Elsevier

Journal of Power Sources

Volume 269, 10 December 2014, Pages 855-865
Journal of Power Sources

Nanostructured F doped IrO2 electro-catalyst powders for PEM based water electrolysis

https://doi.org/10.1016/j.jpowsour.2014.07.045Get rights and content

Highlights

  • Nanostructured IrO2:F electro-catalysts have been wet chemically synthesized.

  • IrO2:10 wt.% F exhibits superior electrochemical activity than pure IrO2.

  • Stability of the IrO2:F nanomaterials is comparable to pure IrO2.

  • High surface area F doped IrO2 are promising OER anode electro-catalysts.

  • Both half-cell and full cell test data show the superior response of IrO2:10 wt.% F.

Abstract

Fluorine doped iridium oxide (IrO2:F) powders with varying F content ranging from 0 to 20 wt.% has been synthesized by using a modification of the Adams fusion method. The precursors (IrCl4 and NH4F) are mixed with NaNO3 and heated to elevated temperatures to form high surface area nanomaterials as electro-catalysts for PEM based water electrolysis. The catalysts were then coated on a porous Ti substrate and have been studied for the oxygen evolution reaction in PEM based water electrolysis. The IrO2:F with an optimum composition of IrO2:10 wt.% F shows remarkably superior electrochemical activity and chemical stability compared to pure IrO2. The results have also been supported via kinetic studies by conducting rotating disk electrode (RDE) experiments. The RDE studies confirm that the electro-catalysts follow the two electron transfer reaction for electrolysis with calculated activation energy of ∼25 kJ mol−1. Single full cell tests conducted also validate the superior electrochemical activity of the 10 wt.% F doped IrO2.

Graphical abstract

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Fluorine doped iridium oxide (IrO2:F) powders with varying F content ranging from 0 to 20 wt.% has been synthesized by using a modification of the Adams fusion method. The precursors (IrCl4 and NH4F) are mixed with NaNO3 and heated to elevated temperatures to form high surface area nanomaterials as electro-catalysts for PEM based water electrolysis. The catalysts were then coated on a porous Ti substrate and have been studied for the oxygen evolution reaction in PEM based water electrolysis. The IrO2:F with an optimum composition of IrO2:10 wt.% F shows remarkably superior electrochemical activity and chemical stability compared to pure IrO2. The results have also been supported via kinetic studies by conducting rotating disk electrode (RDE) experiments. The RDE studies confirm that the electro-catalysts follow the two electron transfer reaction for electrolysis with calculated activation energy of ∼25 kJ mol−1. Single full cell tests conducted also validate the superior electrochemical activity of the 10 wt.% F doped IrO2.

Introduction

Hydrogen has been universally claimed as a potential next generation energy carrier with the tremendous ability to provide clean, reliable and affordable energy to meet the ever-increasing global energy demands [1]. A major barrier limiting the progress towards realization of the hydrogen economy is production, storage and distribution of low cost, carbon-free and ultra-high purity (UHP) hydrogen to meet our sustainability goals. High quality hydrogen can be benignly produced by electrochemical conversion of water using electricity i.e., water electrolysis. The high cost of electricity to date has thus far always hindered the production of electro-catalytic hydrogen [2], [3], [4]. Electricity induced splitting of water despite the cost nevertheless, offers no pollutants or the creation of toxic by-products if the electricity is generated via renewable energy sources such as the use of photovoltaic cells, wind turbines, geothermal and hydropower. If all of the limitations, and the barriers discussed above are overcome, we can envision the hydrogen fuel and hydrogen technology to provide a very plausible and ecologically sustainable option for energy production if the efficiency of water electrolysis can be significantly improved with considerable reduction in costs [5], [6], [7], [8], [9].

The current technologies using proton exchange membrane (PEM) or acid based water electrolysis are very cost intensive. This impedes us from achieving the targeted hydrogen production cost (∼$ 3.0/gasoline gallon equivalent (gge)). High capital costs are encountered due to the expensive noble metal catalysts currently employed combined with the inferior efficiencies and labor intensive fabrication of PEM based electrolyzers [4], [5], [10], [11], [12], [13], [14]. PEM based electrolysis however, display several advantages over alkaline and neutral pH based water electrolysis processes including, but not limited to, high proton conductivity, low gas crossover, compact stack design, higher current densities, high pressure operation and the desired chemical and electrochemical tolerance [6], [10], [15]. At the anode, the over-potential and the ohmic resistance however results in poor electrochemical activity accounting for the sluggish catalytic performance. The durability of the electro-catalysts is also a major issue under the harsh acidic PEM conditions, thereby making the search for efficient and stable catalysts a high priority and a major imperative need if the much desired progress in this area is to be achieved.

Rutile type noble metal oxides, such as IrO2 and RuO2 are well known and accepted as gold standard anode catalysts for the oxygen evolution reaction (OER) in PEM based water electrolysis. Decrease in the noble metal oxide (IrO2/RuO2) loading with improved catalytic activity would enable much reduction in capital costs of PEM electrolyzer cells. It has been reported previously by many researchers [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27] that addition of cheaper metal oxides or diluents (viz., SnO2, Ta2O5, TiO2, Nb2O5) to the parent noble metal oxide resulting in a binary or ternary metal oxide mixture could reduce the overall noble metal oxide content. However, with the addition of the cheaper diluents results in a reduction in the active surface area and electronic conductivity of the mixed oxides [17], [18], [24], [28], [29], [30].

We have shown in various publications [16], [27], [31], [32], [33], [34] the viability and efficacy of fluorine as a dopant combined with the use of solid solutions of binary and ternary systems as efficient electro-catalysts for water electrolysis. Specifically, F doped IrO2 as a thin film electro-catalyst for OER in PEM based electrolysis has been previously reported by us [31]. Fluorine doping resulted in ∼20% increase in electrochemical catalytic activity. The stability of the catalysts was also comparable to pure IrO2. We also performed first principle theoretical calculations using the ab-initio approach [32] that concluded that electrolytic water splitting utilizing F doped IrO2 could contribute to significantly increasing the catalytic activity. This initial study formed the basis for motivating the development of F doped IrO2 catalysts in the form of nanomaterials and also conduct detailed studies on the same.

In the present study, research is carried out to synthesize nanostructured F doped IrO2 electro-catalysts in order to improve the catalytic activity and the corrosion stability of the doped oxide compared to pure noble metal oxide electro-catalyst. In this article, F doped IrO2, denoted as IrO2:x wt.% F or IrO2:F, with x = 0, 5, 10, 15 and 20 have been synthesized using a modification of the Adams fusion approach [35] and tested as an OER electro-catalyst. The catalyst ink is then coated on a porous Ti foil and tested as an anode electro-catalyst for PEM water electrolysis. In order to achieve a better understanding of the fundamental electrochemical reactions or electro-catalytic activity, detailed characterization analyses comprising X-ray diffraction (XRD), transmission electron microscopy (TEM), electrochemical impedance spectroscopy (EIS), Tafel analysis, rotating disk electrode (RDE) experiments, and chronoamperometry (CA) studies have been performed and reported on the synthesized IrO2:F nanostructured powder electro-catalysts.

Section snippets

Electro-catalyst preparation

Iridium tetrachloride [IrCl4, 99.5%, Alfa Aesar], and ammonium fluoride [NH4F, 98%, Alfa Aesar] were used as the precursor sources for Ir and F, respectively. IrO2:F was synthesized using a modification of the Adams fusion method, first reported by Adams et al. [35], and used by other researchers as well [36], [37]. The precursors were taken in stoichiometric amounts and completely dissolved in D.I. water generated by the Milli-Q system [18.2 MΩ cm deionized water; Milli-Q Academic, Millipore].

Structural characterization

X-ray diffraction, specific surface area measurements and transmission electron microscopy characterization was conducted in order to study the phase purity and crystalline nature of the electro-catalysts synthesized. The XRD patterns of the IrO2:F powders after heat treatment to 500 °C are shown in Fig. 1. The XRD patterns show a rutile type tetragonal structure similar to pure IrO2 for all the synthesized electro-catalysts. No additional peak (Example. iridium fluoride) is observed which

Conclusion

In the present experimental study, nanostructured F doped IrO2 was synthesized by a wet chemical approach and studied as an anode catalyst for the OER in PEM based water electrolysis. A modified Adams fusion method using NaNO3, fluoride precursors, and heat treatment of the salt mixture to 500 °C was employed to obtain high specific surface area IrO2 and F doped IrO2 electro-catalysts. The XRD pattern confirmed formation of a complete solid solution and the particle size was ∼5–10 nm confirmed

Acknowledgments

Research supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0001531 and the National Science Foundation fund Award CBET Grant # 0933141. P.N.K. acknowledges the Edward R. Weidlein Chair Professorship Funds and the Center for Complex Engineered Multifunctional Materials (CCEMM) for procuring the electrochemical equipment and facilities used in this research work. The authors also acknowledge Proton OnSite

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