Bidirectional energy & fuel production using RTO-supported-Pt–IrO2 loaded fixed polarity unitized regenerative fuel cells

Abstract

Fixed-polarity unitized regenerative fuel cells (FP-URFCs) are devices for energy conversion coupled with fuel production with low self-discharge, high energy densities, fast start-up/shut down dynamics, and long cycle-life. We have synthesized a bifunctional Pt–IrO₂/RTO (ruthenium–titanium mixed oxide) electrocatalyst for the HOR/OER (hydrogen oxidation reaction/oxygen evolution reaction) wherein IrO₂/RTO acts as an OER electrocatalyst, upon which the HOR active Pt electrocatalyst is deposited. Pt–IrO₂/RTO shows higher OER (η_{@10 mA cm⁻²} = 0.26 ± 0.01 V, i = 67 ± 1.9 mA cm⁻² at η = 0.32 V) and HOR (η_{@2.5 mA cm⁻²} = 0.01 V, i = 2.78 ± 0.11 mA cm⁻² at η = 0.1 V) activity and higher stability in comparison to benchmark OER (RuO₂, η_{@10 mA cm⁻²} = 0.42 ± 0.03 V, i = 3.1 ± 0.2 mA cm⁻² at η = 0.32 V and IrO₂, η_{@10 mA cm⁻²} = 0.33 ± 0.02 V, i = 10.2 ± 0.2 mA cm⁻² at η = 0.32 V) and HOR (Pt/C, η_{@2.5 mA cm⁻²} = 0.05 V, i = 2.58 ± 0.08 mA cm⁻² at 0.1 V vs. RHE) electrocatalysts, respectively. Pt–IrO₂/RTO exhibits higher bifunctional HOR/OER activity as compared to both IrO₂/RTO and RTO, which is achieved through a combined effect of enhanced active site density and favorable electronic interactions (increased metal oxidation states of Ru and Ir) as confirmed by XPS and UPS. The deposition of IrO₂ in Pt–IrO₂/RTO is responsible for high stability by lowering the dissolution of the RTO support, as confirmed by ICP-MS. A FP-URFC tested with Pt–IrO₂/RTO and Pt/C as the anode and cathode, respectively yields a mass-specific activity of 1050 ± 22 A g⁻¹ at 1.8 V and 20 ± 2 A g⁻¹ at 0.9 V under electrolyzer and fuel cell modes, respectively. The FP-URFC shows a round-trip efficiency of 40.2% at 1 A cm⁻² with low Pt-loading on both electrodes. Therefore, FP-URFCs using Pt–IrO₂/RTO as the HOR/OER bifunctional electrocatalyst have significant potential towards efficient and cost-effective bidirectional energy and fuel production.