Uncovering the real active sites of ruthenium oxide for the carbon monoxide electro-oxidation reaction on platinum: The catalyst acts as a co-catalyst
Graphical abstract
Introduction
The electro-oxidation of CO on Pt-based electrocatalytic surfaces is a widely studied reaction [1], [2], [3], [4], [5], [6], [7], [8], owing to its paramount importance for the development of fuel cell systems. CO readily adsorbs on Pt, blocking the electroactive sites towards the relevant electrochemical reaction needed to operate the fuel cell. This is for instance the case at the anode of direct alcohol fuel cells (DAFCs) where CO can be produced as a reaction intermediate during the electro-oxidation of organic molecules (such as methanol or ethanol), hindering the performances of the devices [9], [10], [11]. Although the reactions occurring inside H2 fueled polymer electrolyte fuel cells (PEFCs) do not involve organic molecules, CO is present as trace amounts in the anode when the hydrogen gas is reformed from fossil fuels [12]. The CO-poisoning of the Pt surface lowers the apparent activity for the hydrogen oxidation reaction (HOR), leading to higher anode overpotential and thus to lower PEFC performances [13], [14], [15], [16].
Further understandings of the CO-tolerance of the electrocatalysts is thus important in order to improve the PEFC, either by lowering the adsorption (i.e. poisoning) rate of CO on Pt, or by enhancing the removal of CO adsorbed (COad) on Pt. The combination of Pt with another transition metal to form bimetallic nanoparticles (alloy or core-shell [17]) is a strategy extensively used to improve the CO-tolerance of the electrocatalyst [14]. Among others, the combinations of Fe [18], [19], Sn [20], [21], [22] and Ru [23], [24], [25], [26], [27] metal with Pt have been reported as beneficial for the electro-oxidation of CO, compared to bare Pt surface. Two main explanations are generally invoked to account for this synergetic effect: (i) a ligand effect [28], [29], [30] (electronic and strain effects) where the CO binding strength on Pt is weakened by the influence of the foreign atom, and (ii) a bifunctional effect [23], [25], [31], where the foreign atom, more oxophilic than Pt, provides oxygenated species to oxidize (and thus remove) CO adsorbed on Pt at lower potential than the COad removal potential on bare Pt. Although attractive, these bimetallic electrocatalysts suffer from (i) slow kinetics of the foreign metal for hydrogen electro-oxidation reaction (HOR) and (ii) their relatively low durability. The latter being problematical where the foreign atom, for instance Ru, is generally not stable upon incursions into high potential [14], [32], [33], [34], [35], which occur even at the anode of the fuel cells during start-up and shut-down operations.
An alternative strategy to the bimetallic electrocatalyst is the use of transition metal oxides in combination with Pt metal. Enhanced CO electro-oxidation on Pt has been reported when (hydrous) ruthenium oxide [36], [37], [38], [39], [40], [41], [42], [43], [44] was added as a co-catalyst. In addition to ruthenium oxide particles, it has been shown that hydrous ruthenium oxide nanosheets (chemically exfoliated from layered H0.2RuO2.1·nH2O [45] and noted as RuO2(ns) hereinafter) are also promising co-catalyst for the electro-oxidation of CO on Pt [36], [37] and the CO-tolerance of Pt during hydrogen electro-oxidation reaction (HOR) [43]. There are many advantages of RuO2(ns), for example, the hydrous and (hydr)oxide layers can provide oxygenated species to Pt. In addition, RuO2(ns) bear sufficient electrochemical stability upon potential cycling between H2 and O2 evolution potential regions [45]. Furthermore, the nanosheets are characterized by a high specific surface area (being monoatomic thick), thus the intimate contact between Pt and RuO2 per Ru mass is maximized. These characteristics make RuO2(ns) a well suited material for model electrocatalytic studies.
It has been suggested for practical PtRu bimetallic electrocatalysts that hydrous ruthenium oxide (expressed as RuO2·xH2O or RuOxHy in the literature), and not metallic Ru, is the co-catalyst which enables removing CO adsorbed on Pt at lower potential [46], [47], [48]. The similarities between the beneficial effects towards CO-tolerance of fully oxidized RuO2 and metallic Ru, two different types of material, are quite surprising. Other metal/metal oxide co-catalysts do not have these similarities, i.e. the metal oxides do not behave as effectively as their metal counterparts. This led us to a working hypothesis where the surface of ruthenium oxide can be partially electrochemically reduced on Pt, activating the co-catalytic effect for the removal of CO adsorbed on Pt.
In the present work, RuO2(ns) were deposited on the surface of polycrystalline Pt, glassy carbon (GC) and polycrystalline Au rotating disk electrodes (RDE). The synergetic effect of RuO2(ns) towards (monolayer adsorbed and bulk) CO electro-oxidation is investigated on RuO2(ns)/Pt electrode, while the effects of both the metal substrate and the experimental protocol conditions towards the electrochemical behavior of RuO2(ns) are investigated using RuO2(ns)/GC and RuO2(ns)/Au electrodes. The strategy behind these relatively simple model electrodes is to take advantage of both the high specific surface area and the high crystallinity of RuO2(ns). We expected that an almost full interaction between RuO2(ns) and the RDE surface (Pt, Au or GC) can be achieved with the deposition of the order of only one monolayer coverage RuO2(ns) on the RDE surface. Therefore, the signal ratio is expected to be maximized between the phenomena of interest (synergetic effect towards CO electro-oxidation, modification of the RuO2 by the metal substrate) to the background signal of the model electrode. Furthermore, the high crystallinity of RuO2(ns), leading to more defined voltammetric features than poorly crystalline hydrous ruthenium oxide nanoparticles, enables a more precise analysis of the synergetic effect of RuO2 towards CO electro-oxidation on Pt.
Section snippets
Experimental
Ruthenium oxide nanosheets (RuO2(ns)) were prepared from the exfoliation of layered K0.2RuO2.1·nH2O, as detailed in [45], [49]. Briefly, K0.2RuO2.1·nH2O was synthesized by solid state reaction between K2CO3 and RuO2 (5:8 molar ratio) at 850 °C for 12 h under argon atmosphere. Potassium ion was then exchanged to protons via acid treatment (immersion in 1 M HCl for 3 days). Exfoliation of the acid-treated H0.2RuO2.1·nH2O layered compound into [RuO2.1]0.2− nanosheets (noted as RuO2(ns) hereinafter)
Characterization of RuO2(ns)
The synthesis of ruthenium oxide nanosheets (RuO2(ns)) is confirmed by AFM (Fig. 1). As previously reported by our group, the nanosheets lateral size is of the order of hundreds of nanometers, while the average thickness is ca. 1 nm [45], [49], [51]. Although the crystallographic thickness of a monoatomic RuO2 nanosheet is approximately 0.5 nm [49], the higher average thickness of ca. 1 nm measured by AFM is attributed to adsorbed species on both faces of the nanosheets, such as water molecules or
Conclusions
Using model rotating disk electrodes composed of RuO2 nanosheets (RuO2(ns)) deposited on the surface of polycrystalline Pt, polycrystalline Au and glassy carbon, we uncover a new co-catalytic mechanism regarding the Pt/RuO2 interface towards the electro-oxidation of CO. Two distinct synergetic effects were identified, decreasing the bulk CO electro-oxidation onset potential compared to bare Pt by 50 mV (the ‘high potential’ synergetic effect at 0.85 V vs. RHE) and 250 mV (the ‘low potential’
Acknowledgments
This work was supported in part by the ‘Polymer Electrolyte Fuel Cell Program’ from the New Energy and Industrial Technology Development Organization (NEDO), Japan.
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