Graphene Supported Pt–Ru–Sn Electrocatalyst for Borohydride Oxidation in Membraneless Borohydride Fuel Cell

Abstract

Graphene supported Pt–Ru–Sn trimetallic electrocatalysts are prepared by a modified sodium borohydride reduction method in aqueous solution at room temperature, and used as the anode electrocatalysts for membraneless borohydride fuel cell. The physical and electrochemical properties of the as-prepared electrocatalysts are investigated by X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), cyclic voltammetry (CV), chronoamperometry (CA) and fuel cell test. XRD results show that the diffraction peaks in Pt–Ru–Sn/G catalysts shift slightly to lower 2θ values compared with that of Pt/G catalyst, suggesting the formation of Pt–Ru–Sn alloying. TEM results show that the morphologies of Pt–Ru–Sn trimetallic catalysts are uniformly spherical with the particle size of about 3.5 nm on the graphene surface. Besides, it has been found that the Pt–Ru–Sn catalysts have much higher catalytic activity for the oxidation of sodium borohydride than Pt/G catalyst, especially the Pt–Ru–Sn/G (60:30:10) catalyst presents the highest catalytic activity among all as-prepared catalysts. The membraneless borohydride fuel cell with Pt–Ru–Sn/G (60:30:10) anode catalyst and Pt/G cathode catalyst obtains the maximum power density as high as 39.61 mW cm─2 at room temperature.

Keywords

Graphene; Electrocatalysts; Platinum; Ruthenium; Tin; Membraneless Borohydride Fuel

References

1. 65 V, probably caused by the mixed potential at the anode and cathode from the simultaneous oxidation of sodium borohydride and H 2 at the anode. The maximum output power density for Pt/G (100) is 7.75 mW cm ─2. The addition of Ru is considerably decreasing the sodium borohydride electro -oxidation reaction as observed from the polarization curves. The Ru content that provides maximum activity is in the range 30 at % of Ru: a decrease in the BOR activity for higher Ru c ontents is commonly ascribed to hindering access of the reactant to Pt sites by the presence of Ru oxide and/or low amounts of Pt sites; lower Ru contents depend on the degree of alloying. Above 30 at %, alloyed Ru hinders sodium borohydride adsorption by the ensemble effect. On these bases, the best compromise alloyed Ru in Pt – Ru/G catalysts should be 20 –30 at %. Pt‒Ru/G (50:50) (Ru 50 at %), Pt‒Ru‒Sn/G (60:10:30) (Ru 10 at.%), Pt –Ru–Sn/G (60:20:20) (Ru 20 at.%) and Pt‒Ru‒Sn/G (60:30:10) (Ru 30 at.%) showed OCP of 1.92 V, 2.23 V, 2.26 V and 2.29 V respectively. The comparison of both the bimetallic catalysts showed that peak power density of Pt‒Ru/G (50:50) (16.99 mW cm ─2). The results of MLBFC adapting to different catalysts are summarized in Table 3. When the current was normalized to the geometric area of single cell, it was observed that the cell performance of Pt‒Ru‒Sn/G (60:30:10) catalyst was better than other catalysts. In the low current discharging region, the power drawn from single cell was almo st the same for all catalysts except Pt‒Ru/G (50:50) and Pt/G (100). Table 3. Summary of performance of fuel cell tests using 2 mg cm ─2 catalyst loading, (40 wt% catalysts on graphene) Anode Catalysts Open circuit Potential (V) Maximum power density (mW cm─2) Maximum Current density (mA cm─2) Pt/G (100) 1.65 7.75 25.639 Pt‒Sn/G (50:50) 1.90 12.59 37.524 Pt‒Ru/G (50:50) 1.92 16.79 45.695 Pt–Ru–Sn/G (60:10:30) 2.23 26.47 65.953 Pt–Ru–Sn/G (60:20:20) 2.26 35.53 77.201 Pt–Ru–Sn/G (60:30:10) 2.29 39.61 78.258 The cell voltage of Pt‒Ru‒Sn/G (60:30:10) at a current density of 51.21 mA cm ─2 was 0.69 V which was higher than rest of the catalyst. In addition, there was a rapid initial fall in cell voltage for all catalysts, which was due to the slow initial sodium borohydride electro - oxidation reaction at the electrode surface. After an initial drop of 0.69 V the change in slope of the polarization curve for Pt‒Ru‒Sn/G (60:30:10) decreased, and it started drawing more current. This is attributed to the more effective catalytic ability of Pt‒Ru‒Sn/G (60:30:10), once the sodium borohydride electro- oxidation reaction being initiated. Based on the power density drawn from single cell, Pt‒Ru‒Sn/G (60:30:10) is the best anode catalyst with peak power density value of 39.61 mW cm─2. Moreover, as to Pt‒Ru‒Sn/G (60:30:10) catalyst, the replacement of 40% of Pt by Ru and Sn results in a large decrease of the catalyst cost. Thus Pt‒Ru‒Sn/G catalyst not only improves electrocatalytic activity of BH4 ─ electro- oxidation for MLBFC, but also reduces the cost of the catalyst. Fig. 8 . Polarization and power density curves of different catalyst at 2 mg cm ─2 catalyst loading on anode and cathode at room temperature. Anode feed: 0.15 M sod ium borohydride in 3 M NaOH and Cathode feed: 0.15 M Sodium Perborate + 1.5 M H2SO4. Stream flow rates: 0.3 ml min─1 Conclusions In the present study, Pt/G, Pt‒Ru/G, Pt‒Sn/G, and different Pt‒Ru‒Sn/G catalysts were synthesized according to the conventional sodium borohydride reduction method and examined as potential electrocatalysts for oxidation of borohydride ion. The catalytic activity was assessed by cyclic voltammetry and chronoampherometry under conditions relevant for the anode catalysts layer composition of the membraneless borohydrid e fuel cell. The maximum activity for sodium borohydride oxidation was found for the Pt –Ru–Sn/G (60:30:10) than the Pt –Ru–Sn/G (60:20:20), Pt – Ru–Sn/G (60:10:30), Pt –Ru/G (50:50), Pt –Sn/G (50:50) and Pt/G (100) catalysts. The significantly enhanced catalyti c activity for sodium borohydride oxidation can be attributed to the high dispersion of ternary catalysts and to Sn acting as a promoting agent. XRD results show the homogeneous alloy structure of Pt, Ru and Sn. 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