Hydrogen Bubble-Assisted Electrodeposition of Metal Nanoparticles

Letter pubs.acs.org/journal/ascecg

Hydrogen Bubble-Assisted Electrodeposition of Metal Nanoparticles from Protic Ionic Liquids for Electrocatalysis Majid Asnavandi and Chuan Zhao* School of Chemistry, The University of New South Wales, High St., Sydney, NSW 2052, Australia S Supporting Information *

ABSTRACT: A facile hydrogen evolution-assisted electrodeposition method is proposed for fabrication of metal nanoparticles from protic ionic liquids with high electrocatalytic activity. The controlled evolution of hydrogen bubbles functions as physical spacers to prevent aggregation of nanoparticles. Uniform silver, palladium, and nickel nanoparticles with high surface area have been generated and used as catalysts for oxygen reduction reactions with enhanced performance. KEYWORDS: Dynamic template, Thin film, Palladium, Silver, Nickel, Oxygen reduction reaction

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for electrodeposition of Ag foam.12,13 The approach of a hydrogen template is effective for increasing the catalysts reactive surface area and their catalytic activity for electrochemical applications.10,15 Nevertheless, large amount of metals are required in the method to form thick macroporous metal films, which is impractical for precious metals. Furthermore, the approach is not very applicable to reactive metals such as Ni and Co due to the formation of metal hydroxide precipitates in aqueous electrolytes. Herein, we show a facile hydrogen bubble-assisted electrodeposition method for preparation of metal nanoparticles from protic ionic liquids (PILs). Ionic liquids (ILs) have received significant interest for electrodeposition of metals and alloys due to a unique set of properties such as high ionic conductivity, thermal stability, nonvolatility, wide electrochemical potential window, and absence of detrimental water and water/metal reactions.16,17 PILs formed by a variety of proton transfer and association equilibria between neat Brønsted acids and bases. PILs have attracted attention for electrodeposition owing to their low cost, low environmental impact, high attendant conductivity owing to the mobile protons, and high solubility to simple metal salts.18,19 We report in this study the electrodeposition of metal nanoparticles from a typical PIL, ethylammonium nitrate (EAN), at high negative

INTRODUCTION In electrochemical energy devices, the use of electrocatalysts is essential. For example, fuel cells and metal−air batteries have received significant attention for renewable energy conversion and storage. However, the slow kinetic of the oxygen reduction reaction (ORR) often requires the use of Pt-based electrocatalysts.1,2 Other metals such as Pd and Ag have also been reported as alternative ORR catalysts with high activity at significantly lowered cost.3,4 In general, the activity of electrocatalysts is strongly influenced by their structures and active surface area.3 Therefore, fabrication of nanoparticle electrocatalysts has been a popular approach to achieve enhanced catalytic activity due to dramatically increased active sites and surface areas. However, a common issue for nanostructured electrocatalysts is that they tend to agglomerate to form large particles during the fabrication and/or operation, which limits their electrochemical active surface area.5,6 Electrodeposition is an established method for preparation of thin films and coatings of metals on various conductive current collectors.7 Hydrogen evolution is a common issue for metal electrodeposition from aqueous solutions which compromise the physical quality of the deposited films due to the embrittlement8,9 or particles aggregation.10 Liu et al. developed an elegant approach for electrodeposition from aqueous acid electrolytes at high deposition potentials and current densities using strongly evolved hydrogen bubbles as a template for production of macroporous metals (e.g., Cu and Sn) thin films.11−15 Other researchers also have developed electrolytes © 2016 American Chemical Society

Received: September 14, 2016 Revised: October 30, 2016 Published: October 31, 2016 85

DOI: 10.1021/acssuschemeng.6b02219 ACS Sustainable Chem. Eng. 2017, 5, 85−89

Letter

ACS Sustainable Chemistry & Engineering

Figure 1. (a) Schematic of gas bubbling in electrodeposition from protic ionic liquids. (b) CV of EAN on GC and Pt electrode. (c) RRDE measurement on GC electrode in Ar-saturated dry EAN at rotation speed of 1600 rpm and scan rate of 10 mV s−1.

peak at about +0.65 V during the anodic scan. For Pd (Figure S2c), there are two reduction peaks attributing to Pd2+ to Pd+ and then Pd+ to metallic Pd. These electrochemical behaviors of Ag and Pd in EAN are in accordance with previous studies.4,16 The CV of Ni in EAN also shows two reduction peaks at potentials around +0.5 and −0.3 V (vs FC0/+), which the latter one attributed the electrodeposition of metallic Ni.25 Figure S2b, S2d, and S2f shows the CVs in the wider potential range from 1.5 to −1.5 V vs FC0/+. Comparing to that obtained on a bare GC (Figure 1b), the hydrogen evolution potentials in the cathodic scan all shift positively. This has been observed in the literature26−28 and is attributed to the formation of thin metallic films of Ag, Pd, and Ni during the cathodic scan, which acts as a catalyst for hydrogen evolution processes. Controlled potential electrodeposition is then carried out at different potentials for 1 min to generate the metal nanoparticle catalysts. Figure S3 demonstrates i−t curves for electrodeposition of Ag, Pd, and Ni at different potentials. At deposition potentials lower than −0.5 V but prior to the hydrogen evolution potential, a continuous metallic film is deposited. On the other hand, during electrodeposition at high negative values, e.g., −2.5 and −4.5 V vs Fc0/+, hydrogen bubbles were observed evolving on the electrode. Figure S4 displays X-ray photoelectron spectroscopy (XPS) patterns of the electrodeposited metals at −4.5 V. Scanning electron microscope (SEM) has been utilized to study the effect of electrodeposition potential on surface morphology. Figure 2 shows the SEM images of electrodeposited Ag, Pd, and Ni films from EAN obtained at potentials before and after hydrogen evolution and the electrodeposited particles size distribution. The results suggest that gas coevolution has a strong influence on the morphology of the electrodeposited particles. When the deposition potentials step into the gas evolution region (−1 V), the electrodeposited continuous films transform to fine metallic

potentials by taking advantage of the mobile protons present in PILs. A mild evolution of tiny hydrogen bubbles acts as physical barriers to prevent aggregation (shown in Figure 1a) of nanoparticles and forming a continuous thin film. Uniform Pd, Ag, and Ni nanoparticles thus have been fabricated and used as electrocatalysts for ORR.

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RESULTS AND DISCUSSION The electrochemical behavior of the dried EAN on a glassy carbon (GC) electrode is first studied. The water content after the drying process is measured by Karl Fischer titration to be Pd > Ag > Ni. In particular, the electrodeposited Pd nanoparticles show comparable ORR

nanoparticles uniformly dispersed on the GC substrate surface (Figure 2). The dramatic change in morphology can be attributed to more negative deposition potentials and the evolved hydrogen gas bubbles, which act as a spacer between the metal nanoparticles preventing them from agglomeration and coupling. It is known that more negative deposition overpotentials can lead to morphological changes owing to more nucleation sites. Nevertheless, we found that the influence of hydrogen bubbles on morphology is more dramatic in this case. Before the deposition potentials step into the hydrogen evolution region, increasing the deposition potentials only results in a continuous metallic film formed rather than isolated nanoparticles. It also has been reported that gas bubbling can make a convective effect to increase metallic ions flux (electrolyte) in the vicinity of growing particles.29,30 This phenomenon is more favorable in the protic ionic liquids as they are relatively more viscous with fewer protons contained, in comparison to aqueous electrolytes. As a result, in PILs, the hydrogen evolution rate tends to be mild with smaller hydrogen bubbles, and hence, smaller particles with uniform size distribution can be achieved (Figure 2c,f,i). The effective hydrogen bubble size for nanoparticle deposition in PILs is estimated to be less than 500 nm. 87


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Figure 3. RDE voltammetry curves for oxygen reduction on (a) Ag, (b) Pd, and (c) Ni film electrodeposited from EAN at different potentials. (d) Comparison of electrodeposited catalyst for ORR with GC and Pt/C. All RDE measurements have been done at the rotation rate of 1600 rpm at a scan rate of 10 mV s−1 in O2-saturated 1 M KOH.

tension and mass transport upon the addition of water into ILs, which reduced the viscosity. Nevertheless, it is noted that the change in morphology is insignificant (Figure S10b), and the ORR activity is still significantly higher than the Pd film prepared using the conventional electrodeposition method.

activity performance to Pt/C, with similar onset potential and slightly lower current density. When the RDE voltammograms are normalized to the electrochemical surface area (ECSA), the results (Figure S6) show that the difference between limiting current densities becomes much closer for different electrodeposition potentials, although slightly higher current densities are still observed for higher deposition potentials, especially in hydrogen evolution region. These results suggest that the high catalytic current obtained from hydrogen bubble-assisted electrodeposition (normalized to geometric area, Figure 3) largely stems from the high surface area of electrodeposited nanoparticles. The hydrogen bubble-assisted electrodeposition can also facilitate the exposure of specific crystalline facets that are favorable for ORR. To study the ORR mechanism on the surface of the nanostructured metallic catalysts, RDE and RRDE voltammetry at a rotation speed from 400 to 2500 rpm was carried out (see more details in the Supporting Information and Figures S7− S9). The values of the Tafel slope, number of transferred electrons, and OH2− production percentage obtained with the electrodeposited catalysts were determined and are summarized in Table S1. The obtained results are consistent with Figure 3, with Pd showing the highest catalytic activity and higher ORR performance observed when each metal catalyst was prepared at very negative deposition potentials. Finally, the effect of the water content in EAN on the ORR activity of electrodeposited Pd particles has also been studied. Pd nanoparticles were also electrodeposited under the same deposition conditions from benchtop EAN containing 2.7 wt % H2O. Figure S10a depicts that the ORR activity of Pd electrodeposited from benchtop EAN is lower than that from dried EAN. This is because Pd particles are slightly bigger as a result of increased interfacial

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CONCLUSION In summary, we report a hydrogen bubble-assisted electrodeposition method for preparation of Ag, Pd, and Ni nanoparticles from a PIL (EAN) by electrodeposition at high negative potentials. The formation of hydrogen gas bubbles during electrodeposition is utilized as a template to achieve smaller and finer metallic nanoparticles. It was found that by increasing the electrodeposition potentials to a more negative value and taking advantage of the hydrogen bubbling effect, metal nanoparticles with higher active surface area and ORR catalytic activity can be achieved. In particular, the Pd nanoparticles prepared via this method even approach the benchmark Pt/C catalyst for ORR. The study suggests the hydrogen gas-assisted electrodeposition from PILs as an effective, facile, and “green” method for nanoparticle catalyst fabrication, which could find applications as catalysts for a range of electrochemical energy devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02219. Determination of ECSA, number of transferred electrons, byproduct percentage production and Tafel plots, 88


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benchtop EAN performance, and summarized tables. (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel: +61293854645. Fax: +61293856141. E-mail: chuan. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to UNSW Mark Wainwright Analytical Center for providing access to their SEM and XPS facilities. The study was financed by Australian Research Council (DP150101861).

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Hydrogen Bubble-Assisted Electrodeposition of Metal Nanoparticles

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