Researchers from Lawrence Berkeley National Laboratory and the University of Connecticut have demonstrated high-performance metal-supported solid oxide fuel cells (MS-SOFC) with an integrated high entropy alloy (HEA) internal reforming catalyst (IRC) for transportation applications using ethanol and methanol as fuels. A paper on their work is published in the Journal of Power Sources.

Solid oxide fuel cells (SOFCs) have fuel flexibility; in addition to hydrogen, various kinds of hydrocarbon fuels, such as natural gas (methane), methanol, ethanol, biofuel, and coal can be used via external steam reforming (ESR) or direct internal reforming (DIR). Renewable bio-ethanol produced from agricultural feedstocks and algae offers the benefits of high energy density, easy storage and transport, and a low environmental impact.

Nissan’s vision is to utilize ethanol for electric vehicle range extenders [earlier post]. Metal supported SOFCs (MS-SOFC), utilizing low cost ferritic steels as support, enable fast start and thermal cycling of the MS-SOFC power system, a key requirement for consumer vehicles. Taking these advantages of MS-SOFCs and bio-ethanol, ethanol fueled SOFCs are being developed for electric vehicles using concentrated ethanol (≥45 v% ethanol/water (balance), abbreviated as “45% EtOH”) as a fuel. Recent evaluation of the feasibility of ethanol and gasoline in SOFC vehicles indicates that ethanol fueled SOFCs have more economic, environmental, and social advantages than gasoline fueled vehicles.

—Hu et al.

DIR operation offers a number of benefits:

• The steam reforming reaction and fuel cell reaction proceed in sequence in the anode side, without the extra complexity, volume, and cost associated with external reformers.

• Increased overall efficiency and inherent dynamic stability. As fuel cell power is increased, more heat and water are produced, which are required for internal reforming. Hydrogen consumption by the fuel cell reaction thereby directly drives the steam reforming of ethanol forward.

• Steam produced returns to the outside of reforming catalyst layer and reduces the need for water injection. This allows high concentration of ethanol fuel in the storage tank, thereby increasing the driving range of the electric vehicle.

However, shortcomings of DIR operation include the inability to replace the DIR catalyst, and limited space for catalyst. DIR catalysts thus must be durable and highly efficient. Further, formation of carbon or coke during DIR is a primary concern for cell stability. Carbon deposition in the anode can block the electrode pores, deactivate catalyst sites, and lead to rapid cell breakdown in the worst cases, the researcheres noted.

High entropy alloy (HEA) reforming catalysts, which contains 5 or more elements with concentrations between 5 and 35 atom%, expand conventional alloying approaches for functional catalysts. Lee et al. reported that HEA-GDC reforming catalyst delivered superior operation stability without coking or carbon deposition in the anode using methane fuel for 30 h. The HEA shows advantages (resistance to coke formation and sintering) over conventional bi-alloys of Ni, Co, Cu, Fe and Mn elements. Some of the challenges associated with introducing internal reforming catalysts (other than HEA) into MS-SOFCs were affinity to metal support, loss of surface area due to sintering, and deleterious interaction with SOFC components.

Our team previously developed HEA for methane and successfully integrated it into MS-SOFCs with gaseous fuels. MS-SOFCs with HEA-GDC reforming layer achieved 0.5 W cm⁻² using 97% CH₄/3% H₂O fuel without carbon deposits or coke after 40 h operation. Here, we extended the use of HEA reforming layer on MS-SOFCs to high-concentration ethanol liquid fuel.

This study demonstrates HEA-SDC internal reforming catalyst for improving cell performance and stability. The HEA-SDC catalyst is integrated onto our recently optimized MS-SOFCs (with thin and highly porous metal supports) via infiltration and brush paste methods. The impact of the HEA-SDC reforming catalyst loading and thickness, and the operating temperature on the cell performance are explored. Mass transport of the HEA-SDC/Ni-SDC-based MS-SOFC is quantified and electrode morphologies are analyzed with scanning electron microscopy. Carbon deposition is predicted by thermodynamic analysis and confirmed by cell testing results. The stability of the ethanol-fueled MS-SOFCs are tested for up to 500 h. Degradation factors such as catalyst agglomeration and chromia deposition are analyzed. Alternate fuels such as methanol are also tested with HEA catalysts. This study provides useful information for further development and commercialization of ethanol-fueled MS-SOFCs with IRC.

—Hu et al.

Among the findings:

• Addition of the HEA IRC dramatically improves cell performance and stability when using ethanol/water blend fuel.

• Infiltrated HEA reforming catalyst provides a highly porous structure and low catalyst loading (6 mg cm⁻²).

• The best ethanol concentration (60:40 v% ethanol: water) provides 0.83 W cm⁻¹ at 700 °C, without carbon deposition.

Resources

• Boxun Hu, Grace Lau, Kevin X. Lee, Seraphim Belko, Prabhakar Singh, Michael C. Tucker (2023) “Ethanol-fueled metal supported solid oxide fuel cells with a high entropy alloy internal reforming catalyst,” Journal of Power Sources, Volume 582. doi: 10.1016/j.jpowsour.2023.233544