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Rh and Rh Alloy Nanoparticles as Highly Active H2 Oxidation Catalysts for Alkaline Fuel Cells Hongsen Wang, and Héctor D. Abruña ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00906 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019
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ACS Catalysis
Rh and Rh Alloy Nanoparticles as Highly Active H2 Oxidation Catalysts for Alkaline Fuel Cells Hongsen Wang and Héctor D. Abruña* Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301 KEYWORDS: Rh Nanoparticles, Rh alloy nanoparticles, H2 oxidation catalysts, size effect, synergistic effect, alkaline fuel cells Abstract: When moving from acidic to alkaline media, non-Pt group metal catalysts can be used as the oxygen reduction catalyst due to their high activity and stability. However, in alkaline electrolytes, we meet another challenge slower H2 oxidation kinetics on platinum group metals relative to acidic electrolytes. We require new robust anode catalysts to enhance H2 oxidation kinetics in alkaline anion exchange membrane fuel cells (AEMFCs). In this letter, carbon-supported Rh and Rh alloy nanoparticles such as Pt7Rh3/C, Ir9Rh1/C, Rh9Ru1/C and Rh9Pd1/C were found to be highly active hydrogen oxidation and evolution catalysts in alkaline media. A size effect was observed for Rh/C, i.e. the HOR activity significantly increased on Rh nanoparticles, relative to bulk Rh. A synergistic effect was noted for the hydrogen oxidation on PtRh and IrRh alloy catalysts. These Rh and Rh alloy nanoparticles outperformed Ir/C and Pt/C as electrocatalysts. In particular, Rh/C with an average particle size of ca. 2 nm was identified to be the best HOR catalyst among all catalysts studied in terms of its mass activity, while Pt7Rh3/C outperformed other studied catalysts in terms of its specific activity and exchange current density. Ir9Rh1/C was found to be the most effective electrocatalyst among all studied Ir alloy nanoparticles. These Rh and Rh alloy catalysts could be employed as highly active H2 electrocatalysts in anion exchange membrane fuel cells.
The application of alkaline fuel cells to the Apollo project began in the 1960s. Alkaline fuel cells were used in the Apollo spacecraft because of their high energy efficiency (about 70%). However, alkaline fuel cells are sensitive to CO2, and electrolyte carbonation precluded their commercial deployment. Anion exchange membranes, with immobilized cations, can mitigate carbonate precipitation. In recent years, anion exchange membranes have been successively developed, and thus, studies on alkaline anion exchange membrane fuel cells (AEMFCs) have become an area of intense research.1-6 AEMFCs have some advantages over proton exchange membrane fuel cells (PEMFCs). A key one is that the oxygen reduction reaction (ORR) kinetics are improved on many non-noble cathode electrocatalysts, and these low-cost electrocatalysts are also stable under alkaline conditions.1,712 However, under alkaline conditions, we have to confront another challenge, since hydrogen oxidation reaction (HOR) kinetics on platinum becomes significantly slower relative to acidic media.13-18 Similarly, the HOR on other Ptgroup metals, such as Ir and Pd, is also slowed down, as the pH increases.13,15 Due to these reasons, there is a clear need for new robust hydrogen oxidation catalysts to enhance the HOR kinetics in AEMFCs. So far, the most effective HOR catalysts are still noble metal based nanoparticles. Several years ago, Strmcnik et al. found that the exchange current density of the HOR could be increased by ca. 6 times on Ir and PtRu, instead of
Pt, in alkaline solutions.19 They proposed that an “oxophilic effect” might play a role in the enhancement, analogous to the bifunctional mechanism for CO and methanol oxidation on PtRu catalysts. Electronic effects have also been proposed to account for the enhancement.14,15,20 PdRu was reported to exhibit quite a different HOR activity from PtRu, and the difference was ascribed to the “interplay of bifunctional and ligand effects”.21 However, the enhancement mechanism of HOR on PtRu is still a matter of debate. Other types of PtRu catalysts such as Pt core-Ru shell, and Ru core-Pt shell catalysts have also exhibited high catalytic activity towards the hydrogen oxidation reaction.22,23 The pH effects on the hydrogen oxidation kinetics at noble metal nanoparticle catalysts have been studied by several groups, over the pH range of 1 to 13.13,15,17 Pt was still noted to outperform other noble metals at high pH values, in disagreement with Strmcnik’s results.19 Recently we found that polycrystalline Ir outperformed polycrystalline Pt in the oxidation of hydrogen at pH = 13, though, nanoparticle Pt catalysts were superior to nanoparticle Ir catalysts.24 The disparity could be due to a size effect or to the formation of more oxides on Ir nanoparticle catalyst surfaces. Non-Pt noble metal based catalysts such as IrPd25 and IrRu26 were also investigated as hydrogen oxidation catalysts at high pH, and the alloys performed better than non-alloys (as evidenced from XRD and TEM measurements).26 The size effect of Ru/C catalysts on the hydrogen oxidation reaction activity was studied by Ohyama et al., and the maximal activity was
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were used to synthesize Rh/C catalysts (referred to as Rh/C-1 and Rh/C-2), respectively. We found that the synthesis with K3RhCl6 + KOH as precursor yielded very small and narrowly distributed Rh nanoparticles (Figure 1d).
found for 3 nm particles.27 Recently we systematically studied nanoparticle binary and ternary Ir alloys with different Ir:Ru:Pd atomic ratios, and found that Ir1-xRux/C, Ir1-xPdx/C and Ir1-x-yPdyRux/C alloy catalysts outperformed Pt/C and Ir/C for the hydrogen oxidation at pH = 13, and they were also stable and much less expensive.24 Miller et al. reported that Pd/C-CeO2 could enhance HOR kinetics and stability, in contrast to Pd/C.28 Non-noble metal catalysts were also studied as HOR catalysts.29-32 However, their surfaces were often covered with oxide layers, which inhibited hydrogen dissociation, and thus none of them could outperform noble metal based catalysts (especially PtRu, IrRu, IrPdRu). To search more effective HOR catalysts, we synthesized some Vulcan XC-72R supported Rh and Rh alloy nanoparticles that surprisingly outperformed the best catalysts reported so far, and could work as highly active HOR catalysts for AEMFCs.
X-ray diffraction patterns of Rh/C-1, Rh/C-2, Rh9Pd1/C, Rh9Ru1/C, Ir9Rh1/C and Pt7Rh3/C nanoparticle catalysts are presented in Figures 1a and 1b. X-ray diffraction data of Pt/C and Ir/C are also shown for comparison.24 All studied catalysts exhibited an fcc structure. The lattice parameters of all studied nanoparticle catalysts are presented in Table S1. The lattice parameter of Rh9Pd1/C increased slightly, when compared to Rh/C, due to the larger size of Pd relative to Rh. In contrast, the lattice parameter of Rh9Ru1/C remained essentially the same as that of Rh/C, owing to the virtually same size of Ru and Rh. The lattice parameters of Ir9Rh1/C and Pt7Rh3/C were slightly smaller than those of Ir/C and Pt/C, respectively, suggesting that the smaller sized Rh was incorporated into the lattice of Ir and Pt. X-ray diffraction peaks were used to estimate the average particle sizes (shown in Table S1), and the details of the X-ray diffraction peak analysis are discussed in ref. 24.24 Transmission electron microscopic measurements were carried out to check the nanoparticle’s size distributions. The image in Figure 1d indicates that the Rh/C-2 nanoparticles were well dispersed on Vulcan XC72R, with a very narrow particle size distribution, as well as a small mean particle size of about 2.20.5 nm. In contrast, the Rh/C-1 catalyst had a broader nanoparticle size distribution, with a large mean particle size of about 5.22.5 nm, which is over 2 times larger than Rh/C-2 (Figure 1c). The nanoparticle sizes, determined from transmission electron microscopy, were a bit smaller than those estimated by X-ray diffraction (Table S1). Figures S1 and S2 present transmission electron microscopic images of Pt7Rh3/C and Ir9Rh1/C nanoparticles, together with size distribution histograms, respectively. The mean nanoparticle sizes of Pt7Rh3/C and Ir9Rh1/C were 2.91.0 nm and 3.61.4 nm, respectively. The HOR activity of these nanoparticle catalysts in 0.1 M potassium hydroxide solution was measured via rotating disk electrode (RDE) voltammetry, with a metal loading of 3.5 g/cm2 on a glassy carbon (GC) electrode. The details for preparing a thin even-distributed catalyst film on GC have been described previously.24
Figure 1. (a) X-ray diffraction patterns of Rh/C-1, Rh/C-2, Rh9Ru1/C and Rh9Pd1/C nanoparticles. (b) X-ray diffraction patterns of Pt7Rh3/C-1 and Ir9Rh1/C, compared to Ir/C and Pt/C24 catalysts. The standard peaks are marked for Rh (|) in a, and Ir (|) and Pt (|) in b, respectively. (c) Transmission electron microscopic image of Rh/C-1 and particle size distribution histogram. (d) Transmission electron microscopic image of Rh/C-2 and particle size distribution histogram.
Cyclic voltammograms (CVs) of bulk Rh, Rh/C-1 and Rh/C-2 in a basic solution are compared in Figure S3. Two pairs of less reversible broad hydrogen peaks occurred in the CV of bulk Rh electrode at ca. 0.18 and 0.26 V, respectively. CVs of Rh/C-1 and Rh/C-2 were both different from that of the bulk Rh electrode. Besides the two pairs of H adsorption and desorption peaks observed in the CV of the bulk Rh electrode, an extra sharp H adsorption peak was observed around 0.08 V, and an extra H desorption peak occurred at around 0.35 V. The differences in the CV profiles could be attributed to different crystal facets
A series of Vulcan XC-72R supported nanoparticle catalysts Rh/C, Pt7Rh3/C, Ir9Rh1/C, Rh9Pd1/C and Rh9Ru1/C were synthesized via a surfactant-free impregnation method, which is described in detail in the Supporting Information (SI).24,33 For comparison, other Pt alloy catalysts Pt7Ru3/C, Pt7Pd3/C and Pt7Ir3/C were also synthesized. Two Rh precursors - RhCl3·3H2O and K3RhCl6
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ACS Catalysis and/or size effects. The H adsorption/desorption charges for Rh/C-2 were higher than for Rh/C-1, in agreement with the fact that the nanoparticle size of Rh/C-2 was over 2 times smaller, relative to Rh/C-1.
to the MA, SA and ECD, and was 3 6 times more active than the Rh catalysts reported previously.34 CVs of Rh9Ru1/C, Rh9Pd1/C and Rh/C-1 in 0.1 M potassium hydroxide solution are presented in Figure S5. CVs for Rh9Ru1/C and Rh9Pd1/C had an H adsorption peak at ca. 0.08 V, similar to Rh/C-1. An increase in the oxidation current between 0.05 and 0.15 V appeared for Rh9Ru1/C and Rh9Pd1/C, and we believe that it is caused by the increased activity of hydrogen stripping from these alloys.
We also compared the RDE voltammetric profiles of Rh9Ru1/C and Rh9Pd1/C catalysts in hydrogen-saturated 0.1 M potassium hydroxide with other catalysts, in Figure 2a. The half-wave potentials of the HOR on Rh9Ru1/C and Rh9Pd1/C were slightly negatively shifted, when compared to Rh/C-1. Compared to Rh/C-1 catalysts, the hydrogen oxidation kinetics of these two alloy catalysts were slightly enhanced, in terms of their MA, SA and ECD (Figure 2b). Cyclic voltammetric profiles of both Pt7Rh3/C and Ir9Rh1/C in alkaline media were compared with those of carbon-supported pure metal Ir and Pt catalysts (Figure S6). The CV of the Pt/C electrode exhibited two pairs of peaks at ca. 0.26 and 0.39V, respectively, which were associated with the reversible adsorption and oxidation processes of hydrogen atoms on different crystal facets. In the CV of Pt7Rh3/C, besides H adsorption/desorption peaks on the Pt sites, two pairs of additional peaks occurred at ca. 0.08 and 0.17 V, and were attributed to the reversible adsorption and oxidation of hydrogen atoms at Ptneighboring Rh sites (Figure S3). Ir/C had four pairs of hydrogen adsorption and desorption peaks at ca. 0.14, 0.17, 0.23 and 0.32 V. The CV of Ir9Rh1/C was similar to that of Ir/C, with the fourth pair of peaks shifting from 0.32V to 0.30 V. Figure 2. (a) Rotating disk electrode voltammetric profiles for a bulk Rh, Rh/C-1, Rh/C-2, Rh9Ru1/C and Rh9Pd1/C nanoparticle electrocatalysts at a potential sweep rate of 5 mV/s and a rotation rate of 1600 rpm in a hydrogen saturated 0.1 M potassium hydroxide solution. (b) Comparison of MA, SA at 10 mV and ECD for the HOR on bulk Rh, Rh/C-1, Rh/C2, Rh9Ru1/C and Rh9Pd1/C. Pt/C is also shown for comparison.24
The HOR activity of bulk Rh electrode, Rh/C-1 and Rh/C2 were characterized using RDE (Figure 2a). Both Rh/C-1 and Rh/C-2 exhibited very high HOR activity, especially Rh/C-2, as indicated by the very small overpotentials. The HOR activities in terms of “the mass activity (MA), the specific activity (SA) at 10 mV and the exchange current density (ECD)”24 are compared in Figure 2b. The equations used to calculate the MA, SA and ECD are provided in the SI and ref. 2424 It is evident that the SA and ECD for the HOR on Rh/C-1 and Rh/C-2 were significantly enhanced, relative to the bulk Rh electrode, and that the HOR kinetics on Rh/C-2 were faster than on Rh/C-1. This suggests a nanoparticle size effect for the HOR kinetics on Rh/C. Rh/C-2, with a small nanoparticle size, was much more active than the Pt catalyst (Figure S4), with respect
Figure 3. RDE voltammetric profiles for the HOR on Pt7Rh3/C and Ir9Rh1/C in 0.1 M potassium hydroxide solution at a potential sweep rate of 5 mV/s and a rotation rate of 1600 rpm. For comparison, the voltammetric profiles of Pt/C and Ir/C are also shown.24
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RDE voltammograms for Pt7Rh3/C and Ir9Rh1/C catalysts in a hydrogen-saturated 0.1 M potassium hydroxide solution are presented together with Pt/C and Ir/C in Figure 3. We found that the half-wave potentials of the HOR on Pt7Rh3/C and Ir9Rh1/C were negatively shifted by about 24 mV and 32 mV, relative to Pt/C and Ir/C, respectively. Pt7Rh3/C and Ir9Rh1/C outperformed Pt/C and Ir/C by ca. 2.5 times, in terms of their MA, SA and ECD.24
Binary PtRu, IrPd and IrRu, and ternary IrPdRu alloy catalysts have been reported to outperform Pt and Ir for the HOR under alkaline conditions.19,24,26 Here we systematically compared the hydrogen oxidation kinetics of Pt7Rh3/C and Ir9Rh1/C with other Pt alloy nanoparticle catalysts (Pt7Ru3/C, Pt7Pd3/C and Pt7Ir3/C) and other Ir alloy nanoparticle catalysts (Ir9Ru1/C and Ir9Pd1/C).24 RDE voltammograms of all Pt based binary nanoparticle catalysts, and all Ir based nanoparticle catalysts for the hydrogen oxidation in 0.1 M KOH are presented in Figures S8 and S9, respectively. RDE voltammetric profiles of carbon-supported Pt7Rh3 and Pt7Ru3 nanoparticles were virtually identical, and exhibited the smallest overpotentials for hydrogen oxidation and evolution reactions among the Pt alloy electrocatalysts. Ir9Rh1/C was slightly more active than Ir9Ru1/C, and exhibited the smallest overpotential for hydrogen oxidation and evolution reactions among the Ir alloy electrocatalysts. The hydrogen oxidation activity of all studied Pt and Ir alloy catalysts are compared in Figure 4. All studied Pt binary catalysts outperformed Pt/C, while all studied Ir binary catalysts had a higher activity than Ir/C. Carbon supported Pt7Rh3 and Pt7Ru3 binary catalysts exhibited the highest hydrogen oxidation and evolution activity among all Pt alloy and Ir alloy catalysts studied, and were even more active than the most active catalysts reported in the literature under the same conditions.34 Ir9Rh1/C was a bit more active than Ir9Ru1/C, and much more active than Ir9Pd1/C and Ir/C.24
As discussed previously, a Tafel plot analysis could not be performed, due to very narrow kinetic region.24 The HOR activities of all studied catalysts are compared in Figure S7. As for the MA at 10 mV, all Rh and Rh alloy nanoparticle catalysts outperformed pure Pt and Ir nanoparticle catalysts, with activity decreasing in the order: Rh/C-2 > Pt7Rh3/C > Rh9Ru1/C > Rh9Pd1/C > Ir9Rh1/C > Rh/C-1 > Pt/C > Ir/C. With respect to the SA at 10 mV, the HOR activity decreased in the following order: Pt7Rh3/C > Rh/C-2 > Ir9Rh1/C ≈ Rh9Ru1/C ≈ Rh9Pd1/C > Rh/C-1 > Pt/C > Ir/C. Finally, the ECD for the HOR decreased in the following order: Pt7Rh3C > Ir9Rh1/C > Rh/C-2 ≈ Ir9Rh1/C ≈ Rh9Ru1/C ≈ Rh9Pd1/C > Rh/C-1 > Pt/C > Ir/C.24
Rh nanoparticles behaved quite differently from a bulk Rh electrode in alkaline media (Figure S3). An additional sharp H adsorption peak was evident in the CVs of Rh/C-1 and Rh/C-2 at lower potentials (about 0.08 V) than the other two H adsorption peaks, suggesting that more weakly adsorbed H atoms are formed on Rh nanoparticles. These weakly adsorbed H atoms might cause the HOR enhancement on the nanoparticle catalysts – Rh/C-1 and Rh/C-2 (Figure 2). For the small nanoparticle Rh/C-2 catalyst, H adsorption/desorption kinetics further increased, and thus the HOR kinetics were even more enhanced, when compared to the large nanoparticle Rh/C1 catalyst. Rh alloying with Ru and Pd slightly enhanced the HOR activity (Figure 2) due to the increase in H adsorption/desorption kinetics (Figure S5). The HOR activities of all Pt and Ir binary alloy catalysts were higher than those of the individual pure metal catalysts, suggesting the presence of synergistic effects in these binary catalysts. Pd/C, Ru/C and Ir/C all exhibited lower performance than Pt/C for hydrogen oxidation under alkaline conditions (Figure S4), though they are more oxophilic than Pt/C (Figure S10). Although Pd is less oxophilic than Ir and Rh (Figure S10), alloying Pd with Ir and Rh enhanced the HOR activity (Figures 2, 4 and S9). Similarly, PtPd/C outperformed PtIr/C as alkaline hydrogen oxidation catalysts (Figures 4 and S8), though Ir is a more oxophilic metal than Pd (Figure S10). Due to the above reasons, one may have to amend the “oxophilic
Figure 4. (a) Comparison of the hydrogen oxidation kinetics of Pt7Rh3/C to other Pt based nanoparticle catalysts. (b) Comparison of the hydrogen oxidation kinetics of Ir9Rh1/C to other Ir based catalysts.24
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ACS Catalysis Corresponding Author
effect” argument,19 in order to explain the HOR activity on the binary catalysts studied. The Sabatier principle suggests that the hydrogen oxidation shall show a maximal activity at modest hydrogen adsorption energies, which is the basis for a “volcano plot”.35,36 DFT calculations indicated that the H adsorption on Pt and Ir (group I) are weaker, relative to Ru, Rh and Pd (group II) (Figure S11), so that the formation of alloys between metals of the two groups could lower the H binding strength on the group II metals, and thus the promotion of HOR activity observed on these Pt and Ir alloy catalysts could be ascribed to a change in hydrogen affinity. Moreover, H adsorption energy differences for atop, bridge, hcp and fcc sites of Pt(111) and Ir(111) surfaces are very small (Figure S11), and thus H diffusion on Pt and Ir are favorable. Therefore, we propose a synergistic mechanism for the enhancement of the HOR activity on Pt and Ir alloys with Ru, Rh and Pd, in which H2 dissociates to form adsorbed H atoms at Pt or Ir sites, and then these adsorbed H atoms can easily diffuse to Ru, Pd or Rh sites, where they are subsequently oxidized. The details of a mechanistic study of the HOR on Pt and Ir alloys with Pd, Rh, and Ru will be reported in a future paper.
*
[email protected] Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT This work was supported as part of the Center for Alkaline Based Energy Solutions (CABES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0019445. This work made use of TEM facilities of the Cornell Center for Materials Research (CCMR).
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In summary, we have synthesized Vulcan XC-72R supported Rh and Rh alloy nanoparticle catalysts - Rh/C, Pt7Rh3/C, Ir9Rh1/C, Rh9Pd1/C and Rh9Ru1/C, and Pt based alloy nanoparticle catalysts Pt7Ru3/C, Pt7Pd3/C and Pt7Ir3/C with a controlled nanoparticle size of 26 nm, characterized them by X-ray diffraction and transmission electron microscopy, and evaluated their catalytic activity for the hydrogen oxidation and evolution under alkaline conditions via thin-film rotating disk electrode techniques. We found that all the carbon supported Rh and Rh alloy nanoparticle catalysts outperformed Pt/C and Ir/C for the hydrogen oxidation and evolution reactions in 0.1 M potassium hydroxide solution. Among them, Rh/C-2 remained the most effective in terms of MA due to a size effect. In contrast, Pt7Rh3/C outperformed other studied catalysts in terms of the SA and ECD (comparable to or outperformed Pt7Ru3/C). Ir9Rh1/C was the most active among all studied Ir alloy nanoparticle catalysts; even more active than Ir9Ru1/C for the hydrogen oxidation and evolution reactions.24 Therefore, these carbon-supported Rh and Rh alloy nanoparticles are prospective hydrogen oxidation and evolution electrocatalysts for alkaline fuel cells and water electrolyzers.
ASSOCIATED CONTENT Supporting Information Nanoparticle synthesis procedure, physical characterization, the table of catalyst properties, electrochemical characterization, and H and OH adsorption energies calculated with DFT. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
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