Superior Ethanol Oxidation Electrocatalysis Enabled by Ternary Pd

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Superior Ethanol Oxidation Electrocatalysis Enabled by Ternary Pd−Rh−Te Nanotubes Liujun Jin, Hui Xu, Chunyan Chen, Hongyuan Shang, Yong Wang,* and Yukou Du* College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China

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ABSTRACT: Designing and elaborating cost-efficient Pdbased electrocatalysts for direct ethanol fuel cells is thought to be a significant approach to obliterating the challenge of largescale practical application of fuel cells. Herein, our group creates a novel class of one-dimensional (1D) PdRhTe nanotubes (NTs) by using H2PdCl4 and RhCl3 as metal precursors and Te nanowires (NWs) as the reductant and sacrificial template. Strikingly, the as-obtained PdRhTe ternary nanomaterials with a unique 1D nanotube structure display a high specific activity of 6.53 mA cm−2 and a mass activity of 2039.2 mA mg−1 for the ethanol oxidation reaction (EOR) in alkaline media, which are 1.25 (1.6) and 1.77 (8.0) times those of PdTe/C and (Pd/C), respectively. More significantly, further electrochemical measurements such as CA and successive CV confirm that the optimized PdRhTe NTs display desirable durability and negligible activity decay. Taking advantage of physicochemical characterizations and electrochemical measurements, we reasonably reveal that the outstanding electrocatalytic performances are derived from the unique geometric structure and synergistic effect. The introduction of Rh facilitates the cleavage of C−C bonds, increasing the self-stability of PdRhTe NTs. In general terms, this work should provide new orientations to synthesize cost-efficient electrocatalysts by a sacrificial template method.

1. INTRODUCTION With the massive demand for energy resources (such as coal, oil, and gas) and the development of serious environmental contamination, seeking renewable clean energy as a potential alternative to traditional fossil fuels is an urgent pursuit.1,2 In the past few decades, fuel cells as sustainable energy conversion devices, which convert chemical energy of fuels and an oxidant directly into electric energy, have become a research hot spot and attracted widespread attention because of their high conversion efficiency and low or even zero emission.3,4 Direct ethanol fuel cells (DEFCs) are identified as the most likely practical application because ethanol has unique advantages as a fuel, such as its nontoxicity, high energy density, and environmentally friendliness.5−7 Nanocatalysts have been demonstrated to be a critical part of fuel cells in the process of oxidation of ethanol fuels.8 To date, Pt and Pt-based catalysts, due to their excellent electrocatalytic nature, have been widely investigated as traditional catalysts for DEFCs.9,10 Unfortunately, their fatal drawbacks of sluggish dynamics, poor toxicity resistance, prohibitive cost, and low inventory have severely impeded the practical application of DEFCs.11,12 Therefore, the design and production of high-efficiency and low-cost electrocatalysts for ethanol electrooxidation is significant for the development of DEFCs. To surmount the obstacles mentioned above, it is of paramount importance to fabricate cost-efficient nanomaterials to substitute for Pt and Pt-based catalysts. In previous studies, © XXXX American Chemical Society

a variety of novel and efficient catalysts have been successfully synthesized to promote the electrocatalytic performance of ethanol fuel cells. Compared with Pt, Pd has been regarded as a promising alternative that exhibits outstanding merits in DEFCs, such as a higher CO antitoxic tolerance, a lower cost, and a larger stock.13,14 However, the nanoparticle aggregation, limited surface active sites, and poor long-term stability of pure Pd inevitably affected the catalytic performance.15,16 The strategy of alloying Pd with transition metals (such as Cu, Ag, Ru, and Rh) is generally considered to be an efficient method for constructing highly efficient Pd-based electrocatalysts, which can decrease the cost of the electrocatalyst and simultaneously promote the electrocatalytic quality for EOR through the electronic effect.17−20 So far, various Pd-based bimetallic electrocatalysts have been synthesized, such as PdCu,21 PdAg,22 and PdPb,23 all of which exhibit promoted electrocatalytic performance like pure metal Pd to some extent. Recently, incorporating Rh into Pd to form bimetallic PdRh nanomaterials as an electrocatalyst for fuel cells has attracted a great deal of attention owing to the outstanding C−C bond breaking ability and distinct capacity for the electrooxidation of ethanol to CO2.24,25 The distinctly improved electrocatalytic oxidation activity of the PdRh electrocatalyst is attributed to Received: July 4, 2019

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DOI: 10.1021/acs.inorgchem.9b01976 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

catalysts to expose more active sites. Benefiting from typical physical and chemical characteristics, one-dimensional (1D) Pd-based nanomaterials have attracted a great deal of attention. The 1D Pd-based nanomaterials have the benefits of a high charge/mass transfer efficiency and more easily exposed active sites, which are beneficial for the improvement of catalytic performance.32,33 To the best of our knowledge, there have been few studies of the synthesis of 1D PdRh nanotubes. Taking all of this into consideration, we herein demonstrated a facile two-step and eco-friendly approach to synthesizing 1D PdRhTe nanotubes (NTs) with a uniform diameter by using H2PdCl4 and RhCl3 as metal precursors and Te nanowires (NWs) as a sacrificial template. Via a combination of the physicochemical characterizations and electrochemical results, the newly generated 1D PdRhTe NTs exhibit remarkably excellent performance for the ethanol oxidation reaction (EOR) in basic media, from which the unique tubular structure and electronic effect originate. Additionally, the synergistic effect between the PdRh alloy and Te NWs is also favorable for promoting the electrocatalytic performance of PdRhTe NTs.

Figure 1. Schematic illustration of the synthesis of PdRhTe NTs.

the unique electronic structure of PdRh, the lower energy for binding to COads, and the synergistic effect. Except for the careful regulation of the chemical composition, it is worth noting that the morphology and structure of nanomaterials also play an indispensable role in affecting the performance of electrocatalysts.26 Despite great progress having been achieved in the synthesis of nanomaterials with different morphologies and structures such as nanoparticles, nanosheets, nanocubes, etc.,27−31 all of these special morphologies endowed the electrocatalysts with greatly enhanced electrocatalytic performance through optimizing the atom utilization of

2. EXPERIMENTAL SECTION 2.1. Preparation of 1D Te Nanowires. In a typical synthesis, 500 mg of polyvinyl pyrrolid (PVP) as a surfactant and 46.1 mg of

Figure 2. Morphology and structure of PdRhTe NTs. (a−d) TEM images of PdRhTe NTs. (e) Atomic ratios of PdRhTe from a scanning electron microscopy−EDS spectrum. B


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Inorganic Chemistry

Figure 3. Representative (a) HRTEM image, (b and c) HAADF-STEM images of PdRhTe NTs, and (d) HAADF-STEM elemental mappings of PdRhTe NTs.

Figure 4. XPS spectra of (a) survey scan and (b) Pd 3d, (c) Rh 3d, and (d) Te 3d in 1D PdRhTe NTs. 2.2. Preparation of PdRhTe NTs. In a typical synthesis of PdRhTe NTs, 2 mL of previously prepared Te NWs was dispersed in 10 mL of ethylene glycol (EG). Afterward, 2 mL of 22.5 mM H2PdCl4 and 5 mg of RhCl3 powders were added dropwise to the solution described above under vigorous shaking to obtain a homogeneous mixed solution. Finally, the mixed solution was kept at 50 °C for 12 h in an oil bath without being stirred. The final products were collected by centrifugation (10000 rpm, 10 min) and washed three times with DI water and ethanol. For comparison, PdTe NTs were also synthesized by using a procedure similar to that for PdRhTe NTs but without adding Rh precursors.

sodium tellurite (NaTeO3) as a precursor were dispersed in 17.5 mL of deionized (DI) water to form a uniform solution under vigorous shaking at ambient temperature. Afterward, 1.68 mL of aqueous ammonia (NH3·H2O) and 0.88 mL of hydrazine hydrate (N2H4· H2O) as a reductant were orderly injected into the solution described above. The mixed aqueous solution was transferred into a Tefionlined-stainless steel autoclave, which was sealed and heated from room temperature to 180 °C and maintained for 3 h. After the reaction was completed and the mixture was cooled to ambient temperature, the products were then centrifuged (10000 rpm) and washed several times with ethanol and DI water. Finally, the as-prepared Te NWs were redispersed into 10 mL of DI water for further use. C


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Inorganic Chemistry

Figure 5. (a−-c) Typical TEM images of PdTe NTs. (d) Atomic ratios of PdTe from a scanning electron microscopy−EDS spectrum. 2.3. Physicochemical Characterizations. The morphology and structure were characterized by transmission electron microscopy (TEM) (HITACHI7700 transmission electron microscope operating at an accelerating voltage of 120 kV). High-resolution TEM (HRTEM) was performed on a TECNAI G20 instrument at an accelerating voltage of 200 kV. The atomic ratio of products was investigated by energy dispersive X-ray spectroscopy (EDS), which was performed on a scanning electron microscope (Hitachi, S-4700). Compositions and chemical states were characterized by X-ray photoelectron spectroscopy (XPS), which was conducted on a VG scientific ESCALab 220XL electron spectrometer using 300 W Al Kα radiation. X-ray diffraction (XRD) patterns of the products were recorded on an X’Pert-pro MPD diffractometer (Netherlands PANalytical) equipped with a Cu Kα X-ray source. 2.4. Electrochemical Measurements. Before the electrochemical measurements, the as-obtained PdRhTe NTs were loaded on carbon supports with a loading content of 20 wt %; typically, 2 mg of the PdRhTe catalyst and 8 mg of carbon supports were added to 2 mL of the mixture of DI water and isopropanol (volume ratio of 1:3), after ultrasound for 1 h to prepare a homogeneous catalyst ink suspension. Similarly, the procedure was also employed to prepare a PdTe/C catalyst ink suspension. The CHI760E electrochemical workstation with a standard three-electrode system was employed for all electrochemical measurements. The Pt wire and the Ag/AgCl electrode were employed as the counter and reference electrodes, respectively. The glass carbon electrode (GCE) was selected as the working electrode, which should be carefully polished with alumina powders every time and then washed sequentially with distilled water and ethanol before measurement to remove the impurities on the

surface of GCE. All electrochemical tests were in process with a 1 M KOH solution or a 1 M KOH/1 M CH3CH2OH solution.

3. RESULTS AND DISCUSSION As illustrated in Figure 1, the PdRhTe NTs were acquired via a feasible two-step method, in which RhCl3 and H2PdCl4 were the metal precursors and Te NWs were adopted as the sacrificial template.34 A transmission electron microscope was employed to preliminarily characterize the morphology and structure of the eventual PdRhTe NTs (Figure 2a−d), and from the representative TEM images, the well-retained outline and obvious hollow interiors can clearly be seen, suggesting the successful construction of PdRhTe NTs, which is ascribed to the galvanic replacement reaction between RhCl3, H2PdCl4 and Te NWs. The diameters of prepared PdRhTe NTs (Figure S1) were larger than those of the pure Te NWs owing to the further growth of Pd and Rh on the Te NWs. Meanwhile, from an electrochemical perspective, the unique nanotube structure of PdRhTe NTs was beneficial for supplying abundant surface active sites and convenient transportation for ethanol molecules.35 Further (EDS) analysis was performed to determine the Pd/Rh/Te atomic ratio in the resulting PdRhTe NTs (Figure 2e), which also confirmed the successful generation of ternary PdRhTe nanomaterials. Aiming to understand the crystal structure of PdRhTe NTs, we also conducted an X-ray diffraction (XRD) test. As shown D


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Figure 6. Electrocatalytic performance of PdRhTe NTs/C, PdTe NTs/C, and commercial Pd/C catalysts. (a) CV curves recorded in a 1 M KOH solution and (c) a 1 M KOH/1 M ethanol solution. (b) Corresponding cartogram of ECSA values. (d) Comparison of mass and specific activities of three kinds of catalysts.

Pd 3d XPS spectrum of PdRhTe NTs underwent a positive shift versus that of standard Pd, indicating the modification of the electronic structure of PdRhTe NTs.36 Figure 4c displays the XPS spectrum of Rh 3d of NTs; the peaks of the binding energy located at 307.2 and 311.9 eV can be attributed to the zero-valent Rh 3d5/2 and Rh 3d3/2, respectively, in addition, the peaks at 308.4 and 313.1 eV are attributed to the oxidized metal Rh(III). Compared to the pure elemental Pd and Rh, via the binding energies of Pd and Rh, there has been a slight deviation, which indicated that the electronic structure was changed (Figure S3). In addition, the Te 3d5/2 and 3d3/2 signal that also appears in PdRhTe NTs, as illustrated in Figure 4d, the typical signals located at 574.9 and 585.7 eV are derived from zero-valent Te; the other two peaks located at 573.5 and 583.8 eV are assigned to TeOx. Combined with the XPS data of all of the investigated elements, it was demonstrated that the Pd and Rh precursors did undergo a displacement reaction with the Te nanowire. We investigated the mechanism of formation of such PdRhTe NTs with a particular morphology by conducting a series of controlled experiments. For instance, we synthesized the PdTe NTs without adding RhCl3. The representative TEM images of PdTe NTs were acquired. As illustrated in Figure 5, it is clearly observed that all of the tubular products have rough walls and hollow interiors, indicating that the displacement reaction between PdCl42− and Te on the surface. Reasonably, this method was feasible for the synthesis of well-defined nanotubelike catalysts.

in Figure S2, all of the XRD patterns strongly testified to the successful formation of the particular ternary PdRhTe NTs. To gain more comprehensive information about the crystal structure of PdRhTe NTs, HRTEM measurements were also employed. Figure 3a shows the HRTEM image of PdRhTe NTs; the continuous lattice fringe suggests its single crystallinity. The lattice spacing of nanotubes is calculated to be around 0.230 nm, which is consistent with the (111) lattice spacing of Pd/Rh, suggesting the successful formation of an alloy phase in ternary PdRhTe NTs.24 Moreover, the HAADFSTEM images (Figure 3b,c) also strongly demonstrated the successful fabrications of PdRhTe NTs with a well-retained outline and obvious hollow interiors. The elemental distributions of PdRhTe NTs were also confirmed by HAADF-STEM elemental mappings (Figure 3d), where Pd, Rh, and Te were distributed uniformly throughout the whole nanotube, further suggesting the formation of PdRhTe NTs. The compositions and chemical states on the surface of PdRhTe NTs were also verified by XPS in Figure 4. Figure 4a displays the results of XPS analysis, where the characteristic signals at binding energies of 284, 335, and 532 eV correspond to Te 3d, Pd 3d, and Rh 3d, respectively. With regard to the Pd 3d spectrum of the PdRhTe NTs (Figure 4b), the two peaks located at binding energies of 335.3 and 340.5 eV exactly belong to Pd 3d5/2 and Pd 3d3/2, respectively. However, the relatively low-intensity signals at 336.2 and 341.1 eV are attributed to bivalent Pd(II). According to the intensity of the peaks, metallic Pd is the majority component. More significantly, the E


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Inorganic Chemistry

Figure 7. (a) Chronoamperometry curves of PdRhTe NTs/C, PdTe NTs/C, and commercial Pd/C in a 1 M ethanol/1 M KOH solution at the −0.15 V potential. (b) Cartogram of the residual catalytic activities of different electrocatalysts after chronoamperometry measurement. (c) Longterm cycling stability curves of different electrocatalysts for EOR during 500 potential cycles. (d) Comparison of the mass activity and normalized current percentage of different catalysts after 500 sweeping cycles toward EOR at a scan rate of 50 mV s−1.

electrocatalytic activity (Figure 6c). Obviously, the mass activity of PdRhTe NTs/C is calculated to be 2039.2 mA mg−1, which is 6.8- and 1.77-fold higher than those of commercial Pd/C (298.8 mA mg−1) and PdTe NTs/C (1154.8 mA mg−1), respectively. In Table S1, one can clearly see that the electrocatalytic activity of PdRhTe NTs/C is also much higher than those of recently reported Pd-based electrocatalysts. Meanwhile, compared with commercial Pd/C and PdTe NTs/C, the PdRhTe NTs/C show a negative onset potential for EOR, indicating that the electrooxidation of ethanol is more easily triggered on the surface of PdRhTe NTs/C (Figure S4) than those of PdTe NTs/C and Pd/C. Additionally, as illustrated in Figure 6d, the specific activity of PdRhTe NTs/C normalized to ECSA is measured to be 6.53 mA cm−2, for which these electrocatalysts were also investigated. The long-term stability of the catalyst is another crucial parameter for evaluating the qualities of the catalysts, because this is also a serious obstacle for the development of the electrocatalysts toward EOR in practical applications.41 It is predicted the PdRhTe NTs/C that were integrated with the compositional and structural advantages could display desirable electrocatalytic activity and durability for alcohol electrooxidation reaction. The CA measurements were first employed to evaluate the prepared electrocatalysts (PdRhTe NTs/C, PdTe NTs/C, and Pd/C). Apparently, as shown in Figure 7a, in the initial period electrocatlytic activity is rapidly lost for all of the

Extensive efforts, in the past decades, have verified the fact that the composition and morphology are crucially significant for the electrocatalystic performance of catalysts.37 The excellent electrochemical properties of electrocatalysts are the precondition for large-scale commercial applications. In these terms, the electrochemical measurements of ethanol oxidation were performed to evaluate the electrocatalytic performance of the resulting PdRhTe NTs. The electrocatalytic activity of PdRhTe NTs in combination with compositional and structural merits was primarily evaluated by CV measurement (Figure 6a) in a 1 M KOH solution at a scan rate of 50 mV s−1 (Figure 6b). The electrochemical surface areas (ECSAs) were calculated to be 34.15 m2/gPd for PdRhTe NTs/C, which was higher than that of PdTe NTs/C (27.30 m2/gPd) and that of commercial Pd/C (38.2 m2/gPd). The largest ECSA value is principally derived from the distinctive tubular structure with a larger surface area.38−40 The electrocatalytic ethanol oxidation reaction (EOR) performance of PdRhTe NTs/C, PdTe NTs/C, and commercial Pd/C was also characterized by CV measurement in a 1 M KOH and 1 M CH3CH2OH solution at room temperature. It is widely accepted that the electrocatalytic activity of catalyst can be evaluated by two important parameters, namely, current density and onset potential in the working process. The catalytic activity for EOR was normalized with Pd mass; among the three kinds of prepared catalysts, the PdRhTe NTs/C achieved the apparently highest F


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ACKNOWLEDGMENTS Y.D.’s group thanks the National Natural Science Foundation of China (Grant 51873136), the project of scientific and technologic infrastructure of Suzhou (SZS201708), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

prepared catalysts, but it is worth noting that the PdRhTe NTs/C have the slowest descending rate and the highest value among the three catalysts in the whole CA measurements, indicating the high stability of PdRhTe NTs/C for poisonous intermediates such as COads.42 Additionally, the residual mass activities of the three catalysts are also shown in Figure 7b. Remarkably, the PdRhTe NTs/C could maintain the highest mass activities of 165.48 mA mg−1, further indicating the remarkable enhancement of the stability for EOR. Apart from the CA test, the measurement of catalytic durability was investigated by successive CV scans from −0.9 to 0.3 V (vs RHE) at a scan rate of 50 mV s−1 (Figure 7c,d). Similarly, the peak current density of PdRhTe NTs/C remained at 80.48% of its initial mass activity after 500 potential cycles for EOR. By contrast, the peak current density of commercial Pd/C and PdTe NTs/C remained at 35.22% and 67.56%, respectively. The results obviously showed that the PdRhTe NTs had impressive stability in contrast to commercial Pd/C and PdTe NTs, which can be attributed to the robust nanotube-like structure, optimized PdRhTe composition, and strong resistance to poisonous intermediates.43,44

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01976.

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REFERENCES

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4. CONCLUSIONS In summary, 1D PdRhTe NT electrocatalysts were successfully synthesized via a typical galvanic replacement reaction between metal precursors (PdCl42− and Rh3+) and a reductive template (Te NWs). Owing to the robust formation mechanism of PdRhTe NTs, this method can be reproduced and is conveniently scalable and eco-friendly. We determine that the asprepared PdRhTe NTs showed remarkable promotion of the electrocatalystic performance toward EOR compared to commercial Pd/C and PdTe NTs, which is attributed to the electronic effect between Pd and Rh, and the synergistic effect derived from the PdRh alloy and Te nanowire. More significantly, the 1D nanotube structure is also devoted to the larger amount of active sites, being beneficial for promoting electrocatalystic activity and stability. We firmly believe that this class of novel PdRhTe NTs can serve as a potential electrocatalyst for DEFCs and possess a bright application prospect due its excellent electrocatalytic activity and stability.

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Article

Histograms of the diameters of Te NWs and PdRhTe NTs, XRD pattern of PdRhTe NTs, Pd XPS spectra of PdRhTe and PdTe, CV curves of PdRhTe NTs, PdTe NTs, and Pd/C for EOR, and a table of catalytic activities of Pd-based electrocatalysts for EOR (PDF)

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Corresponding Authors

*Telephone: 86-512-65880089. Fax: 86-512-65880089. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yong Wang: 0000-0002-1481-5118 Yukou Du: 0000-0002-9161-1821 Notes

The authors declare no competing financial interest. G


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Superior Ethanol Oxidation Electrocatalysis Enabled by Ternary Pd

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