Facile Hydrothermal Synthesis of Highly Efficient and Durable Ternary PtPdCu Electrocatalysts for the Methanol Oxidation Reaction

Precious metal Pt‐based electrocatalysts have been widely used in the methanol catalytic oxidation anodes in direct methanol fuel cells. However, decreasing their cost and improving their efficiency and durability have still been challenging. Herein, ternary PtPdCu nanocatalysts are synthesized through a facile one‐step hydrothermal synthesis method. When KI is present with a suitable amount in the synthesis, PtPdCu nanospheres with surface‐embedded CuI clusters (CuI/PtPdCu) are fabricated. However, without KI, the prepared PtPdCu catalysts show a distinct hollow structure (h‐PtPtCu). CuI/PtPdCu displays the highest specific activity with enhancement 4 times higher than commercial Pt/C for the methanol oxidation reaction in an alkaline medium. The superior activity can be attributed to two aspects: 1) the electronic effect originating from the highly alloyed PtPdCu; 2) the synergetic effect resulting from surface inlaid CuI clusters, which can promote the CO intermediate removal. Furthermore, because of the stable Pt–Pd‐rich surface and its special linked hollow structure, the h‐PtPtCu catalyst exhibits good durability with only a 3.6% decay in the specific activity.


Introduction
As a fuel, methanol has the properties of feasible production and transportation, high energy density, and operation at almost ambient temperature. [1]Meanwhile, due to the rapid development of improved alkaline exchange membranes, the possibility of using a cost-effective transition metal in catalysts and brightening catalytic mechanisms revealed by advanced techniques, the direct alkaline methanol fuel cell (DAMFC) has attracted increasing attention. [2,3]However, the sluggish reaction kinetics of methanol oxidation on the anode side is still one of the most critical challenges. [4,5]t has been used as the conventional and commercial catalyst for the methanol oxidation reaction (MOR).However, Pt shows the weakness of its high cost, low antipoisoning capability, and improvable durability, which, in all, limit the large-scale commercialization of the DAMFC. [3,6]ne way to overcome this weakness is to introduce other metals and fabricate Pt-based polymetallic catalysts. [7]In the alkaline medium, transition metals which are usually active and cost-effective can retain good stability. [3]As most of them have empty d-band electrons, mixing them with noble metals such as Pd and Pt can lower the d-band centers of the latter, thus improving the catalytic efficiency. [7]ollowing this idea, efficient catalysts such as PtNi, [8] PtCu, [9,10] PdCu, [11] PtCo, [12] PdIr, [13] PtCuNi, [14] etc. have been fabricated.To enhance the anti-CO poisoning capability, Pd and Ru have been introduced to mix with Pt due to their substantial bifunctional role. [15,16][19] Trimetallic PtPdCu porous nanodendrites obtained through an aqueous-phase one-step way showed an enhanced MOR activity, durability, and a superior CO-poisoning tolerance due to their multifunctional effect and special Precious metal Pt-based electrocatalysts have been widely used in the methanol catalytic oxidation anodes in direct methanol fuel cells.However, decreasing their cost and improving their efficiency and durability have still been challenging.Herein, ternary PtPdCu nanocatalysts are synthesized through a facile one-step hydrothermal synthesis method.When KI is present with a suitable amount in the synthesis, PtPdCu nanospheres with surface-embedded CuI clusters (CuI/ PtPdCu) are fabricated.However, without KI, the prepared PtPdCu catalysts show a distinct hollow structure (h-PtPtCu).CuI/PtPdCu displays the highest specific activity with enhancement 4 times higher than commercial Pt/C for the methanol oxidation reaction in an alkaline medium.The superior activity can be attributed to two aspects: 1) the electronic effect originating from the highly alloyed PtPdCu; 2) the synergetic effect resulting from surface inlaid CuI clusters, which can promote the CO intermediate removal.Furthermore, because of the stable Pt-Pdrich surface and its special linked hollow structure, the h-PtPtCu catalyst exhibits good durability with only a 3.6% decay in the specific activity.
morphology. [20]Ultrafine trimetallic PtPdCu alloy nanoparticles decorated on carbon nanotubes (CNTs) could be used for the catalysis of ethanol oxidation and oxygen reduction reaction.The improved performance of this catalyst is due to a combination of factors, including the ultrafine metallic particles with a high rate of Pt/Pd utilization, the fine-tuned electron structure of catalysts with a lowered d-band center, Pd/Cu-induced bifunctional effects, and the CNTs carriers with high conductivities. [21]owever, it is still a great challenge to synthesize highly efficient and durable catalysts with controlled structure and composition using facile and environmentally friendly methods.
In catalyst synthesis, some special additives are quite useful and efficient in controlling the morphology and structure of the catalyst. [22,23]Halide ions have a strong coordination ability to form complexes with Pt nþ , thus lowering their reduction potential. [24]At the same time, they can selectively adsorb on a specific crystal plane during the reduction nucleation process to control the specific crystal face that covers the efficient catalyst.It is found that I or Br is the key factor for the formation of nanocubes.When the growth time of the nanoparticles is adjusted, the concave nanocubes can be further obtained. [25]However, the strong interaction between copper and precious metal promotes the co-formation of Cu II /Cu I nanosites as clusters or subnanoparticles.We recently reported on a PtCu nanocube catalyst (PtCu-NCb) fabricated using KI as a shape control additive.The prepared catalyst showed enhanced electrocatalytic activity (4.15 mA cm À2 ) and high durability when introducing graphene as the support.CuI in the form of clusters was found to be inlaid onto the crystalline surface of the nanocubes.The synergistic effect induced between PtCu and CuI was supposed to contribute partially to the enhancement of catalytic activity. [18]A similar effect of Cu þ /Cu 2þ sites on improving catalytic performance was reported by Janina et al.They fabricated an N-doped carbonaceous decorated with Cu II /Cu I nanoclusters.Cu I species were found to be the active sites that facilitate O 2 binding during the oxygen reduction reaction. [26]n this study, ternary PtPdCu catalysts were fabricated using a simple hydrothermal method.The prepared catalysts show different structures and surface compositions by controlling the amount of KI additive used in the synthesis.Structureactivity/durability correlations have been investigated.

Morphological and Structural Characterization
All three ternary PtPdCu catalysts (CuI/PtPdCu, h-PtPdCu, and CuI-PtPdCu) were fabricated by a one-pot hydrothermal method with similar synthesis conditions.The only variation in the amount of additive KI in the reacting solution results in distinct differences in the structure and morphology of the catalysts obtained.Figure 1 shows the transmission electron microscopy (TEM) images of CuI/PtPdCu prepared using 20 mg KI.As shown in Figure 1a,b, CuI/PtPdCu has a spherical morphology (avg.29.4 nm in diameter) with subnanoclusters (darker dots, avg.1.9 nm in diameter) homogenously distributed on the surface of the sphere.The high-resolution transmission electron microscopy (HRTEM) image (Figure 1c) shows clear lattice fringes and a fast Fourier transform (FFT) pattern; the interplanar spacings are measured at 0.51 and 0.23 nm, which may correspond to the {400} and {111} facet of Pt and was further confirmed by the FFT pattern. [18]The HRTEM image recorded at a different site is shown in Figure 1d; similar inlaid CuI clusters and a lattice distance of 0.51 nm can be observed.
The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figure 1f ) shows small particles with a bit brighter color on the surface of the nanospheres, and those small particles are supposed to be CuI clusters.The elemental analysis of Pt, Pd, Cu, and I is shown in Figure 1g-i and Figure S4, Supporting Information.The select area electron diffraction (SAED) pattern (Figure 1e) shows two groups of diffraction rings underlined with red and yellow dashed lines, which can be assigned to the lattice planes of polycrystalline face-centered cubic (fcc) Pt(Pd) and crystalized CuI, respectively.In addition to the rings, diffraction spots are also periodically observed on the specific rings that are attributed to the crystallized CuI.Therefore, it is assumed that CuI in the form of a single crystal also exists in the material.As shown in Figure S1, Supporting Information, when the amount of KI is doubled, the CuI-PtPdCu obtained is also spherical particles and more CuI particles can be observed.
In the absence of KI in the reacting solution during the synthesis, h-PtPdCu was obtained.As shown in Figure 2a,b, most particles have a distinct hollow structure with a brighter interior and darker exterior layer.The average size of the particles is around 17 nm and particles are connected via side chains.This special hollow chain structure contributes to the improvement of electrocatalytic activity and stability. [27]A similar brighter area appears in the middle of the particle shown in the HRTEM image (Figure 2c), which is consistent with the hollow structure configuration.Careful observation shows that these hollow structures are not entirely closed but have some channels, which is conducive to fully utilizing the two surface active sites inside and outside.The interplanar spacings of about 0.23 nm are measured on both the exterior and interior parts of the hollow particle; they can correspond to the {111} facets of Pt(Pd), which is further confirmed by the FFT pattern.The 0.45 nm spacing was also measured in the area where the Moiré pattern appears in Figure 2c, and it can be ascribed to interferences from two randomly oriented PtPdCu crystals. [28]he SAED pattern (Figure 2d), showing a group of diffraction rings, confirms the presence of the fcc structure of Pt(Pd).Two random particles displayed in the HAADF-STEM image (Figure S2, Supporting Information) demonstrate a bit of difference in the structure; the particle on the right side with an irregular shape shows a brighter area in the middle, implying its hollow interior.Another particle shows the solid particle configuration.Indeed, there are still several particles present in the form of solid particles rather than hollow ones, and this is consistent with the existence of some solid particles in the TEM image of Figure 2b.When the particle on the right side, which is suggested to have a hollow structure is viewed, it is still recognized that there is a small blank area in the middle, more Cu appears in the interior area, and a bit more Pt and Pd are distributed on the exterior part.Also, for the particle on the left side, Cu and Pd show a denser signal in the core area than Pt.From this point, we can conclude that in the h-PtPdCu sample, most particles have a hollow structure with more Pt and Pt on the surface.The surface enrichment of active elements helps improve the electrocatalytic activity of the catalysts. [29]he crystal phase structure of the prepared catalysts was determined from X-ray diffraction (XRD) patterns (Figure 3a).Diffraction peaks located at around 40°, 46°, 67°, and 82°can be ascribed to (111), ( 200), (220), and (311) of fcc Pt(Pd), respectively. [30]Both CuI/PtPdCu and CuI-PtPdCu display a group of diffraction peaks corresponding to crystallized cubic CuI (PDF#24-0363).The additional peak at around 40°can be assigned to trigonal CuI. [31]The relative diffraction intensity of Pt(Pd) to CuI in CuI-PtPdCu is slightly higher than that in CuI/PtPdCu, indicating a greater amount of CuI formed when using the double amount of KI during synthesis.Furthermore, as shown in the enlarged peak of Pt(Pd) (111) at around 40°, CuI/PtPdCu and CuI-PtPdCu show a 0.3°shift to a higher 2-theta angle compared to that in the sample h-PtPdCu, indicating a formation of PtPdCu with a higher degree of alloying in the former two samples prepared using KI.
It should be noted that according to Scherer's formula, for materials of the same system, the half-peak width of the diffraction peak is closely related to its particle size. [32]Here, the particle size of CuI is much smaller than PtPdCu, but its diffraction peak is sharper, probably because the two belong to different systems and CuI crystal is easier to form. [18]Our earlier work also found that the XRD peaks of smaller, nanoscale CeO 2 precipitated from La 0.3 Ce 0.1 Sr 0.5 Ba 0.1 TiO 3 are sharper than those of the bulk material. [33]The small particle size of CuI can be ascribed to the equilibrium of the formation and consumption of CuI.
The surface chemical composition and elemental valence state of CuI/PtPdCu were investigated by X-ray photoelectron spectroscopy (XPS).In the spectra of Pt 4f (Figure 3b) and Pd 3d (Figure 3c), two peaks located at binding energies of 71.54 eV (Pt 4f 7/2 ) and 74.86 eV (Pt 4f 5/2 ) can be assigned to metallic Pt. [34] Doublet located at 336.61 eV (Pd 3d 5/2 ) and 341.85 eV (Pd 3d 3/2 ) can be assigned to metallic Pd. [35] The signal of a small amount of oxidized Pt (II) and Pd (II) has also been detected due to inevitable oxidation during the synthesis process.For Cu, except for the strong deconvoluted peaks belonging to Cu 0 , two weaker peaks at 933.7 and 952.8 eV can be attributed to the oxidized state of Cu þ , [26,36] indicating the possible existence of CuI on the surface.The XPS survey scan profile and I 3d spectra of CuI/PtPdCu are shown in Figure S3, Supporting Information.The composition of the bulk CuI/PtPdCu and h-PtPdCu samples was determined by energy-dispersive spectroscopy (EDS) elemental analysis (Figure S4, Supporting Information).The atomic ratio of Pt:Pd in both samples is around 1:1, which is consistent with the molar ratio of Pt and Pd precursors in the reacting solution.Cu shows a much higher ratio due to using the Cu grid as the TEM sample substrate.
The schematic illustration of ternary CuI/PtPdCu and h-PtPdCu is shown in Figure 3e.As reported in the previous binary study of PtCu, when KI was absent, PtCu with hexagonal nanosheet morphology was obtained.When 20 mg of KI was used during synthesis, the prepared PtCu displayed a uniform nanocube shape. [18]In this work, the main difference is the introduction of the third element Pd in synthesis.However, for h-PtPdCu, synthesized without adding KI shows a mainly hollow structure.It is proposed that under high temperature and pressure hydrothermal conditions, the Cu, Pd, and Pt precursor ions are supposed to be reduced simultaneously, even though Cu has a reduction potential much lower than that of the other two.When it comes to the stage of particle growth, the galvanic replacement reaction (GRR) starts. [37,38]Reduced Cu 0 tends to bind unreduced Pt 4þ and Pd 2þ and acts as a reductant.As observed in the growth process of the PtCu-NCb catalyst, all those reactions would occur in a confined domain. [18]In addition to the GRR, the nanoscale Kirkendall effect may also accelerate the migration of the atoms; the reduced PtPdCu would move into a surrounding shell, thus forming a hollow structure. [39]nlike h-PtPdCu, 20 mg of KI was added as an additive to synthesize CuI/PtPdCu.The morphology of CuI/PtPdCu is mainly nanospheres rather than uniform cubic shapes such as those of PtCu-NCb.Moreover, CuI appears in the form of nanoclusters with an average size of about 2 nm, while in PtCu-NCb, no clear CuI particles or clusters can be observed on the surface of the nanocube.When a wide range of observations are taken in TEM images, a few nanocubes mixing with the spheres can still be found (Figure S5, Supporting Information).Therefore, we hypothesize that the introduction of Pd would invalidate the shape-controlling function of I to a large extent, thus resulting in a spherical morphology.It has been proposed that the cubic shape control is mainly due to the iodine ion that exhibits a strong coordination capability of I that tends to adsorb on the specific crystal facet of Pt {100}. [40]In the current case, the presence of Pd might weaken the adsorption strength of I on the {100} facet.Most particles would have overgrowth along the energy-favored directions, thus forming the spherical morphology.The considerable amount of unabsorbed I À tends to bind with Cu 2þ to form visible CuI subnanoclusters inlaid on the spherical PtPdCu surface.Furthermore, due to the strong coordination of I À , I À can replace Cl À to form the coordination complex PtI x Cl (6Àx) 4À and PdI y Cl (4Ày) 2À (0 ≤ x ≤ 6, 0 ≤ y ≤ 4), which shows a much lower reduction potential than that of PtCl 6 4À (PtCl 6 4À /Pt, 1.47 V vs standard hydrogen electrode (SHE) and PdCl 4 2À (PdCl 4 2À /Pd, 0.62 V vs SHE). [24]The reduction potential gap between Cu 2þ and the coordinated Pt, Pd complex would contribute to the formation of highly alloyed PtPdCu, which is in good agreement with the upward shift on the peak of Pt(Pd) (111) in XRD patterns.

Electrochemical Properties
Considering the special material properties of the two types of PtPdCu catalysts, both catalysts are expected to exhibit promising catalytic activity toward MOR.To have a systematic comparison, the electrochemical properties of CuI-PtPdCu and commercial Pt/C catalysts were also investigated.First, cyclic voltammetry (CV) measurements were performed as the catalyst activation procedure.As shown in Figure 4a, all the catalysts studied show the typical CV curves of Pt(Pd)-based catalysts with a distinct reduction peak of Pt(Pd) oxides between À0.4 and 0 V.However, the reduction peak potential shifts to lower potentials when increasing the amount of KI used in the synthesis of the corresponding catalysts.CuI-PtPdCu has the lowest reduction  S1, Supporting Information.peak potential of approximately À0.4 and À0.2 V for h-PtPdCu, which was prepared without adding KI.It indicates that the existence of CuI on the material surface would decrease in the surface oxide reduction peak, suggesting a modification of the redox properties and the strengthening of the interaction with the oxygen adsorbate. [41]Furthermore, different from catalysts synthesized using KI, h-PtPdCu has a region of hydrogen adsorption/desorption similar to that of Pt/C, which implies the presence of the Pt-Pd-enriched surface in h-PtPdCu. [42]To assess the poisoning tolerance to CO species during methanol oxidation, CO stripping voltammograms were collected.Figure 4b shows that the CO stripping peak potential (E CO ) of CuI/PtPdCu is À0.28 V, which is lower than that of h-PtPdCu (À0.21 V), CuI/PtPdCu (À0.26 V), and Pt/C (À0.16 V), demonstrating a better tolerance to poisoning of CuI/PtPdCu to CO intermediates.A suitable amount of CuI on the catalyst surface is proposed to weaken the CO adsorption strength and thus promote CO oxidation, which is regarded as an essential factor in the MOR efficiency.The CO stripping peak is also used to determine the electrochemically active surface area (ESA) of the catalysts.The detailed procedure and calculation description can refer to the description in our previous work. [16,18]The ESA values computed for all studied catalysts and Pt/C are summarized in Table 1.Among the three binary PtPdCu catalysts, h-PtPdCu displays an ESA value that is a bit larger, probably because of its hollow structure.
The MOR activity was then evaluated by performing CV measurements in N 2 -saturated 1 M NaOH þ 1 M CH 3 OH with a sweep rate of 50 mV s À1 .Figure 4c shows the representative CV curves for MOR.The onset potential (E 0 ) of the studied catalysts is derived from CVs in Figure 3c and summarized in Table 1.All the three PtPdCu catalysts display lower E 0 than Pt/C.Moreover, the catalyst sample with more CuI (i.e.CuI-PtPdCu) shows lower E 0 .From this point, it can be concluded that surface CuI can boost MOR efficiency in the initial process.Specific activity (SA) has been regarded as the straightforward index to evaluate the intrinsic activity of the catalysts. [43]The SA of CuI/PtPdCu derived from Figure 4b is 5.73 mA cm À2 , which is 1.2, 1.9, and 4.1 times larger than that of h-PtPdCu (4.72 mA cm À2 ), CuI-PtPdCu (3.08 mA cm À2 ), and Pt/C (1.41 mA cm À2 ), respectively.As shown in Figure 4f, both CuI-PtPdCu and h-PtPdCu display competitive and promising MOR catalytic activity in an alkaline medium compared to other reported PtCu-and PtPd-based nanocatalysts.Although the proposed mechanism of onset potential decreasing does not seem to work, CuI-PtPdCu, which was prepared using the most amount of KI, shows the highest CuI content in the catalyst (see from the XRD result); however, the lowest SA is displayed compared to CuI/PtPdCu and h-PtPdCu catalysts.It is suggested that only the appropriate amount of CuI clusters on the surface of the catalyst may accelerate the methanol molecule adsorption oxidation process on the Pt surface, thus enhancing the catalytic kinetics of MOR.In the case of CuI-PtPdCu, there may be too much CuI on the PtPdCu surface, which may even bind to the catalytic active sites.The lowest ESA value of CuI-PtPdCu could be used as direct evidence.The above results indicate that appropriate CuI has a pronounced co-catalytic effect on the MOR properties of the PtPdCu catalyst.
It is proposed that CuI can be a cocatalyst that can react with the intermediate CO products to accelerate CO desorption.This type of synergistic effect induced by PtCu and CuI was suggested in our previous study. [18]Additionally, a similar effect of Cu þ /Cu 2þ sites on improving catalytic performance was also reported, where N-doped carbonaceous decorated with Cu II /Cu I nanoclusters was prepared.The Cu I species were found to be the active sites that facilitate the binding of O 2 during the oxygen reduction reaction. [24]Furthermore, the electronic surface structure of CuI/PtPdCu could be significantly tuned due to the much higher alloying degree.The induced electronic effect would contribute to the enhancement of the activity to a great extent. [44,45]he stability of the catalysts is equally significant as the activity from the practical application point of view.Chronoamperometry (CA) measurements are performed in 1 M NaOH þ 1 M CH 3 OH at À0.1 V for 1800 s.As shown in Figure 4d, due to the rapid formation of metal oxide species on the surfaces of the catalysts at the high potential at the beginning of the CA measurement, all CA curves drop sharply from the high current density in the initial stage and gradually undergo a slight decrease. [16,46]After 1800 s, the retained current density of CuI/PtPdCu and h-PtPdCu is much higher than that of CuI-PtPdCu and Pt/C, suggesting that the former two are more stable under these CA test conditions.In addition, the accelerated stability test (AST) was conducted on all of the catalysts studied.CVs before and after 500 cycles were recorded, as shown in Figure S6, Supporting Information.The h-PtPdCu catalyst surprisingly exhibits the best durability, with only 3.4% decay in SA.With more CuI, CuI-PtPdCu shows a significant decay of 20%, even more than that of CuI/PtPdCu (12%).It is believed that the surface CuI might not be that stable during the ADT test, probably because of the significant lattice mismatch between CuI and PtPdCu.A similar fast decay of SA was also found in our previously reported PtCu catalysts.More detailed reasons for the fast decay will be investigated in the future.As deduced from TEM, EDS mapping, and CV analysis, the ultrahigh durability of h-PtPdCu might be attributed to the more stable Pt-Pd-rich surface layer.Furthermore, the overall linked hollow structure is believed to have the advantage of preventing particles from collapsing or further agglomeration. [47,48]In addition, it can be seen from the amplified TEM image of h-PtPdCu (Figure S7, Supporting Information) that the outer layer of this hollow structure is not completely closed.The active parts of the inner layer of the hollow structure can be gradually utilized in the long-term cycle test. [49]his is also one of the reasons why h-PtPdCu shows more stable MOR performance.Compared with other catalysts rich in Pt/Pd surface, the simple one-step hydrothermal synthesis method is   also one of its advantages in addition to its special hollow structure (chain-shaped, partially closed), which helps to improve its stability.

Conclusions
Two types of ternary PtPdCu nanocatalysts are successfully synthesized by a one-pot hydrothermal method.The presence of KI in the synthesis demonstrates a significant influence on both material properties and the corresponding MOR catalytic performance.CuI/PtPdCu, prepared with 20 mg of KI, consists of CuI clusters inlaid on PtPdCu nanospheres, exhibiting the best anti-CO poisoning capability and the highest SA of 5.73 mA cm À2 .h-PtPtCu, produced without adding KI, has a hollow structure, showing the highest durability with only 3.6% decay in SA after the AST.The enhanced MOR activity of CuI/PtPdCu can be attributed to the electronic effect originating from the highly alloyed ternary PtPdCu.Moreover, the inlaid CuI clusters are supposed to promote the desorption of CO intermediates, thus improving the catalytic efficiency.The superior durability of h-PtPdCu can be ascribed to the stable Pt-Pd-rich surface and its special linked hollow structure that does not easily collapse.This work provides the result of using iodide to tailor the composition and structure of the ternary PtPdCu catalysts with improved activity and stability.
Methods: The detailed synthesis steps are as follows: 500.0mg of P123 was dissolved in 20.0 mL of ultrapure water, and then 1.6 mL H 2 PtCl 6 •6H 2 O (20 mM), 10.0 mg of K 2 PdCl 4 , 4.0 mg of CuCl 2 , and 20.0 mg of KI were mixed and added there until it was completely dissolved in 25.0 mL.The mixture was transferred to a high-pressure hydrothermal polytetrafluoroethylene (PTFE) reactor and kept at 180 °C for about 12 h.After centrifugation and washing with an ethanol solution (50% v/v), the catalyst was redispersed in 2.0 mL of ethanol and named CuI/PtPdCu.Regarding the synthesis of h-PtPdCu and CuI-PtPdCu, the other conditions were kept unchanged but 0 and 40.0 mg KI were added, respectively.
Electrochemical Tests: Electrocatalytic tests were performed on an AUTOLAB electrochemical workstation (PGSTAT 12) using a conventional three-electrode system.The glassy carbon electrode (GCE, 5 mm in diameter) supported by the PtPdCu catalyst was used as the working electrode (WE), the Pt electrode plated with Pt black (1 Â 1 Â 0.01 cm) was used as the counter electrode (CE), and the saturated calomel electrode (SCE) was used as the reference electrode (RE).Before preparing the catalyst ink, 2.0 mL of the above catalyst-ethanol suspensions was dried in a vacuum drying oven at 50°C.1.0 mg of the dried catalyst powder was weighed and redispersed in 1.0 mL of ethanol solution (50% v/v).The 10.0 μL solution was dropped onto the surface of GCE and dried at room temperature to obtain WE.The amount of Pt on the GCE was aimed at 20 μg cm À2 .Then, to prevent the catalyst from falling during the test, 10.0 μL of 0.5 wt Nafion solution was dropped onto the electrode surface to fix the catalyst.For CO stripping measurements, high purity (99.999%)CO was slowly bubbled in 1.0 M NaOH solution for 30 min to obtain a complete CO adsorption layer (CO ads ) at Pt/Pd active sites between À0.9 and À0.8 V. Subsequently, the residual dissolved CO was excluded by purging into N 2 for 30 min and then two consecutive CV curves were recorded to confirm complete oxidation of CO ads .Electrochemical tests were completed at room temperature and pressure.
Material Characterization: XRD tests were performed on a Rigaku powder X-ray diffractometer (Ultima IV) using a Cu Kα X-ray source (λ = 1.5405Å).TEM and HRTEM were tested on the FEI TeGNai-F30 (300 kV), where EDS was performed with the device attached.The EDS was recorded on JEM-2010 (HR) (200 kV).XPS was measured on a PHI Quantum 2000 XPS system (Physical Electronics, Inc.).

Figure 1 .
Figure 1.TEM and XRD characterizations of CuI/PtPdCu prepared using 20 mg KI. a,b) TEM images; inset in (b) shows the size distribution of PtPdCu spheres and the embedded CuI subnanoparticles, which have been highlighted with yellow dash circles.c) HRTEM image with FFT patterns inserted.d) XRD patterns with standard cards for Pt, Pd, and CuI.e) SAED pattern.f ) HADDF-STEM image and elemental mappings, g) Pt, h) Pd, and i) Cu.

Figure 4 .
Figure 4. a) Stabilized CV curves and b) representative CO stripping curves of CuI/PtPdCu, h-PtPdCu, CuI-PtPdCu, and commercial Pt/C collected in N 2saturated 1 M NaOH solution at a scan rate of 100 mV s À1 .c) CV curves for MOR recorded in N 2 -saturated of 1 M NaOH þ 1 M CH 3 OH solution at a scan rate of 50 mV s À1 .d) CA curves obtained in N 2 -saturated of 1 M NaOH þ 1 M CH 3 OH solution at À0.1 V for 1800 s. e) AST; CV of h-PtPdCu before and after 500 cycles.f ) SA of CuI/PtPdCu and h-PtPdCu compared to that in the literature; the MOR test was all conducted in an alkaline medium, 1.0 M KOH (or NaOH) þ 1.0 M CH 3 OH solution, at a scan rate of 50 mV s À1 ; for more detailed information, see TableS1, Supporting Information.