Anionic engineering of the Ruddlesden-Popper oxide La₂NiO_4+δ: Targeted enhancement of the electrocatalytic activity of air electrodes via chemical fluorination for solid oxide electrolysis cells

Ruddlesden-Popper (RP) perovskite oxides (A_n+1B_nO_3n+1) are promising air electrodes for solid oxide electrolysis cells (SOECs) due to their excellent electrochemical performance and unique structure. Herein, a method of anionic engineering for the oxygen sites of air electrode materials has been proposed in order to further improve the electrocatalytic activity and thermal stability of La₂NiO_4+δ (LNO). La₂NiO_4+δ-xF_x (x = 0, 0.05, 0.1, 0.15, denoted as LNOF0, LNOF005, LNOF01, LNOF015) perovskite oxides are synthesized and evaluated as the air electrode for SOECs. Introducing electronegative F⁻ weakens the metal-oxygen bond, optimizes the oxidation environment (e.g., lattice oxygen activity), and improves surface oxygen exchange. The increased ratio of reactive oxygen species accelerates the oxygen evolution reaction (OER) process. Moreover, the polarization resistance of LNOF01 is 33.3 % lower than that of the LNO in the same test condition at 800 °C. The current densities of the full cell prepared using LNOF01 air electrode for CO₂ and H₂O electrolysis at 800 °C are 1.23 A cm⁻² (70 % CO₂–30 % CO, 1.5 V) and 1.04 A cm⁻² (70 % H₂O-30 % H₂, 1.3 V), which are enhanced by 25.51 % and 44.44 % compared to the LNO, respectively. In the stability tests, the LNOF01 half-cell maintains stability for more than 170 h at 0.5 A cm⁻² and 800 °C, and the full cell tests with the electrolysis of CO₂ and H₂O maintains stability for more than 50 h at as high as 1 A cm⁻² and 800 °C, respectively. This study provides a rational design strategy for constructing high-performance SOECs air electrodes and demonstrates the potential of air electrode anion engineering for SOECs applications.