Carbon Supported Oxide-rich Pd-cu Bimetallic Electrocatalysts for Ethanol Electrooxidation in Alkaline Media Enhanced by Cu/cuo X

Different proportions of oxide-rich PdCu/C nanoparticle catalysts were prepared by the NaBH 4 reduction method, and their compositions were tuned by the molar ratios of the metal precursors. Among them, oxide-rich Pd 0.9 Cu 0.1 /C (Pd:Cu = 9:1, metal atomic ratio) exhibits the highest electrocatalytic activity for ethanol oxidation reaction (EOR) in alkaline media. X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) confirmed the existence of both Cu and CuO x in the as-prepared Pd 0.9 Cu 0.1 /C. About 74% of the Cu atoms are in their oxide form (CuO or Cu 2 O). Besides the synergistic effect of Cu, CuO x existed in the Pd-Cu bimetallic nanoparticles works as a promoter for the EOR. The decreased Pd 3d electron density disclosed by XPS is ascribed to the formation of CuO x and the spill-over of oxygen-containing species from CuO x to Pd. The low Pd 3d electron density will decrease the adsorption of CH 3 CO ads intermediates. As a result, the electrocatalytic activity is enhanced. The onset potential of oxide-rich Pd 0.9 Cu 0.1 /C is negative shifted 150 mV compared to Pd/C. The oxide-rich Pd 0.9 Cu 0.1 /C also exhibited high stability, which indicated that it is a candidate for the anode of direct ethanol fuel cells (DEFCs).


Introduction
Fuel cell is a device of translating the chemical energy into electrical energy [1].Hydrogen fueled proton exchange membrane fuel cells have attracted widespread attention, owing to their high energy conversion efficiency and low or zero emissions [2].However, the production, storage and distribution of hydrogen are facing huge technical challenges [3].The storage or transportation of methanol and ethanol are easier than that of hydrogen.Although the methanol fuel cells are expected to replace batteries as portable power sources [4], there are still some severe obstacles unsolved, such as methanol toxicity and the fuel crossover problem in Nafion based membranes [5].In contrast, direct ethanol fuel cells (DEFCs) are considered as possible power source for electric vehicles and portable applications, due to their high efficiency, high energy density and low operating temperature [6].Ethanol is non-toxic [7].It is an interesting alternative renewable fuel, which can be produced in large quantity from the fermentation process.Moreover, the penetration efficiency of ethanol through the Nafion membrane is much lower than that of methanol [4].
Pt-based catalysts are most widely used for ethanol oxidation reaction (EOR) in acidic media, e.g., Pt [8,9], Pt-Rh [10], Pt-Ir [11], and so on.The history of using platinum as anode for EOR can be traced back to 1940s [12].However, Pt-based catalysts are easily poisoned in acid medium by ethanol dissociation intermediate species particularly CO-like intermediates [13].Platinum is rare and expensive, which hinders its wide application in ethanol fuel cells [14][15][16].In recent years, the electrocatalysts for alcohol oxidation operating in alkaline electrolytes have attracted great interests [17].In alkaline media, kinetics of alcohol oxidation process are significantly increased and corrosion is decreased [18,19].Furthermore, the selectivity for ethanol electro-oxidation to CO 2 could be improved significantly in alkaline media [20].
Palladium is known to be a good electrocatalyst for ethanol oxidation in alkaline solution [21,22], and it is at least fifty times more abundant than Pt on earth [23].However, the activity and durability of Pd catalyst are urgent to improve because of the rapid deactivation of the catalyst [13].Alloying Pd with other elements based on d-band theory [24] is a common method to improve the activity and stability.Successful examples of Pd-alloy electrocatalysts include but not limit to Pd-Au [25], Pd-Ni [26], Pd-Co [27], Pd-Ag [28], Pd-Cu [29], Pd-Ni-P [30], and so on.Stoichiometry of the electrocatalysts is of great significance to improve the catalytic activity [31].Meanwhile, oxides also affect the electrocatalytic performance of Pd-based electrocatalysts for EOR.Transition metal oxides such as Co 3 O 4 , SnO 2 , CeO 2 and NiO [18,32] have been used as promoters to improve the electrocatalytic performance.In summary, both the second metals and the metal-oxides can be used to promote the electrocatalytic activity of Pd-based catalysts for EOR.Naturally a question appears: Can the combination of the second metal and its coexisted oxides improve the electrocatalytic performance in EOR better?We examined the related references, and noticed an interesting example about Pd-Ni system.The Ni catalyst for methanol oxidation has been reported since 2004 [33], and Pd-Ni for EOR was reported in 2010 [26].However, the energy efficient role of Ni/NiO in Pd-Ni catalysts has not been discussed until the report published in 2014 [32].This enlightened us to focus on the new oxide-rich Pd-M systems (M: second metal) and the role of M/MO x .
More and more reports about Pd-Cu system for EOR appeared [4,29,31,[39][40][41][42][43][44][45][46] and the synergistic effect of Cu has been confirmed.A preliminary comparison of some Pd-Cu electrocatalysts is provided in Table 1.As is well known, Cu is easily oxidized by the ambient air [47,48].CuO and Cu 2 O have aroused much attention in many research fields such as lithium-ion batteries [49][50][51], photoelectrocatalytic processes [52][53][54], electrochromic devices [55,56], supercapacitors [57][58][59], dye-sensitized solar cells [60,61], gas sensors [62,63], non-enzymatic amperometric sensors of glucose and hydrogen peroxide [64][65][66], and so on.CuO has been used as the electrocatalyst for the oxidation reactions concerned with nitrite, hydrazine, water, glucose and ascorbic acid [67][68][69][70][71]. Cu 2 O has been used as the electrocatalyst for oxygen reduction reaction and glucose oxidation reaction [72,73].When we start this work, there is no report about CuO or Cu 2 O used as the promoter for EOR.After we had started the research, a paper by Rostami and his coworkers appeared that reported that Cu 2 O promotes the EOR at Pd [74].Compared with Pd loaded on multi-walled carbon nanotube (Pd/MWCNT), the onset potential of Pd/Cu 2 O/MWCNT shifted toward negative value by 120 mV.In this paper, carbon supported oxide-rich Pd-Cu bimetallic electrocatalysts containing both Cu and its oxides are prepared under mild conditions.Compared to Pd/C, the onset potential of oxide-rich Pd 0.9 Cu 0.1 /C (Pd:Cu = 9:1, metal atomic ratio) is negative shifted 150 mV.The negative shift of onset potential indicates a higher catalytic activity.Though either the synergy of Cu or the promotion of Cu 2 O alone for the EOR at Pd has been reported, the catalysts enhanced by the combination of Cu and CuO x have not been reported.Compared with Pd, the shift of oxide-rich Pd 0.9 Cu 0.1 (150 mV) for EOR is more negative than almost all the Pd-Cu electrocatalysts mentioned above (Table 1) and the Pd/Cu 2 O [74], which indicates that the combination of Cu/CuO x also exhibits excellent promoting effect.

Materials Characterization
The X-ray diffraction (XRD) patterns for the as-synthesized Pd/C, oxide-rich Pd 0.6 Cu 0.4 /C, oxide-rich Pd 0.7 Cu 0.3 /C, oxide-rich Pd 0.8 Cu 0.2 /C, oxide-rich Pd 0.9 Cu 0.1 /C and oxide-rich Pd 0.95 Cu 0.05 /C catalysts are shown in Figure 1.There are four typical diffraction peaks on the XRD diffractograms.The peak located at about 25 ˝is associated with the Vulcan XC-72 carbon (002) lattice plane.The other three peaks around 40 ˝, 46 ˝and 68 ˝are characteristic of face centered cubic (fcc) crystalline Pd, corresponding to the diffraction peaks of Pd crystal faces (111), ( 200) and (220), respectively.The XRD patterns do not show any peak corresponding to the Cu [75], CuO [49] or Cu 2 O [58].These indicated that Cu(0), CuO and Cu 2 O have not existed in the form of crystals as the other studies described [29,39].All the diffraction peaks of the oxide-rich PdCu/C catalysts shift toward higher 2θ values compared with the corresponding reflection of Pd/C catalyst.The shifts of the diffraction peaks were caused by the difference in size between Pd and Cu, which further indicated that Cu atoms have entered into the Pd lattice and formed Pd-Cu alloy [76].The size of the catalyst particles can be estimated according to Scherrer's equation [77].The calculated average particle sizes of Pd/C, oxide-rich Pd 0.6 Cu 0.4 /C, oxide-rich Pd 0.7 Cu 0.3 /C, oxide-rich Pd 0.8 Cu 0.2 /C, oxide-rich Pd 0.9 Cu 0.1 /C, and oxide-rich Pd 0.95 Cu 0.05 /C are 5.5, 5.3, 5.1, 4.9, 4.5, and 4.8 nm, respectively.The size of oxide-rich PdCu/C nanoparticles is smaller than that of Pd/C.

Materials Characterization
The X-ray diffraction (XRD) patterns for the as-synthesized Pd/C, oxide-rich Pd0.6Cu0.4/C,oxiderich Pd0.7Cu0.3/C,oxide-rich Pd0.8Cu0.2/C,oxide-rich Pd0.9Cu0.1/Cand oxide-rich Pd0.95Cu0.05/Ccatalysts are shown in Figure 1.There are four typical diffraction peaks on the XRD diffractograms.The peak located at about 25° is associated with the Vulcan XC-72 carbon (002) lattice plane.The other three peaks around 40°, 46° and 68° are characteristic of face centered cubic (fcc) crystalline Pd, corresponding to the diffraction peaks of Pd crystal faces (111), ( 200) and (220), respectively.The XRD patterns do not show any peak corresponding to the Cu [75], CuO [49] or Cu2O [58].These indicated that Cu(0), CuO and Cu2O have not existed in the form of crystals as the other studies described [29,39].All the diffraction peaks of the oxide-rich PdCu/C catalysts shift toward higher 2θ values compared with the corresponding reflection of Pd/C catalyst.The shifts of the diffraction peaks were caused by the difference in size between Pd and Cu, which further indicated that Cu atoms have entered into the Pd lattice and formed Pd-Cu alloy [76].The size of the catalyst particles can be estimated according to Scherrer's equation [77]  The transmission electron microscopy (TEM) patterns and particle size distribution histograms of the Pd/C and oxide-rich Pd0.9Cu0.1/Ccatalysts were shown in Figure 2. Oxide-rich Pd0.9Cu0.1/Cwas chosen as the representative because it exhibits the best performance for the anode oxidation of ethanol, which will be discussed later.The average diameters of the Pd/C and oxide-rich Pd0.9Cu0.1/Ccatalysts observed by TEM images are 5.5 and 4.5 nm, respectively.Furthermore, the high resolution transmission electron microscopy (HRTEM) patterns of oxide-rich Pd0.9Cu0.1/Ccatalyst (Figure 3a)  The transmission electron microscopy (TEM) patterns and particle size distribution histograms of the Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts were shown in Figure 2. Oxide-rich Pd 0.9 Cu 0.1 /C was chosen as the representative because it exhibits the best performance for the anode oxidation of ethanol, which will be discussed later.The average diameters of the Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts observed by TEM images are 5.5 and 4.5 nm, respectively.Furthermore, the high resolution transmission electron microscopy (HRTEM) patterns of oxide-rich Pd 0.9 Cu 0.1 /C catalyst (Figure 3a) show well-resolved fringes with spacing of 0.223 and 0.193 nm, which are ascribed to the (111), (200) planes of Pd crystal, respectively.Most of the oxide-rich Pd 0.9 Cu 0.1 nanoparticles are in the size of less than 8 nm (Figure 2d).We did not find any diffraction fringes corresponding to Cu or CuO in the nanoparticles with the size less than 8 nm, which is consistent with the result of XRD.In rarely appearing large Pd-Cu nanoparticles (with the size of ~10 nm), we found the CuO diffraction fringes of CuO (002) plane with spacing of 0.254 nm [57] (Figure 3b).This provides visible evidence of the coexisting CuO x in Pd-Cu system.
Catalysts 2016, 6, 62 4 of 13 planes of Pd crystal, respectively.Most of the oxide-rich Pd0.9Cu0.1 nanoparticles are in the size of less than 8 nm (Figure 2d).We did not find any diffraction fringes corresponding to Cu or CuO in the nanoparticles with the size less than 8 nm, which is consistent with the result of XRD.In rarely appearing large Pd-Cu nanoparticles (with the size of ~10 nm), we found the CuO diffraction fringes of CuO (002) plane with spacing of 0.254 nm [57] (Figure 3b).This provides visible evidence of the coexisting CuOx in Pd-Cu system.The X-ray photoelectron spectroscopy (XPS) [78,79] was used to further explore the chemical states and surface compositions of Pd/C and oxide-rich Pd0.9Cu0.1/Ccatalysts (Figure 4).All the XPS curves are fitted by the Gaussian-Lorentzian (20% Gaussian) method after background subtraction using Shirley's method [80].Figure 4a is composed of Pd 3d signal.For Pd/C catalyst, the peaks of the Pd 3d5/2 and Pd 3d3/2 (deconvoluted into Pd and PdOy (0 < y < 2) [81]) are located at 336.0 and 341.1 eV, respectively.The Pd 3d5/2 and Pd 3d3/2 binding energy values of oxide-rich Pd0.9Cu0.1/Cshift by +0.2 eV.In the Cu 2p range (Figure 4b), the peak at 932.1 eV corresponds to zero-valent copper, while the peaks Catalysts 2016, 6, 62 4 of 13 planes of Pd crystal, respectively.Most of the oxide-rich Pd0.9Cu0.1 nanoparticles are in the size of less than 8 nm (Figure 2d).We did not find any diffraction fringes corresponding to Cu or CuO in the nanoparticles with the size less than 8 nm, which is consistent with the result of XRD.In rarely appearing large Pd-Cu nanoparticles (with the size of ~10 nm), we found the CuO diffraction fringes of CuO (002) plane with spacing of 0.254 nm [57] (Figure 3b).This provides visible evidence of the coexisting CuOx in Pd-Cu system.The X-ray photoelectron spectroscopy (XPS) [78,79] was used to further explore the chemical states and surface compositions of Pd/C and oxide-rich Pd0.9Cu0.1/Ccatalysts (Figure 4).All the XPS curves are fitted by the Gaussian-Lorentzian (20% Gaussian) method after background subtraction using Shirley's method [80].Figure 4a is composed of Pd 3d signal.For Pd/C catalyst, the peaks of the Pd 3d5/2 and Pd 3d3/2 (deconvoluted into Pd and PdOy (0 < y < 2) [81]) are located at 336.0 and 341.1 eV, respectively.The Pd 3d5/2 and Pd 3d3/2 binding energy values of oxide-rich Pd0.9Cu0.1/Cshift by +0.2 eV.In the Cu 2p range (Figure 4b), the peak at 932.1 eV corresponds to zero-valent copper, while the peaks The X-ray photoelectron spectroscopy (XPS) [78,79] was used to further explore the chemical states and surface compositions of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts (Figure 4).All the XPS curves are fitted by the Gaussian-Lorentzian (20% Gaussian) method after background subtraction using Shirley's method [80].Figure 4a is composed of Pd 3d signal.For Pd/C catalyst, the peaks of the Pd 3d 5/2 and Pd 3d 3/2 (deconvoluted into Pd and PdO y (0 < y < 2) [81]) are located at 336.0 and 341.1 eV, respectively.The Pd 3d 5/2 and Pd 3d 3/2 binding energy values of oxide-rich Pd 0.9 Cu 0.1 /C shift by +0.2 eV.In the Cu 2p range (Figure 4b), the peak at 932.1 eV corresponds to zero-valent copper, while the peaks at 933.1 and 934.6 eV correspond to the Cu 2 O and CuO, respectively.The compositions of the as-prepared catalysts determined by XPS are displayed in Table 2.The metal atomic ratio (Pd:Cu) of oxide-rich Pd 0.9 Cu 0.1 /C catalyst calculated from XPS spectra is Pd 0.92 Cu 0.08 , which is similar with that of the metal precursor.PdO has been reported to be a good catalyst for EOR [82].The coexisted CuO and Cu 2 O are helpful for the formation of oxygen containing species (OH ´ads ) [31].OH ´ads can react with acetaldehyde and CO-type intermediate species, thus the active sites on Pd surface can be released [83], which will benefit for the ethanol oxidation.The positive shift of the Pd 3d binding energy indicates that the Pd 3d electron density is decreased.This positive shift is quite different with the negative shifts reported in other references about Pd-based bimetallic catalysts such as Pd-Ni [84] and Pd-Co [85] electrocatalysts.Their negative shifts resulted from the slight electrons transfer from Ni to Pd or Co to Pd, consistent with their electronegativity.Though the electronegativity of Cu is also less than Pd, XPS analysis indicates that 74% of the copper atoms are in their oxide form (CuO or Cu 2 O) (Table 2), thus the positive shift can be ascribed to the formation of CuO x and the spill-over of oxygen-containing species from CuO x to Pd.The positive shifts can also be found in the literature of Pd-Cu-Fe [86], Pd-P [87], and so on.The shift of binding energies results in a decrease of the intermediate adsorption energy [30,88], thus the active sites of Pd can be kept or be released.XPS is regarded to be a surface-analysis method [78].In this paper, inductively coupled plasma-atomic emission spectrometer (ICP-AES) is used for the accurate determination of the bulk composition of the catalysts (Table 3).
Catalysts 2016, 6, 62 5 of 13 of the metal precursor.PdO has been reported to be a good catalyst for EOR [82].The coexisted CuO and Cu2O are helpful for the formation of oxygen containing species (OH − ads) [31].OH − ads can react with acetaldehyde and CO-type intermediate species, thus the active sites on Pd surface can be released [83], which will benefit for the ethanol oxidation.The positive shift of the Pd 3d binding energy indicates that the Pd 3d electron density is decreased.This positive shift is quite different with the negative shifts reported in other references about Pd-based bimetallic catalysts such as Pd-Ni [84] and Pd-Co [85] electrocatalysts.Their negative shifts resulted from the slight electrons transfer from Ni to Pd or Co to Pd, consistent with their electronegativity.Though the electronegativity of Cu is also less than Pd, XPS analysis indicates that 74% of the copper atoms are in their oxide form (CuO or Cu2O) (Table 2), thus the positive shift can be ascribed to the formation of CuOx and the spill-over of oxygencontaining species from CuOx to Pd.The positive shifts can also be found in the literature of Pd-Cu-Fe [86], Pd-P [87], and so on.The shift of binding energies results in a decrease of the intermediate adsorption energy [30,88], thus the active sites of Pd can be kept or be released.XPS is regarded to be a surface-analysis method [78].In this paper, inductively coupled plasma-atomic emission spectrometer (ICP-AES) is used for the accurate determination of the bulk composition of the catalysts (Table 3).

Electrochemical Performance
It is generally accepted that the Pd-Cu alloy can boost catalytic activity for EOR compared to monometallic Pd or Cu catalyst.It can be seen from the polarization curves of ref. [89] that the onset potential of EOR at Cu electrode is about 0.47 V vs. Ag/AgCl.Obviously, the activity of Cu for EOR is much lower than that of Pd.Cu 2 O does not exhibit electrocatalytic activity for EOR, though it can be used as a promoter [74].In this paper, we focused on the Pd-Cu bimetallic catalysts.The cyclic voltammograms (CVs) of Pd/C, oxide-rich Pd 0.6 Cu 0.4 /C, oxide-rich Pd 0.7 Cu 0.3 /C, oxide-rich Pd 0.8 Cu 0.2 /C, oxide-rich Pd 0.9 Cu 0.1 /C and oxide-rich Pd 0.95 Cu 0.05 /C catalysts were measured in 1 M NaOH solution at a scan rate of 10 mV¨s ´1 (shown in Figure 5).The shape of the CVs is the typical behavior of the Pd modified electrode in alkaline media [90].The peaks (A 1 , B 1 ) at the potential range from ´0.8 to ´0.5 V versus Hg/HgO are assigned to the desorption/adsorption of hydrogen [91].The conspicuous peak (A 2 ) of the catalysts at ´0.4 V is due to the absorption of OH ´on the electrocatalyst surface while those peaks (A 3 , B 2 ) from ´0.4 to 0.1V correspond to the oxidation of surface metal and the reduction of thus formed oxides [92].The peaks (A 2 ) of the Pd/C, oxide-rich Pd 0.6 Cu 0.4 /C, oxide-rich Pd 0.7 Cu 0.3 /C, oxide-rich Pd 0.8 Cu 0.2 /C, oxide-rich Pd 0.9 Cu 0.1 /C and oxide-rich Pd 0.95 Cu 0.05 /C were at ´0.42, ´0.37, ´0.40, ´0.44, ´0.47, and ´0.45 V, respectively.In a former investigation, Dong and his coworkers [39] indicated that a negative shifted absorption peak of OH ´would give rise to a greater amount of OH ´and OH ads species to react with CH 3 CO ads in ethanol electrooxidation process, which leads to higher electroactivity and better tolerance to intermediate species by refreshing surface active sites of Pd.It is generally accepted that the Pd-Cu alloy can boost catalytic activity for EOR compared to monometallic Pd or Cu catalyst.It can be seen from the polarization curves of ref. [89] that the onset potential of EOR at Cu electrode is about 0.47 V vs. Ag/AgCl.Obviously, the activity of Cu for EOR is much lower than that of Pd.Cu2O does not exhibit electrocatalytic activity for EOR, though it can be used as a promoter [74].In this paper, we focused on the Pd-Cu bimetallic catalysts.The cyclic voltammograms (CVs) of Pd/C, oxide-rich Pd0.6Cu0.4/C,oxide-rich Pd0.7Cu0.3/C,oxide-rich Pd0.8Cu0.2/C,oxide-rich Pd0.9Cu0.1/Cand oxide-rich Pd0.95Cu0.05/Ccatalysts were measured in 1 M NaOH solution at a scan rate of 10 mV•s −1 (shown in Figure 5).The shape of the CVs is the typical behavior of the Pd modified electrode in alkaline media [90].The peaks (A1, B1) at the potential range from −0.8 to −0.5 V versus Hg/HgO are assigned to the desorption/adsorption of hydrogen [91].The conspicuous peak (A2) of the catalysts at −0.4 V is due to the absorption of OH − on the electrocatalyst surface while those peaks (A3, B2) from −0.4 to 0.1V correspond to the oxidation of surface metal and the reduction of thus formed oxides [92].The peaks (A2) of the Pd/C, oxide-rich Pd0.6Cu0.4/C,oxide-rich Pd0.7Cu0.3/C,oxiderich Pd0.8Cu0.2/C,oxide-rich Pd0.9Cu0.1/Cand oxide-rich Pd0.95Cu0.05/Cwere at −0.42, −0.37, −0.40, −0.44, −0.47, and −0.45 V, respectively.In a former investigation, Dong and his coworkers [39] indicated that a negative shifted absorption peak of OH − would give rise to a greater amount of OH − and OHads species to react with CH3COads in ethanol electrooxidation process, which leads to higher electroactivity and better tolerance to intermediate species by refreshing surface active sites of Pd.    3. The onset potential of the oxide-rich Pd 0.95 Cu 0.05 /C, oxide-rich Pd 0.9 Cu 0.1 /C, oxide-rich Pd 0.8 Cu 0.2 /C, oxide-rich Pd 0.7 Cu 0.3 /C, and oxide-rich Pd 0.6 Cu 0.4 /C catalysts are 68, 150, 66, 48 and 34 mV, respectively, more negative than that at the Pd/C catalyst electrode.The results demonstrate that oxide-rich Pd 0.9 Cu 0.1 /C exhibits the highest activity toward ethanol oxidation in terms of onset potential and peak-current density.As mentioned above, the onset potential of EOR at Pd/Cu 2 O/MWCNT has a negative shift of 120 mV compared with Pd/MWCNT [74], and the onset potential at the Pd 0.9 Cu 0.1 /C has a negative shift of 15 mV compared with Pd/C [31].Our oxide-rich Pd 0.9 Cu 0.1 /C exhibits a negative shift of 150 mV compared with Pd/C.XPS analysis indicates that there are Cu and CuO x coexisted in the oxide-rich Pd 0.9 Cu 0.1 nanoparticles.Thus, we draw a conclusion that the combination of Cu/CuO x greatly improves the EOR at Pd.  Figure 7 displays the cyclic voltammograms (CV) of the Pd/C and oxide-rich Pd0.9Cu0.1/Ccatalysts in 1.0 M NaOH and 1.0 M CH3CH2OH solution.The peak in the forward scan corresponds to ethanol oxidation, while the peak in the reverse scan is mainly assigned to the removal of incompletely oxidized carbonaceous species rather than the oxidation of freshly chemisorbed species coming from ethanol adsorption [39].The forward anodic peak current density (If) and the reverse anodic peak current density (Ib) of Pd/C and oxide-rich Pd0.9Cu0.1/Care shown in Table 3.The ratio (If/Ib) of Pd/C and oxide-rich Pd0.9Cu0.1/Care 0.40 and 0.54, respectively.The catalyst tolerance for intermediate carbonaceous species accumulated on the electrode surface in the reaction can be measured by the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib) [93].The higher ratio of oxide-rich Pd0.9Cu0.1/Cimplied that the ethanol was more completely oxidized and there was less carbonaceous residue accumulated on the electrode surface [39].The peak in the forward scan corresponds to ethanol oxidation, while the peak in the reverse scan is mainly assigned to the removal of incompletely oxidized carbonaceous species rather than the oxidation of freshly chemisorbed species coming from ethanol adsorption [39].The forward anodic peak current density (I f ) and the reverse anodic peak current density (I b ) of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C are shown in Table 3.The ratio (I f /I b ) of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C are 0.40 and 0.54, respectively.The catalyst tolerance for intermediate carbonaceous species accumulated on the electrode surface in the reaction can be measured by the ratio of the forward anodic peak current density (I f ) to the reverse anodic peak current density (I b ) [93].The higher ratio of oxide-rich Pd 0.9 Cu 0.1 /C implied that the ethanol was more completely oxidized and there was less carbonaceous residue accumulated on the electrode surface [39].
The electrochemical stability of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts for ethanol electro-oxidation was investigated by chronoamperometry in 1 M NaOH containing 1 M C 2 H 5 OH aqueous solution at ´0.5 V for 3600 s (Figure 8) [90].The initial currents drop quickly for both the oxide-rich Pd 0.9 Cu 0.1 /C and Pd/C catalysts, and then the current densities of the catalysts become relatively stable.The phenomenon is similar to the earlier report [4].The current density at oxide-rich Pd 0.9 Cu 0.1 /C catalyst is higher than Pd/C catalyst with time in oxidation of ethanol, which indicates that the stability of the oxide-rich Pd 0.9 Cu 0.1 /C is better than that of Pd/C.current density (Ib) of Pd/C and oxide-rich Pd0.9Cu0.1/Care shown in Table 3.The ratio (If/Ib) of Pd/C and oxide-rich Pd0.9Cu0.1/Care 0.40 and 0.54, respectively.The catalyst tolerance for intermediate carbonaceous species accumulated on the electrode surface in the reaction can be measured by the ratio of the forward anodic peak current density (If) to the reverse anodic peak current density (Ib) [93].The higher ratio of oxide-rich Pd0.9Cu0.1/Cimplied that the ethanol was more completely oxidized and there was less carbonaceous residue accumulated on the electrode surface [39].The electrochemical stability of Pd/C and oxide-rich Pd0.9Cu0.1/Ccatalysts for ethanol electrooxidation was investigated by chronoamperometry in 1 M NaOH containing 1 M C2H5OH aqueous solution at −0.5 V for 3600 s (Figure 8) [90].The initial currents drop quickly for both the oxide-rich Pd0.9Cu0.1/Cand Pd/C catalysts, and then the current densities of the catalysts become relatively stable.The phenomenon is similar to the earlier report [4].The current density at oxide-rich Pd0.9Cu0.1/Ccatalyst is higher than Pd/C catalyst with time in oxidation of ethanol, which indicates that the stability of the oxide-rich Pd0.9Cu0.1/C is better than that of Pd/C.

Chemical Reagents
PdCl2 was purchased from Sinopharm Chemical Reagent Co.Ltd (Shanghai, China).PdCl2 was dissolved in a 0.2 M HCl solution before use.The Vulcan carbon powder XC-72R was obtained from Cabot Corporation (Cabot Corp., Billerica, MA, USA).Nafion solution (5%) was obtained from DuPont (Delaware, DE, USA).All other chemicals were of analytical grade and used as acquired unless otherwise noted.Triple-distilled water was used through-out.

Preparation of the Catalysts
The Pd/C and oxide-rich PdCu/C catalysts with the appropriate alloy composition and a metal loading of 20 wt% were prepared by chemical reduction with NaBH4 as a reducing agent that we have used before [80,94].Briefly, we prepared the carbon mixture with carbon black and triple-distilled water according to the proportion of 160 mg:100 mL and agitated under ultrasonication for 30 min.Then appropriate volume of 0.2 M PdCl2 and 0.2 M CuSO4 used as the precursors were added to the carbon suspension under stirring for 30 min.Next, 1 M NaOH solution was added to this mixture to adjusted the pH to 10 and 3 mL of 5 wt% freshly prepared sodium borohydride solution was added.The mixture was left to stand overnight after stirring for 15 min, and filtered in air at room temperature.The electrocatalysts were thoroughly washed with triple-distilled water to fully remove all residual reducing agents and salts.Finally, the products were dried in a vacuum oven at 403 K for 6 h.The catalysts were stored in tubes with screw caps without further treatment.The air was not removed from the tubes.They were used as the Pd/C and oxide-rich PdCu/C in this paper.The final compositions of the oxide-rich PdCu/C electrocatalysts were adjusted by the molar ratios of the metal precursors.

Chemical Reagents
PdCl 2 was purchased from Sinopharm Chemical Reagent Co.Ltd (Shanghai, China).PdCl 2 was dissolved in a 0.2 M HCl solution before use.The Vulcan carbon powder XC-72R was obtained from Cabot Corporation (Cabot Corp., Billerica, MA, USA).Nafion solution (5%) was obtained from DuPont (Delaware, DE, USA).All other chemicals were of analytical grade and used as acquired unless otherwise noted.Triple-distilled water was used through-out.

Preparation of the Catalysts
The Pd/C and oxide-rich PdCu/C catalysts with the appropriate alloy composition and a metal loading of 20 wt% were prepared by chemical reduction with NaBH 4 as a reducing agent that we have used before [80,94].Briefly, we prepared the carbon mixture with carbon black and triple-distilled water according to the proportion of 160 mg:100 mL and agitated under ultrasonication for 30 min.Then appropriate volume of 0.2 M PdCl 2 and 0.2 M CuSO 4 used as the precursors were added to the carbon suspension under stirring for 30 min.Next, 1 M NaOH solution was added to this mixture to adjusted the pH to 10 and 3 mL of 5 wt% freshly prepared sodium borohydride solution was added.The mixture was left to stand overnight after stirring for 15 min, and filtered in air at room temperature.The electrocatalysts were thoroughly washed with triple-distilled water to fully remove all residual reducing agents and salts.Finally, the products were dried in a vacuum oven at 403 K for 6 h.The catalysts were stored in tubes with screw caps without further treatment.The air was not removed from the tubes.They were used as the Pd/C and oxide-rich PdCu/C in this paper.The final compositions of the oxide-rich PdCu/C electrocatalysts were adjusted by the molar ratios of the metal precursors.

Preparation of the Working Electrode
To prepare the catalysts modified working electrodes, an ink of the sample was prepared by dispersing 4 mg of the catalyst powder in a mixture of 0.2 mL water and 0.2 mL ethanol with ultrasonic stirring for 15 min.The glassy carbon electrode (GCE, 3 mm in diameter) was carefully polished with 0.05 µm alumina (Al 2 O 3 ) powder, and washed with the triple-distilled water before use.Then, 10 µL (2 µL ˆ5 times) of this ink was coated on the GCE and the ink was allowed to dry in air. 3 µL of Nafion solution (5 wt%) was added onto the GCE to fix the catalyst and dried at room temperature.

Materials Characterization
The metal phases and crystalline particle sizes of the products were obtained with a Bruker D8 advance X-ray diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) operating at 40 keV and 30 mA with Cu Kα radiation source, λ = 0.15406 nm.The morphology of the samples was characterized by TEM on a JEOL JEM-1011 (JEOL, Beijing, China).HRTEM was carried out on a JEOL JEM 2100 (JEOL, Beijing, China).The chemical states and surface compositions were determined by X-ray photoelectron spectroscopy.The X-ray photoelectron spectroscopy was obtained with an ESCALAB 250 (ThermoFisher SCIENTIFIC, Shanghai, China) spectrometer using Al Kα (1486.71eV) X-ray radiation for excitation.The accurate compositions were measured with an inductively coupled plasma-atomic emission spectrometer (Optima 2000DV, PERKINELMER, Massachusetts, MA, USA).

Electrochemical Measurements
The electrochemical measurements were performed with a CHI832B electrochemical workstation (CH Instruments, Austin, TX, USA) and a conventional three-electrode electrochemical cell.A carbon-rod was used as the auxiliary electrode, a GCE was used as working electrode, and Hg/HgO electrode was used as a reference electrode.All electrolyte solutions have been deaerated with N 2 for 30 min just before use.N 2 atmosphere was used throughout the electrochemical measurements.The electrochemical performance of Pd/C and oxide-rich PdCu/C catalysts was examined by cyclic voltammetry (CV), linear sweep voltammetry (LSV) and chronoamperometry methods.The CV curves of all catalysts were measured in 1 M NaOH solution, at a scan rate of 10 mV¨s ´1.The LSV curves of all catalysts were measured in a solution comprised of 1 M NaOH and 1 M C 2 H 5 OH, at a scan rate of 10 mV¨s ´1.The cyclic voltammograms curves of the Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C were measured in a solution comprised of 1 M NaOH and 1 M C 2 H 5 OH, at a scan rate of 10 mV¨s ´1.The chronoamperometric curves of the Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts were measured in the same solution comprised of 1 M NaOH and 1 M C 2 H 5 OH at an applied potential of ´0.5 V (vs.Hg/HgO) for 3600 s.All electrochemical tests were carried out at room temperature.

Conclusions
In summary, we prepared carbon loaded Pd-Cu bimetallic nanoparticles with the existence of Cu and its oxides (Cu 2 O and CuO).The combination of Cu/CuO x greatly improves the electrocatalytic performance of Pd-Cu bimetallic system for ethanol electrooxidation.The oxide-rich Pd 0.9 Cu 0.1 /C also exhibited high stability, which indicated that it is a candidate for the anode of direct ethanol fuel cells.

Figure 2 .
Figure 2. The transmission electron microscopy (TEM) patterns and particle size distribution histograms of: Pd/C (a,b) and oxide-rich Pd 0.9 Cu 0.1 /C (c,d).
voltammetry (LSV) curves of the Pd/C and oxide-rich PdCu/C catalysts in 1.0 M NaOH and 1.0 M CH 3 CH 2 OH solution are shown in Figure 6.The peak in the forward scan corresponds to the anodic oxidation of ethanol.The forward anodic peak current densities (I f ) and the onset potential of the Pd/C and oxide-rich PdCu/C catalysts were shown in Table .1 nanoparticles.Thus, we draw a conclusion that the combination of Cu/CuOx greatly improves the EOR at Pd.

Figure 7
Figure7displays the cyclic voltammograms (CV) of the Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts in 1.0 M NaOH and 1.0 M CH 3 CH 2 OH solution.The peak in the forward scan corresponds to ethanol oxidation, while the peak in the reverse scan is mainly assigned to the removal of incompletely oxidized carbonaceous species rather than the oxidation of freshly chemisorbed species coming from ethanol adsorption[39].The forward anodic peak current density (I f ) and the reverse anodic peak current density (I b ) of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C are shown in Table3.The ratio (I f /I b ) of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C are 0.40 and 0.54, respectively.The catalyst tolerance for intermediate carbonaceous species accumulated on the electrode surface in the reaction can be measured by the ratio of the forward anodic peak current density (I f ) to the reverse anodic peak current density (I b )[93].The higher ratio of oxide-rich Pd 0.9 Cu 0.1 /C implied that the ethanol was more completely oxidized and there was less carbonaceous residue accumulated on the electrode surface[39].The electrochemical stability of Pd/C and oxide-rich Pd 0.9 Cu 0.1 /C catalysts for ethanol electro-oxidation was investigated by chronoamperometry in 1 M NaOH containing 1 M C 2 H 5 OH aqueous solution at ´0.5 V for 3600 s (Figure8)[90].The initial currents drop quickly for both the oxide-rich Pd 0.9 Cu 0.1 /C and Pd/C catalysts, and then the current densities of the catalysts become relatively stable.The phenomenon is similar to the earlier report[4].The current density at oxide-rich Pd 0.9 Cu 0.1 /C catalyst is higher than Pd/C catalyst with time in oxidation of ethanol, which indicates that the stability of the oxide-rich Pd 0.9 Cu 0.1 /C is better than that of Pd/C.

Table 1 .
Comparison of electrocatalytic performance of the Pd-Cu system with Pd for ethanol oxidation reaction (EOR).

Table 3 .
Activity of Pd/C and oxide-rich PdCu/C for EOR and compositions determined by inductively coupled plasma-atomic emission spectrometer (ICP-AES).