Achieving excellent and durable CO₂ electrolysis performance on a dual-phase fuel electrode in solid oxide electrolysis cells

Highlights

• •A dual-phase Pr(Ca)Fe(Ni)O₃-Ca₂Fe₂O₅ is synthesized by the one-pot method

• •The presence of Ca₂Fe₂O₅ improves the CO₂ adsorption capability

• •The dual-phase sample achieves CO₂-RR rate constant of 1.10 × 10⁻⁴ cm s⁻¹ at 800 °C

• •A current density of 0.648 A cm⁻² at 1.5 V and 800 °C for pure CO₂ electrolysis

• •The SOEC exhibits excellent durability for 300 h

Abstract

Conversion of CO₂ into valuable chemicals through solid oxide electrolysis cells (SOECs) is a promising technology towards efficient utilization of CO₂ and reducing its emission. However, the well-established Ni-cermet fuel electrode is not applicable for high-concentration or pure CO₂ electrolysis due to the Ni oxidation and coking issues. Here, we report that a novel nominal Pr_0.2Ca_0.8Fe_0.8Ni_0.2O_3-δ perovskite fuel electrode, which is self-assembled into Pr(Ca)Fe(Ni)O_3-δ perovskite and Ca₂Fe₂O₅ brownmillerite dual-phase composite during the calcination process, possesses efficient CO₂ electrolysis activity. The presence of Ca₂Fe₂O₅ with abundant oxygen vacancies significantly improves CO₂ chemical adsorption and activation and Pr(Ca)Fe(Ni)O_3-δ serves charge carrier matrix, and consequently leads to a high CO₂ reduction reaction rate constant of 1.104 × 10⁻⁴ cm⁻¹ at 800 °C as determined by the electrical conductivity relaxation method. The electrochemical performance of pure CO₂ electrolysis is investigated in model SOECs, exhibiting an excellent current density of 0.648 Acm⁻² at 1.5 V and 800 °C. Moreover, the cell shows no noticeable degradation under two constant current densities of 0.421 Acm⁻² at 800 °C and 0.3 Acm⁻² at 700 °C for nearly a total of 300 h. The present study reveals a novel strategy to develop dual-phase Pr(Ca)Fe(Ni)O_3-δ-Ca₂Fe₂O₅ materials as a reliable electrode for pure CO₂ electrolysis.

Introduction

A large amount of CO₂ is poured into the air due to the large consumption of fossil fuels, leading to a series of problems such as global warming, which have aroused wide concern in the world [[1], [2], [3]]. In this context, the effective capture and conversion of CO₂ to valuable chemicals are urgently required, which would be appreciated to combine with renewable energy power, such as solar energy. Solid oxide electrolysis cells (SOECs) are considered to be one of the most viable systems to reduce CO₂ emissions, because of their capability to efficiently electrolyze CO₂ into value-added CO for a wide of industrial applications [1,2,[4], [5], [6]]. In fact, SOECs have been demonstrated on the stack level in a few groups [5,7,8]. Moreover, in the future, the decentralized SOEC plants can produce CO only when needed, improving plant/energy safety, and reducing large storage/transportation costs [9,10].

An archetype of SOECs consists of a dense electrolyte membrane for ion transportation and two separated porous electrodes for CO₂ reduction reaction (CO₂-RR) and oxygen evolution reaction (OER), respectively. When operated at elevated temperatures (600–800 °C), SOECs perform the considerably facilitated reaction kinetics by reducing cell activation resistance, while achieving the Faradaic efficiency close to 100% because the oxygen ion migration number is close to the unity [[11], [12], [13]]. Usually, CO₂-RR occurring at the cathode (also named as fuel electrode since producing CO fuel) is the nominal rate-determining step because of the largest resistance to the cell which is majorly contributed by CO₂ chemical adsorption and activation. Therefore, more recent efforts have been focused on developing high-performing materials as cathodes for accelerating the CO₂-RR process [[14], [15], [16]].

The traditional cermet Ni-YSZ (Y₂O₃-stabilized ZrO₂) cathodes are borrowed from the state-of-the-art of anodes in solid oxide fuel cells (SOFCs) [17], which exhibit superior CO₂ electrolysis performance in the presence of safe gas (H₂ or other reducing gas) even if CO₂-RR only occurs at three-phase boundaries (TPBs) [18]. However, the introduction of safe gas avoiding Ni oxidation increases the cost and system complexity. Besides, carbon deposited on super active Ni near the electrolyte will damage the cell durability and integration [19,20]. In response to this, redox stable perovskite oxides (ABO₃) have been exploited as alternative cathodes for direct CO₂ electrolysis without the addition of reducing gas (CO or H₂). Some of them exhibit prevailing mixed ionic-electronic conducting properties and adequate CO₂ tolerance [[21], [22], [23]], making them suitable and promising candidates because of the availability of the entire electrode surface for CO₂-RR. However, their adsorption and activation capability of CO₂ are unsatisfactory at intermediate temperatures (500–800 °C), limiting the CO₂-RR kinetics at the cathodes [24,25]. Recently, Xie and co-workers proposed that doping Mn cations into La(Sr)TiO_3+δ could enhance the CO₂ chemical adsorption by 1–2 order of magnitude, achieving the current density from 0.12 to 0.25 A cm⁻² at 2.0 V and 800 °C [24], and a similar result was also reported on Cu-doped La(Sr)Ti(Mn)O_3+δ by Sun's group [26]. In our previous work, it's found that fluorine doping could also enhance CO₂ adsorption capability because of creating additional oxygen vacancies by weakening Fe-O bonds [21]. However, CO₂ adsorption is still insufficient. In the recent few years, more research on perovskite oxides as alternative cathodes has been going, towards improving CO₂ adsorption and activation [23,[26], [27], [28]]. Although the electrolysis current densities reported in recent studies are encouraging, there is still room for improving the performance in different ways.

Recently, Rojo and co-workers reported that the brownmillerite-type phase (e.g., Ca₂Fe₂O₅) appeared in excess Ca-doped Ln_1-xCa_xFe_0.8Ni_0.2O_3-δ (Ln = La or Pr) compositions by a single-step synthesis route [29,30]. The new Ca₂Fe₂O₅ phase was advantageous for oxygen reduction reaction in SOFCs because of abundant oxygen vacancy concentrations, showing a level competitive with typical La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ [31]. Recently, Brownmillerite-structured Ca₂Fe₂O₅ also showed spontaneous CO₂ capture with remarkable reversibility at the temperature from room temperature up to 900 °C because of the accommodation of abundant oxygen defects [32]. Based on these two points, the presence of the secondary brownmillerite-type phase with considerable CO₂ adsorption and activation capability at elevated temperature would improve the cathode activity for CO₂ electrolysis in SOECs.

Inspired by abundant oxygen vacancy concentrations of Ca₂Fe₂O₅, we presume that the introduction of Ca₂Fe₂O₅ can be also applied to perovskite oxide cathodes for improving CO₂ adsorption and thus promoting CO₂-RR kinetics. In this work, we attempted to synthesize a Ca₂Fe₂O₅ contained composite with a nominal composition of Pr_0.2Ca_0.8Fe_0.8Ni_0.2O_3-δ by the one-pot solution-combustion method. As expected, this material was self-assembled into two separated phases with different functionalities during the calcination process, consisting of perovskite-structured Pr(Ca)Fe(Ni)O_3-δ and brownmillerite-structured Ca₂Fe₂O₅. This dual-phase composite is explored as an alternative cathode or potential symmetrical electrodes for CO₂ electrolysis in SOECs.