Reduced graphene oxide as efficient carbon support for http://dx.doi.org/10.5599/jese.1643 771 J. Electrochem. Sci. Eng. 13(5) (2023) 771-782; http://dx.doi.org/10.5599/jese.1643 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper Reduced graphene oxide as efficient carbon support for Pd-based ethanol oxidation catalysts in alkaline media Sigrid Wolf1,, Michaela Roschger1, Boštjan Genorio2, Nejc Hodnik3, Matija Gatalo3, Francisco Ruiz-Zepeda3 and Viktor Hacker1 1Institute of Chemical Engineering and Environmental Technology, Graz University of Technology, Inffeldgasse 25/C, 8010 Graz, Austria 2Faculty of Chemistry and Chemical Technology, University of Ljubljana, Večna pot 113, 1000 Ljubljana, Slovenia 3Department of Materials Chemistry, National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia Corresponding author: sigrid.wolf@tugraz.at Received: December 21, 2022; Accepted: January 24, 2023; Published: February 8, 2023 Abstract The sluggish kinetics of the ethanol oxidation reaction (EOR) and the related development of low-cost, highly active and stable anode catalysts still remains the major challenge in alkaline direct ethanol fuel cells (ADEFCs). In this respect, we synthesized a PdNiBi nanocatalyst on reduced graphene oxide (rGO) via a facile synthesis method. The prepared composite catalyst was physicochemically characterized by SEM, STEM, EDX, ICP-OES and XRD to analyze the morphology, particle distribution and size, elemental composition and structure. The electrochemical activity and stability towards EOR in alkaline media were examined using the thin-film rotating disk electrode technique. The results reveal well-dispersed and strongly anchored nanoparticles on the rGO support, providing abundant active sites. The PdNiBi/rGO presents a higher EOR activity and stability compared to a commercial Pd/C ascribed to a high ECSA and synergistic effects between Pd, Ni and Bi and the rGO material. These findings suggest PdNiBi/rGO as a promising anode catalyst in ADEFC applications. Keywords Synergistic effects; electrochemical active surface area; rotating disk electrode; alkaline direct ethanol fuel cell Introduction Fuel cells have emerged as one of the most promising renewable energy technologies of the future, as they convert chemical energy directly into electrical energy [1,2]. Alkaline direct ethanol fuel cells (ADEFCs) have gained significance among the various fuel cell types, especially for portable and transportation applications, as they have a high theoretical energy density (8 kWh kg-1), they http://dx.doi.org/10.5599/jese.1643 http://dx.doi.org/10.5599/jese.1643 http://www.jese-online.org/ mailto:sigrid.wolf@tugraz.at J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 PDNIBI/RGO AS EOR CATALYST IN ALKALINE MEDIA 772 are environmentally friendly, and ethanol can be easily handled compared to other fuels such as hydrogen [3,4]. Ethanol has proven significant advantages over methanol as it can be produced from agricultural products, is non-toxic and features a lower crossover from the anode to the catho- de [4,5]. However, one of the major problems of ADEFCs remains the development of cost-efficient, highly active and stable electrocatalysts for the ethanol oxidation reaction (EOR) at the anode. Pd- based catalysts are currently reported to be most suitable and show better EOR performance in alkaline media than their Pt counterparts [2,6]. The C-C bond of ethanol is difficult to break and the EOR does not proceed via a complete oxidation (equation (1)) with the formation of CO2 and 12 e-, but via incomplete oxidation to acetate and 4 e- (equation (2)) [1,4]. Complete oxidation: CH3CH2OH + 12 OH- → 2 CO2 + 9 H2O + 12 e- (1) Incomplete oxidation: CH3CH2OH + 5 OH- → CH3COO- + 4 H2O + 4 e- (2) To overcome this problem, alloying Pd with oxophilic co-catalyst elements such as Ni, Bi, Cd or Co is very effective for increasing the EOR activity and stability due to enhanced -OH- adsorption while reducing costs [7-11]. Deposition of the metal nanoparticles on a suitable support material offers another possibility to increase the efficiency of such catalysts, as it features the capability to control the size, distribution, morphology and stability of the immobilized particles. Carbon powders such as carbon black, carbon nanotubes or graphene are commonly used as support [4,8,11]. Among these carbon materials, reduced graphene oxide (rGO) has proven to be particularly suitable for this purpose, as its unique sp2-hybridized carbon structure provides properties such as a large specific surface area (SSA), high thermal and chemical stability, and excellent electronic conductivity. Usually rGO still retains important oxygen-containing functionalities (e.g. epoxy, hydroxyl, and carboxyl groups), causing strong C-O-metal bridges that enhance the stability and prevent agglomeration of the nanoparticles [4,6,8,12]. Krishna et al. [13] have described, for example, that a Pd@NixB/RGO nanocomposite exhibited improved catalytic activity and stability compared with the unsupported nanoparticles. Another study by Alfi et al. [9] also showed that a Pd-Cd/rGO catalyst outperforms the commercial Pd/C in terms of EOR performance. A combination of adding oxophilic co-catalysts to the Pd active material and the deposition on a suitable support material such as rGO therefore appears to be a promising approach for the development of highly active and stable EOR catalysts. Based on the above findings, in this work we synthesized a composite catalyst of PdNiBi nano- particles on rGO for the first time and investigated the electrochemical activity and stability towards EOR in alkaline media. The PdNiBi/rGO catalyst was comprehensively characterized by various physicochemical techniques, which confirmed the successful deposition of the metal particles on the rGO support. Finally, the electrochemical rotating disk electrode (RDE) experiments revealed a promising catalytic EOR performance of the prepared nanocomposite attributed to synergistic effects between Pd, Ni and Bi and the rGO support material. Experimental Materials The following chemicals were used for material preparation and electrochemical tests without further purification: graphene oxide (GO) precursor (Timrex KS44) was acquired from Imerys (Bodio, Switzerland), sodium borohydride (NaBH4, 97 %) was delivered by Alfa Aesar (Haverhill, MA, USA) and sodium hydroxide (NaOH, ≥98 %, ACS, pellets) was purchased from Honeywell Fluka (Charlotte, NC, USA). Hydrazine hydrate (N2H4H2O, reagent grade), palladium chloride (PdCl2, anhydrous, 59-60 % Pd basis), nickel(II) nitrate hexahydrate (Ni(NO3)26H2O, 99 % trace metal basis), bismuth(III) chloride S. Wolf et al. J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 773 (BiCl3, reagent grade, ≥98 %) and potassium hydroxide (KOH, 1.0 M Fixanal 1 L Ampoule) were purchased from Sigma-Aldrich (Darmstadt, Germany). Hydrochloric acid (HCl, ROTIPURAN®37 % fuming, p.a., ACS, ISO), ethanol (EtOH, 99.9 % p.a.) and isopropyl alcohol (2-propanol, ≥99.9 %, UV/IR-grade) were supplied by Carl Roth (Karlsruhe, Germany). NafionTM Solution (5 wt.% in H2O) from Quintech (Göppingen, Germany) and an alumina suspension (Al2O3, 0.05 µm particle size) from MasterPrep® Bühler (Lake Bluff, IL, USA) were used. All solutions were prepared with ultrapure water (Barnstead NANOpureWater Purification System, 18 MΩ cm). A commercial Pd/C (40 wt.%) from Fuel Cell Store (College Station, TX, USA) was used as a benchmark for the electrochemical results. Catalyst preparation The preparation of rGO was carried out using a previously published chemical reduction method [5,14]. First, the GO precursor and ultrapure water were mixed in a 2 L-round bottom flask. The dispersion was slowly heated to 100 °C under reflux and stirred at 550 rpm using an oil bath and a PTFE magnetic stir bar. Hydrazine hydrate was then slowly added to perform the reduction process at 105 °C for 24 h. After successful reduction, indicated by a color change from brown to black, filtration of the hot reaction mixture and intensive washing with hot ultrapure water and ethanol was performed. The rGO was obtained by drying at air (24 h) and subsequently under vacuum at 80 °C (overnight) and was used as catalyst support material without further purification. The PdNiBi/rGO (40 wt.% of metal and 60 wt.% of carbon support) composite catalyst was synthesized according to the modified instant reduction method [7,15]. The as-prepared rGO was dispersed in ultrapure water under a nitrogen atmosphere and ice cooling using an ultrasonic probe (Hilscher, UP440s). Afterwards, the following three metal precursor salt solutions were added to the rGO dispersion: PdCl2 and Ni(NO3)26H2O were both mixed with ultrapure water and 1 M HCl and BiCl3 was dissolved in ultrapure water and a few drops of HCl (concentrated). The pH was then adjusted to 10, an aqueous solution of NaBH4 (in ultrapure water and 1 M NaOH) was slowly added and the reduction was carried out at 60 °C for 4 h. Finally, the precipitate was filtered, washed with ultrapure water and dried at 40 °C for 24 h. Characterization The morphology and particle size of the metal nanoparticles were studied in a probe Cs-corrected Jeol ARM 200 CF scanning transmission electron microscope (STEM) system equipped with an SDD Jeol Centuria Energy-dispersive X-ray (EDX) spectrometer at 80 kV. For the sample preparation, the catalyst was dispersed in ethanol, dropped on a Cu-grid and dried. Scanning electron microscopy (SEM) was carried out on a Zeiss Ultra+ field emission scanning electron microscope at 1 kV with a secondary electron detector to further analyze the catalyst morphology. A conductive carbon tape on an Al stub served as a sample holder. EDX spectroscopy was performed at 7 kV within the SEM using an SDD X-MAX 50 EDX spectrometer to determine the elemental composition. Metal concen- tration was additionally examined by inductively coupled plasma optical emission spectrometry (ICP-OES) on an Acros SOP system by Spectro using detection wavelengths of 223.061 (Bi), 231.604 (Ni) and 340.458 (Pd). The sample preparation was as follows: microwave-assisted pressurized acid digestion on a Multiwave 3000 of the sample (10 mg) in 7 mL of concentrated HNO3, 0.2 mL HClO4 and 0.2 mL HF (40 %) was done at 195 °C and 1500 W for 25 min. The crystal structure of the com- posite catalyst was characterized by X-ray diffractometry (XRD). The measurement was conducted on a PANalytical X’Pert PRO MPD diffractometer with a fully opened X’Celerator detector using a Cu Kα1 radiation (λ = 0.15406 nm) within a 2 range from 20 to 60° and a step size of 0.034° step per 100 s. The sample was placed on a zero-background Si holder. http://dx.doi.org/10.5599/jese.1643 J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 PDNIBI/RGO AS EOR CATALYST IN ALKALINE MEDIA 774 Electrochemical measurements Electrochemical characterization of the Pd-based composite catalysts was performed in a three- electrode cell by using a rotating disk electrode (RDE) and a workstation from Pine Research Instru- mentation. A glassy carbon (GC) disk electrode (AFE5T0GC) coated with a thin film of the catalyst material, a platinized titanium rod (Bank Elektronik - Intelligent Controls GmbH) and a reversible hydrogen electrode (RHE, HydroFlex®, gaskatel) were used as working, counter and reference electrode, respectively. The GC disk was polished with an Al2O3 suspension (0.5 µm) and rinsed with ultrapure water before the catalyst layer was applied. 10 µL of an ultrasonically dispersed (30 min) ink containing 8.1 mg catalyst powder, 1.75 mL 2-propanol, 0.737 mL ultrapure water and 13 µL of a Nafion solu- tion (5 wt.%) were pipetted onto the GC disk (0.196 cm-2) and dried at 700 rpm for 1 h to form a thin-film with a final Pd loading of 56 µg cm-2. Measurements were carried out using a Reference 600TM potentiostat/galvanostat/ZRA and software from GAMRY Instruments was utilized for data analysis. A constant temperature of 30 °C and de-aerated (N2-purging for 30 min) electrolyte solution was used for all experiments. First, cyclic voltammograms (CVs) in 1 M KOH at 50 mV s-1 (until a reproducible CV curve was obtained) for cleaning purpose and at 10 mV s-1 for the analysis of the redox processes were recorded in a potential range from 0.05 to 1.50 V vs. RHE. Then, CVs at a scan rate of 10 mV s-1 were conducted from 0.05 to 1.20 V vs. RHE in 1 M KOH (base CVs) and a mixture of 1 M KOH / 1 M EtOH to evaluate the electrochemical active surface area (ECSA) and the EOR activity of the Pd-based nanocatalysts, respectively. For each measurement (except cleaning CVs) three cycles were performed and the third was used for investigation. The CVs for the EOR were corrected with the base CVs to exclude the redox processes not linked to EOR. Chronoamperometry (CA) tests were carried out in the ethanol-containing electrolyte solution (1 M KOH / 1 M EtOH) at a potential of 0.83 V vs. RHE for 3600 s to examine the catalytic EOR stability. Results and discussion Structural, morphological and elemental characteristics of PdNiBi/rGO Comprehensive physicochemical characterization of the prepared PdNiBi/rGO composite was performed to analyze the morphology, particle size and distribution of the nanoparticles, the ele- mental composition and the crystal structure. Figure 1 shows the STEM, SEM and EDX elemental mapping images of the catalyst. The STEM images in Figure 1a-c and 1e-g indicate metal nanoparticles anchored on the rGO sheets and the SEM image of PdNiBi/rGO (Figure 1d) evidences the two-dimensional wrinkled sheet morphology of the sample [2,6,9]. The particles are uniformly and well dispersed and only a few agglomerates are noticed. The remaining oxygen-containing functional groups, such as hydroxyl (-OH) of the rGO support, are crucial for holding the metal particles by strong C-O-M bridges and act as a capping agent to prevent strong agglomeration and ensure the formation of very small particles [13]. Therefore, a large specific surface area and abundant active sites are provided. The particle size distribution (PSD) is shown in the inset of Figure 1f. The statistical analysis results in an average particle diameter of 2.6 nm. From the EDX elemental mapping (Figure 1h), the presence of Pd, Ni and Bi and the homogeneous distribution of the elements can be proved. The elemental composition of PdNiBi/rGO determined by EDX and ICP-MS is given in Table 1. S. Wolf et al. J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 775 Figure 1. (a,b,c,e,f,g) BF and ADF STEM (with PSD inset), (d) SEM and (h) EDX elemental mapping images of PdNiBi/rGO Table 1. Elemental composition of PdNiBi/rGO derived by EDX or ICP-OES compared with calculated values Material Content, wt.% C O Pd Ni Bi Calculated 60.0 - 33.9 2.2 3.9 EDX 46.3 12.0 34.5 2.5 3.2 ICP-OES - - 27.7 2.0 3.7 The ratio between support and the active metal material is approx. 60:40. It can also be observed that the support material is composed of C (46.3 wt.%) and O (12.0 wt.%), which confirms the presence of the oxygen-containing functional groups within the rGO. The active material (40 wt.%) is composed of Pd, Ni and Bi and is well comparable with the calculated values. This results in an atomic proportion of approx. 85 wt.% Pd, 10 wt.% Ni and 5 wt.% Bi (calculated from the active material only). The crystal structure of the PdNiBi/rGO composite was analyzed by XRD. The XRD pattern of the prepared material and comparison with graphite (ICSD #18838) and Pd (ICSD #64922) standard data is shown in Figure 2. Figure 2. XRD pattern of PdNiBi/rGO with Pd and graphite ICSD standard data http://dx.doi.org/10.5599/jese.1643 J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 PDNIBI/RGO AS EOR CATALYST IN ALKALINE MEDIA 776 The broad peak at a 2 value of 25.5° can be related to the rGO support material and belongs to the (002) plane of graphite sp2 carbon structure, signifying the successful synthesis of rGO [8,9,13,16]. The peaks at 2 values of 39.7 and 43.9° indexed to the (111) and (200) facets, respectively, can be assigned to the face-centered cubic crystalline structure of Pd [8,11]. The shift to lower angles is caused by lattice expansion due to nanosized particles and alloying with Ni and Bi [7,13,15]. The nanocrystalline nature is further indicated by the broad peak shape. The crystallite size of the nanoparticles can be estimated by using the Scherrer equation: D = kλ b cos (3) where D is the crystallite size (nm), k is the shape factor for spherical particles (0.9), λ refers to the X-ray wavelength (0.154 nm), b is the full width at half maximum of the (111) plane and is the half angle [16,17]. The estimated crystallite size was calculated to be approx. 3.8 nm and is in the same range as observed from the TEM analysis. Electrochemical performance of the EOR catalysts The electrochemical properties of the PdNiBi/rGO catalyst were investigated by CV and CA measurements and compared with a commercial Pd/C catalyst. The most important results, such as the ECSA and the parameters for the EOR activity and stability, are listed in Table 2. Table 2. Electrochemical results of PdNiBi/rGO compared with a commercial Pd/C Catalyst ECSAa, cm2 mg-1 Eonsetb / V vs. RHE jfc / mA mg-1 jbc / mA mg-1 jdd / % PdNiBi/rGO 455.4 0.234 2390 1942 85 Pd/C 410 0.266 2018 2150 87 aElectrochemical active surface area; bonset potential of the ethanol oxidation at 0.02 mA; cpeak current density of forward and backward scan; dcurrent density decrease after 3600 s. CVs in de-aerated 1 M KOH electrolyte solution at 10 mV s-1 were recorded in a potential range of 0.05 to 1.50 V vs. RHE and 0.05 to 1.20 V vs. RHE to investigate the oxidation and reduction processes of the materials and to determine the ECSA. The voltammograms are shown in Figure 3. Figure 3. CV curves in N2-saturated 1 M KOH electrolyte solution at a scan rate of 10 mV s-1 of (a) PdNiBi/rGO in a potential range of 0.05 - 1.50 V vs. RHE and (b) PdNiBi/rGO and a commercial Pd/C in a potential range of 0.05 - 1.20 V vs. RHE with ECSA evaluation (horizontal segments indicate region for reduction charge determination) The CVs of PdNiBi/rGO present that oxidation of Bi to Bi2O3 in alkaline media is observed in the anodic scan at approx. 0.9 V vs. RHE [18], whereas the oxidation of Pd to PdO is a broad peak, which S. Wolf et al. J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 777 is not clearly visible due to overlapping [11]. The peaks between 1.2 to 1.5 V vs. RHE describe the oxidation of Ni(OH)2 to NiOOH and the corresponding reduction of NiOOH to Ni(OH)2 (Figure 3a) [19]. The peaks in the cathodic scan between 0.9 V and 0.6 V vs. RHE are associated with Bi oxide reduction and the reduction of PdO to Pd [7]. A comparison with the CV (Figure 3b) of the commercial Pd/C shows that the hydrogen ad/absorption peaks for the PdNiBi/rGO are suppressed, which can be attributed to the presence of Bi [18,20,21]. Consequently, the ECSA was estimated from the PdO reduction peak by using equation (4): ECSA= QPd QPd * ( 1 cL ) ( 1 AGC ) (4) where QPd is the determined reduction charge of the PdO to Pd (received using Echem AnalystTM Software-Gamry for baseline correction and integration of the region indicated by horizontal segments), Q*Pd is the theoretical PdO to Pd reduction charge (405 µC cm-2), cL is the catalyst loading (56 µgPd cm-2) and AGC is the GC electrode area (0.196 cm-2) [22-24]. PdNiBi/rGO shows a large ECSA of 455 cm2 mg-1 due to good distribution and small particle size (determined by STEM) as well as the overall high surface area provided by the rGO support [11]. The commercial Pd/C catalyst exhibits an ECSA of 410 cm2 mg-1. A high ECSA is very important for ensuring high catalytic activity, as it ensures abundant active sites for the ethanol oxidation reaction [25]. The activity of the PdNiBi/rGO catalyst towards EOR was investigated in a mixture of de-aerated 1 M KOH and 1 M EtOH electrolyte solution at a scan rate of 10 mV s-1 in a potential range of 0.05 to 1.20 V vs. RHE and was compared with the commercial Pd/C catalyst. The CV curves in Figure 4 show two characteristic oxidation peaks, one in the forward and one in the backward scan. Figure 4. CV curves of PdNiBi/rGO in N2-saturated 1 M KOH electrolyte solution containing 1 M EtOH at a scan rate of 10 mV s-1 compared with the commercial Pd/C The forward peak is attributed to the primary oxidation of ethanol according to the generally accepted EOR reaction mechanism of Pd-based catalysts in alkaline media (equations (5) to (8)) as described by Zhang et al. [19] and several other studies [2,26-28]: Pd + CH3CH2OH ⇆ Pd – (CH3CH2OH)ads (5) Pd – (CH3CH2OH)ads + 3OH- ⇆ Pd – (CH3CO)ads + 3H2O + 3e- (6) Pd – (CH3CO)ads + Pd – OHads ⇆ Pd – CH3COOH + Pd (7) Pd – CH3COOH + OH- ⇆ Pd + CH3COO- + H2O (8) http://dx.doi.org/10.5599/jese.1643 J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 PDNIBI/RGO AS EOR CATALYST IN ALKALINE MEDIA 778 In the EOR process, ethanol adsorption and further dissociation (equations (5) and (6)) proceed rapidly in the lower potential region of the anodic scan and the carbonaceous intermediates such as CH3COads will strongly adsorb on the Pd surface. As Pd begins to adsorb OH- with increased potential, the oxidative removal of the adsorbed carbonaceous species (ascribed as the rate-deter- mining step) happens and the current increases (equations (7) and (8)) [2,19,26,29]. At higher potential, the current rapidly drops when inactive PdO is formed. The backward peak results from the oxidation of freshly adsorbed ethanol molecules after the reduction of the PdO and, thus, recovery of the active sites [8,15,19]. The most important parameters for the activity of an EOR catalyst are the onset potential (Eonset), and the maximum peak current density in the forward and backward scan (jf and jb). PdNiBi/rGO exhibits an Eonset of 0.234 V vs. RHE and a jf and jb of 2390 mA mg-1 and 1942 mA mg-1, respectively. These results are compatible with the literature, e.g., Chowdhury et al. [8] described that a jf of 2223 mA mg-1 was obtained with a PdNiP/N-rGO. The EOR activity of PdNiBi/rGO is higher compared to the commercial Pd/C, which is partly caused by the large ECSA generated by the well-distributed, nanosized particles on the rGO support providing abundant active sites [25]. To evaluate the specific activity of the electrocatalysts, the jf values were normalized by the ECSA. It was found that the PdNiBi/rGO (5.25 mA cm-2) still outperforms the commercial Pd/C (4.92 mA cm-2). Therefore, it can be concluded that not only the higher ECSA but also the presence of the oxophilic elements Ni and Bi additionally ensures the enhancement of the EOR activity, as it has also been shown in other studies in the literature [20,22,24,29]. Ni and Bi are not active for the EOR in the applied potential range, but they have the capability to generate OHads at the surface at lower potentials due to their oxophilic character. The oxidative desorption of the intermediate carbonaceous species (equa- tion (7)), which is the rate-determining step of the EOR, can thus be facilitated, leading to lower onset potentials and higher current densities [8,15,29,30]. The catalytic stability of the PdNiBi/rGO and commercial Pd/C catalysts towards EOR is determined by performing CA tests in de-aerated 1 M KOH and 1 M EtOH solution for 3600 s. The curves are shown in Figure 5. Figure 5. CA measurement of PdNiBi/rGO and the commercial Pd/C in N2-saturated 1 M KOH electrolyte solution containing 1 M EtOH at 0.83 V vs. RHE The current density decrease of EOR catalysts with time can generally be attributed to the poisoning of the active sites on the catalyst metal surface by the adsorption of carbonaceous intermediates (e.g., CH3COads) during ethanol oxidation, as also described in the literature S. Wolf et al. J. Electrochem. Sci. Eng. 13(5) (2023) 771-782 http://dx.doi.org/10.5599/jese.1643 779 [8,13,16,29,31]. Lović et al. [29] have shown, for example, by recording CVs after the CA, that although the activity is initially lower than before the CA, the loss can be recovered with further cycling. This indicates the stability of the catalyst surface composition and the regeneration of the active sites by the removal of CO-like adsorbates. The results of Figure 5 reveal that PdNiBi/rGO and the commercial Pd/C display a different decrease in current density. Both catalysts exhibit a sharp decay at the beginning, however, the PdNiBi/rGO can then stabilize at a pseudo-steady state condition due to a gradual self-cleaning process [13,29]. The poisonous intermediates can be oxidatively removed by the generation of OHads, favored by the presence of oxophilic elements (equation (7)) [8,31]. In comparison, the current density of the Pd/C steadily decreases with time as the catalyst is gradually poisoned [8,19]. Therefore, the PdNiBi/rGO composite shows a slightly higher remaining current density after 3600 s than the commercial Pd/C. In summary, the combination of Pd, Ni and Bi with rGO and their synergistic effects proves to be an effective method and highlights the PdNiBi/rGO catalyst as a promising catalyst with high activity and stability for EOR. Conclusions In this study, PdNiBi nanoparticles were successfully anchored on a reduced graphene oxide support using the modified instant reduction method. The physicochemical analyses have shown a two-dimensional wrinkled sheet morphology characteristic for graphene-based materials and uniformly and well-distributed metal particles with an average diameter of 2.6 nm on the carbon support caused by strong C-O-M bridges due to the remaining oxygen functionalities of the rGO. The electrochemical tests revealed that the PdNiBi/rGO composite outperformed commercial Pd/C in terms of EOR activity and stability with an ECSA of 455 cm2 mg-1, an onset potential of 0.234 V vs. RHE, a maximum peak current density of 2390 mA mg-1 and a current density decrease of 85 % after 3600 s. This enhancement of the electrocatalytic activity could be attributed to the generation of abundant active sites due to the rGO support and the presence of the oxophilic Ni and Bi elements. Based on these physicochemical and electrochemical results, PdNiBi/rGO is highlighted as a promising anode catalyst in alkaline direct ethanol fuel cell applications. Acknowledgements: The financial support from the Austrian Science Fund (FWF) [grant number I 3871-N37] in the frame of the project »Graphene Oxide based MEAs for the Direct Ethanol Fuel Cell« and the Slovenian Research Agency (ARRS) through the Research Programms P1-0175, P2- 0423, P2-0393 and I0-0003 and the Bilateral Research Funding Projects N2-0087, NC-0007, NC-0016, N2-0155 and N2-0257 are gratefully acknowledged. Special thanks go to Norbert Kienzl for performing the ICP-OES measurements. Conflicts of Interest: The authors declare no conflict of interest. References [1] L. An, T. S. Zhao, Y. S. Li, Carbon-neutral sustainable energy technology: Direct ethanol fuel cells, Renewable and Sustainable Energy Reviews 50 (2015) 1462-1468. https://doi.org/10.1016/j.rser.2015.05.074 [2] J. Zhang, Z. Zhao, Y. Wang, J. Wang, D. Peng, B. Li, T. Bo, K. Zheng, Z. Zhou, L. Lv, Z. Xin, B. Zhang, L. 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