High Activity pH-Universal Oxygen Reduction Reaction Catalyst: Tri-Doped Porous Carbon Material Derived from Agricultural and Mining Wastes
Authors: Nyangau Edwin Osebe , Nakayasu Yuta , Abe Hiroya , Ooka Chie , Haga Kazutoshi , Watanabe Masaru 
Published: December 2024

Introduction

Fuel cells still command great attention even as the need for an alternative source of energy increases annually. Following the unison conclusion by various researchers on the use of platinum and precious group metals (PGM) catalysts, as an expensive venture, many researchers are still committed to alternative and effective catalysts for fuel cell application. First-row transitional metals (TM) have been proposed as effective alternatives for oxygen reduction reaction (ORR) applications due to their promising high ORR activity and durability besides their abundance and cheaper unit cost compared to PGM [1]. In the last decade in particular, many researchers have reported catalysts derived from carbon doped with iron (Fe) and nitrogen (N). Such catalysts have shown high ORR activity, especially under alkaline conditions. The formation of Fe-N4 moieties in the carbon lattice through the interaction between the doped heteroatoms has been reported as an effective active site during the ORR process [2,3]. However, most of these catalysts have only shown high activity under alkaline conditions while their activity in neutral and acidic mediums still lags hindering wide applications in fuel cells [4]. This negative trend has been attributed to the limited number of active sites and the ease of catalyst degradation in acidic or neutral medium hence low durability in these media. To address these challenges, an increase in the number and concentration of dopants has been considered and shown positive results towards increasing the activity. However, such ventures have a net effect on the sustainability of this technology. Herein an eco-friendly and sustainable approach is presented. In this approach, we harness waste rice husk biomass rich in silica as a source of carbon (C) and self-doped silica (Si) together with pyrite, an abundant byproduct of chalcopyrite smelting following the increased demand of copper for the electric vehicle (EV) industry, as a source of Fe. These materials are then synthesized through a chain of hydrothermal and template carbonization synthesis processes to yield a tri-doped catalyst which is referred to as Fe-N-Si-C in this work. The synthesized catalyst was characterized for both physiochemical properties using TEM, SEM, XAFS and DFT while the electrochemical properties were analyzed using linear sweep voltammetry, cyclic voltammetry, and durability in both alkaline, neutral, and acidic mediums.

Results and discussion

The template carbonization method adopted in the synthesis in this work, resulted in a porous carbon material with successfully doped heteroatoms as confirmed by the TEM and SEM analysis. A homogenous and evenly distributed of C, Fe, N and Si elements was further confirmed by the EDS mapping. The significance of this heteroatom doping that was achieved from wastes was exhibited in the Fourier transform EXAFS where the coexistence of Fe-N4 moieties and Fe clusters with surface oxides was confirmed by the presence of Fe-N and Fe-O peaks were detected on the catalyst spectrum. Similarly, a Si-K-edge X-ray absorption revealed the formation of an amorphous SiO2 peak on the Fe-N-Si-C catalyst. These Fe-N4 porphyrin-like moieties and the SiO2 provided effective active sites for ORR as demonstrated in the LSV analysis in all universal pH media, where Fe-N-Si-C showed an onset potential of 1.02V, 0.96V and 0.86V which was close to those of commercial PtC ;1.04V, 0.99V and 0.91V in ORR analysis conducted in 0.1M KOH (alkaline),0.1M PB (neutral) and 0.1M HClO4 (acidic) respectively as highlighted in Table 1. Motivated by superior ORR performance in alkaline conditions, a Zn-air battery (ZAB) was assembled with 6M KOH and 0.2M Zinc acetate. Fe-N-Si-C cell achieved a 103mW/cm2 power density, nearly 50% higher than what the PtC (78mW/cm2) cell achieved. This work not only showcases value addition to unused environmental waste resources considered pollutants but also introduces a cost-effective and environmentally friendly catalyst, serving as a substitute for precious and expensive metal-derived electrocatalysts.

References

[1] H Wang et al., Small,19 (46),2304863 (2023).
[2] Wong, C. Nature., 623(7988):674-675 (2023).
[3] Jin Zhaoyu et al., Nature Catalysis, 4(7),615-622(2021)
[4] Rauf, M.Z. Electrochemistry Communication, 73, 71-74 (2016).

Figure 1