Direct-Hydrocarbon Proton-Conducting Solid Oxide Fuel Cells

Fan Liu,Chuancheng Duan

doi:10.3390/su13094736

Abstract

Solid oxide fuel cells (SOFCs) are promising and rugged solid-state power sources that can directly and electrochemically convert the chemical energy into electric power. Direct-hydrocarbon SOFCs eliminate the external reformers; thus, the system is significantly simplified and the capital cost is reduced. SOFCs comprise the cathode, electrolyte, and anode, of which the anode is of paramount importance as its catalytic activity and chemical stability are key to direct-hydrocarbon SOFCs. The conventional SOFC anode is composed of a Ni-based metallic phase that conducts electrons, and an oxygen-ion conducting oxide, such as yttria-stabilized zirconia (YSZ), which exhibits an ionic conductivity of 10−3–10−2 S cm−1 at 700 °C. Although YSZ-based SOFCs are being commercialized, YSZ-Ni anodes are still suffering from carbon deposition (coking) and sulfur poisoning, ensuing performance degradation. Furthermore, the high operating temperatures (>700 °C) also pose challenges to the system compatibility, leading to poor long-term durability. To reduce operating temperatures of SOFCs, intermediate-temperature proton-conducting SOFCs (P-SOFCs) are being developed as alternatives, which give rise to superior power densities, coking and sulfur tolerance, and durability. Due to these advances, there are growing efforts to implement proton-conducting oxides to improve durability of direct-hydrocarbon SOFCs. However, so far, there is no review article that focuses on direct-hydrocarbon P-SOFCs. This concise review aims to first introduce the fundamentals of direct-hydrocarbon P-SOFCs and unique surface properties of proton-conducting oxides, then summarize the most up-to-date achievements as well as current challenges of P-SOFCs. Finally, strategies to overcome those challenges are suggested to advance the development of direct-hydrocarbon SOFCs.

Highlights

The rapid growth in fossil fuel extraction, transportation, and consumption is leading to significant anthropogenic climate change and global warming accompanied by the principal CO2 emission [1,2]
Surface-enhanced Raman Spectrology (SERS) was employed to better understand the coke mitigation mechanisms of Ni-based anodes coated with BaO, BZY, and BCZYYb, respectively [41], revealing that abundant –OH is absorbed on all three materials
Notable Achievements on Coking-Tolerant Direct-Hydrocarbon proton-conducting SOFCs (P-SOFCs). Both P-SOFCs and oxygen-ion SOFCs (O-SOFCs) are capable of directly converting methane and other hydrocarbon fuels (e.g., C2 H6 and C3 H8 ) into electricity

Summary

Introduction

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Fossil energy sources, including coal, oil, and natural gas, are currently accounting for >80% of the global energy consumption. SOFCs are, one of the most promising power sources, which eliminates the fuel processor units and could be integrated into the current power grid, drastically enhancing energy conversion efficiency and reducing emissions. Robust SOFC anodes, which are catalytically active at intermediate operating temperatures, coking and sulfur-tolerant, as well as chemically stable, are essential for direct-hydrocarbon SOFCs. Proton-conducting oxides have been developed for numerous electrochemical devices because of high proton conductivity at 300–650 ◦ C [15,16,17,18,19]. The current challenges and future perspectives of these promising P-SOFCs are highlighted

Proton-Conducing Oxides

Unique Surface Properties of Proton-Conducting Oxides

The Rationale for Developing Direct-Hydrocarbon P-SOFCs

Notable Achievements on Coking-Tolerant Direct-Hydrocarbon P-SOFCs

Findings

Conclusions and Perspectives