High-Performing Unitized Regenerative Fuel Cell Using PGM-Free Bifunctional Catalysts from Waste-Derived Carbon

Abstract

Unitized regenerative alkaline-exchange membrane fuel cells (UR-AEMFCs) are a promising technology for energy storage, attending to their ability to operate in both galvanic (FC) and electrolytic (WE) modes. However, their round-trip efficiency (RTE) is greatly dependent on the selected electrodes toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). As both mechanisms have slow kinetics, developing innovative and efficient bifunctional catalysts is critical to increasing UR-AEMFC performance. In this work, bifunctional precious group metal-free (PGM-free) catalysts were prepared by nucleating either Fe, Ni, or FeNi on nitrogen-doped carbons; the prepared catalysts have a maximum metal loading of up to 3 wt %. To align with circular economy principles, a sustainable approach was employed by using a phenolic resin-based waste product as a carbon precursor. The carbonization process, conducted in the presence of H3PO4 and melamine, yielded nitrogen-doped carbon supports with enhanced properties. Notably, in single-cell operation, the prepared PGM-free catalysts─NiNC, FeNC, and FeNiNC─generated RTEs of 29%, 38%, and 35%, respectively, under corrosion-free alkaline electrolytes (0.5 M KOH); similar results were reported for benchmark commercial catalysts such as IrO2, which displayed ca. 30% of RTE. Moreover, NiNC catalyst showed a half-wave potential of 0.74 V, an onset potential of 0.88 V, and an overpotential of 420 mV at 10 mA cm–2. In turn, the FeNC displayed a half-wave potential of 0.72 V, an onset potential of 0.90 V, and an overpotential of 510 mV at 10 mA cm–2. Regarding the FeNiNC, this catalyst demonstrates a half-wave potential of 0.76 V, an onset potential of 0.86 V, and an overpotential of 380 mV at 10 mA cm–2. These results are particularly noteworthy considering the significantly lower metal loadings in the produced PGM-free catalysts. Besides, to evaluate their long-term performance, the single cell was submitted to several potential cycles within the potential window range for FC and WE modes, i.e., between 0.6 and 2.0 V. The noble Pt/C catalysts deactivated completely after the stress test, while the novel FeNiNC and NiNC catalysts, containing an ultralow loading of critical raw materials (CRMs), demonstrated an outstanding compromise between activity and stability, proving their potential to be integrated in UR-AEMFC.