Elsevier

Catalysis Today

Volume 262, 15 March 2016, Pages 170-180
Catalysis Today

Oxygen and hydrogen evolution reactions on Ru, RuO2, Ir, and IrO2 thin film electrodes in acidic and alkaline electrolytes: A comparative study on activity and stability

https://doi.org/10.1016/j.cattod.2015.08.014Get rights and content

Highlights

  • We study catalytic performance of iridium and ruthenium metal and oxide electrodes.

  • Identical electrode's morphologies and experimental parameters are employed.

  • Metals are more active in catalyzing oxygen and hydrogen evolution but less stable.

  • Oxides are completely intact during hydrogen evolution.

  • Iridium oxide is very stable during oxygen evolution.

Abstract

Metallic iridium and ruthenium as well as their oxides are among the most active oxygen evolution (OER) electrocatalysts in acidic media, and are also of interest for the catalysis of the hydrogen evolution (HER). The stability of these materials under different operating conditions is, however, still not fully understood. In the current work, activity and stability of well-defined Ru, RuO2, Ir, and IrO2 thin film electrodes are evaluated in acidic and alkaline electrolytes using an electrochemical scanning flow cell (SFC) connected to an inductively coupled plasma mass spectrometer (ICP-MS). Identical experimental protocols are intentionally employed for all electrodes and electrolytes, to obtain unambiguous and comparable information on intrinsic activity and stability of the electrodes. It is found that independent of the electrolyte, OER activity decreases as Ru > Ir  RuO2 > IrO2, while dissolution increases as IrO2  RuO2 < Ir  Ru. Moreover, dissolution of these metals in both solutions is 2–3 orders of magnitude higher compared to their respective oxides, and dissolution is generally more intense in alkaline solutions. Similarly to the OER, metallic electrodes are more active catalysts for HER. They, however, suffer from dissolution during native oxide reduction, while IrO2 and RuO2 do not exhibit significant dissolution. The obtained results on activity and stability of the electrodes are discussed in light of their potential applications, i.e. water electrolysers or fuel cells.

Introduction

The electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are two reactions of special importance in both fundamental and applied electrochemistry. Actually, the first concepts of electrochemical kinetics originate from the seminal works on the rate of the relatively simple HER on different electrodes [1]. The OER was fundamentally less studied, as it is much more complex in terms of the variety of possible reaction mechanisms and due to the surface instability at typical reaction potentials [2]. Both reactions are continuously discussed in applied electrochemistry, with the most notable example of electrochemical water splitting. Besides, there are numerous examples in which only one reaction out of the two is of main interest. Electrowinning of metals or cathodic electrochemical organic synthesis for instance rely on the sluggish OER as a counter reaction, which leads to high energy losses in the processes. On the other hand, HER is a counter reaction to chlorine evolution or the oxidation of organic species in electrolyte, e.g. methanol.

Considering the industrial importance of the topic, it is not surprising that comparative studies on electrocatalytic properties of Ir- and Ru-based materials, the best performing catalysts in acidic conditions, are numerous. There is a large body of literature on OER performance of the monometallic and the mixed IrO2 and RuO2 oxides [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], as also recently discussed in several reviews [24], [25]. With the exception of some reports, it is found that addition of RuO2 to IrO2 results in enhanced activity, but in general also lower stability. Typically, the stability is estimated in an accelerated degradation test at relatively high potentials under uncontrolled electrocatalyst corrosion or mechanical degradation due to vigorous oxygen gas evolution. Materials showing relatively fast increase in OER overpotenial (usually with high Ru concentration) are assigned as unstable [26], [27]. The obvious drawback of this approach is that the exact degradation mechanism is not known, which inhibits the designated development of mitigation strategies. Only a few more specific studies could distinguish between different contributions to material degradation, as for instance dissolution, while evaluating the dissolved species in the electrolyte [19], [20]. Recently, a number of comparative studies on the performance of nanostructured Ir and Ru electrodes in aqueous electrolytes, in particular in the region of OER, were published. A comparison between activity and stability of Ru and RuO2 nanoparticles in acidic electrolyte was for instance performed by Paoli et al. [28] and Hodnik et al. [29]. In the former work, the dissolution of metallic Ru nanoparticles was observed by a clear disappearance of nanoparticles from the electrode as seen in potentiodynamic STM images, measured by rotating ring disk electrode (RRDE) with detection of RuO4, and inductively coupled plasma mass spectrometry (ICP-MS) post-analysis. It was also shown that already after 15 min of electrolysis at 1.5 VRHE ca. 90% of Ru was dissolved from the metal electrode, while for the oxide electrode this number was ca. 30% after 15 h of electrolysis. Oxide nanoparticles are therefore much more stable than metallic ones, similar to bulk material [26], [30], [31], [32], [33]. In general, the results presented by Hodnik et al. are in line with the previous work [29]. Unlike Paoli et al., however, these authors presented additional time- and potential-resolved Ru dissolution curves for the exact identification of dissolution potentials and dissolution kinetics. Moreover, the importance of cathodic dissolution, as previously shown for metallic electrodes [34], was highlighted by the authors. From a recent electrochemical quartz crystal microbalance (EQCM) study, it seems that dissolution of RuO2 during OER occurs at a relatively constant rate [35]. Similar results were shown recently also for an Au electrode [36] and are probably related to the known parallelism between OER and dissolution [37]. The stability of rutile RuO2 and IrO2 was found to be superior to metallic nanoparticles in acidic as well as in alkaline electrolytes [38]. The rate of metallic Ru performance deterioration, especially in alkaline electrolyte, was, however, much higher than that of metallic Ir. An analogous conclusion was drawn in a similar study by Reier et al. [39]. In a similar manner to Ru electrodes, iridium oxides are much more stable than the metal [27], [40], [41], [42]. Note, that in all those studies the utilization of electrodes of different morphology under different operating conditions renders a direct comparison of results obtained by different groups difficult. At best, only qualitative trends can be drawn.

Conductive oxides can also be considered as potential catalysts for the HER [43]. The reported advantage of such electrodes is that, unlike their metallic counterparts and most prominently Pt, they are not prone to poisoning by underpotential deposition of less active metals that are always present as impurities in technological electrolytes [44]. The first report on the application of RuO2 for HER appeared 40 years ago [45]. Since then numerous works were published by Trasatti et al., Guay et al., Burke et al., and others on the performance of RuO2 in acid [44], [46], [47], [48], [49], [50], [51], [52] and base [53], [54], [55], IrO2 in acid [47], [48], [51], [56], [57], [58] and base [53], [58], and mixed oxides in both electrolytes [8], [48]. This comprises by no means a complete list, but it clearly represents that the amount of work that has been done in the area of HER on Ru- and Ir-based electrodes is extensive. As numerously mentioned in the literature, these oxides are thermodynamically unstable in the potential range of the HER. The Pourbaix diagram reveals that the only stable phase at these potentials should be metallic Ir or Ru [59]. Nevertheless, due to the high electronic conductivity and the absence of an electric field in such oxides, the migration of protons in the oxide lattice and thus the bulk oxide reduction is inhibited [43]. More recent X-ray diffraction and X-ray photoelectron spectroscopy results by Guay et al. [47], [49], [52] revealed that during the cathodic polarization some hydrogen absorbs in the RuO2 and IrO2 lattice, although most of it desorbs again after the treatment, since lattice parameters relax back almost to their original values. The authors also show that similar to bulk [49], the surface of thermally prepared RuO2 is also not reduced to metallic Ru [52]. It was mentioned that only on the hydrous ruthenium oxide a partial surface reduction takes place [52]. Despite the reported stability, as mentioned by Trasatti, long-term tests of the HER on RuO2 and IrO2 are lacking [43]. The same can be said for metallic electrodes. Dissolution of Ru at low (cathodic) potentials is of high industrial importance due to its application as a co-catalyst in hydrogen or alcohol fuel cell anodes. The topic was addressed in several studies [60], [61], [62], [63], in which, however, only anodic processes were considered. It is now well established that dissolution of noble metals during oxide reduction is of similar, or even higher, importance [34], [64].

In this work, we aim to assess the performance of metallic and oxidized Ir and Ru thin film electrodes in two important pH ranges (acidic and alkaline) during OER and HER. The choice of low pH is mainly dictated by the practical consideration of a need for a stable and active material for OER and HER in acidic electrochemical water splitting. In contrast, the use of these electrodes in alkaline electrolytes is questionable, as cheaper materials can be used in most applications. Nevertheless, there are numerous very recent studies on alternative OER catalysts for water electrolyzers and air-metal batteries reporting performance of ill-defined Ir and/or Ru metal and oxide for supported and supportless electrodes in alkaline solution, often used without care as a reference or a benchmark material [65], [66], [67], [68], [69], [70], [71], [72], [73]. Often, performance of non-noble element based catalysts in an alkaline electrolyte is compared to the activity of IrO2 and RuO2 in an acidic media [74]. Hence, OER and HER in alkaline electrolyte is also included in the study. The main difference of the current work to the huge body of literature is that identical experimental conditions are used to test activity and stability of well-defined model polycrystalline thin film electrodes in parallel, providing thus directly comparable information in one place. The goal is achieved by employing a setup based on the scanning flow cell (SFC) directly coupled to an ICP-MS, allowing real time on-line detection of dissolved species.

Section snippets

Electrodes preparation

Ir and Ru thin films were deposited in a combinatorial sputter system having 5 confocal magnetron cathodes (DCA Instruments, Finland). Single crystal Si wafers (100 mm diameter) with 1.5 μm thick thermal SiO2 as a buffer layer against silicide formation were used as substrates. These wafers were cleaned with acetone and isopropanol in an ultrasonic bath and dried with compressed dry air immediately prior to loading into the deposition chamber. The base vacuum before deposition was 2.5 × 10−6 Pa and

Activity of the electrodes towards OER

Fig. 3 shows the positive-going linear sweep voltammograms (LSV) of the metal and thermal oxide Ru and Ir electrodes in 0.1 M H2SO4 and 0.05 M NaOH electrolytes. Prior to recording LSVs, the metal electrodes were polarized successively at 0.03 and 1.2 VRHE for 3 min. to reduce the native oxide and to electrochemically form a thin layer of oxide, respectively. The formation of the thin oxide layer prior to the LSVs is required as otherwise the study of OER is disturbed by metal oxidation processes.

Oxygen evolution reaction

In both electrolytes metallic Ru has the smallest overpotential for OER, however, the dissolution rate is also the highest. It was even reported that in alkaline media no oxygen evolution occurs at these potentials on an Ru electrode, as no visual oxygen bubbles were observed, and the anodic current is dominated by dissolution [30]. Indeed, a dissolution efficiency of 100% was shown for concentrated bases in the potential range of 0.45–0.55 V vs. standard hydrogen electrode (SHE) [83]. The

Conclusions

Activity and stability of sputtered model Ru and Ir thin films, as well as the corresponding thermally oxidized RuO2 and IrO2 thin films, were evaluated in a SFC/ICP-MS based setup in the potential range of water electrolysis. It was found that both metals show higher activity for OER than their oxides, however, the dissolution is also ca. 2–3 orders of magnitude higher, respectively. Moreover, IrO2 was more stable than RuO2, with a difference in dissolution amounts of ca. 30 times under

Acknowledgements

The authors thank A. Mingers for experimental assistance and acknowledge the BMBF (Kz: 033RC1101A) for financial support. K.M. acknowledges financial support from the DFG under the project number MA4819/4-1. O.K. acknowledges financial support from the Alexander von Humboldt Foundation.

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