High-entropy materials for solid oxide cells

Abstract

Solid oxide cells (SOCs), which include solid oxide fuel cells (SOFCs), symmetrical solid oxide cells (S-SOCs), and reversible solid oxide cells (R-SOCs), are considered key technologies for driving low-carbon and green revolution in the energy sector. Because of their clean, low-cost, and high-efficiency characteristics, SOCs have great potential for energy conversion and storage. However, the further development of SOC technologies faces challenges, such as a lack of long-term operational stability of the cell system, high material cost under high-temperature operating conditions, and limited catalytic effects at low temperatures. Recently, high-entropy materials (HEMs) have demonstrated excellent performance and wide application prospects in catalytic reactions, energy storage, supercapacitors, and other fields owing to their unique atomic arrangement and the four core effects (high mixed entropy stabilization effect, sluggish diffusion effect, lattice distortion effect, and “cocktail” effect). HEMs provide a new perspective for solving the aforementioned problems in the field of SOCs. This comprehensive review summarizes the applications of HEMs in the three fundamental components of SOCs: electrodes, electrolytes, and interconnects, focusing on the role of HEMs in enhancing catalytic activity and conductivity while mitigating harmful gas poisoning. In addition, this review proposes possible development directions for HEMs in SOCs based on the current research progress, providing valuable reference for high-entropy designs aimed at further enhancing the performance of SOCs.

1. Introduction

To meet the diverse energy demands of social and economic development while achieving the “dual carbon” goals, transforming the energy system from traditional fossil energy to green and low-carbon energy has become an urgent and essential strategic priority [1–2]. Solid oxide cells (SOCs), as an effective device for energy transformation, have attracted increasing attention [3–5]. SOCs include solid oxide fuel cells (SOFCs), symmetrical solid oxide cells (S-SOCs), and reversible solid oxide cells (R-SOCs). SOFCs represent the most traditional type of SOCs and can be classified into oxygen ion-conducting SOFCs and proton-conducting SOFCs according to the type of ions conducted by the electrolyte [6]. SOFCs convert the chemical energy of fuels into electrical energy without the efficiency limitations of the Carnot cycle. Compared with traditional power generation equipment that relies on the combustion of fossil fuels, SOFCs can increase power generation efficiency from 45% to over 70% [7–8]. SOFCs have a wide range of fuel options, including hydrogen, natural gas, carbon monoxide, and methane [9]. In addition, their clean operation, low noise, and quick start-up and shutdown capabilities [10–12] position SOFCs as the fourth-generation energy source following thermal, hydraulic, and nuclear power [13]. Thus, the development of SOFCs is vital for advancing energy transition strategies.

S-SOCs, which use the same material for both cathodes and anodes, can reduce manufacturing costs and time compared to traditional SOCs that require different materials for these electrodes. R-SOCs combine power generation and electrolysis functions, enabling a switch between fuel cell and electrolytic cell modes. In summary, SOCs are advanced energy conversion devices that not only convert clean energy directly into electricity but can also electrolyze H2O or CO2 to produce H2 and hydrocarbon fuels. A single SOC primarily comprises solid-state electrodes and solid-state electrolytes. However, its output voltage is typically less than 1 V. In practical applications, multiple cells are usually linked in series via interconnects, forming a power stack to obtain a high output. Compared to high-temperature SOCs operating at 800–1000°C, medium-temperature (650–800°C) SOCs have been extensively studied in the past decade, as they do not require costly high-temperature resistant materials [14]. The reduction in the operating temperature of SOCs also facilitates the widespread utilization of low-cost stainless steel (SS) interconnects. However, as the operating temperature decreases, the electrode polarization loss of SOCs gradually increases, which inevitably deteriorates the performance of the cell [15–16]. During the operation of a power stack, volatilization of Cr from the SS can cause serious oxidation of the interconnect and poison the cathode [17], degrading the performance and shortening the service life of the power stack.