Cobalt-Based Perovskite Electrodes for Solid Oxide Electrolysis Cells

Abstract

Recently, many efforts and much attention has been paid to developing environmentally friendly energy. Solid oxide electrolyte cells (SOECs) process in reverse to solid oxide fuel cells (SOFCs) producing hydrogen gas as a green energy source. However, in this application, high-performance catalysts are usually required to overcome the sluggish oxygen evolution reactions (OER) during water decomposition. For this reason, discovery of catalysts with high performance is a crucial issue for the wide application of SOECs. Owning to their inherent activity and adequate stability in electrochemical conditions, perovskite oxides have been intensively employed in SOECs. In this mini review, we summarize the currently available studies concerning the applications of cobalt-based perovskite oxide catalysts in SOECs. Particularly, their structural properties and corresponding electronic structures are discussed based on their electrochemical performance, both experimentally and theoretically.

1. Introduction

In recent years, the methods for hydrogen gas production have attracted much attentions due to the increasing concerns of environmental protection. As the reverse process of solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs) have been considered as effective devices for hydrogen production with low cost, high efficiency, and environmental friendly [1]. High working temperatures could accelerate the reaction speed in SOECs, thus improving their electrocatalytic capacity [2]. However, even if SOECs take the advantages from elevated temperatures, it is still essential to decrease the working temperatures over long device duration times [3]. However, unfortunately, low working temperatures would also decrease the electrocatalytic activity of the oxygen electrodes and thus result in the degradation of the catalytic performance. Therefore, the development of low-temperature oxygen electrodes with high electrochemical activities is essential research concerning SOECs [4]

Oxygen electrodes applied in SOECs are mainly reference the extensively studied cathodes for SOFCs [5,6,7,8]. In SOEC processes, the oxygen evolution reactions (OER) are the most important reactions during water splitting into oxygen and hydrogen gases, in this case, the study of OER, therefore, have become a main topic in the study of hydrogen production in SOECs. Trasatti et al. [9] investigated the OER activities of different perovskite oxides and emphasized a volcano-shaped relationship combining the enthalpy and the corresponding OER overpotential. At the same time, Bockris et al. [10] clarified that the current density at a constant overpotential was contrarily dependent on the formation enthalpy of M(OH)₃ hydroxide. The previous activity and mechanism study became the basis of the subsequent design of efficient oxide electrocatalysts [11]. The perovskite structure, possessing various types of rare/alkaline earth metal cations (A site) and 3D transition-metal cations (M sites), has been reported to exhibit different architectures, some of them possessing diffusion channels for oxygen ions that would be advantageous in the OER process [12]. Partial metal substitution either at A or M sites would significantly influence not only the charge state and distribution of ions in the perovskite structure, but also its electric conductivity, and finally the OER performance [13].

At high temperatures, the defective structure of oxides and their related chemical activities have been extensively detected. Ln₂NiO_4-δ (Ln = Pr, La, Nd) series have been investigated as oxygen electrodes for SOECs [14]. Former data has indicated that La₂NiO_4-δ (LNO) exhibited obvious superiority electrocatalytic activities [15] and performance of surface oxygen exchange [16]. Liu and coworkers confirmed that LNO applied as an SOEC oxygen electrode showed excellent electrochemical catalytic properties [17]. However, former research has also indicated that LNO electrodes would decompose in SOEC operating situations if Ce_xGd_1-xO_2-δ (GDC) was used as the oxide electrolyte [18]. This resulted in a rapid degeneracy in performance [16]. Flua’s group further proved that the decomposition of LNO on GDC was induced by element diffusion at the LNO–GDC interphase [19]. This decomposition phenomenon was also found at the La_0.5Sr_0.5MnO₃ (LSMO) electrode [20]. The LSMO gradually cracked into nanoparticles due to oxygen ions migrating through the electrolyte during anodic polarization, which led to an alteration in valence of manganese ions and thereafter resulted in lattice shrinkage and consequent breakdown into particles [21,22]. Due to this concern, improved stability of perovskite oxides is a critical point in SOEC research. In addition, although LNO and LSMO have superior performance in oxygen surface exchange [16], they have weakness in electronic conductivity. With the aim to overcome this shortcoming, a La₂NiO_4-δ-coated PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_5-δ electrode was prepared which showed good electrochemical performances [23,24]. At the same time, La_1-xSr_xCo_1-yNi_yO_3-δ is a common cathode for SOFCs [25,26], moreover, La_1-xSr_xCo_1-yNi_yO_3-δ (LSCNO), La_1-xSr_xCo_1-yFe_yO_3-δ (LSCFO) [27], and Ba_xSr_1-xCo_yFe_1-yO_3-δ (BSCFO) [28] have also been widely investigated as electrodes for SOECs [29,30]. High electronic conductivity was found for Co-based electrodes [31,32], however this was accompanied by an increasing thermal expansion coefficient [33]. In this case, preparing Co-based perovskites with high thermal stability is a challenge.

Perovskites have been proven to be the one of best cathodes for the SOEC devices, in particular, Co-based ones usually exhibit extremely high catalytic performances due to the multivalent properties of Co ions [34]. In this mini review, the application of Co-based perovskite in hydrogen production in SOEC devices is summarized. The effect of A- and M-site doping and crystal distortion in thin-film structures on OERs is discussed, in addition, the composite perovskite is reviewed providing insights into mending the weakness of single perovskite, and thereafter provide better electrochemical catalysts.

2. Crystal and Electronic Structure of Perovskite Oxides

The crystal structure of perovskite oxide (AMO₃), taking LaCoO₃ (LCO) as example, is shown in Figure 1a, which easily describes the prototypical ion-covalent, and the CoO₆ octahedra connected by the corners [35,36]. It features good divisibility in that the A site can be located in any rare-earth or alkaline earth metals, while the M site can be any transition metal, as shown in Figure 1b for the Sr-substituted LaCoO₃. In addition, the oxygen stoichiometry can also be varied based on the ratio of different kinds of metal ions. This structural adaptability has been successfully applied to tune the electronic structure of M site cations, specifically recognized as the OER active center, and therefore a target to manipulate the electrocatalytic activities. For example, Cheng and coworkers partially doped Sr²⁺ at the A site in LCO and then obtained increasing activity in OERs, which was attributed to the enhancement in overlap between the unoccupied Co 3d conduction bands and the occupied O 2p valence bands [37]. Zhang et al. replaced Ni with Fe in LaNiO₃ (LNO) to promote its OER activity, resulting from the influence of the Fe doping in strengthening the Ni–O bonding and thus restraining the generation of Ni²⁺ ions on the LaNi_1-xFe_xO₃ surface [38]. By employing a reductive heating treatment, Yang et al. produced high oxygen-deficient CaMnO_2.5, which exhibited great enhancement in OER performance when compared with unreduced CaMnO₃. The increase in OER activity was assigned primarily to the propitious electronic configuration of high-spin Mn³⁺ ions induced by sufficient oxygen vacancies in CaMnO_2.5 [39].

A heterogeneous catalysis greatly relied on the surface electronic structure models associated with the catalytic properties. In this case, the surface oxygen reactivity is thus an important factor for oxidation chemical reactions, such as OERs [40,41]. Due to their excellent flexibility in accepting a high variability of cations in A and/or M locations, the electronic conductivity of perovskites can be manipulated from insulating to metallic. Through careful material design, electronic structures in perovskites can be tailored to the thermodynamic energies of a variety of reactions to minimize electrochemical reaction barriers. Before describing the role of the active sites in OERs, common ideas on the electronic structure of perovskites should be given. The charge transfer energy gained from density functional theory (DFT) calculations has been proven to be the most proper descriptor, which leads to a model of the charge transfer energy between the adsorbates and transition metals (assigned as Δ′ in Figure 2). A recently proposed descriptor, charge transfer energy (Δ), displays a linear relationship with the polarization potential (η). This is because that the Δ values are obtained from both theoretical calculations and spectroscopic data. This finding proposes that Δ is the most helpful descriptor in the design of OER perovskite oxide catalysts, as Δ decreases, the Δ′ may also decrease and lower the energy. The O 2p band center relative to the Fermi energy and e_g electron number of the transition metal ions at the Msites is not necessarily adequate for the most part of perovskite oxides [42].

3. Co-Based Perovskite for SOECs

3.1. Pure or Doped Perovskite

Co-based perovskite oxides are promising electrocatalysts for OERs applied in various devices; however, a shortage of fundamental comprehension in oxide surfaces obstructs their rational design in order to further enhance their electrochemical performance. In this case, further understanding of the factors which influence OERs is essential. The properties of perovskite electrocatalysts greatly depend on their electronic configurations, especially for electronic conductivity and oxygen vacancy. Figure 3a reports a control method for lattice orientation of LCO films using sol–gel technology, and Figure 3b–d show their corresponding high-resolution transmission electron microscope (HR-TEM) images. The LCO films with different lattice orientations induced different distortion degrees of the CoO₆ octahedron, which results in a spin-state transition of Co ions from a low spin-state (t_2g⁶e_g⁰) to an intermediate spin-state (t_2g⁵e_g¹), as proven by the Co L-edge and O K-edge X-ray absorption spectroscopy (XAS). The OER ability of the LCO films has been ascribed to the synergetic effects of the e_g electron filling degree, electronic conductivity, and adsorption free energy of the intermediates. Judging by these descriptions, the LCO(100) film possesses the optimal e_g electron filling degree, which is as high as 0.87, lower adsorption free energy, and higher conductivity as indicated in Figure 4. It shows the best electrocatalytic ability for OERs, which is 2.9 and 6.7 times greater in mass density compared with the LCO(110) and LCO(111) films under an overpotential of 470 mV [43]. This is because that the (100) film appears to possess a JT-like distortion in the CoO₆ octahedrons, consequently elongating the Co–O covalent bonding and thus stabilizing the intermediate spin-state Co³⁺ (t₂g⁵e_g¹) ions. Furthermore, (100) film exposes more Co–O bonds, and the largest tensile extension of Co–O bonds contributes to weakening of the Co–O covalent bonding, which also assists in intermediate adsorption and release of O₂ generated in the electrolyte, empowering a better OER electrocatalytic performance.

Electrochemical studies of the epitaxial La_1−xSr_xCoO_3−δ (LSCO)(001) films have been carried out using in situ and operando ambient pressure X-ray photoelectron spectroscopy (XPS) to detect the effects of surface stoichiometry and electronic structure on OERs. Electrochemical results indicate that both lattice strain and oxygen vacancies play an important role in determining OER performance. In a humid environment, hydroxyl and carbonate affinity increase with Sr content (up to 40%), leading to an increase in the binding energy of metal core levels and the valence band edge, which then induces the formation of a surface dipole. Formation of OH might be impelled by the re-filling of oxygen vacancies and as well as the reduction of Co⁴⁺ to Co³⁺ ions, as it agrees with the correlation of OH coverage with hole mobility. The combination of in situ and in operando XPS results on thin films has brought a deeper understanding of the oxide surface and electrochemical interface, which will guide future modeling of the realistic electrocatalyst surfaces and OER mechanisms as well [44].

The effects of lattice strain and oxygen vacancies of oxide catalysts in OERs have been extensively investigated, which are generally treated separately due to the limitations in experimental setup. Recently, the conjugated impacts of both lattice strain and oxygen vacancy on the electrocatalytic activity of LSCO films prepared on the LaAlO₃ (LAO) and SrTiO₃ (STO) single-crystal substrates have been investigated [45]. Electrochemical experiments showed that the OER activity of the LSCO films was higher under compression rather than under tension and were diminished when oxygen vacancies were introduced. Figure 5 shows the electronic structures of LSCO films with various strain statuses and oxygen deficiencies obtained from the DFT calculations, including density of states (DOS) (Figure 5a), the occupancy of e_g states (Figure 5b), the energy gap between O 2p and Co 3d band center (Δ) (Figure 5c), and oxygen vacancy formation energy on LAO and STO substrates (Figure 5d). Perovskite catalysts with lower Δ values were observed to exhibit greater electronic conductivity and an easier charge transfer process in OERs. A smaller Δ or a higher O 2p band center energy relative to the Fermi level could enhance the participation of lattice oxygen during OERs, thus leading to higher activity. Compared with the effects of strain on the e_g filling degree and Δ value of LSCO for a δ value, the e_g occupancy for LSCO/STO with tensile strain and LSCO/LAO with compressive strain are quite close. Contrarily, the Δ value has proven to be conspicuously greater for LSCO/LAO than LSCO/STO with same δ, which suggests a lower electronic conductivity, a difficult charge transfer process, and less covalent bonding between metal and oxygen.

Such strain induces extra oxygen defects in the LSCO/STO interphase increasing the e_g state filling degree and as well as the energy difference between the O 2p and Co 3d orbitals, resulting in a reduction in OER activity. Both the LSCO/STO and the LSCO/LAO showed inferior OER activities as more oxygen defects are created via vacuum heating treatment. This degradation in OER performance could be reinstated by re-locating oxygen ions back to the films during OER operation. Engineering lattice strain and tuning defect chemistry could be regarded as the effective methods to develop high-performance catalysts. Such regulation could also be applied to other metal oxides and be helpful in piloting the rational design of new kinds of high-performance catalysts for various energy conversion devices. Depending on the calculation results of e_g occupancy and Δ value, one may predict similar or even worse OER activity for LSCO/LAO compared with LSCO/STO if given the same δ value, which in contrast has a noticeably better OER performance for LSCO/LAO than LSCO/STO when observed experimentally. Such alterations in electronic properties are high, possibly due to be the origins of the degraded OER activity for the vacuum-reduced LSCO, observed in electrochemical characterization [45].

Ionic conductor GdBaCo₂O_5.5±δ (GBCO) thin film has been grown epitaxially and applied to OERs in electrochemical devices. Time-resolved XRD combined with voltage step chrono-amperometric results provided a deeper understanding of chemical expansion (CE) mechanisms at the GBCO electrode. The CE coefficient at the c-axis showed a negative value during compound oxidation with a remarkable asymmetry from α_c = −0.037 to −0.014 at both δ < 0 and δ > 0. This alteration in CE indicated structural changes associated with the alterable Co cation oxidation valence from Co²⁺ to Co³⁺ and then to Co⁴⁺. The reaction dynamics gradually decelerated in reduction when δ < 0; however, they still maintained a high catalytic activity when δ > 0, consistent with the structural variations and the electronic carrier delocalization when crossing δ = 0 [46]. Then, Sr-doped GBCO (GdBa_1-xSr_xCo₂O_6-δ, GBSCO) was also investigated, which possesses an enhanced oxygen evolution performance and higher electronic conductivity compared to GBCO. XRD patterns combined with Rietveld analysis indicated that the electrodes with higher Sr-content dopant crystallized with higher tetragonal P4/mmm symmetry. Excellent electrical conductivity of 980 S cm⁻¹ was detected for GBSCO, in agreement with the better OER activity at 1.51 V (the onset polarization) versus a reversible hydrogen electrode (RHE). This outstanding electrochemical characteristic was explained based on the wider carrier bandwidth, which is a function of Co–O–Co bending angles and Co–O bonding lengths. Moreover, the higher oxygen evolution activity was observed in spite of the formation of non-lattice oxides and enrichment of alkaline ions at the electrode surface [47]. Thorough understanding of the crystallographic occupancy ordering and surface characteristics are fundamental for optimizing the electronic conductivity, ionic mobility, and electrocatalytic performances, which is critical for designing electrochemical devices with high efficiency.

An easy sol–gel approach was developed to prepare M site ordered double perovskites Ba₂Bi_xSc_0.2Co_1.8-xO_6-δ (BBSCO), in which Co ions are located in two different environments. The BBSCO electrodes were demonstrated to be superior electrocatalysts for OERs in alkaline solution with long-term test, no obvious performance degradation was observed based in the continuous chronopotentiometry results. The critical role of the ordered [Co²⁺] and [Sc³⁺, Bi⁵⁺, Co³⁺] dual environments in improving the OER activity was approved. Compared with BSCFO, the presence of ordered Co dual-site environments explained the high OER activity of BBSCO [48]. A promising Ca-doping method was also used to enhance the durability and electrocatalytic OER activity of Pr_0.5Ba_0.5CoO_3-δ (PBCO). XRD data indicated that phase structures progressively altered from layered perovskite PBCO to the conventional cubic one with rising Ca content. Compared to pristine PBCO, the electrocatalytic activity of Ca-doped PBCO perovskite was increased by about 90%. More importantly, after one thousand cycles of OER electrolysis tests, the activity of PBCO without Ca doping was degraded by approximately 60%. In contrast, there was almost no activity degradation for the Ca-doped one. These results indicated that the OER durability could be significantly improved by Ca doping, and thus makes it a promising electrocatalyst for electrolysis cells [49].

3.2. Composite Perovskite

Many composite electrodes have been studied as OER catalysts for SOECs, such as LSCFO–LaSrGaO₄(LSGO) oxygen electrodes [50], in which LSGO was used as the scaffold, and the LSCFO nanoparticles (~100 nm) were homogeneously coated on to the LSGO scaffold surface. The cathode polarization resistance for the LSCFO–LSGO cathode was lower than that of pure LSCFO. The enhancement in electrochemical performance of the nano-LSCFO–LSGO cathode could be attributed to the improvement of the triple phase boundaries. Qiu and coworkers reported impregnation of LSMO into Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCFO), which greatly improved the CO₂-poisoning resistance of the BSCFO electrodes [36]. The core constituted a porous BSCFO backbone with high oxygen ion conductivity, while the dense shell was composed of LSMO. The presence of the dense LSMO shell prevented the BSCFO core cathode from being poisoned by CO₂, and therefore increased the electrochemical performance of the electrode [51].

By taking advantage of the charge transfer kinetics at the junction of RuO₂/LBSCFO perovskite, the SOEC demonstrated an outstanding electrochemical performance [52], which delivered a current density of ~2.26 A cm⁻² at 1.6 V and 600 °C, and was maintained after 90 h. The built-in charge transfer in the RuO₂/LBSCFO junction revealed the electron donation from the Co–O bonding orbitals to the RuO₂ clusters. In addition, RuO₂ further reinforced the electrocatalytic performance by enhancing the mass transport during OERs. Sharma and coworkers studied Cr deposition on LSCO as the contact layer [53], and concluded that Cr deposition occurred throughout the whole LSCO contact layer after polarization at SOEC conditions for 2000 h by the formation of La–Cr–O phases at the electrode’s outermost surface. Later on, the influence of Cr deposition on the electrochemical activity of LSCFO electrodes under SOEC operating conditions was also investigated [54]. The formation of a major SrCrO₄ phase on the LSCFO surface and rapid degradation in the electrochemical activity were observed. Fabrication of perovskite oxide with various carbon materials to form composite catalysts is a widely accepted strategy to improve OER catalytic activities. Porous Sm_0.5Sr_0.5CoO_3-δ (SSCO) hollow nanofibers composited with cross-linked, three-dimensional, and N-doped graphene [55] exhibited a remarkable enhancement in OER activity in alkaline media. The onset potential for OERs is only 1.53 V with a Tafel slope of 115 mV dec⁻¹. The synergistic effect between SSCO and graphene components in the OER process were revealed by the DFT calculations, which suggested that the electron transport from graphene to O₂ and SSCO enhanced the electrocatalytic reaction activity by activating the O₂ via increasing the covalent bonding of lattice oxygen. The significance of this result suggests a direction for the design of perovskite-type bifunctional catalysts to improve OERs.

4. Conclusions & Perspective

Co-based perovskites applied in SOECs has been reviewed. The previous research data indicated that the excellent electrochemical properties of Co-based perovskites in hydrogen production strongly depend on their electronic and crystal structures. In addition, the solid interface would also enhance the catalytic capability by increasing the electron transport. The previous reports indicated that an enhancement in the transport properties is usually linked to an increasing mismatch in the thermal expansion; therefore, the suitable design of a Co-based perovskite is important. There is a common way to limit the thermal expansion of Co-based perovskite by surface coating of ceria-based oxide or impregnating them into LNO, which is an impactful way to design high-performance SOEC cathodes. The electronic structural design could be realized from the chemical flexibility of perovskites by replacing or partial replacing A and/or M metals. Moreover, the different crystal structures of a perovskite could be prepared on different substrates, and then exhibit various electronic properties which could influence the OER process. The literature data indicates that surface and interfacial engineering by altering the crystal strain and heterointerfaces presents exciting opportunities to enhance the electrocatalytic properties of perovskites.

We mainly have two perspectives for future research. First, to draw a reliable picture concerning the impacts of lattice strain, surface electronic and crystal structures on OER activity, which is necessary to combine advanced theoretical calculations and experimental research. The results obtained, both theoretically and experimentally, could be used to better understand the role of surface and interfacial engineering on the design of proper perovskites for OERs. Based on this understanding, one can rationally design an electrocatalyst with high activity and stability for different applications. Secondly, besides the activity, the long-term stability of oxide catalysts is also essential for the practical application of oxide catalysts. To understand the reasons of performance reduction with time, in situ characterization techniques are essential to gain valuable information on trancing the material properties under real operating conditions. The in situ measurement results during electrochemical tests will provide useful information, beneficial to the rational design of catalysts with high activity. In this case, developing new in situ techniques to monitor true catalysts with a combination of theoretical calculations would make it possible to determine OER active sites and comprehensively understand the OER mechanism. This would facilitate the design of high-efficiency and robust OER catalysts which could be applied in high-performance SOECs.