Titanium–tantalum oxide as a support for Pd nanoparticles for the oxygen reduction reaction in alkaline electrolytes
We report a facile synthetic method for the preparation of titanium–tantalum oxide by means of a modified Adam’s method. This new method allowed obtaining Ti0.8Ta0.2O2 with a high surface area (234 m2 g−1), to be used as catalyst support for Pd nanoparticles. Cyclic voltammetry and linear sweep voltammetry measurements confirm the noticeable oxygen reduction reaction (ORR) activities of the Pd/Ti0.8Ta0.2O2 electrocatalyst in alkaline electrolytes, along with a high-selectivity towards a 4e− pathway. The good ORR performance for the Pd/Ti0.8Ta0.2O2 could arise from both the strong metal-support interaction and the contribution of the Ti0.8Ta0.2O2 in facilitating the ORR process, acting as co-catalyst. However, the stability of this catalyst seems insufficient for practical applications.
KeywordsTitanium Tantalum Oxides Oxygen reduction reaction Alkaline electrolyte
Oxygen reduction is the most challenging electrochemical reaction for many electrochemical devices, such as fuel cells and metal-air batteries [1, 2, 3, 4, 5]. The high potential of this reaction (1.23 V vs. reversible hydrogen electrode, RHE) forces the use of highly active and stable catalysts, being Pt-based ones the most employed [6, 7, 8, 9, 10]. However, it is widely known the high cost of this metal; thus, several strategies are sought to replace it [11, 12, 13]. There has been a huge increment of papers dealing with alternatives to Pt, like the use of other noble metals like Pd, Ag, Au and/or their alloys [14, 15, 16, 17, 18, 19, 20] or the use of non-noble metal catalysts, usually based on transition metals disperse onto a carbon matrix (phthalocyanines, ferrocenes, Co-based catalysts, doped-graphene, etc.) [12, 21, 22, 23, 24]. Additionally, the dispersion of noble metal nanoparticles on high surface area carbon supports is acknowledge as a good strategy to reduce the amount of expensive active phases [25, 26, 27, 28]. However, under the conditions of a fuel cell or a metal-air battery, carbon-based catalysts suffer from the corrosion of the support [29, 30].
There has been a great interest in developing alternative supports, such as the ones based on titanium dioxide. TiO2 has been recognized as a versatile material, easy to produce and with a wide variety of applications (solar cells, degradation of organic pollutants, electrocatalysts supports, etc.) [31, 32, 33, 34, 35]. It features relatively low cost, non-toxicity, photo-stability, and inertness [31, 34]. However, depending on the application, increasing the electrical conductivity of TiO2 is necessary. Heating TiO2 in a reducing atmosphere or doping with cations of higher valence (transition metals) are strategies usually pursued to enhance the electrical conductivity; however, at the expense of the specific surface area [34, 35, 36, 37, 38]. Several authors have introduced different dopants such as Nb, W, V, Ta, etc., by different methods to enhance the electronic conductivity maintaining an adequate surface area [34, 35, 37, 39, 40]. For example, Beauger et al. synthesized TiO2 aerogels and xerogels doped with Nb, V and Ta as alternative to conventional carbon black supports for PEMFCs . Wang et al. obtained TaNbTiO2 and C–TaNbTiO2 as hybrid supports for Pt–Pd nanoparticles for the ORR . Stassi et al.  supported a Pt–Co alloy on Ta-doped TiO2 and Siracusano et al. studied bared and doped TiO2 as catalysts supports for fuel cells . In all cases, doped-TiO2 showed an increased resistance to corrosion.
The different properties of TiO2, such as surface area, crystallographic structure, etc., depend on the synthesis method. There have been many synthetic methods studied in literature, like sol–gel processes, hydrothermal and solvothermal routes, reverse microemulsion methods, etc. [33, 40].
Herein, we propose a new synthetic method to prepare TiO2 doped with Ta, having a porous structure, to be used as the support for Pd nanoparticles. The prepared Pd supported on TiTa-oxide electrocatalyst was investigated, in the present work, for the oxygen reduction reaction in alkaline media. Pd nanoparticles have been already demonstrated as a suitable catalyst for the ORR in basic electrolytes [14, 43, 44, 45]. The advantages of this method are the easy scalability and simplicity, leading to the preparation of a highly porous TiO2-based support doped with Ta, what increases its electrical conductivity. The stability and activity towards the ORR in alkaline electrolyte of the Pd/TiTa-oxide electrocatalyst was compared to a commercial Pd/C based material.
Materials and methods
Titanium–tantalum oxides were synthesized by the Adams fusion method using the procedure modified by Marshall [46, 47]. TiCl4 (98%, Fluka) and TaCl5 (99.8%, Sigma-Aldrich) metal precursors were added to isopropanol (99.5%, Sigma-Aldrich) to obtain a total metal concentration of 0.08 M. This solution was magnetically stirred at room temperature for 1 h to ensure the complete dissolution of the precursors. Then NaNO3 (99.0%, Sigma-Aldrich), previously grounded in ball-milling, was added to the isopropanol solution under vigorous stirring. The slurry was then heated at 90 °C under constant stirring, until obtaining a humid paste, which was dried in a ventilated oven at 90 °C for 24 h. The dry salt was then placed in a furnace at 500 °C for 30 min. The fused salt oxide was washed with distilled water to remove the remaining salts, filtered and dried in an oven at 80 °C for 12 h.
The so-obtained TiTa-oxide was employed as the support for Pd nanoparticles synthesized by a sulphite complex methodology. Firstly, a Pd-sulphite complex was prepared by reaction of PdCl2 (99.9%, Strem Chemicals) with sodium bisulfite (99.995%, Aldrich). Once obtained, the Pd-sulphite salt was first dissolved in acidic solution and subsequently decomposed with H2O2 (40% p/v, Titolchimica) to form a colloidal dispersion of PdOx that was impregnated on the oxide support. The as-formed catalyst was reduced in H2 atmosphere (10 wt% in Ar) at 25 °C to obtain Pd metal nanoparticles supported on the TiTa-oxide with a loading of 60 wt% of Pd .
Several characterization techniques were employed to investigate the different physico-chemical features of both the support, TiTa-oxide, and the catalyst, Pd/TiTa-oxide. X-ray diffraction (XRD) was performed with Cu Kα radiation operating at 40 kV and 20 mA in a Philips X-pert 3710 X-ray diffractometer. The diffraction patterns were fitted to Joint Committee on Powder Diffraction Standards (JCPDS). The peaks broadening was used to calculate the crystallite size by the Debye–Scherrer equation after correction for the instrumental broadening. The morphology of the samples and their composition were studied by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis, carried out by a FEI XL30 SFEG microscope. The instrument was operated at 25 kV and the EDX probe was used to determine the bulk elemental composition of the samples. Transmission electron microscopy (TEM) analysis was performed with a FEI CM12 microscope by depositing some drops of the samples dispersed in isopropyl alcohol on carbon film-coated Cu grids. The specific surface area of the oxide support was calculated by the Brunauer–Emmett–Teller (BET) equation and nitrogen adsorption–desorption isotherms, measured at − 196 °C, using an ASAP 2020 M Micrometrics.
The electrochemical studies were carried out using a standard three-electrode cell and an Autolab potentiostat/galvanostat with GPES software and connected to a rotating disk electrode. A glassy carbon electrode (GC, 5 mm in diameter) was used as the working electrode, a platinum mesh was used as the counter and an Hg|HgO electrode was used as the reference electrode. Typically, 5 mg of the catalyst was dispersed in 5 mL of a mixture of isopropanol (99.5%, Sigma-Aldrich) and water (Milli Q water, 18.2 MΩ cm) in a 3:1 v/v ratio and sonicated for 30 min. 30 wt% of Nafion (5%, Ion Power) was added as polymer binder. Then, 15 µL of the catalyst ink was dropped onto the glassy carbon electrode to obtain a metal loading of 50 µg cm−2 of Pd and allowed to dry at room temperature for 15 min to obtain a uniform carbon film. All electrochemical experiments were carried out at room temperature and ambient pressure using 1 M KOH (90%, Sigma-Aldrich) as the electrolyte solution. Linear sweep voltammetries from 0.65 V vs. RHE to 0.25 V vs. RHE were performed at a scan rate of 5 mV s−1 at different rotation rate: 100, 200, 400, 1000, 1600 and 2500 rpm, bubbling pure O2 in the electrolyte. Before the measurements, the electrode was repeatedly potentiodynamically swept, with a scan rate of 100 mV s−1, from 0 to 1.2 V vs RHE in the deaerated (degassed with He) 1 M KOH solution until a steady voltammogram was obtained. The durability of the TiTaOx-based catalyst was assessed by potential cycling between 0.6 and 1.2 V vs. RHE, bubbling He, until 5000 cycles were completed. The activity of the catalyst was evaluated before and after the cycles by linear sweep voltammetries at different rotation speeds (as previously detailed).
The Pd/TiTa-oxide catalyst was compared to a commercial catalyst employing a carbonaceous support, 30% Pd/C (E-TEK).
Results and discussion
Textural parameters obtained by N2-physisorption on TiTa-oxide
Specific surface area (SB.E.T.)/m2 g−1
Micropore surface area/m2 g−1
Pore volume (Vp)/cm3 g−1
Micropore volume (Vmp)/cm3 g−1
The behavior of the Pd/Ti0.8Ta0.2O2 catalyst in comparison to Pd/C (E-TEK) was investigated for the oxygen reduction reaction (ORR) in an alkaline solution with a rotating disk electrode (RDE). Titanium-based materials have been widely studied in literature as supports for noble metal catalysts such as Pt or Au and mainly in acid media [50, 51, 52]. Recently, there has been a growing interest of this type of catalysts for alkaline media [53, 54], also in combination with carbon materials [55, 56]. Up to our knowledge, Pd supported on Ta-doped titanium oxide has not been studied in alkaline electrolytes until now. For example, Elezovic et al. studied a Nb–TiO2 supported Pt catalyst in comparison with Vulcan supported one, showing similar catalytic activity towards oxygen reduction. Maltanava et al. studied the electrocatalytic activity of both bare high-ordered TiO2 nanotubes (TNTs) and gold nanoparticles (Au NPs) loaded TNTs toward ORR. They determined that the overpotential of O2 reduction on the surface of Au NPs with a definite size increases with increasing the annealing temperature of TiO2 support. In general, it has been thoroughly proved that titanium-based supports seem to enhance the catalytic activity of noble metals towards the ORR in alkaline media.
The electrochemically active surface area (ECSA) was first evaluated in a deaerated 1 M KOH solution by cyclic voltammetry from 0.05 to 1.2 V vs. RHE, at a scan rate of 100 mV s−1 (not shown) as previously described in . Briefly, ECSA was determined from the integration of the peak related to Pd-oxide reduction (between 0.4 and 0.8 V vs. RHE), assuming 405 µC cm−2 for the reduction of a monolayer of Pd-oxide .
The ECSA for the Pd/Ti0.8Ta0.2O2 catalyst was of 25.8 m2 g−1, whereas for the Pd/C was 69.7 m2 g−1, significantly higher due to the lower concentration of metal (30 wt %) on the carbonaceous support and, thus, a better nanoparticle dispersion .
Cyclic voltamograms in Fig. 9 show the H-adsorption/desorption peaks around 0.2 V vs. RHE both in the anodic and the cathodic sense. The Pd oxidation peak is visible in the range 1.0–1.2 V vs. RHE (anodic sense) and the reduction of the Pd-oxide formed is visible at around 0.7 V vs. RHE (cathodic sense). The size of this peak is significantly higher for the commercial catalyst in comparison to the TiTaOx-based one, indicating a higher ECSA for the Pd/C (E-TEK), as previously described. Besides, in the case of the commercial catalyst (Fig. 9a), there is a shoulder at 0.6 V vs RHE that could be ascribed to the oxidation of the carbon support. After 5000 cycles, the width of the voltammogram (red line) is severely reduced for both catalysts. ECSA calculated from the charge associated to the reduction of the Pd-oxide peak (0.7 V vs RHE) was 7.6 m2 g−1 for the Pd/C (E-TEK) catalyst (Fig. 9a) and 0.44 m2 g−1 for Pd/Ti0.8Ta0.2O2 (Fig. 9b). This explains the considerable decay in performance towards the ORR measured by linear sweep voltammetry in previously shown Fig. 8. From these results, it is clear that, although Ti0.8Ta0.2O2 synthesized by this new method presents a good activity for the ORR, this support is not stable enough for a practical application.
A new method for the synthesis of Ta-doped titanium oxides was proposed. The Adam’s method permits obtaining considerable amounts of oxide in a reproducible way with a good compromise between surface area and crystallinity.
Pd nanoparticles were supported on the as-prepared Ti0.8Ta0.2O2 and studied as a catalyst for the oxygen reduction reaction in alkaline media. The activity of Pd/Ti0.8Ta0.2O2 was very similar to that of a commercial Pd/C catalyst. The activity of the titanium-based catalyst was ascribed to both the strong metal support interaction effect and to the intrinsic activity of the Ti0.8Ta0.2O2 towards the ORR in alkaline solution. Stability tests were carried out by potential cycling. Results determined that the Ti0.8Ta0.2O2 synthesized by this new method is not stable, leading to a loss of performance of the catalyst of a 25% in terms of current density. The lack of stability of the support was attributed to the crystallographical structure, being semi-crystalline anatase less stable than other crystallographic phases of titanium-based materials. Future studies will center on the study of the calcination temperature, to increase both conductivity and crystallinity notwithstanding the excellent textural properties of the oxide support.
The research leading to these results has received funding from the ``Accordo di Programma CNR-MiSE, Gruppo tematico Sistema Elettrico Nazionale e Progetto: Sistemi elettrochimici per l’accumulo di energia’’.
- 2.Dresp, S., Luo, F., Schmack, R., Kühl, S., Gliech, M., Strasser, P.: An efficient bifunctional two-component catalyst for oxygen reduction and oxygen evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes. Energy Environ. Sci. 9, 2020–2024 (2016). https://doi.org/10.1039/C6EE01046F CrossRefGoogle Scholar
- 9.Mani, P., Srivastava, R., Strasser, P.: Dealloyed binary PtM3 (M = Cu Co, Ni) and ternary PtNi3 M (M = Cu Co, Fe, Cr) electrocatalysts for the oxygen reduction reaction: performance in polymer electrolyte membrane fuel cells. J. Power Sources 196, 666–673 (2011). https://doi.org/10.1016/j.jpowsour.2010.07.047 CrossRefGoogle Scholar
- 10.Sebastián, D., Serov, A., Artyushkova, K., Gordon, J., Atanassov, P., Aricò, A.S., Baglio, V.: High performance and cost-effective direct methanol fuel cells: Fe-N-C methanol-tolerant oxygen reduction reaction catalysts. Chemsuschem 9, 1986–1995 (2016). https://doi.org/10.1002/cssc.201600583 CrossRefGoogle Scholar
- 12.Chen, Z., Higgins, D., Yu, A., Zhang, L., Zhang, J., Heller, A., Hui, S.Q., Zhang, J.J., Ota, K., Campbell, S.A., Dahn, J.R., Olson, T., Pylypenko, S., Atanassov, P., Ustinov, E.A.: A review on non-precious metal electrocatalysts for PEM fuel cells. Energy Environ. Sci. 4, 3167 (2011). https://doi.org/10.1039/c0ee00558d CrossRefGoogle Scholar
- 14.McKerracher, R., Alegre, C., Baglio, V., Aricò, A.S., Ponce de León, C., Mornaghini, F., Rodlert, M., Walsh, F.C.: A nanostructured bifunctional Pd/C gas-diffusion electrode for metal-air batteries. Electrochim. Acta 174, 508–515 (2015). https://doi.org/10.1016/j.electacta.2015.06.001 CrossRefGoogle Scholar
- 15.Miller, H.A., Lavacchi, A., Vizza, F., Marelli, M., Di Benedetto, F., D’Acapito, F., Paska, Y., Page, M., Dekel, D.R.: A Pd/C-CeO 2 Anode catalyst for high-performance platinum-free anion exchange membrane fuel cells. Angew. Chemie Int. Ed. 55, 6004–6007 (2016). https://doi.org/10.1002/anie.201600647 CrossRefGoogle Scholar
- 16.Félix-Navarro, R.M., Beltrán-Gastélum, M., Reynoso-Soto, E.A., Paraguay-Delgado, F., Alonso-Nuñez, G., Flores-Hernández, J.R.: Bimetallic Pt–Au nanoparticles supported on multi-wall carbon nanotubes as electrocatalysts for oxygen reduction. Renew. Energy. 87, 31–41 (2016). https://doi.org/10.1016/j.renene.2015.09.060 CrossRefGoogle Scholar
- 22.Jaouen, F., Proietti, E., Lefèvre, M., Chenitz, R., Dodelet, J.-P., Wu, G., Chung, H.T., Johnston, C.M., Zelenay, P.: Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuelcells. Energy Environ. Sci. 4, 114–130 (2011). https://doi.org/10.1039/C0EE00011F CrossRefGoogle Scholar
- 24.Alegre, C., Busacca, C., Di Blasi, O., Antonucci, V., Aricò, A.S., Di Blasi, A., Baglio, V.: A combination of CoO and Co nanoparticles supported on electrospun carbon nanofibers as highly stable air electrodes. J. Power Sources 364, 101–109 (2017). https://doi.org/10.1016/j.jpowsour.2017.08.007 CrossRefGoogle Scholar
- 27.S. L. Suib, F. Maillard, N. Job, M. Chatenet, Chapter 14—Approaches to Synthesize Carbon-Supported Platinum-Based Electrocatalysts for Proton-Exchange Membrane Fuel Cells, New Futur. Dev. Catal., 2013: pp. 407–428. https://doi.org/10.1016/b978-0-444-53880-2.00019-3
- 28.S.L. Suib, F. Maillard, N. Job, M. Chatenet, Chapter 17 – Basics of PEMFC Including the Use of Carbon-Supported Nanoparticles, in: New Futur. Dev. Catal., 2013: pp. 401–423. https://doi.org/10.1016/b978-0-444-53874-1.00018-4
- 30.Castanheira, L., Dubau, L., Mermoux, M., Berthomé, G., Caqué, N., Rossinot, E., Chatenet, M., Maillard, F.: Carbon corrosion in proton-exchange membrane fuel cells: from model experiments to real-life operation in membrane electrode assemblies. ACS Catal. 4, 2258–2267 (2014). https://doi.org/10.1021/cs500449q CrossRefGoogle Scholar
- 34.Wang, Y.-J., Wilkinson, D.P., Neburchilov, V., Song, C., Guest, A., Zhang, J.: Ta and Nb co-doped TiO2 and its carbon-hybrid materials for supporting Pt–Pd alloy electrocatalysts for PEM fuel cell oxygen reduction reaction. J. Mater. Chem. A. 2, 12681 (2014). https://doi.org/10.1039/C4TA02062F CrossRefGoogle Scholar
- 36.Beauger, C., Testut, L., Berthon-Fabry, S., Georgi, F., Guetaz, L.: Doped TiO2 aerogels as alternative catalyst supports for proton exchange membrane fuel cells: a comparative study of Nb, v and Ta dopants. Microporous Mesoporous Mater. 232, 109–118 (2016). https://doi.org/10.1016/j.micromeso.2016.06.003 CrossRefGoogle Scholar
- 38.Siracusano, S., Baglio, V., D’Urso, C., Antonucci, V., Aricò, A.S.: Preparation and characterization of titanium suboxides as conductive supports of IrO2 electrocatalysts for application in SPE electrolysers. Electrochim. Acta 54, 6292–6299 (2009). https://doi.org/10.1016/j.electacta.2009.05.094 CrossRefGoogle Scholar
- 39.C. Hao, H. Lv, B. Li, H. Xin, J. Ma, Investigation of mesoporous vanadium doped TiO2 support for anode catalyst of SPE electrolyzer, Taiyangneng Xuebao/Acta Energiae Solaris Sin. 34 (2013) 1464–1470. http://www.scopus.com/inward/record.url?eid=2-s2.0-84886859703&partnerID=tZOtx3y1
- 43.McKerracher, R.D., Figueredo-Rodríguez, H.A., Ponce de León, C., Alegre, C., Baglio, V., Aricò, A.S., Walsh, F.C.: A high-performance, bifunctional oxygen electrode catalysed with palladium and nickel–iron hexacyanoferrate. Electrochim. Acta 206, 127–133 (2016). https://doi.org/10.1016/j.electacta.2016.04.090 CrossRefGoogle Scholar
- 50.Bauer, A., Chevallier, L., Hui, R., Cavaliere, S., Zhang, J., Jones, D., Rozière, J.: Synthesis and characterization of Nb-TiO2 mesoporous microsphere and nanofiber supported Pt catalysts for high temperature PEM fuel cells. Electrochim. Acta 77, 1–7 (2012). https://doi.org/10.1016/J.ELECTACTA.2012.04.028 CrossRefGoogle Scholar
- 56.Jukk, K., Kozlova, J., Ritslaid, P., Sammelselg, V., Alexeyeva, N., Tammeveski, K.: Sputter-deposited Pt nanoparticle/multi-walled carbon nanotube composite catalyst for oxygen reduction reaction. J. Electroanal. Chem. 708, 31–38 (2013). https://doi.org/10.1016/J.JELECHEM.2013.09.009 CrossRefGoogle Scholar
- 59.A. Więckowski, Interfacial electrochemistry : theory, experiment, and applications, Marcel Dekker, 1999Google Scholar
- 61.Castegnaro, M.V., Paschoalino, W.J., Fernandes, M.R., Balke, B., Alves, M.C.M., Ticianelli, E.A., Morais, J.: Pd–M/C (M = Pd, Cu, Pt) Electrocatalysts for Oxygen reduction reaction in alkaline medium: correlating the electronic structure with activity. Langmuir 33, 2734–2743 (2017). https://doi.org/10.1021/acs.langmuir.7b00098 CrossRefGoogle Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.