CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201400129
Palladium and Gold Nanotubes as Oxygen Reduction Reaction and Alcohol Oxidation Reaction Catalysts in Base Shaun M. Alia,[a, b] Kathlynne Duong,[b] Toby Liu,[b] Kurt Jensen,[a, b] and Yushan Yan*[a, b] Palladium (PdNTs) and gold nanotubes (AuNTs) were synthesized by the galvanic displacement of silver nanowires. PdNTs and AuNTs have wall thicknesses of 6 nm, outer diameters of 60 nm, and lengths of 5–10 and 5–20 mm, respectively. Rotating disk electrode experiments showed that the PdNTs and AuNTs have higher area normalized activities for the oxygen reduction reaction (ORR) than conventional nanoparticle catalysts. The PdNTs produced an ORR area activity that was 3.4, 2.2, and 3.7 times greater than that on carbon-supported palladium nanoparticles (Pd/C), bulk polycrystalline palladium, and
carbon-supported platinum nanoparticles (Pt/C), respectively. The AuNTs produced an ORR area activity that was 2.3, 9.0, and 2.0 times greater than that on carbon-supported gold nanoparticles (Au/C), bulk polycrystalline gold, and Pt/C, respectively. The PdNTs also had lower onset potentials than Pd/ C and Pt/C for the oxidation of methanol (0.236 V), ethanol (0.215 V), and ethylene glycol (0.251 V). In comparison to Pt/C, the PdNTs and AuNTs further demonstrated improved alcohol tolerance during the ORR.
Introduction Solid polymer hydroxide exchange membrane fuel cells (HEMFCs) have recently been developed as a potential alternative to polymer proton exchange membrane fuel cells. HEMFCs are particularly enticing because more cost-effective catalysts can be used instead of platinum (Pt) for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction. Several studies have examined Pd and Au catalysts for ORR in base.[2] Chen et al. studied the effect of Pd particle size on ORR activity, finding a gradual decline in area activity with decreasing diameter.[2a] Chen et al. further compared Pd nanoparticle catalysts to Pt; although the analysis was largely qualitative, Pd produced higher ORR activity and a lower activation energy.[2b] Pd catalyst developments have focused on the use of Pd alloys for ORR.[2c] In the case of Au, Adzic et al. and Jttner characterized different facets for ORR, and they found that there was increasing activity among low-index facets in the order: (111) < (11 0) < (1 0 0).[2d–f] McFarland et al. examined Au particle size effects on ORR; although a particle size effect was not observed, the ORR activity and Au utilization was particularly low for the polymer-encapsulated nanoparticles.[2g] Solla-Gulln et al. studied quasi-extended surfaces in the form of Au nanocubes; the ORR activity, however, was modest, and the examination was [a] S. M. Alia, K. Jensen, Prof. Y. Yan Department of Chemical Engineering University of Delaware Newark, DE 19716 (USA) Fax: (+ 1) 302-831-1048 E-mail: [email protected] [b] S. M. Alia, K. Duong, T. Liu, K. Jensen, Prof. Y. Yan Department of Chemical and Environmental Engineering University of California, Riverside Riverside, CA 92521 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201400129.
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void of ORR mass and area activity measurements.[2h] Likewise, Geng et al. examined gold icosahedra with limited activity and did not determine ORR mass or area activities.[2i] Direct alcohol hydroxide exchange membrane fuel cells (DAHEMFCs) are also of interest, as the switch from acid to base reduces the alcohol oxidation overpotential, alcohol permeation rate, and catalyst cost.[3] Pd and Au have been studied previously as methanol (MOR), ethanol (EOR), and ethylene glycol oxidation reaction (EGOR) catalysts.[4] In the case of MOR, Pd and Au are typically utilized as a Pt alloy and have not exceeded the activity of Pt as single metal catalysts. Lamy et al. studied PtPd electrodes in acidic, neutral, and basic environments; they found that the alloy provided a synergistic effect and improved the MOR activity in the alkaline electrolyte.[4a] Watanabe and Motoo characterized PtPd and PtAu electrodes for MOR and found improved activity following the addition of each ad-metal.[4b] Pd is often utilized as an EOR catalyst and was previously found to be more active (in terms of peak current density) than Pt.[4c] Liu et al. demonstrated a fivefold improvement in activity by using Pd nanoparticles in place of Pt.[4d] Xu et al. also examined Pd nanowires that produced significantly higher EOR activity than a Pd film. The nanowire diameter, however, was very large (80 nm) and the synthesis was template based, an approach that faces significant challenges in large-scale synthesis.[4e] Au was studied as an EGOR catalyst by Moussa et al., who found that Au electrodes had a higher peak activity than Pt and Pd, although the onset potential was delayed.[4f] Although Au is typically not mentioned, Pd is frequently studied for methanol and ethanol tolerance.[2b, 5] Although Pd alcohol tolerance is generally poor, the Pd ORR shifts are typically lower than Pt. In this study, Pd nanotubes (PdNTs) and Au nanotubes (AuNTs) were examined for the first time for activity in ORR ChemSusChem 2014, 7, 1739 – 1744
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CHEMSUSCHEM FULL PAPERS and in alcohol oxidation in base. This work was motivated by findings that extended network catalysts (Pt nanotubes), showed dramatically improved ORR and MOR activity in acid.[6]
Results and Discussion The PdNTs and AuNTs were synthesized by the galvanic displacement of silver (Ag) nanowires (AgNWs). The AgNWs were synthesized with a 60 nm diameter and a length of 10– 500 mm, yielding PdNTs and AuNTs with outer diameters of 60 nm and wall thicknesses of 6 nm (Figure 1 and Figure S1,
www.chemsuschem.org ported Pd nanoparticles (Pd/C), 19.3 for AuNTs, 2.7 for carbonsupported Au nanoparticles (Au/C), 59.6 for carbon supported Pt nanoparticles (Pt/C), and 60.1 m2 g 1 for carbon-supported PtRu nanoparticles (PtRu/C). ECSAs were validated by measurements on bulk polycrystalline Pd (BPPd) and Au (BPAu), which had surface roughness of 1.22 and 1.33, respectively. PdNTs produced an ORR area activity that exceeded that of Pd/C, BPPd, and Pt/C by 3.4, 2.2, and 3.7 times, respectively (Figure 2 a and Figure S6 a). The extended surface, expressed
Figure 1. SEM and TEM images of a, b) PdNTs and c, d) AuNTs.
Figure 2. Mass and area activities of a) PdNTs, Pd/C, BPPd, and Pt/C and b) AuNTs, Au/C, BPAu, and Pt/C. All activities were determined at 0.9 V vs. RHE in a 0.1 m potassium hydroxide (KOH) electrolyte during an anodic polarization scan at 20 mV s 1 and 1600 rpm.
Supporting Information). The PdNTs and AuNTs were further found to have lengths of 5–10 and 5–20 mm, respectively. Selected area electron diffraction (SAED) patterns were utilized to confirm the nanotube growth directions (Figures S2 and S3); the SAED patterns showed the [0 0 1] zone ({1 0 0} and {11 0} reflections) superimposed on the [1, 1, 2] zone ({111} and {11 0} reflections). The (1,1, 1) lattice spacing in the PdNTs and AuNTs was approximately 0.235 and 0.238 nm, respectively, which matches that of AgNWs (0.237 nm, Figures S2 and S3). It was anticipated that the identical crystallographic structure (face-centered cubic) and similar atomic size accounted for the templating of the growth directions and lattice spacing during the displacement process. Electrochemically active surface areas (ECSAs) were determined from the charge associated with the oxidation of an adsorbed carbon monoxide layer (Figure S4). Catalyst ECSAs (normalized to the mass of Pd, Au, Pt, or Ru, denoted PM in figures) were found to be: 20.9 for PdNTs, 69.7 for carbon-sup-
facets, and lattice strain were examined as potential contributors to the high ORR area activity of the PdNTs. Galvanic displacement of the AgNWs produced long, continuous PdNTs; the nanotube morphology potentially allowed for the PdNTs to avoid a Pd particle-size effect.[2a] PdNTs also maintained the fivefold twinned growth directions of the AgNWs. Assuming smooth nanotubes, the PdNT surface would consist of the (1 0 0) facet. Although surface roughness potentially diluted the prominence of the (1 0 0) facet, its increased prevalence potentially improved the ORR activity.[7] Previous studies on Pd nanocubes similarly attributed the improved ORR activity to preferential orientation of the (1 0 0) facet.[8] Previous studies also calculated that a Pd overlayer on Ag shifted the Pd d-band center toward the Fermi level, which would strengthen the oxygen binding energy and reduce the ORR activity in acid.[9] Singleelement results were similar in base, for which Adzic et al. found that Pd/C and Pd(111) had an oxygen binding energy that was too strong, which decreased the ORR activity.[10]
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CHEMSUSCHEM FULL PAPERS AgNW templating and potentially remnant Ag (10 % by mass, confirmed by energy-dispersive X-ray spectroscopy) electronically tuned the PdNTs by expanding the Pd lattice. Lattice expansion would increase the oxygen binding strength of Pd, and this would decrease the ORR activity; it therefore seemed unlikely that the high ORR activity of the PdNTs could be attributed to electronic tuning. Furthermore, although Ag is active for the ORR, the AgNWs expressed a lower activity than the PdNTs (1 % of PdNT mass activity, 5 % of PdNT area activity) and were not believed to significantly affect the ORR performance solely by their presence (Figure S8). The high area activity of the PdNTs also allowed for mass activity that was 30 % greater than that of Pt/C and 3 % greater than that of Pd/ C, but it had an ECSA that was 2.9 (compared to Pt/C) and 3.3 (compared to Pd/C) times less. AuNTs produced an ORR area activity that exceeded that of Au/C, BPAu, and Pt/C by 2.3, 9.0, and 2.0 times, respectively (Figure 2 b and Figure S6 b). The expressed facets and electronic tuning were examined as potential contributors to the high ORR activity of the AuNTs. The AuNT surface contained a high proportion of the (1 0 0) facet and high-index facets in the < 11 0 > zone axis; given that the AgNW displacement yielded smooth nanotubes, the AuNTs were believed to maintain a high proportion of these facets, which were previously shown to be highly active for ORR.[2d–f] AuAg alloys were also previously found to slightly weaken the binding energy of Au (+ 0.0 eV at 5 % Ag, + 0.1 eV at 50 % Ag). No significant change, however, was found in the (1,1 1) lattice spacing of the AuNTs (0.238 nm) in comparison to Au metal (0.236 nm); it was therefore unlikely that the AgNW template and remnant Ag (15 % by mass, confirmed by energy-dispersive X-ray spectroscopy) electronically tuned the AuNTs.[11] Although AgNWs were active for ORR, they expressed a lower activity (2 % of AuNT mass activity, 8 % of AuNT area activity) and were not believed to significantly affect the ORR performance solely by their presence (Figure S8). The AuNT ORR mass activity further exceeded that of Au/C by 16.3 times and came within 65 % of that of Pt/C. Differences were also noted during the study of Pd and Au benchmark catalysts for the ORR. As anticipated, the ORR activity of BPPd was larger than that of Pd/C; this confirmed previous studies that found a Pd particle size effect.[2a] The BPPd electrode had grain sizes of several micrometers and was a statistical average of low- and high-index facets; the Pd nanoparticles, however, contained a higher proportion of less-active fringe facets.[2a] In contrast, BPAu had a lower ORR area activity than Au/C. Previous studies found that BPAu had a weaker binding energy than Au nanoparticles (BPAu: + 0.17 eV compared to 4 nm particles).[12] Studies have also found large variations in the ORR activity of low-index Au facets.[2d–f, 13] It is possible that facet configuration contributed to the disproportionately low ORR activity of BPAu. Quasi-steady-state activities of all the Pd, Au, and Pt catalysts were determined for MOR, EOR, and EGOR (Figure 3 and Figure S10). Although the voltammograms (0.05–1.1 V vs. a reversible hydrogen electrode, RHE) were completed, the onset potential region was focused upon owing to the typical demand of DAHEMFC anodes (Figure S11).[14] PtRu/C was included in 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 3. Quasi-steady-state activities of Pd catalysts for alcohol oxidation, performed at 1 mV s 1 in a 0.1 m KOH electrolyte containing 1.0 m alcohol. a) Methanol, b) ethanol, and c) ethylene glycol oxidation of PdNTs, Pd/C, BPPd, PtRu/C, and Pt/C.
the analysis because of a benchmark status in acidic direct alcohol fuel cells. The PdNTs produced an earlier onset potential than all other catalysts in MOR and EGOR and it either met or exceeded the area activity of Pt/C at a low overpotential. In comparison to Pd/C, PdNTs expressed an onset potential that was 150 to 200 mV lower and a peak area activity that was 3–9-fold higher; in the low overpotential region, the PdNTs also produced an area activity at least one order of magnitude greater for all alcohol oxidation experiments. Of particular interest was that the PdNTs exceeded the mass activity of Pd/C for all alcohols and potentials, in spite of the ECSA discrepancy. In carbon monoxide experiments, the PdNTs also required a potential that was 0.18 V less than that required for Pd/C for peak oxidation (Figure 4). Although it expressed a higher peak potential (0.70 V) than Pt/C (0.67 V), the onset of carbon monoxide oxidation on PdNTs was 0.38 V, which is comparable to PtRu/C. Surface roughness and electronic tuning were examined as potential factors in the alcohol oxidation activity of PdNTs. Wang et al. studied carbon-supported PdAg nanoparticles for ethanol oxidation in base; Wang et al. determined that the lattice expansion (due to the presence of Ag) electronically tuned ChemSusChem 2014, 7, 1739 – 1744
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www.chemsuschem.org Table 1. Alcohol tolerance for the ORR. Catalyst AuNT Au/C BPAu PdNT Pd/C BPPd Pt/C
Figure 4. Carbon monoxide oxidation voltammograms of PdNTs, Pd/C, BPPd, PtRu/C, and Pt/C normalized to the ECSA. Catalyst exposure was set to 10 min at 0.2 V vs. RHE in a carbon monoxide saturated 0.1 m KOH electrolyte.
the Pd, which improved the ethanol oxidation activity and lowered the onset potential.[15] Chung et al. also studied PdAu nanowires for ethanol oxidation in base; although the lattice expansion was not experimentally confirmed, they hypothesized that an expanded lattice electronically tuned Pd, which improved the ethanol oxidation activity.[16] AgNW templating clearly increased the PdNT lattice spacing, which potentially strengthened the Pd alcohol/carbon monoxide binding energy. The electronic tuning of Pd potentially contributed to the high activity and low onset potential for alcohol oxidation. Studies in acidic electrolytes previously found that Pt nanoparticles suffered less intermediate adsorption and poisoning during the MOR than bulk Pt.[17] The PdNTs consisted of agglomerated Pd particles (< 3 nm), which yielded a rough, electrodispersed surface (Figure S2) that potentially aided alcohol oxidation. AuNTs required an onset potential that was 50–100 mV lower and produced a larger area activity than Au/C for alcohol oxidation; AuNTs further produced mass activities that were approximately eightfold higher. Unlike that of the PdNTs, the surface of the AuNTs was smooth, possibly as a result of the match in atomic radii (Au and Ag 144 pm, Pd 137 pm; Figure S3). It is possible that the smooth AuNT surface limited alcohol oxidation activity; in comparison to the PdNTs and Pt catalysts, the onset potential was too high to compete without further modification. Alcohol tolerance for the ORR was also studied, and an improved tolerance of Pd and Au in comparison to Pt was demonstrated (Table 1 and Figure S12). As anticipated from the alcohol oxidation data, the tolerance improved in the order: Pt < Pd < Au. The type of alcohol introduced further affected the degree of tolerance, and it increased in the order: ethanol ethylene glycol < methanol. The AuNTs expressed larger half-wave potential (E1/2) shifts than Au/C, and conversely, PdNTs expressed smaller E1/2 shifts than Pd/C.
Conclusions PdNTs and AuNTs were developed and demonstrated for the first time to have enhanced activity in ORR and in alcohol oxidation reactions in base. The PdNTs and AuNTs exceeded the ORR area activity of Pt/C and their respective bulk polycrystal 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
E1/2 [V] KOH[a] 0.844 0.800 0.716 0.863 0.917 0.842 0.894
methanol[b] 45 30 0 209 365 111 380
E1/2 shift [mV] ethanol[c] ethylene glycol[d] 283 158 39 343 469 334 538
303 161 34 347 362 182 466
[a] ORR E1/2 in a 0.1 m KOH electrolyte. [b] ORR E1/2 shift following the addition of 1.0 m methanol normalized to the reference electrode. [c] ORR E1/2 shift following the addition of 1.0 m ethanol normalized to the reference electrode. [d] ORR E1/2 shift following the addition of 1.0 m ethylene glycol normalized to the reference electrode.
line electrodes. This study is also the first to present a Au catalyst that exceeds the ORR area activity of commercial Pt. The use of an alkaline environment reduces the ORR activity gap between Pt and non-Pt catalysts, which increases the number of viable ORR catalysts. In particular, PdNTs were shown to be a promising ORR catalyst for HEMFCs with high activity and low cost. In alcohol oxidation, the PdNTs were found to produce a higher peak area activity and a lower onset potential than Pd/C and BPPd for alcohol oxidation; furthermore, the PdNTs required a lower onset potential for the MOR and the EGOR than all of the examined catalysts. A lower onset potential can translate into a lower overpotential and a higher cell voltage for HEMFCs. Notably, the PdNTs demonstrated an onset potential that was significantly lower than that found in acidic direct alcohol fuel cells.
Experimental Section The PdNTs and AuNTs were synthesized by the galvanic displacement of AgNWs.[18] The AgNWs were synthesized by the ethylene glycol reduction of Ag nitrate.[18b, 19] In the synthesis of the AgNWs, ethylene glycol was heated at reflux (197.3 8C) for 4 h to remove impurities with a low boiling point. The refluxed ethylene glycol (15 mL) was added to a threenecked, round-bottomed flask equipped with an addition funnel, a condenser, and a thermocouple. After 10 min at 170 8C, chloroplatinic acid (0.4 mm in ethylene glycol, 1.25 mL) was injected into the flask. The reduction of Pt seeds (required to induce wire formation) proceeded for 5 min to ensure temperature stabilization. An ethylene glycol solution (18 mL) containing 0.05 m Ag nitrate and 0.1 m polyvinyl pyrrolidone (molecular weight 40 000) was then added dropwise over a period of 19 min. The synthesis proceeded for an additional 10 min, after which the flask was immersed in an ice bath. Aliquots (5 mL) of the flask contents were separated and washed in ethanol and acetone. This method for the synthesis of the AgNWs was similar to methods available in the literature.[12b, 13] A higher temperature was utilized in this study, which was found to reduce the Ag particle content. Prior to nanotube synthesis, two aliquots (10 mL, 37.8 mg) of cleaned AgNWs were dispersed in an aqueous solution (200 mL) of ChemSusChem 2014, 7, 1739 – 1744
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CHEMSUSCHEM FULL PAPERS 16.74 mm polyvinyl pyrrolidone (molecular weight 40 000), saturated with sodium chloride. This solution was added to a threenecked, round-bottomed flask equipped with a thermocouple, an addition funnel, and a condenser. Under an atmosphere of argon, the flask contents were brought to 100 8C. For PdNT synthesis, an aqueous solution of sodium tetrachloropalladate (1.75 mm, 100 mL, saturated sodium chloride) was added dropwise once the flask contents reached reflux. In the case of AuNTs, an aqueous solution of chloroauric acid (3.50 mm, 100 mL, saturated sodium chloride) was added dropwise once the flask contents reached reflux. In the case of both nanotube syntheses, the source metal addition occurred over a period of 15 min. The flask contents were then heated at reflux for 1 h, at which point the reaction was quenched in an ice bath. The methods for the synthesis of the PdNTs and AuNTs in this study were similar to those available in the literature.[18] Differences in the methodology were as follows: saturated sodium chloride solutions were utilized to ensure Ag chloride formation and to increase the solubility following the synthesis; and polyvinyl pyrrolidone was included to aid in the removal of Pd chloride during cleaning. The concentration of sodium tetrachloropalladate used was low to ensure growth directions were maintained during the synthesis; the concentration of chloroauric acid used was higher to reduce the Ag content following displacement. The PdNTs and AuNTs were washed with a saturated sodium chloride solution, followed by water. Morphological measurements were made with SEM images taken with a Philips XL30-FEG microscope at a potential of 20 kV and TEM images taken with a Philips CM300 microscope at a potential of 300 kV. SAED patterns were taken at a length of 24.5 cm. The TEM samples were pipetted (2propanol suspension) onto holey-carbon copper grids (Ted Pella) and allowed to dry in air. Rotating disk electrode (RDE) experiments were conducted with a mercury/mercurous oxide reference electrode (Hg/HgO, Koslow Scientific Company), a platinum wire counter electrode, and a 5 mm outer diameter glassy carbon working electrode equipped with a modulated speed rotation controller (Pine Instrument Company). Electrochemical data was collected with a multichannel potentiostat (VMP2, Princeton Applied Research). Commercial electrocatalysts were characterized as benchmark materials: Pd/C (20 wt %, Premetek Company), Au/C (20 wt %, Premetek Company), Pt/C (20 wt %, ETEK), and PtRu/C (50 wt % Pt, 25 wt % Ru, Johnson Matthey). Catalysts were pipetted (20 mL) onto the glassy carbon electrode with aqueous suspensions (0.392 mg mL 1, metal basis) at a loading of 40 mgM cm 2. Kinetic ORR activities were determined by RDE experiments in an oxygen-saturated 0.1 m KOH electrolyte at a rotation speed of 1600 rpm during 20 mV s 1 anodic scans. The ORR activities were calculated at 0.9 V versus RHE; area activities derived through ECSAs as determined by carbon monoxide oxidation voltammograms, assuming a Coulombic charge of 420 mC cm 2. Catalyst ECSAs were verified with cyclic voltammograms: Pt/C and PtRu/C ECSAs by the charge associated with hydrogen adsorption; Pd and Au ECSAs by the charge associated with metal oxide reduction. Coulombic charges utilized for Pt, Pd, and Au ECSAs were 210, 420, and 543 mC cm 2, respectively.[20] The validity of these calculations was confirmed with bulk polycrystalline electrode testing, in which BPPd and BPAu expressed a surface roughness of 1.22 and 1.33, respectively. The electrode potential was also corrected for internal resistance during the ORR experiments with impedance spectroscopy data collected between 10 kHz and 0.1 mHz. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Alcohol oxidation voltammograms were conducted at a scan rate of 5 mV s 1 in an argon-saturated 0.1 m KOH electrolyte containing 1.0 m alcohol. Quasi-steady-state experiments were performed during anodic scans at 1 mV s 1 in an argon-saturated 0.1 m KOH electrolyte containing 1.0 m alcohol. A slower scan rate was used to minimize the charge contributions of the carbon supports and metal redox. KOH electrolytes were used for a minimal amount of time to limit the possibility of electrolyte deterioration.[21] Potential values were converted into RHE by potentiostat measurements between the Hg/HgO electrodes and bulk polycrystalline Pt (BPPt) in a hydrogen-saturated 0.1 m KOH electrolyte.[22] Potential values were reported with reference to RHE to compare the results to those in acidic media. Key elements of electrochemical data were performed in RDE experiments by using a Au mesh counter electrode (Figures S13– S16). This data were collected to ensure that the examined catalysts received no benefit from the Pt wire counter electrode.
Acknowledgements Financial support was provided by the EERE program of the US Department of Energy. Keywords: electrochemistry nanostructures · palladium
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Received: January 24, 2014 Published online on April 23, 2014
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Palladium and Gold Nanotubes as Oxygen Reduction Reaction and Alcohol Oxidation Reaction Catalysts in Base Shaun M. Alia,[a, b] Kathlynne Duong,[b] Toby Liu,[b] Kurt Jensen,[a, b] and Yushan Yan*[a, b] Palladium (PdNTs) and gold nanotubes (AuNTs) were synthesized by the galvanic displacement of silver nanowires. PdNTs and AuNTs have wall thicknesses of 6 nm, outer diameters of 60 nm, and lengths of 5–10 and 5–20 mm, respectively. Rotating disk electrode experiments showed that the PdNTs and AuNTs have higher area normalized activities for the oxygen reduction reaction (ORR) than conventional nanoparticle catalysts. The PdNTs produced an ORR area activity that was 3.4, 2.2, and 3.7 times greater than that on carbon-supported palladium nanoparticles (Pd/C), bulk polycrystalline palladium, and
carbon-supported platinum nanoparticles (Pt/C), respectively. The AuNTs produced an ORR area activity that was 2.3, 9.0, and 2.0 times greater than that on carbon-supported gold nanoparticles (Au/C), bulk polycrystalline gold, and Pt/C, respectively. The PdNTs also had lower onset potentials than Pd/ C and Pt/C for the oxidation of methanol (0.236 V), ethanol (0.215 V), and ethylene glycol (0.251 V). In comparison to Pt/C, the PdNTs and AuNTs further demonstrated improved alcohol tolerance during the ORR.
Introduction Solid polymer hydroxide exchange membrane fuel cells (HEMFCs) have recently been developed as a potential alternative to polymer proton exchange membrane fuel cells. HEMFCs are particularly enticing because more cost-effective catalysts can be used instead of platinum (Pt) for the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction. Several studies have examined Pd and Au catalysts for ORR in base.[2] Chen et al. studied the effect of Pd particle size on ORR activity, finding a gradual decline in area activity with decreasing diameter.[2a] Chen et al. further compared Pd nanoparticle catalysts to Pt; although the analysis was largely qualitative, Pd produced higher ORR activity and a lower activation energy.[2b] Pd catalyst developments have focused on the use of Pd alloys for ORR.[2c] In the case of Au, Adzic et al. and Jttner characterized different facets for ORR, and they found that there was increasing activity among low-index facets in the order: (111) < (11 0) < (1 0 0).[2d–f] McFarland et al. examined Au particle size effects on ORR; although a particle size effect was not observed, the ORR activity and Au utilization was particularly low for the polymer-encapsulated nanoparticles.[2g] Solla-Gulln et al. studied quasi-extended surfaces in the form of Au nanocubes; the ORR activity, however, was modest, and the examination was [a] S. M. Alia, K. Jensen, Prof. Y. Yan Department of Chemical Engineering University of Delaware Newark, DE 19716 (USA) Fax: (+ 1) 302-831-1048 E-mail: [email protected] [b] S. M. Alia, K. Duong, T. Liu, K. Jensen, Prof. Y. Yan Department of Chemical and Environmental Engineering University of California, Riverside Riverside, CA 92521 (USA) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201400129.
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void of ORR mass and area activity measurements.[2h] Likewise, Geng et al. examined gold icosahedra with limited activity and did not determine ORR mass or area activities.[2i] Direct alcohol hydroxide exchange membrane fuel cells (DAHEMFCs) are also of interest, as the switch from acid to base reduces the alcohol oxidation overpotential, alcohol permeation rate, and catalyst cost.[3] Pd and Au have been studied previously as methanol (MOR), ethanol (EOR), and ethylene glycol oxidation reaction (EGOR) catalysts.[4] In the case of MOR, Pd and Au are typically utilized as a Pt alloy and have not exceeded the activity of Pt as single metal catalysts. Lamy et al. studied PtPd electrodes in acidic, neutral, and basic environments; they found that the alloy provided a synergistic effect and improved the MOR activity in the alkaline electrolyte.[4a] Watanabe and Motoo characterized PtPd and PtAu electrodes for MOR and found improved activity following the addition of each ad-metal.[4b] Pd is often utilized as an EOR catalyst and was previously found to be more active (in terms of peak current density) than Pt.[4c] Liu et al. demonstrated a fivefold improvement in activity by using Pd nanoparticles in place of Pt.[4d] Xu et al. also examined Pd nanowires that produced significantly higher EOR activity than a Pd film. The nanowire diameter, however, was very large (80 nm) and the synthesis was template based, an approach that faces significant challenges in large-scale synthesis.[4e] Au was studied as an EGOR catalyst by Moussa et al., who found that Au electrodes had a higher peak activity than Pt and Pd, although the onset potential was delayed.[4f] Although Au is typically not mentioned, Pd is frequently studied for methanol and ethanol tolerance.[2b, 5] Although Pd alcohol tolerance is generally poor, the Pd ORR shifts are typically lower than Pt. In this study, Pd nanotubes (PdNTs) and Au nanotubes (AuNTs) were examined for the first time for activity in ORR ChemSusChem 2014, 7, 1739 – 1744
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CHEMSUSCHEM FULL PAPERS and in alcohol oxidation in base. This work was motivated by findings that extended network catalysts (Pt nanotubes), showed dramatically improved ORR and MOR activity in acid.[6]
Results and Discussion The PdNTs and AuNTs were synthesized by the galvanic displacement of silver (Ag) nanowires (AgNWs). The AgNWs were synthesized with a 60 nm diameter and a length of 10– 500 mm, yielding PdNTs and AuNTs with outer diameters of 60 nm and wall thicknesses of 6 nm (Figure 1 and Figure S1,
www.chemsuschem.org ported Pd nanoparticles (Pd/C), 19.3 for AuNTs, 2.7 for carbonsupported Au nanoparticles (Au/C), 59.6 for carbon supported Pt nanoparticles (Pt/C), and 60.1 m2 g 1 for carbon-supported PtRu nanoparticles (PtRu/C). ECSAs were validated by measurements on bulk polycrystalline Pd (BPPd) and Au (BPAu), which had surface roughness of 1.22 and 1.33, respectively. PdNTs produced an ORR area activity that exceeded that of Pd/C, BPPd, and Pt/C by 3.4, 2.2, and 3.7 times, respectively (Figure 2 a and Figure S6 a). The extended surface, expressed
Figure 1. SEM and TEM images of a, b) PdNTs and c, d) AuNTs.
Figure 2. Mass and area activities of a) PdNTs, Pd/C, BPPd, and Pt/C and b) AuNTs, Au/C, BPAu, and Pt/C. All activities were determined at 0.9 V vs. RHE in a 0.1 m potassium hydroxide (KOH) electrolyte during an anodic polarization scan at 20 mV s 1 and 1600 rpm.
Supporting Information). The PdNTs and AuNTs were further found to have lengths of 5–10 and 5–20 mm, respectively. Selected area electron diffraction (SAED) patterns were utilized to confirm the nanotube growth directions (Figures S2 and S3); the SAED patterns showed the [0 0 1] zone ({1 0 0} and {11 0} reflections) superimposed on the [1, 1, 2] zone ({111} and {11 0} reflections). The (1,1, 1) lattice spacing in the PdNTs and AuNTs was approximately 0.235 and 0.238 nm, respectively, which matches that of AgNWs (0.237 nm, Figures S2 and S3). It was anticipated that the identical crystallographic structure (face-centered cubic) and similar atomic size accounted for the templating of the growth directions and lattice spacing during the displacement process. Electrochemically active surface areas (ECSAs) were determined from the charge associated with the oxidation of an adsorbed carbon monoxide layer (Figure S4). Catalyst ECSAs (normalized to the mass of Pd, Au, Pt, or Ru, denoted PM in figures) were found to be: 20.9 for PdNTs, 69.7 for carbon-sup-
facets, and lattice strain were examined as potential contributors to the high ORR area activity of the PdNTs. Galvanic displacement of the AgNWs produced long, continuous PdNTs; the nanotube morphology potentially allowed for the PdNTs to avoid a Pd particle-size effect.[2a] PdNTs also maintained the fivefold twinned growth directions of the AgNWs. Assuming smooth nanotubes, the PdNT surface would consist of the (1 0 0) facet. Although surface roughness potentially diluted the prominence of the (1 0 0) facet, its increased prevalence potentially improved the ORR activity.[7] Previous studies on Pd nanocubes similarly attributed the improved ORR activity to preferential orientation of the (1 0 0) facet.[8] Previous studies also calculated that a Pd overlayer on Ag shifted the Pd d-band center toward the Fermi level, which would strengthen the oxygen binding energy and reduce the ORR activity in acid.[9] Singleelement results were similar in base, for which Adzic et al. found that Pd/C and Pd(111) had an oxygen binding energy that was too strong, which decreased the ORR activity.[10]
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CHEMSUSCHEM FULL PAPERS AgNW templating and potentially remnant Ag (10 % by mass, confirmed by energy-dispersive X-ray spectroscopy) electronically tuned the PdNTs by expanding the Pd lattice. Lattice expansion would increase the oxygen binding strength of Pd, and this would decrease the ORR activity; it therefore seemed unlikely that the high ORR activity of the PdNTs could be attributed to electronic tuning. Furthermore, although Ag is active for the ORR, the AgNWs expressed a lower activity than the PdNTs (1 % of PdNT mass activity, 5 % of PdNT area activity) and were not believed to significantly affect the ORR performance solely by their presence (Figure S8). The high area activity of the PdNTs also allowed for mass activity that was 30 % greater than that of Pt/C and 3 % greater than that of Pd/ C, but it had an ECSA that was 2.9 (compared to Pt/C) and 3.3 (compared to Pd/C) times less. AuNTs produced an ORR area activity that exceeded that of Au/C, BPAu, and Pt/C by 2.3, 9.0, and 2.0 times, respectively (Figure 2 b and Figure S6 b). The expressed facets and electronic tuning were examined as potential contributors to the high ORR activity of the AuNTs. The AuNT surface contained a high proportion of the (1 0 0) facet and high-index facets in the < 11 0 > zone axis; given that the AgNW displacement yielded smooth nanotubes, the AuNTs were believed to maintain a high proportion of these facets, which were previously shown to be highly active for ORR.[2d–f] AuAg alloys were also previously found to slightly weaken the binding energy of Au (+ 0.0 eV at 5 % Ag, + 0.1 eV at 50 % Ag). No significant change, however, was found in the (1,1 1) lattice spacing of the AuNTs (0.238 nm) in comparison to Au metal (0.236 nm); it was therefore unlikely that the AgNW template and remnant Ag (15 % by mass, confirmed by energy-dispersive X-ray spectroscopy) electronically tuned the AuNTs.[11] Although AgNWs were active for ORR, they expressed a lower activity (2 % of AuNT mass activity, 8 % of AuNT area activity) and were not believed to significantly affect the ORR performance solely by their presence (Figure S8). The AuNT ORR mass activity further exceeded that of Au/C by 16.3 times and came within 65 % of that of Pt/C. Differences were also noted during the study of Pd and Au benchmark catalysts for the ORR. As anticipated, the ORR activity of BPPd was larger than that of Pd/C; this confirmed previous studies that found a Pd particle size effect.[2a] The BPPd electrode had grain sizes of several micrometers and was a statistical average of low- and high-index facets; the Pd nanoparticles, however, contained a higher proportion of less-active fringe facets.[2a] In contrast, BPAu had a lower ORR area activity than Au/C. Previous studies found that BPAu had a weaker binding energy than Au nanoparticles (BPAu: + 0.17 eV compared to 4 nm particles).[12] Studies have also found large variations in the ORR activity of low-index Au facets.[2d–f, 13] It is possible that facet configuration contributed to the disproportionately low ORR activity of BPAu. Quasi-steady-state activities of all the Pd, Au, and Pt catalysts were determined for MOR, EOR, and EGOR (Figure 3 and Figure S10). Although the voltammograms (0.05–1.1 V vs. a reversible hydrogen electrode, RHE) were completed, the onset potential region was focused upon owing to the typical demand of DAHEMFC anodes (Figure S11).[14] PtRu/C was included in 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 3. Quasi-steady-state activities of Pd catalysts for alcohol oxidation, performed at 1 mV s 1 in a 0.1 m KOH electrolyte containing 1.0 m alcohol. a) Methanol, b) ethanol, and c) ethylene glycol oxidation of PdNTs, Pd/C, BPPd, PtRu/C, and Pt/C.
the analysis because of a benchmark status in acidic direct alcohol fuel cells. The PdNTs produced an earlier onset potential than all other catalysts in MOR and EGOR and it either met or exceeded the area activity of Pt/C at a low overpotential. In comparison to Pd/C, PdNTs expressed an onset potential that was 150 to 200 mV lower and a peak area activity that was 3–9-fold higher; in the low overpotential region, the PdNTs also produced an area activity at least one order of magnitude greater for all alcohol oxidation experiments. Of particular interest was that the PdNTs exceeded the mass activity of Pd/C for all alcohols and potentials, in spite of the ECSA discrepancy. In carbon monoxide experiments, the PdNTs also required a potential that was 0.18 V less than that required for Pd/C for peak oxidation (Figure 4). Although it expressed a higher peak potential (0.70 V) than Pt/C (0.67 V), the onset of carbon monoxide oxidation on PdNTs was 0.38 V, which is comparable to PtRu/C. Surface roughness and electronic tuning were examined as potential factors in the alcohol oxidation activity of PdNTs. Wang et al. studied carbon-supported PdAg nanoparticles for ethanol oxidation in base; Wang et al. determined that the lattice expansion (due to the presence of Ag) electronically tuned ChemSusChem 2014, 7, 1739 – 1744
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www.chemsuschem.org Table 1. Alcohol tolerance for the ORR. Catalyst AuNT Au/C BPAu PdNT Pd/C BPPd Pt/C
Figure 4. Carbon monoxide oxidation voltammograms of PdNTs, Pd/C, BPPd, PtRu/C, and Pt/C normalized to the ECSA. Catalyst exposure was set to 10 min at 0.2 V vs. RHE in a carbon monoxide saturated 0.1 m KOH electrolyte.
the Pd, which improved the ethanol oxidation activity and lowered the onset potential.[15] Chung et al. also studied PdAu nanowires for ethanol oxidation in base; although the lattice expansion was not experimentally confirmed, they hypothesized that an expanded lattice electronically tuned Pd, which improved the ethanol oxidation activity.[16] AgNW templating clearly increased the PdNT lattice spacing, which potentially strengthened the Pd alcohol/carbon monoxide binding energy. The electronic tuning of Pd potentially contributed to the high activity and low onset potential for alcohol oxidation. Studies in acidic electrolytes previously found that Pt nanoparticles suffered less intermediate adsorption and poisoning during the MOR than bulk Pt.[17] The PdNTs consisted of agglomerated Pd particles (< 3 nm), which yielded a rough, electrodispersed surface (Figure S2) that potentially aided alcohol oxidation. AuNTs required an onset potential that was 50–100 mV lower and produced a larger area activity than Au/C for alcohol oxidation; AuNTs further produced mass activities that were approximately eightfold higher. Unlike that of the PdNTs, the surface of the AuNTs was smooth, possibly as a result of the match in atomic radii (Au and Ag 144 pm, Pd 137 pm; Figure S3). It is possible that the smooth AuNT surface limited alcohol oxidation activity; in comparison to the PdNTs and Pt catalysts, the onset potential was too high to compete without further modification. Alcohol tolerance for the ORR was also studied, and an improved tolerance of Pd and Au in comparison to Pt was demonstrated (Table 1 and Figure S12). As anticipated from the alcohol oxidation data, the tolerance improved in the order: Pt < Pd < Au. The type of alcohol introduced further affected the degree of tolerance, and it increased in the order: ethanol ethylene glycol < methanol. The AuNTs expressed larger half-wave potential (E1/2) shifts than Au/C, and conversely, PdNTs expressed smaller E1/2 shifts than Pd/C.
Conclusions PdNTs and AuNTs were developed and demonstrated for the first time to have enhanced activity in ORR and in alcohol oxidation reactions in base. The PdNTs and AuNTs exceeded the ORR area activity of Pt/C and their respective bulk polycrystal 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
E1/2 [V] KOH[a] 0.844 0.800 0.716 0.863 0.917 0.842 0.894
methanol[b] 45 30 0 209 365 111 380
E1/2 shift [mV] ethanol[c] ethylene glycol[d] 283 158 39 343 469 334 538
303 161 34 347 362 182 466
[a] ORR E1/2 in a 0.1 m KOH electrolyte. [b] ORR E1/2 shift following the addition of 1.0 m methanol normalized to the reference electrode. [c] ORR E1/2 shift following the addition of 1.0 m ethanol normalized to the reference electrode. [d] ORR E1/2 shift following the addition of 1.0 m ethylene glycol normalized to the reference electrode.
line electrodes. This study is also the first to present a Au catalyst that exceeds the ORR area activity of commercial Pt. The use of an alkaline environment reduces the ORR activity gap between Pt and non-Pt catalysts, which increases the number of viable ORR catalysts. In particular, PdNTs were shown to be a promising ORR catalyst for HEMFCs with high activity and low cost. In alcohol oxidation, the PdNTs were found to produce a higher peak area activity and a lower onset potential than Pd/C and BPPd for alcohol oxidation; furthermore, the PdNTs required a lower onset potential for the MOR and the EGOR than all of the examined catalysts. A lower onset potential can translate into a lower overpotential and a higher cell voltage for HEMFCs. Notably, the PdNTs demonstrated an onset potential that was significantly lower than that found in acidic direct alcohol fuel cells.
Experimental Section The PdNTs and AuNTs were synthesized by the galvanic displacement of AgNWs.[18] The AgNWs were synthesized by the ethylene glycol reduction of Ag nitrate.[18b, 19] In the synthesis of the AgNWs, ethylene glycol was heated at reflux (197.3 8C) for 4 h to remove impurities with a low boiling point. The refluxed ethylene glycol (15 mL) was added to a threenecked, round-bottomed flask equipped with an addition funnel, a condenser, and a thermocouple. After 10 min at 170 8C, chloroplatinic acid (0.4 mm in ethylene glycol, 1.25 mL) was injected into the flask. The reduction of Pt seeds (required to induce wire formation) proceeded for 5 min to ensure temperature stabilization. An ethylene glycol solution (18 mL) containing 0.05 m Ag nitrate and 0.1 m polyvinyl pyrrolidone (molecular weight 40 000) was then added dropwise over a period of 19 min. The synthesis proceeded for an additional 10 min, after which the flask was immersed in an ice bath. Aliquots (5 mL) of the flask contents were separated and washed in ethanol and acetone. This method for the synthesis of the AgNWs was similar to methods available in the literature.[12b, 13] A higher temperature was utilized in this study, which was found to reduce the Ag particle content. Prior to nanotube synthesis, two aliquots (10 mL, 37.8 mg) of cleaned AgNWs were dispersed in an aqueous solution (200 mL) of ChemSusChem 2014, 7, 1739 – 1744
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CHEMSUSCHEM FULL PAPERS 16.74 mm polyvinyl pyrrolidone (molecular weight 40 000), saturated with sodium chloride. This solution was added to a threenecked, round-bottomed flask equipped with a thermocouple, an addition funnel, and a condenser. Under an atmosphere of argon, the flask contents were brought to 100 8C. For PdNT synthesis, an aqueous solution of sodium tetrachloropalladate (1.75 mm, 100 mL, saturated sodium chloride) was added dropwise once the flask contents reached reflux. In the case of AuNTs, an aqueous solution of chloroauric acid (3.50 mm, 100 mL, saturated sodium chloride) was added dropwise once the flask contents reached reflux. In the case of both nanotube syntheses, the source metal addition occurred over a period of 15 min. The flask contents were then heated at reflux for 1 h, at which point the reaction was quenched in an ice bath. The methods for the synthesis of the PdNTs and AuNTs in this study were similar to those available in the literature.[18] Differences in the methodology were as follows: saturated sodium chloride solutions were utilized to ensure Ag chloride formation and to increase the solubility following the synthesis; and polyvinyl pyrrolidone was included to aid in the removal of Pd chloride during cleaning. The concentration of sodium tetrachloropalladate used was low to ensure growth directions were maintained during the synthesis; the concentration of chloroauric acid used was higher to reduce the Ag content following displacement. The PdNTs and AuNTs were washed with a saturated sodium chloride solution, followed by water. Morphological measurements were made with SEM images taken with a Philips XL30-FEG microscope at a potential of 20 kV and TEM images taken with a Philips CM300 microscope at a potential of 300 kV. SAED patterns were taken at a length of 24.5 cm. The TEM samples were pipetted (2propanol suspension) onto holey-carbon copper grids (Ted Pella) and allowed to dry in air. Rotating disk electrode (RDE) experiments were conducted with a mercury/mercurous oxide reference electrode (Hg/HgO, Koslow Scientific Company), a platinum wire counter electrode, and a 5 mm outer diameter glassy carbon working electrode equipped with a modulated speed rotation controller (Pine Instrument Company). Electrochemical data was collected with a multichannel potentiostat (VMP2, Princeton Applied Research). Commercial electrocatalysts were characterized as benchmark materials: Pd/C (20 wt %, Premetek Company), Au/C (20 wt %, Premetek Company), Pt/C (20 wt %, ETEK), and PtRu/C (50 wt % Pt, 25 wt % Ru, Johnson Matthey). Catalysts were pipetted (20 mL) onto the glassy carbon electrode with aqueous suspensions (0.392 mg mL 1, metal basis) at a loading of 40 mgM cm 2. Kinetic ORR activities were determined by RDE experiments in an oxygen-saturated 0.1 m KOH electrolyte at a rotation speed of 1600 rpm during 20 mV s 1 anodic scans. The ORR activities were calculated at 0.9 V versus RHE; area activities derived through ECSAs as determined by carbon monoxide oxidation voltammograms, assuming a Coulombic charge of 420 mC cm 2. Catalyst ECSAs were verified with cyclic voltammograms: Pt/C and PtRu/C ECSAs by the charge associated with hydrogen adsorption; Pd and Au ECSAs by the charge associated with metal oxide reduction. Coulombic charges utilized for Pt, Pd, and Au ECSAs were 210, 420, and 543 mC cm 2, respectively.[20] The validity of these calculations was confirmed with bulk polycrystalline electrode testing, in which BPPd and BPAu expressed a surface roughness of 1.22 and 1.33, respectively. The electrode potential was also corrected for internal resistance during the ORR experiments with impedance spectroscopy data collected between 10 kHz and 0.1 mHz. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Alcohol oxidation voltammograms were conducted at a scan rate of 5 mV s 1 in an argon-saturated 0.1 m KOH electrolyte containing 1.0 m alcohol. Quasi-steady-state experiments were performed during anodic scans at 1 mV s 1 in an argon-saturated 0.1 m KOH electrolyte containing 1.0 m alcohol. A slower scan rate was used to minimize the charge contributions of the carbon supports and metal redox. KOH electrolytes were used for a minimal amount of time to limit the possibility of electrolyte deterioration.[21] Potential values were converted into RHE by potentiostat measurements between the Hg/HgO electrodes and bulk polycrystalline Pt (BPPt) in a hydrogen-saturated 0.1 m KOH electrolyte.[22] Potential values were reported with reference to RHE to compare the results to those in acidic media. Key elements of electrochemical data were performed in RDE experiments by using a Au mesh counter electrode (Figures S13– S16). This data were collected to ensure that the examined catalysts received no benefit from the Pt wire counter electrode.
Acknowledgements Financial support was provided by the EERE program of the US Department of Energy. Keywords: electrochemistry nanostructures · palladium
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Received: January 24, 2014 Published online on April 23, 2014
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