Low-Noble-Metal-Loading Hybrid Catalytic System for Oxygen Reduction Utilizing Reduced-Graphene-Oxide-Supported-Platinum Aligned with Carbon-Nanotube-Supported Iridium

Hybrid systems composed of the reduced graphene oxide-supported platinum and multiwall carbon nanotubes-supported iridium (both noble metals utilized at low loadings on the level of 15 and<5 microg cm-2, respectively) have been considered as catalytic materials for the reduction of oxygen in acid media (0.5 mol dm-3 H2SO4). The electrocatalytic activity toward reduction of oxygen and formation of hydrogen peroxide intermediate have been tested using rotating ring-disk electrode voltammetric experiments. The efficiency of the proposed catalytic systems has also been addressed by performing galvanodynamic measurements with gas diffusion electrode half-cell at 80 {\deg}C. The role of carbon nanotubes is to improve charge distribution at the electrocatalytic interface and facilitate the transport of oxygen and electrolyte in the catalytic systems by lowering the extent of reduced graphene oxide restacking during solvent evaporation. The diagnostic electrochemical experiments reveal that at iridium-containing systems not only higher disk currents, but also much smaller ring currents have been produced (compared to reduced graphene oxide-supported platinum and its composite with bare carbon nanotubes), clearly implying formation of lower amounts of the undesirable hydrogen peroxide intermediate. The enhancement effect coming from the addition of traces of iridium (supported onto carbon nanotubes) to Pt, utilized at low loading, may originate from the high ability of Ir to induce decomposition of the undesirable hydrogen peroxide intermediate. There is a competition between activation (due to the presence of small amounts of Ir) and dilution (by carbon nanotubes) of Pt active centers in hybrid systems, therefore special attention is paid to the adjustment of their composition.


Introduction
Despite appreciable progress in the development of proton exchange membrane fuel cells, their commercialization still requires the lowering of the costs and more durable materials.
The problem of electrochemical stability is crucially important for the cathode operating under oxidative conditions which include low pH, high potentials (especially during startup/shut-down operations) and the generation of reactive oxygen species from hydrogen peroxide (the intermediate product of oxygen reduction) [1]. The latter issue may become even more serious when lower amount of expensive Pt catalyst is utilized, which on the one hand enables to reduce the costs of devices but on the other hand results in production of higher quantity of undesirable hydrogen peroxide (due to dilution of active centers). Further, high potential of the cathode may cause both the oxidative degradation of carbon support and the dissolution or sintering of platinum nanoparticles. Additionally, it is well recognized that platinum accelerates the corrosion rate of the carbon, what is particularly evident in a case of commonly used carbon blacks [2]. Therefore, there is need for searching alternative more corrosion-resistant carriers. In this regard, highly graphitized carbon materials like multiwalled carbon nanotubes (MWCNT) or more recently graphene-related materials are considered as promising alternatives for carbon blacks [3,4]. From the family of graphenerelated materials the most perspective and the most widely used (due to facility of manufacturing and relatively low costs) are graphene oxide (GO) and reduced graphene oxide (rGO). The presence of surface oxygen-containing groups onto corrugated graphene layers facilitates their exfoliation and excellent dispersion of metal nanoparticles with narrow range of sizes what should allow for high utilization of the catalyst. But at the same time, large population of oxygen functionalities decrease the electronic conductivity of the support which, together with its restacking during slow evaporation of solvents, cause charge and mass (oxygen and water) transport limitations in catalytic layer [5]. One has to be aware that in the catalytic layer for oxygen electroreduction, where the reaction proceeds at three-phase boundary and under harsh conditions, the compromise in many planes should be assured. It has been proven for instance that for rGO-supported Pt the graphitization level and the content of oxygenated functionalities should be optimized not only in terms of proper electronic conductivity and stability of the rGO itself, but also to ensure the balance between hydrophobic and hydrophilic properties which in turn influence the dispersion of catalytic centers, the strength of their binding and the gas/water transport within catalytic material [6].
Further, several studies showed that the hybrid systems of two dimensional graphene-based materials with carbon blacks or carbon nanotubes (acting as separators between graphene sheets) allow the preparation of highly porous three dimensional structures with substantially improved mass transport and durability [7][8][9][10][11][12][13][14][15]. What is more, even an expansion of the dspacing of the graphene layers in graphene nanosheets after assembling with carbon nanotubes or fullerene C60 has been observed [16]. Such composites, when applied as supports for platinum nanoparticles, have considerably increased the utilization of metallic centers during oxygen electroreduction process and the stability of the catalysts [11][12][13][14][15].
There have been many attempts of application of iridium and iridium(IV) oxide as the components of cathodic materials in polymer electrolyte membrane fuel cells (PEMFCs).
Although their intrinsic activity is far away from the platinum-based catalysts, they have been effectively utilized as additives to platinum [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. In this regard, depending on the route of coupling of Ir-based systems with Pt, different effects and mechanisms of their functioning were elucidated. In a case of metallic iridium additive, better efficiency of unsupported Pt-Ir alloys [18] or Pt-Ir nanoporous structures [19] than of Pt was attributed to the increase of the Pt 5d vacancy resulting in increased O 2 adsorption and weakening the O-O double bond. It was also observed that Ir(core)@Pt(shell) nanodendrites [20] and mixed platinum-iridium monolayers deposited on the surfaces of Pd(111) single crystals or carbon-supported Pd nanoparticles [21] owe their higher activity (compared to that of pure Pt counterparts) to lowering the coverage of Pt with OH as a result of lateral repulsion between the OH or O adsorbed on Ir and the OH adsorbed on a neighboring Pt atom. Similar activating effects (relative to Pt) were observed for PtIr dendritic tripods [22], PtIr sputtered thin films [23,24] and thin films of Pt deposited on Ir by thermal evaporation in vacuum [25]. It has also been reported more recently that PtIr alloy deposited by spray pyrolysis onto three dimensional crumpled rGO exhibited even higher activity than a commercially available Pt/C catalyst [26].
On the other hand, on the basis of the experiments conducted with carbon-supported coreshell structures (with Pt monolayer shell) and DFT calculations, it has been revealed that sole Ir core cause too strong contraction in Pt-Pt bonding and thus too weak binding energy of oxygen to Pt to dissociate it, therefore lower activity than Pt [27]. Also Popov et al. [28] reported decreased ORR efficiency of physical mixture of Pt/TiO 2 and Ir/TiO 2 in relation to pure Pt/TiO 2 . It is worth mentioning, that none of described above research have discussed the possible changes in ORR mechanism on Pt due to its coupling with Ir. It has been shown that Ir nanostructures, despite of lower efficiency (in terms of less positive potentials), are more selective in the 4-electron ORR pathway with respect to the Pt [29,37]. It was observed that the addition of Ir to Pt promote the selectivity of the ORR in the 4-electron mechanism, which was ascribed to higher population of Pt active sites free of oxygen-based adsorbates [29].
Concerning IrO 2 utilization, there is general agreement in the literature that, regardless of the preparation method (physical mixture of Pt and IrO 2 or Pt nanoparticles deposited onto oxide support), the platinum activity decreases toward oxygen reduction in relation to unmodified Pt [30][31][32][33][34][35][36]. What is more, it has been shown that synthetized system of Pt/IrO 2 was less efficient than the physical mixture of Pt and IrO 2 blacks, what was ascribed to stronger adsorption of oxides on the platinum surface in the former case [31].
Finally, it is also well documented that the addition of Ir or IrO 2 to the cathodes of PEMFCs may considerably improve their durability [17,22,25,26,28,[32][33][34][35][36]38], wherein the stability of IrO 2 was reported to be higher than metallic iridium [39]. It was proposed that these highly efficient oxygen evolution reaction catalysts suppress the electrochemical carbon corrosion due to decomposition of water around the carbon supports at high potentials [34][35][36].
In the present contribution, we explore the hybrid systems composed of the reduced graphene oxide-supported platinum and multiwall carbon nanotubes-supported iridium (both noble metals utilized at low loadings on the level of 15 and ≤ 5 µg cm -2 under RRDE studies, respectively) as catalytic materials for the reduction of oxygen in acid medium (0.5 mol dm -3 H 2 SO 4 ). The role of carbon nanotubes is to improve charge distribution at the electrocatalytic interface and facilitate the transport of oxygen and electrolyte in the catalytic systems by lowering the extent of reduced graphene oxide restacking during solvent evaporation. Another important issue is the ability of the proposed carrier (carbon nanotubes decorated with Ir) to act as the efficient system of decomposition of the hydrogen peroxide intermediate, generated when Pt catalyst is utilized at low loading. The diagnostic electrochemical experiments reveal that at iridium-containing systems not only higher disk currents, but also much smaller ring currents have been produced (compared to reduced graphene oxide-supported platinum and its composite with bare carbon nanotubes), clearly implying formation of lower amounts of the undesirable hydrogen peroxide intermediate. Due to the competition between activation (by small amounts of Ir) and dilution (by carbon nanotubes) of Pt active centers in hybrid systems, special attention is paid to the adjustment of their composition.
Nitrogen and oxygen gases (purity, 99.999%) were from Air Products (Poland). The solutions for electrochemical experiments were prepared from doubly-distilled and subsequently deionized (Millipore Milli-Q) water. Multi-walled carbon nanotubes (CNT) were activated in 2 mol dm -3 HNO 3 under reflux for 2 h and washed with water until pH was near 6.

Materials preparation
Graphene oxide (GO) was prepared using a modified Hummer's method from commercial graphite powder (ACROS ORGANICS). In this process, 10 g of graphite powder was added to 230 ml of concentrated sulfuric acid (98 wt.%) and stirred for 30 minutes. Next 4.7 g of sodium nitrate and 30 g of potassium permanganate were slowly added to the mixture and the temperature was kept below 10 ºC in an ice bath. Then the mixture was slowly heated up to ~33 ºC and was controlled so as not to exceed 35 ºC for 2 hours under stirring. In the next step, 100 ml of water was added to the mixture and the temperature reached ~120 ºC. Finally, the mixture was treated with 10 ml of H 2 O 2 (30 wt.%). Obtained slurry was kept in an ultrasonic bath for 1 hour. For purification, the slurry was filtered using ceramic membranes with 0.2 micron pore size and washed with deionized water in order to remove the byproducts of the synthesis till the pH of the filtrate reached 6.5.
Reduced graphene oxide (rGO) was obtained in the course of the hydrazine reduction method in an analogous manner as described in ref [40]. In brief, 10 ml of 50 % hydrazine water solution was added to 100 ml of 0.5 wt.% GO water dispersion. The mixture was heated up to 100 ºC and kept under stirring for 2 h. After reduction, the product was filtered using polyethersulfone (PES) filter with 0.8 µm pore size.
Reduced graphene oxide-supported platinum (Pt/rGO) catalyst (with final Pt loading at the level of 20% wt.) was synthetized in the course of the borohydride reduction method as described in ref [41]. Briefly, a proper amount of the Pt precursor (K 2 PtCl 6 ) was added to 0. Before reflux, the suspension of CNT and solution of IrCl 3 were subjected to sonication for 1 h. After reflux the sample was washed out with water by centrifugation (3-4 times) and dried on a hot plate at 50 °C.

Equipment and characterization of materials
Electrochemical measurements were performed using CH Instruments (Austin, TX, USA) 920D workstation. A glassy carbon rod served as a counter electrode and the reference electrode was a mercury/mercurous sulfate electrode, Hg/Hg 2 SO 4 (all potentials were recalculated and expressed versus the reversible hydrogen electrode (RHE) scale). The rotating ring-disk electrode (RRDE) voltammetric experiments were conducted via variable speed rotator (Pine Instruments, USA). RRDE assembly included a glassy carbon disk (diameter 5.61 mm) and a platinum ring (inner and outer diameters were 6.25 and 7.92 mm, respectively) and its collection efficiency (determined with the procedure reported previously [37]) was equal to 0.39. Prior the experiments the working electrode was polished with aqueous alumina slurries (grain size 0.05 μm) on a Buehler polishing cloth.
The inks for fabricating the electrocatalytic layers for thin-film RRDE studies (with constant platinum loading of 15 μg cm −2 ) were prepared by sonicating and mixing at magnetic stirrer of 5 mg of catalysts (or hybrid systems), 1 ml of 2-propanol and 10 -30 %ww of Nafion®.
Due to different densities of carbon supports it was necessary to optimize the content of Nafion in each case (in terms of the highest possible homogeneity of the layers and obtaining proper ionic conductivity and mass transport). Therefore, for rGO-containing samples its optimum content was 10%, for CNT -30% and for the mixtures it proportionally depended on the ratio of rGO and CNT. The inks were introduced on the GC electrode surface and, subsequently, dried at room temperature, 22±2 °C. The electrodes, covered with the catalytic layers, were washed out with the stream of water and subjected to potential cycling in the range between 0.05 V and 0.8 V (vs RHE) at the 100 mV s −1 in the N 2 -saturated 0.1 mol dm −3 0.5 mol dm -3 H 2 SO 4 until a stable stationary voltammetric responses were obtained. Before electrochemical examination, the hybrid systems were produced by subjecting Pt/rGO to mixing (sonicated and mixed at magnetic stirrer) with Ir/CNT or CNT. The mass ratio of Pt/rGO to Ir/CNT was 2.5 : 1 , 2 : 1 and 1.5 : 1. For comparative studies, a sample of Pt/rGO mixed in the mass ratio of 2 : 0.9 with CNT was prepared (the amount of CNTs was the same as in the sample Pt/rGO+Ir/CNT 2 : 1).
The performance of examined catalysts was also tested at 80 °C with a gas diffusion electrode (GDE, geometric area of active part, 3 cm 2 ) mounted into Teflon Flex Cell (Gaskatel GmbH, Germany). A spiral PtIr wire and Hg/Hg 2 SO 4 electrode (the latter placed in a salt bridge) served as counter and reference electrodes, respectively. The gas diffusion backing layer for electrocatalysts was a carbon cloth with microporous layer (W1S1005, Fuel Cell Store, TX, U.S.A.). Catalytic layers were brush painted from the inks (using vacuum table heated up to 80 ºC), in which the ratio of catalyst to Nafion® was the same as in the course of RRDE thin film studies, but the level of dilution was higher (1 mg catalyst per 1 ml of 2-propanol).

Results and discussion
The Pt/rGO, Ir/CNT and the hybrid system of Pt/rGO+Ir/CNT (with 2 : 1 mass ratio) were examined using transmission and scanning electron microscopies. It is apparent from Fig. 1A that the diameters of platinum nanoparticles are on the level of 7-8 nm, but except the areas with homogenous distribution of "single" particles, there are also regions where their agglomerates can be found (inset to Fig. 1A and Fig. 1B).   [25,38]. The peak positions for both: metallic and oxidized iridium are consistent with other literature findings [44]. The relative intensity between metallic Ir and oxidized Ir components (Ir metal /Ir oxide = 2.51) points that only a little bit more than 1/3 of the overall iridium amount was oxidized. This is highly probable, since the iridium creates agglomerates which are oxidized only at their surface leaving the core area unaltered in its metallic form.
The iridium is known from its high affinity to oxygen species, therefore the results are consistent with the view that small nanoparticles (as evident form TEM and XRD analyses) are prone to surface oxidation under exposure to air. As a confirmation to above statement the inset to Fig. 3A presents the Ir 4f region for control sample containing 2% of Ir (prepared in the same manner as the one with 10% of Ir). The decomposition revealed, according to peak binding energy position, that the whole amount of the iridium is present in its oxidized form [44]. Since from additional TEM experiments (data not shown) we could observe that for the 2% sample the Ir agglomerates were of significantly smaller dimensions, we assume that the whole volume of iridium was penetrated by oxidizing agents.   For comparative purposes also the results for the Ir-free system, namely Pt/rGO admixed with CNT at 2:0.9 ratio (curve e), are presented in Fig. 5. It is evident that the disk current densities have increased in a case of the system containing carbon nanotubes Pt/rGO+CNT (2:0.9) (curve e), when compared to the bare Pt/rGO catalyst (curve a). This may imply that the CNT not only improve charge distribution at the electrocatalytic interface, but also facilitate the transport of oxygen and electrolyte in the layer by lowering the extent of rGO restacking during slow solvent evaporation. Meanwhile, the ring currents have also increased to some extent (especially at less positive potentials), therefore it is evident that the presence of CNT slightly alters the mechanism of the process, probably due to "dilution" of Pt catalytic centers.
On the other hand, at iridium-containing systems (curves b-d) not only higher disk currents (Fig. 5A), but also much smaller ring currents (Fig. 5B) have been produced (compared to bare Pt/rGO and Pt/rGO+CNT) clearly implying formation of lower amounts of the undesirable H 2 O 2 intermediate. Finally, it is apparent from Fig. 5A that the current densities of oxygen reduction reach maximum values for the hybrid system of Pt/rGO+Ir/CNT(2:1) (curve c) and further increase of Ir/CNT additive (like in a case of Pt/rGO+Ir/CNT(1.5:1), curve d) does not improve the current densities. It is reasonable to expect that there is a competition between activation and dilution of Pt active centers in hybrid systems, therefore their composition should be adjusted with caution.
To estimate the percentage of H 2 O 2 (X H2O2 ) produced during oxygen reduction at catalytic films and number of electrons (n e ) participating in the electrocatalytic reaction, we have used the equations given below [50][51][52]: In the equations (1) and (2) I R is ring current, I D stands for disk current and N is the collection efficiency of the RRDE assembly. It becomes apparent from Figs 6A and B, where X H2O2 and n e , both plotted versus potential applied to the disk electrode are presented, that the selectivity of the process toward 4-electron reduction (with formation of water as the major product) is increased in the presence of Ir additive.
When compared to platinum-based materials, iridium nanostructures are considered as relatively weak catalyst toward oxygen reduction [29,37], while their high activity in the electrochemical/chemical decomposition of hydrogen peroxide has been postulated [37,53].

Conclusions
We demonstrate here that the addition of carbon nanotubes decorated with iridium nanoparticles results in the enhancement of electrocatalytic activity of graphene oxide-