
The electrocatalytic performance of various catalysts were preliminarily investigated by CV at the scanning rate of 50 mV s À1 in 1.0 M KOH and 0.5 M EG solution. The CV curves of EGOR (Fig. 5(B)) contains two clear oxidation peaks in both the front and back scans, the forward scan is the result of the oxidation of chemisorbed EG molecules, while the other one in the backward scan may be attributed to the further oxidation of new intermediates formed in the previous scan. 41  and Pd/C (-0.33 V). The above results fully demonstrate that the PdPbAg@rGO/In 2 O 3 catalyst has the highest catalytic activity for electro-oxidation, the lowest overpotential and activation energy. This is due to the strong electronic effect of the PdPbAg nanoalloy, which increases the electron density of the Pd-d band. Because the adsorption of carbon-based intermediates is alleviated, the connection between Pd and poisoning species is weakened. Thus, its antitoxicity is enhanced and the electrocatalytic oxidation of EG is promoted. 36,42 Meanwhile, a large amount of oxide species are adsorbed on the catalyst surface. This accelerates the oxidative removal of toxic intermediates, exposing more EGOR active centers. 42,43 The kinetics of PdPbAg@rGO/In 2 O 3 catalyst toward EG oxidation reaction was investigated. As shown in Fig. 6(A), the peak current density (j P ) increases when the potential scan rate increases from 50 mV s À1 to 250 mV s À1 . In the meantime, the peak potential (n) of the CV curves shis continuously, indicating that the electrocatalytic oxidation of EG on the PdPbAg@rGO/In 2 O 3 catalyst electrode is an irreversible electrode process. 44 Furthermore, the linear relationship between the square root scan velocity (n 1/2 ) and the forward peak current density is shown in Fig. 6(B). The linear correlation diffusion factor (R 2 ) of the diffusion is 0.99, demonstrating that EGOR on PdPbAg@rGO/In 2 O 3 is a diffusion-controlled irreversible electrode process. 45 Higher slope values indicate better electrooxidative kinetics. It is not difficult to nd that among all the catalysts, the PdPbAg@rGO/In 2 O 3 trimetallic catalyst has the best EGOR kinetics.
The kinetics of charge transfer and diffusion at the electrode/ electrolyte interface were analyzed by electrochemical impedance spectroscopy (EIS). 21 Fig. 7(A) is the Nyquist diagram and an equivalent circuit diagram of the electrocatalyst reaction process analyzed by electrochemical impedance spectroscopy. The charge transfer resistance of the catalyst was evaluated as a semicircular diameter. Compared with PdPb@rGO/In 2 O 3 (9.52 U), PdAg@rGO/In 2 O 3 (16.05 U), Pd@rGO/In 2 O 3 (19.21 U) and Pd/C (35.19 U), PdPbAg@rGO/In 2 O 3 (8.65 U) has minimal impefance. It is well known that a smaller R ct value is benecial to the charge transfer from electrode to fuel, reducing the activation barrier of fuel oxidation and electrode reaction overpotential. 44 Therefore, PdPbAg@rGO/In 2 O 3 has the most superior charge transport characteristics and the largest reaction driving force. 46 Table 1 The results of CV measurements of all catalysts modified electrodes.

Samples
ECSA a (cm 2 )   To investigate the long-term stability of the electrocatalysts, we conducted the analysis by the chronoamperometry (CA) test operating in 1.0 M KOH + 0.5 M EG for 3600 s at -0.1 V, as shown in Fig. 7(B). The current densities of the ve catalysts drop rapidly in the initial stage due to the accumulation of toxic intermediates, resulting in a severe drop in catalytic activity. 41 The current density then decreases slowly until it reaches a relatively stable state. It can be seen that among the electrocatalysts, PdPbAg@rGO/In 2 O 3 always maintains the highest EG electrooxidation activity. The results show that the PdPbAg@rGO/In 2 O 3 has the best catalytic durability for EG. The retained current density of PdPbAg@rGO/In 2 O 3 (64.79 mA cm À2 ) is superior to that of Pd/C (4.03 mA cm À2 , 20.15%), Pd@rGO/In 2 O 3 (20.96 mA cm À2 ), PdAg@rGO/In 2 O 3 (31.81 mA cm À2 ) and PdPb@rGO/In 2 O 3 (44.89 mA cm À2 ). Among them, compared with Pd/C (20.15%), the current retention ratio of the thrimetallic electrocatalyst (78.15%). This is due to the fact that PdPbAg@rGO/In 2 O 3 catalyst has sufficient metal oxides and the highest effective activation specic surface area.
To further clarify the excellent electrocatalytic activity of PdPbAg@rGO/In 2 O 3 , we propose the electrocatalytic oxidation mechanism of EG and the removal process of CO (ads) on Pd sites, as the following steps: 36,42,43 (CH 2 -OH) 2 / (CH 2 -OH) 2(ads) (1) 2Pd + (CH 2 -OH) 2 + 6OH À / 2Pd-CO (ads) + 6H 2 O + 6e À (6) Pb + OH À / Pb-OH (ads) + e À PbO + OH À / PbO-OH (ads) + e À Pd-CO (ads) + Pb-OH (ads) + 3OH À / Pd + Pb + CO 3 Pd-CO (ads) + PbO-OH (ads) + 3OH À / Pd + PbO + CO 3 From the above reaction mechanism, it can be found that the more active sites of the multi-metallic catalyst, the more oxygencontaining species adsorbed, and the more conducive to the electrocatalytic oxidation of EGOR. Compared with Pd/C and bimetallic catalysts, PdPbAg@rGO/In 2 O 3 trimetallic catalyst contains more active sites. The strong synergistic effect of Pd-Pb-Ag alloy NPs can adsorb more OH À (ads) and (CH 2 OH) 2(ads) , thus exhibiting the best electro-oxidative performance. From this, we can infer that the intermediate CO (ads) on the electrode surface is easily converted to CO 2 . 47 On the one hand, the toxic intermediates in the active site of Pd are eliminated in time, so that fresh Pd can further absorb and oxidize EG molecules. On the other hand, Pb can form OH functional groups on the catalyst surface. This promotes the oxidation of carbonaceous species, thereby improving the oxidative stability of EG. Meanwhile, the high electronic conductivity of the supported rGO/ In 2 O 3 is benecial for metal loading and electrocatalysis. From the above analysis, it can be further proved that PdPbAg@rGO/ In 2 O 3 has faster electron transfer and better catalysis for the electro-oxidation of EG.

Conclusions
A novel composite GO/In 2 O 3 was prepared by hydrothermal and calcination method, and the well-dispersed Pd-Pb-Ag alloy NPs were immobilized on GO/In 2 O 3 by impregnation reduction method. Compared with PdPb@rGO/In 2 O 3 , PdAg@rGO/In 2 O 3 , Pd@rGO/In 2 O 3 and Pd/C, the nally synthesized anode catalyst PdPbAg@rGO/In 2 O 3 has the highest catalytic activity, the lowest E onset (À0.56 V), the smallest impedance for EG (8.65 U), and better resistance to toxicity. Among them, the I p,f of the ternary electrocatalyst PdPbAg@rGO/In 2 O 3 (213.89 mA cm À2 ) for the electrooxidation of EG is 7.89 times higher than that of Pd/C (27.07 mA cm À2 ). Aer 3600 s, PdPbAg@rGO/In 2 O 3 still maintained the highest catalytic activity and durability, the retained current density of PdPbAg@rGO/In 2 O 3 (64.79 mA cm À2 ) reaches 78.15% of the initial value, which is superior to that of Pd/C (4.03 mA cm À2 , 20.15%). This outstanding electrocatalytic performance is related to the surface modication of the support rGO/In 2 O 3 . This structure improves the specic surface area and electron transfer rate. And its good electrical conductivity provides abundant active sites for the Pd-Pb-Ag alloy NPs. Meanwhile, the strong synergistic effect of the formed Pd-Pb-Ag alloy NPs can signicantly enhance the adsorption of oxygencontaining species on the Pd surface. Thereby accelerating the oxidation of intermediate carbon species on the Pd surface and the removal of adsorbed CO species. The design of this composite and its efficient catalytic performance will provide important ideas for the development of a wide range of energy catalysts.

Author contributions
Zhirui Wu: conceptualization, experimental, investigation, formal analysis, writing-original dra and writing-review & editing. Yuting Zhong: conceptualization and investigation. Zhiguo Wang: resources and project administration. Ling Li: project administration and resources. Xiaoguang Liu: supervision and writing-review & editing.

Conflicts of interest
There are no conicts to declare.