Metal Oxide/Gold Hybrid Nanocomposites as Electrocatalysts for Alkaline Air Electrodes

Abstract

MnCo₂O₄/Au was investigated as an electrocatalyst for the oxygen reduction and evolution in alkaline media. Polarization curves showed an unprecedented activity for the oxygen reduction due to a synergistic effect between MnCo₂O₄ and Au, involving Co^II/Co^III-Mn^III/Mn^IV, as revealed by in situ X-ray absorption near edge structure (XANES). Additionally, results showed favorable interaction between the oxide and Au, which enhanced the activity for the oxygen evolution reaction.

Keywords:
fuel cells; oxygen reduction and evolution; hybrid metal oxide nanocomposites

Introduction

Fuel cells (FCs) and unitized regenerative fuel cells (URFCs) are interesting energy conversion/storage systems for a sustainable society. An important impulsion for the development of alkaline fuel cells was associated with the recent discovery of the CO₂ self-purging effect, which takes place when they are operated with anion exchange electrolytes and above 80 ºC.^1-3 This effect minimizes the problem with the atmospheric CO₂, and permits long-term operation.

Mn_yO_x-based materials usually present high activity for the oxygen reduction reaction (ORR) in alkaline media, but still present high overpotentials.^4-6 On the other hand, the MnCo₂O₄ spinel oxide results in a more stable and active material.⁵ Particularly for URFCs, the difficulty is also related to the choice of a conductive and stable electrocatalyst support. In the case of non-conductive catalysts, such as MnCo₂O₄, the electrochemical reaction will not occur over the full surface of the catalyst. It will be constrained to triple phase boundary regions where the oxide makes a contact with a possible conductive phase and the electrolyte. Therefore, it is desirable to increase the surface area of the oxide or of the conductive support in order to enhance the electrocatalytic performance.⁷

In this study, carbon-supported and unsupported MnCo₂O₄/Au hybrid nanocomposites were synthesized and investigated as electrocatalysts for the ORR and oxygen evolution reaction (OER). SnO₂/Au was also prepared in order to separate the contribution of pure Au nanoparticles (SnO₂ is relatively inactive for these reactions in the investigated potential domains).

Experimental

The investigated electrocatalysts consisted of MnCo₂O₄/C/Au or MnCo₂O₄/Au with 20 and 50 wt.% of gold, and 20 wt.% of Mn + Co on carbon (Vulcan XC-72R). MnCo₂O₄/C was synthesized according to previous studies.^5,8 Briefly, 132.8 mg of Mn(NO₃)₂ and 332.8 mg of Co(NO₃)₂ were dissolved in an aqueous carbon suspension (50 mL of water + 80 mg of carbon, for 100 mg of catalyst), followed by the water evaporation at 80 ºC, under magnetic stirring. The dried powder was submitted to a thermal decomposition at 220 ºC for 1 h. For the unsupported material, carbon was eliminated by thermal treatment at 900 ºC, in air, for 2 h. For the deposition of gold nanoparticles (GNs) on MnCo₂O₄ or SnO₂ (Sigma-Aldrich, 99.99%, < 100 nm), the oxides (80 or 50 mg) and HAuCl₄ (71.8 or 179.7 mL for 20 or 50 wt.% of gold, respectively) (Sigma-Aldrich) were kept in water (180 mL), and a solution containing 20 mL of water + 5.83 mL of absolute ethanol + 0.82 g of NaOH was dropped into it. This system was maintained under stirring for 2 h and the precipitate was filtered, washed, and dried at 60 ºC overnight.

X-Ray diffraction (XRD, RIGAKU Ultima IV diffractometer, Cu Kα radiation), X-ray energy dispersive spectrometry (XEDS, Zeiss-Leica/LEO 440 model), transmission electron microscopy (TEM, Jeol GEM 2100, 200 kV), and in situ X-ray absorption near edge structure (XANES) (at Laboratório Nacional de Luz Síncrotron (LNLS)) measurements were performed in order to characterize the materials. The electrochemical experiments were conducted in 1.0 mol L^-1 KOH solution (98.0%); a platinum screen was utilized as counter and Hg/HgO/OH^– as reference electrodes. The working electrodes were prepared by the deposition of the electrocatalyst ink on a glassy carbon disk of a rotating disk electrode (RDE),⁹ followed by the addition of diluted Nafion (0.05 wt.%). Cyclic voltammetry (CV), and polarization curves for ORR, and OER were performed using an AUTOLAB PGSTAT 128N potentiostat.

Results and Discussion

XRD results (Supplementary Information) showed the tetragonal structure of SnO₂ and the spinel structure of MnCo₂O₄, which matched with those of the standard data (Joint Committee on Powder Diffraction Standards (JCPDS)) for these compounds. The average crystallite sizes, calculated by using the Scherrer equation,¹⁰ (d = kl / bcosQ, where d is the average crystallite size; k is the shape factor, here 0.9; l is the X-ray wavelength (Cu Kα1, l = 0.15406 nm); b is the line broadening at half the maximum intensity (in radians); Q is the Bragg angle) resulted in 80.7 and 27.4 nm for MnCo₂O₄ and SnO₂, respectively. GNs deposited on MnCo₂O₄ presented 13.0 and 14.9 nm, and those deposited on SnO₂ presented 20.9 and 22.1 nm, for the loadings of 20 and 50 wt.%, respectively. The composition of gold, determined by XEDS, was ca. 19 and 50 wt.% on MnCo₂O₄, which were close to the nominal values; but it was ca. 10 and 30 wt.% on SnO₂. The differences observed in crystallite sizes and loads of Au on MnCo₂O₄ and on SnO₂ might be related to distinctive metal-support interactions.

The CVs for the carbon-supported materials (Figure 1a) present anodic and cathodic peaks that become even more evident in the presence of gold. For the pure spinel oxide, the CV is featureless, given that it is a non-conductive material. However, the CV for the MnCo₂O₄/Au (Figure 1b) reveals the redox processes from MnCo₂O₄, which indicate they can be accessed due to the GNs. Our experiments indicated that the GNs have to touch or be sufficiently close to each other, forming a continuum conductive phase to the surface of the current collector, in order to observe the redox process from the spinel oxide. A similar result was obtained for Pt deposited on Nb-doped SnO_2-δ.¹¹ Therefore, GNs may increase electrical conductivity and the redox processes of the spinel oxide can be highlighted.

Figure 1
(a) CVs for Au/C, MnCo₂O₄/C, and MnCo₂O₄/C/Au; (b) SnO₂/Au and MnCo₂O₄/Au; 1.0 mol L^-1 N₂-saturated KOH, at 10 mV s^-1.

Figures 2a and 2b show that the XANES main edges of Mn and Co shift to lower values when the electrode potential decreases. The combination of the CVs with the in situ XANES results, as well as the comparison between the obtained spectra and those of standard metal oxides, revealed that both Mn and Co change their oxidation state from IV to II.^4,12 Previous results of XPS of Mn_xCo_3–xO₄ electrodes indicated that the progressive substitution of Co by Mn atoms in spinel oxide structures increased the Co²⁺/Co³⁺ ratio.¹³ The in situ XANES spectra at the Co edge showed, indeed, the electroreduction to Co^II, starting at ca. –0.1 V, whereas the Mn edge showed the electroreduction to Mn^III just below this potential. Therefore, the combination of the CV and XANES results indicates that the anodic current peak at 0.2 V is ascribed to a parallel oxidation from Mn^III to Mn^IV, and Co^II to Co^III species. The small shoulder at 0.55 V, on the CV, may be related to the oxidation of cobalt species to Co^IV, although it is difficult to identify the difference between Co^IV and Co^III spectra. Accordingly, the small cathodic peak (0.15 V) is related to the reduction to Co^III, and the broader cathodic peak (from –0.2 to –0.8 V) is associated with a superimposed electroreduction to Co^II and to Mn^III, with an additional reduction to Mn^II at lower potentials.

Figure 2
In situ XANES spectra for MnCo₂O₄/C at (a) Mn and (b) Co K edges for different potentials in 1.0 mol L^-1 KOH.

The ORR on MnCo₂O₄/C/Au (Figure 3) presented a high ORR onset potential (similar to that of Au/C, and, interestingly, close to that of Pt/C), and a high ORR limiting current (resembling that of MnCo₂O₄/C). Thus, it is proposed the occurrence of a synergistic effect between the metal oxide and the gold nanoparticles. Similarly, the ORR on MnCo₂O₄/Au (Figure 4a) presented a gold-like onset potential, and exhibited higher limiting current, owing to the presence of the MnCo₂O₄.

Figure 3
ORR (1600 rpm) on Vulcan carbon, Au/C, Pt/C, MnCo₂O₄/C, and MnCo₂O₄/C/Au. O₂-saturated 1.0 mol L^-1 KOH, and at 1 mV s^-1.

Figure 4
(a) ORR at 1600 rpm, and (b) OER on SnO₂/Au and MnCo₂O₄/Au. O₂-saturated 1.0 mol L^-1 KOH, and at 1 mV s^-1; (c) TEM image of MnCo₂O₄/Au with 50 wt.% of gold.

Gold presents low oxygen binding energy,^14,15 thus it is hindered to provide 4 electrons during the ORR. On the other hand, the oxide material may produce the O–O bond breaking or the peroxide ion disproportionation.¹⁶ In this case, the ORR on MnCo₂O₄/Au may initiate on the Au atoms surface, producing peroxide ions, at high potentials. Sequentially, the intermediate species move to the spinel active sites (spillover effect) and react again, following the disproportionation reaction or O–O bond breaking, which increases the number of exchanged electrons per O₂ molecule. Furthermore, the insertion of Mn into the Co oxide could contribute to reduce Co^III, during the ORR, which shows higher electrochemical current than the pure Co_yO_x/C.⁵

The OER on MnCo₂O₄/Au presents higher current than SnO₂/Au (Figure 4b, SnO₂ being inactive), and much higher current than pure MnCo₂O₄ (not shown). This evidences that the deposition of GNs is also advantageous for the OER. According to previous studies,^17-19 this increase in activity is associated with (i) change in the OER pathway via the hydrogen acceptor concept¹⁸ or (ii) larger amount of Co^IV (OER active species), induced by the presence of Au. The interaction with gold results in more negative values of oxygen binding energy, which facilitates the O–OH bond formation and OOH deprotonation, so enhancing the OER.¹⁹ One can observe in the TEM image of MnCo₂O₄/Au with 50 wt.% of Au (Figure 4c), that the gold particles (darker domains) possess a heterogeneous distribution in size. Therefore, further improvements are expected by adopting alternative methods of gold deposition with higher surface area.

Conclusions

The results showed an unprecedented activity of the MnCo₂O₄/Au electrocatalyst for the ORR, with high onset potential and high number of electrons, and this was attributed to the synergistic effect between MnCo₂O₄ and Au. For the OER, the high activity of the unsupported MnCo₂O₄/Au nanocomposite was associated with the hydrogen acceptor concept or an increase in the amount of Co^IV species, induced by the presence of Au, that are active for the OER.

Acknowledgments

The authors acknowledge financial support from FAPESP, FHBL (No. 2011/50727-9 and 2013/16930-7), AMP (No .2013/03408-0); CNPq, FHBL (No. 306213/2013-3); and CAPES. Thanks are due to LNLS for assisting with the XAS experiments.

References

• ¹
  Adams, L. A.; Poynton, S. D.; Tamain, C.; Slade, R. C. T.; Varcoe, J. R.; ChemSusChem2008, 1, 79.
• ²
  Fukuta, K.; Inoue, H.; Watanabe, S.; Yanagi, H.; ECS Trans.2009, 19, 23.
• ³
  Lang, C. M.; Kim, K.; Kohl, P. A.; Electrochem. Solid-State Lett.2006, 9, A545.
• ⁴
  Lima, F. H. B.; Calegaro, M. L.; Ticianelli, E. A.; Electrochim. Acta2007, 52, 3732.
• ⁵
  Queiroz, A. C.; Lima, F. H. B.; J. Electroanal. Chem.2013, 707, 142.
• ⁶
  McBreen, J.; Electrochim. Acta1975, 20, 221.
• ⁷
  Oh, T.; Kim, J. Y.; Shin, Y.; Engelhard, M.; Weil, K. S.; J. Power Sources2011, 196, 6099.
• ⁸
  Nissinen, T.; Leskela, M.; Gasik, M.; Lamminen, J.; Thermochim. Acta2005, 427, 155.
• ⁹
  Schmidt, T. J.; Gasteiger, H. A.; Stab, G. D.; Urban, P. M.; Kolb, D. M.; Behm, R. J.; J. Electrochem. Soc.1998, 145, 2354.
• ¹⁰
  Cullity, B. D.; Stock, S. R.; Elements of X-Ray Diffraction, 3^rd ed.; Prentice Hall: Upper Saddle River, 2001.
• ¹¹
  Senoo, Y.; Kakinuma, K.; Uchida, M.; Uchida, H.; Deki, S.; Watanabe, M.; RSC Adv.2014, 4, 32180.
• ¹²
  Li, C.; Han, X.; Cheng, F.; Hu, Y.; Chen, C.; Chen, J.; Nat. Comm.2015, 6, 7345.
• ¹³
  Restovic, A.; Rios, E.; Barbato, S.; Ortiz, J.; Gautier, J. L.; J.Electroanal. Chem.2002, 522, 141.
• ¹⁴
  Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L.; Kitchin, J. R.; Bligaard, T.; Jonsson, H.; J. Phys. Chem. B2004, 108, 17886.
• ¹⁵
  Christoffersen, E.; Liu, P.; Ruban, A.; Skriver, H. L.; Nørskov, J. K.; J. Catal.2001, 199, 123.
• ¹⁶
  Mao, L.; Zhang, D.; Sotomura, T.; Nakatsu, K.; Toshiba, N.; Ohsaka, T.; Electrochim. Acta2003, 48, 1015.
• ¹⁷
  Gorlin, Y.; Chung, C.-J.; Benck, J. D.; Nordlund, D.; Seitz, L.; Weng, T.-C.; Sokaras, D.; Clemens, B. M.; Jaramillo, T. F.; J. Am. Chem. Soc.2014, 136, 4929.
• ¹⁸
  Frydendal, R.; Busch, M.; Halck, N. B.; Paoli, E. A.; Krtil, P.; Chorkendorff, I.; Rossmeisl, J.; ChemCatChem2015, 7, 149.
• ¹⁹
  Yeo, B. S.; Bell, A. T.; J. Am. Chem. Soc.2011, 133, 5587.

Data availability

https://minio.scielo.br/documentstore/1678-4790/mvFCDRwKh48mcSvB75gRZnb/99ecab03b6d27d1c8952b586a76203f8e12522e0.pdf

Publication Dates

• Publication in this collection
  Nov 2015

History

• Received
  14 July 2015
• Accepted
  09 Sept 2015