The transformation of chemical energy to electrical energy can be realized through solid oxide fuel cells (SOFCs) with the advantages of high conversion efficiencies, extensive fuel sources, and low emission, as compared with other energy technologies^[1-2]. Over the last few decades, the common operating condition of SOFCs is 800-1000 ℃. High operating temperature of SOFCs gave rise to some challenges regarding sealing, electrode stability, chemical compatibility, start-up and thermal cycling performance, thus limiting the commercialization of SOFCs. To achieve longer life of cell and faster start-up, the operating temperature of the fuel cell must be shifted towards intermediate-to-low temperature (500-800℃). Nevertheless, the oxygen reduction reaction (ORR) and oxygen surface exchange processes are greatly limited by the decline in temperature^[3]. A large amount of polarization loss occurs at the cathode. Therefore, the development of high-performance cathode materials for IT-SOFCs is essential.

LnBaCo₂O_5+δ (Ln=Pr, Nd, Sm, Gd, La and Y) oxides with excellent ion-electron mixed conduction, high oxygen surface exchange (K) and bulk diffusion (D) coefficient have attracted much attention, which can be used promising cathode materials for IT-SOFCs^[4⇓-6]. In these perovskite oxides, rare earth ions and alkaline ions took up positions at A site of the lattice^[7]. The structure is orderly arranged in sequence of [BaO][CoO₂][LnO_δ][CoO₂] along c-axis. This unique arrangement of ions allows the binding energy of oxygen and rare metal elements to be effectively reduced. But LnBaCo₂O_5+δ perovskite oxides have high coefficients of thermal expansion (CTE) similar to cobalt-based compounds, resulting in the thermal incompatibility with the other SOFC components. The average CTE of LnBaCo₂O_5+δ perovskite oxides are generally 17.6×10^-6 to 21.5×10^-6 K^-1 at 30-800 ℃^[5], which is significantly higher than that of electrolytes (e.g. Gd_0.1Ce_0.9O_2-δ (GDC, 13×10^-6 K^-1), (Y₂O₃)_0.08(ZrO₂)_0.92 (YSZ, 10.5×10^-6 K^-1))^[8-9] and cobalt-free cathodes (such as NdBa_0.5Sr_0.5Cu₂O_5+δ, 14.5×10⁻⁶ K^-1)^[10]. The high CTE of LnBaCo₂O_5+δ mainly comes from two aspects. On the one hand, the spin transition of Co³⁺ leads to an increase in the volume of the CoO₆ octahedron; on the other hand, plenty of oxygen vacancies are generated during the thermal reduction of Co ions, resulting in lattice expansion^[11-12]. Therefore, partially or completely replacing Co with transition metals (Fe, Mn, Cu and Ni) is a feasible approach to depress CTE of LnBaCo₂O_5+δ^[13⇓-15]. Zhang et al.^[16] reported that CTE of GdBaFeNiO_5+δ is 14.7×10^‒6 K^-1 at 30-800 ℃, which is lower than that of GdBaCo₂O_5+δ (GBCO, 20.1×10^‒6 K^-1) in the same temperature range. But this brings about a decrease in the electrochemical catalytic activity of the oxides.

High-entropy ceramics (HECs) were firstly proposed and joined in high-entropy materials in 2015^[17]. HECs are composed of multi-component ceramic compounds^[18], and their high structure entropy is beneficial to the formation of HECs. HECs are demonstrated a homogeneous, crystalline, single-phase structure, which makes their properties superior to those of traditional ceramics^[19]. Han et al.^[20] reported that LaMn_0.2Fe_0.2Co_0.2Ni_0.2Cu_0.2O_3-δ(HE-LMO) materials effectively enhanced the electrochemical performance of Sr-free cathode (LaMnO_3-δ). Yang et al.^[21] found that La_0.2Pr_0.2Nd_0.2Sm_0.2Sr_0.2MnO_3-δ (HE-LSM) suppressed performance degradation caused by Sr segregation. Ling et al.^[22] researched SmBa(Mn_0.2Fe_0.2Co_0.2Ni_0.2Cu_0.2)₂O_5+δ (HE-SBO) double perovskite oxides, which grievously reduced the CTE of cobalt-based perovskite oxides. Therefore, development of new high-entropy materials to obtain suitable CTE and high-performance cathode for IT-SOFC is feasible. In this work, GdBa(Fe_0.2Mn_0.2Co_0.2Ni_0.2Cu_0.2)₂O_5+δ (HE-GBO) oxide was obtained by uniformly doped with various transition elements at B site to improve ORR reaction activity at intermediate temperature via high-entropy effect, its structure and relatively electrochemical properties were explored. Subsequently, GDC was introduced to fabricate HE-GBO-GDC composite cathode for further improving the electrochemical performance of the single cells.

Various metal cation compounds: Gd₂O₃, BaCO₃, Co(NO₃)₂·6H₂O, C₄H₆NiO₄·4H₂O, (Mn(NO₃)₃), Cu(NO₃)₂·3H₂O, and Fe(NO₃)₃·9H₂O were employed to synthesize GdBa(Co_0.2Mn_0.2Fe_0.2Cu_0.2Ni_0.2)₂O_5+δ (HE-GBO) by the self-propagating combustion method, and the specific preparation process has been introduced in our previous work^[23]. Then, to obtain the pure phase of this powder, the primary powder was sintered at 1150 ℃ for 3 h.

The dendritic anode was prepared using phase inversion technology^[24]. Commercial NiO and YSZ were blended with 0.216 g of poly-vinylpyrrolidone and 14.16 g solution of 1-methyl-2-pyrrolidinone (NMP) and polyether sulfone (PESF) (mass ratio 17.7:100) to acquire a homogenous slurry. Two parts of the slurries were placed in a mold and separated by a stainless-steel mesh (ϕ150 μm), and water was poured into the mold at a constant speed. After being left for 2.5-3.0 h, the raw substrate was taken out of the mold, soaked in water for 12 h and dried at 50 ℃. The dried raw substrate was sintered at 1050 ℃ for 2 h. Then YSZ electrolyte slurry was spin-coated 3 times on the anode substrate, and sintered at 1400 ℃ for 10 h. The YSZ slurry was fabricated with 10 g YSZ, 0.2 g polyester/polyamine copolymer (KD-1) and some acetone as solvent with ball milling for 10 h. After the addition of 20 g terpineol ethyl cellulose (5%), the sample was further milled for 12 h. The preparation method of GDC slurry is the same as the above-mentioned YSZ slurry, coated on the YSZ and sintered at 1250 ℃ for 3 h. Subsequently, cathode slurry was coated on GDC, and sintered at 950 ℃ for 3 h. The active area of cathode was 0.24 cm². HE-GBO slurry was manufactured by hand milling 1 g HE-GBO powder with 1.5 g terpineol ethyl cellulose (10%). The synthesized HE-GBO powder was mixed with 30% (in mass) GDC to acquire composite cathode slurry.

X-ray diffractometer (XRD, Bruker 8 ADVANCE, Germany) was used to analyze the characteristic of GBCO, HE-GBO and the relatively chemical compatibility with YSZ and GDC, respectively. GBCO and HE-GBO powders were compressed to form cuboid bars in mold, respectively, and calcined at 1150 ℃ for 5 h for CTE measurement by a thermal dilatometer (DIL402PC, Germany). The electrochemical data of cells were collected using an electrochemical workstation (ZAHNER, Germany). The structures of fuel cells were observed by a scanning electron microscope (SEM, Gemini-300, Germany). The energy dispersive spectroscopy (EDS) mapping was explored by X-ray spectroscope (Oxford Instruments AZtecEnergy, England).

XRD patterns of HE-GBO and GBCO oxides are shown in Fig. 1. The result indicate that HE-GBO is a single-phase double perovskite without any impurities after calcined at 1150 ℃ for 3 h. Previous studies demonstrated that GBCO belongs to an orthogonal structure^[25-26]. The difference in the XRD patterns of GBCO and HE-GBO comes from the change in crystal structures, which suggests that the crystal structure of HE-GBO transform into a tetragonal structure(P4/mmm)^[16]. Schematic diagram of the HE-GBO double perovskite structure is shown in Fig. 1(c).

Chemical compatibility of HE-GBO and YSZ was explored by sintering HE-GBO-YSZ mixtures (mass ratio 1 : 1) powder at 950 ℃ for 3 h, as shown in Fig. 2(a). There are some new reaction phases except HE-GBO and YSZ, indicating that HE-GBO is chemically incompatible with YSZ at 950 ℃. In Fig. 2(b), all diffraction peaks of HE-GBO-GDC mixtures (mass ratio 1 : 1) come from the peaks of HE-GBO and GDC, and no new diffraction peak and position shifts are observed, suggesting that HE-GBO is chemically compatible with GDC. Therefore, GDC can be a good barrier layer between YSZ and HE-GBO to prevent side reactions.

Thermal expansion curves of GBCO and HE-GBO from 25 ℃ to 800 ℃ measured under the same condition are shown in Fig. 3. CTE of HE-GBO is 15.7×10^-6 K^-1, which presents relatively reduced CTE compared with GBCO (20.1×10^-6 K^-1). In HE-GBO, Co is partially replaced by other transition metal elements (Fe, Mn, Ni and Cu), which weakens the influence of the spin state transition of Co ions. Meanwhile, the serious lattice distortion impedes the oscillation amplitude of constituent atoms, resulting in a significant decrease in the CTE of HE-GBO^[27]. Preparation of HE-GBO-GDC composites by addition of GDC can further meet the requirements of thermal expansion matching with electrolytes. Related reports have demonstrated the feasibility of GDC composite electrode materials^[28].

Electrochemical impedance spectroscopy (EIS) plots of symmetrical cells using HE-GBO and HE-GBO-GDC as cathodes in air are shown in Fig. 4. The intersection point of the high-frequency and low-frequency impedance arc with x-axis represents Ohmic impedance (R_Ω) and total impedance (R_t), respectively. Polarization impedance (R_p) can be obtained by subtracting R_Ω from R_t. R_Ω is not discussed and set to zero. R_p of HE-GBO are 1.68, 2.80, 8.28, 27.20 and 98.53 Ω·cm², and R_p of HE-GBO-GDC electrode are 0.23, 0.53, 0.82, 4.00 and 12.58 Ω·cm² at 800-600 ℃, respectively, indicating that HE-GBO-GDC composite cathode possesses higher electrochemical performance than HE-GBO. Because the introduction of the oxygen-ion conductive phase (GDC) in HE-GBO can increase the three-phase interface (TPB) for the oxygen reduction reaction^[29].

The gas mass transfer can be greatly improved using vertical microscopic channels Ni-YSZ anodes^[30]. The microstructures of the cell with HE-GBO and HE-GBO-GDC cathodes after 100 h of long-term testing exhibit in Fig. 5(a, b), respectively. Both single cells consist of 4 components (Ni-YSZ, YSZ, GDC and LSCF-GDC), and components are in good contact.

YSZ electrolyte achieves densification to distinguish the anode and cathode gas atmospheres, and its thickness is about 14 μm. The thickness of the GDC barrier is around 8 μm. Both cells have similar microstructures. SEM microstructures of HE-GBO and HE-GBO-GDC cathodes are given in Fig. 5(c, d) to perform the stability after testing, respectively. HE-GBO displays a loose and porous structure, and the porosity of HE-GBO composite cathode decreases with the addition of GDC. GDC particles uniformly cover the surface of HE-GBO, enlarging the effective area of three-phase interface and reducing R_p of the single cell. The associated EDS spectra of HE-GBO cathode after long-term testing can be observed in Fig. 6. Gd, Ba, Co, Fe, Ni, Mn, and Cu elements are distributed homogeneously.

The electrochemical performance of the cell with HE-GBO and HE-GBO-GDC cathodes is characterized by using wet hydrogen (~3% H₂O) as fuel at 800-600 ℃ in Fig. 7(a, b), respectively. The open circuit voltages (OCVs) of the cells with HE-GBO and HE-GBO-GDC in wet H₂ atmosphere are 1.086 and 1.071 V at 800 ℃, respectively. The maximum power densities (P_max) of single cell with HE-GBO as cathode are 972.12, 638.76, 417.39, 211.53, and 105.23 mW/cm² at 800-600 ℃, respectively. P_max of single cell with HE-GBO-GDC composite cathode are 1057.06, 745.19, 456.09, 249.66 and 122.28 mW/cm² at 800-600 ℃. P_max of single cell with HE-GBO-GDC composite cathode is 8% higher than that of single cell with HE-GBO single-phase cathode at 800 ℃, attributed to lower R_p and CTE of HE-GBO-GDC composite cathode compared to HE-GBO. Lower CTE of the composite cathode is beneficial to improving the bonding performance with the electrolyte and enhancing the oxygen ion migration ability, thereby increasing the electrochemical performance. Due to its high performance and fine thermal expansion compatibility with the electrolyte, HE-GBO-GDC is a further promising cathode for IT-SOFC.

The relevant EIS plots of single cell with HE-GBO and HE-GBO-GDC cathodes measured at OCV exhibit in Fig. 8(a, b), respectively. R_Ω and R_p increase with the decrease of temperature. It is known that R_p of the anode-supported single cells which mainly derived from the polarization process of the cathode can directly reflect the cathode ORR activity^[31]. R_p of single cells with HE-GBO cathode increases from 0.26 to 7.83 Ω·cm² at 800-600 ℃. For HE-GBO-GDC cathode, R_p increases from 0.22 to 4.52 Ω·cm² at 800-600 ℃(Fig. 8(c)). HE-GBO-GDC composite cathode can enhance the electrochemical performance of the cell, which is due to the effect of GDC as mixed conductor of ions and electrons. It improves the diffusion rate of oxygen ions and increase the three-phase interface of ORR. The results of activation energy (E_a) for R_p by Arrhenius equation are shown in Fig. 8(d), E_a of HE-GBO and HE-GBO-GDC cathode are 134.43 and 118.81 kJ/mol, respectively.

To further evaluate the difference in R_p of the cell with HE-GBO and HE-GBO-GDC cathodes, distribution of relaxation time (DRT) is employed to analyze EIS in Fig. 9(a, b). DRT curves are divided into five peaks in the measured frequency field and defined as P1, P2, P3, P4 and P5. Among them, P1 may be related to hydrogen and oxygen diffusion within electrodes at low frequency^[32]. P2 peak is taken as the gas adsorption in the electrode. P3 and P4 peaks are associated with gas adsorption as well as oxygen reduction and diffusion of oxygen ions into electrolyte^[33-34]. P5 is ascribed to gas adsorption and diffusion of oxygen ions to TPB. As shown in Fig. 9(b), R_p of single cell with HE-GBO-GDC decreases at each rate-limiting step, suggesting that the addition of GDC is beneficial, and the improvement in catalytic performance of the HE-GBO-GDC becomes more pronounced with decreasing temperature. Weakening of P4 and P5 peaks may be attributed to the addition of the ionic conductive phase GDC.

The long-term testing of the single cell with HE-GBO cathode in H₂ (~3% H₂O) atmosphere was detected (Fig. 10). The voltages of the single cell under 200 mA/cm² show no attenuation after 100 h long-term measurement, which indicates that the high-entropy oxide as cathode has good stability on the foundation of high performance and owns a proper prospect for the application of IT-SOFCs.

This work proposes a high-entropy double perovskite cathode of GdBa(Fe_0.2Mn_0.2Co_0.2Ni_0.2Cu_0.2)₂O_5+δ (HE-GBO) to solve the conflict problem of thermal compatibility and catalytic activity in IT-SOFCs.

1) HE-GBO material is successfully synthesized via self-propagating combustion method and matches with GDC at operating temperatures. GDC is introduced into HE-GBO to prepare composite cathode.

2) Through doping multiple transition metal cations at B-site, HE-GBO material presents superior CTE (15.7×10^-6 K^-1) compared with GdBaCo₂O_5+δ, suggesting that the high entropy effect can significantly reduce CTE, and matches state-of-the-art electrolytes.

3) P_max of the single cells with HE-GBO-GDC is 8% higher than that of HE-GBO at 800 ℃, indicating the high-entropy cathodes are promising for compatibility- activity balance in IT-SOFC.