Electrocatalysis in Solid Oxide Fuel Cells and Electrolyzers

Abstract

Interest in energy-to-X and X-to-energy (where X represents green hydrogen, carbon-based fuels, or ammonia) technologies has expanded the field of electrochemical conversion and storage. Solid oxide electrochemical cells (SOCs) are among the most promising technologies for these processes. Their unmatched conversion efficiencies result from favorable thermodynamics and kinetics at elevated operating temperatures (400−900 °C). These solid-state electrochemical systems exhibit flexibility in reversible operation between fuel cell and electrolysis modes and can efficiently utilize a variety of fuels. However, electrocatalytic materials at SOC electrodes remain nonoptimal for facilitating reversible operation and fuel flexibility. In this Review, we explore the diverse range of electrocatalytic materials utilized in oxygen-ion-conducting SOCs (O-SOCs) and protonconducting SOCs (H-SOCs). We examine their electrochemical activity as a function of composition and structure across different electrochemical reactions to highlight characteristics that lead to optimal catalytic performance. Catalyst deactivation mechanisms under different operating conditions are discussed to assess the bottlenecks in performance. We conclude by providing guidelines for evaluating the electrochemical performance of electrode catalysts in SOCs and for designing effective catalysts to achieve flexibility in fuel usage and mode of operation. CONTENTS1. Introduction 8234 2. Thermodynamics of Reactions in Solid Oxide/ Proton Conducting Electrochemical Cells (O-SOC/ H-SOCs) 8237 3. Electrochemical Kinetics in SOCs 8241 4. Electrocatalysis in Oxygen Ion Conducting Solid Oxide Electrochemical Cells (O-SOCs) 8244 4.1. Electrochemical Fuel Oxidation Catalysis in O-SOFCs 8244 4.1.1. Electrochemical Oxidation of H 2 in O-SOFCs 8244 4.1.2. Hydrocarbon Fueled O-SOFCs 8248 4.1.3. Ammonia Fueled O-SOFCs 8252 4.2. H 2 O Electrolysis in O-SOECs 8253 4.2.1. Introduction to H 2 O-Electrolysis Kinetics in O-SOECs 8253 4.2.2. Catalyst Structure/Performance Trends 8253 4.3. CO 2 Electrolysis and Co-electrolysis of H 2 O/ CO 2 in O-SOECs 8255 4.3.1. Introduction to CO 2 Electrolysis and H 2 O-CO 2 Co-electrolysis Kinetics in O-SOECs 8255 4.3.2. Catalyst Structure/Performance Trends 8259 4.4. Oxygen Reduction/Evolution Reactions (ORR/OER) in O-SOCs 8262