Introduction The development of high-performance fuel electrode materials suitable for reversible operation is crucial for the application of reversible solid oxide cells (RSOCs). The fuel electrode should have excellent catalytic activity such as hydrogen oxidation reaction and hydrogen evolution reaction in the solid oxide fuel cells/solid oxide electrolysis cells (SOFC/SOEC) dual mode. Conventional Ni-YSZ fuel electrode materials are susceptible to sulfur poisoning when using fuel gas containing sulfur impurities and carbon buildup when using hydrocarbon fuel. LaxSr1-xTiO3 perovskite materials have a superior resistance to coking and sulfur poisoning, which is widely used in RSOCs fuel electrodes in recent years. However, the poor ionic conduction and the non-ideal catalytic ability of fuel gas affect the further application of LaxSr1-xTiO3 fuel electrodes. It is thus necessary for the improvement of the catalytic activity and the electrochemical performance of the fuel electrodes to optimize the microstructure and surface modification. In this paper, La0.2Sr0.8TiO3-?(LST)/YSZ-based fiber fuel electrode by Ce0.9M0.1O2- (M=Fe,Co,Ni) impregnation modification was prepared, and the effect of impregnation on the electrochemical properties of fiber fuel electrode in SOFC/SOEC dual mode was investigated.Methods La0.2Sr0.8TiO3-?(LST)/YSZ composite fibers were directly prepared by an electrospinning method. In the preparation, stoichiometric amounts of La(NO3)3·6H2O, Sr(NO3)2 and C16H36O4Ti were weighed and dissolved with N, n-dimethylformamide (DMF) as a solvent and polyvinylpyrrolidone (PVP) as binder and one-dimensional template. The YSZ suspension was added to the LST precursor solution to obtain the LST/YSZ electrospinning solution for electrospinning. The high catalytic activity of Ce0.9M0.1O2- (M=Fe, Co, Ni, CMO) was loaded on LST/YSZ composite fiber skeleton by an ion impregnation method to prepare the fiber fuel electrode. The electrolyte-supported single cells (CMO@LST/YSZYSZYSZGDCLSCF/GDC) were fabricated to evaluate the electrochemical performance of the fiber fuel electrode. The phase composition of the prepared composite fiber and the fuel electrode after immersion and calcination was determined by X-ray diffraction (XRD). The composite fiber morphology and the cross-section morphology of the tested fuel electrode were characterized by scanning electron microscopy (SEM). The two-phase distribution of the composite fiber was determined by high resolution transmission electron microscopy (HRTEM). The electrochemical performance of the fiber fuel electrodes in SOFC/SOEC dual-mode was tested via assembling a single cell. The impedance data were analyzed via the distribution of relaxation time (DRT) , and the relationship between the microstructure and the electrochemical process was analyzed. Results and discussion The electrospinning solutions with stable spinning are prepared via adjusting the LST:YSZ mass ratio. The LST:YSZ with a mass ratio of 6:4 shows a criss-cross fiber morphology and smooth surface with uniform diameters (i.e., 150-200 nm) through the electrospinning and calcination. The composite fibers consist of the LST skeleton with YSZ particles dispersed and embedded, which is prepared directly by electrospinning, thus forming a three-dimensional network of mixed conductor channels with electronic channels as the main path and ionic channels interspersed. The LST:YSZ composite fiber was used to prepare a fuel electrode skeleton, and CMO precursor solution is prepared to impregnate and modify the fuel electrode. Compared with the fiber morphology, the morphology of fiber fuel electrode is smaller in length and diameter, showing a short rod-like morphology. The surface of the fiber is evenly covered by the impregnated CMO nanoparticles (i.e., 50-80 nm), still retaining abundant and interconnected pores. The three-dimensional network fiber electrode is constructed. The impregnate uniformly wraps on the fiber surface to strengthen the interfacial bonding between the fuel electrode and the YSZ transitional layer. The different impregnation modification has an effect on the catalytic activity of the fuel electrode in SOFC/SOEC dual mode. The fuel electrode using Ce0.9Ni0.1O2- as an impregnate has more Ni nanoparticles exsolution after the testing, effectively increasing the length of the three-phase interface and the active sites for the catalytic reaction. Based on the comprehensive analysis of EIS and DRT, the modified fiber fuel electrode can significantly affect the interfacial charge transfer process corresponding to the middle- and low- frequency. Ni nanoparticles exsolution can adsorb/dissociate hydrogen, and have a tendency to form an activated complex with water molecules, so that the fiber fuel electrode has great hydrogen oxidation capacity and hydrogen reduction capacity.Conclusions LST/YSZ composite?fiber with rich connected pores was directly?prepared?by?an electrospinning?technique. This fiber had a three-dimensional network hybrid conductor pathway with LST skeleton as the main electronic channel and YSZ particles as ion channel interspersed. The interface bond between the fuel electrode and the electrolyte layer was further enhanced via modifying the fuel electrode skeleton with CMO nanoparticles impregnation. The results showed that the fuel electrode with Ce0.9Ni0.1O2- impregnation had a high hydrogen oxidation and reduction capacity. Under the combined action of the impregnation and the precipitation of Ni nanoparticles, the single cell had the maximum power density of 342?mW/cm2 at 850?℃, 3% (in volume fraction) H2O/H2, and the current density of 0.313 A/cm2 under 46.8% H2O/H2 for electrolysis at 1.3 V.