Optimization And Electrochemical Properties Of Cobalt-based Perovskite Electrocatalysts

With the gradual depletion of fossil fuels?e.g.,oil,natural gas and coal?,growth of global warming,deterioration of environment,and increase of energy demands,the employment of electrochemical storage and conversion processes coupled with renewable energy such as regenerative fuel cells and water splitting systems has been one of the most effective routes for creating a sustainable energy system.The employment of efficient and low-cost electrocatalysts for hydrogen evolution reaction?HER?is of critical importance.Although,perovskite oxides have recently gained intensive attention owing to the excellent activity and stability towards oxygen reduction/evolution reactions,their HER activity has been rarely reported.In this thesis,perovskite oxides have been optimized to improve the HER activity and stability by doping and modification.Firstly,we demonstrate that Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_0.95P_0.05O_3-?(BSCFP_0.05)and BSCFP-? with a mass ratio of 150(NaH₂PO₂ to Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-?),obtained via either in situ lattice doping or ex situ surface modification with phosphorus?P?could be efficient electrocatalysts for HER.BSCFP_0.05 and BSCFP-? exhibit onset overpotentials of³37 and²47 mV and low overpotentials of ⁴15 and³86 mV at 10 mA cm^-2 in 0.1M KOH,the lowest among the perovskite oxides for HER.Besides,BSCFP_0.05 and BSCFP-? show the high HER stability based on the accelerated cyclic voltammetry for 1000 cycles and the chronoamperometry.The dramatically enhanced activity of BSCFP_0.05 and BSCFP-? is attributed to an improved electrochemical active surface area,fast charge transfer kinetics,a high-oxidation state of Co cations within the Co³⁺/Co⁴⁺redox couple,a large amount of lattice oxygen species,and a strong adsorption of H₂O.In addition,the presence of an amorphous layer of BSCFP-? is beneficial for HER.These findings suggest that perovskite electrocatalysts could be optimized via either in situ lattice doping or ex situ surface modification.Secondly,we demonstrate that the HER activity of GdBa_1-xSr_xCo₂O_5+??x=0,0.2,0.4,0.6,0.8 and 1?could be optimized by tuning the sintering temperature and atomic ratio of A sites.We found that GdCoO_3-?appears for GdBa_1-xSr_xCo₂O_5+? calcined at 1000°C(GdBa_1-xSr_xCo₂O_5+?-1),while a pure phase could be obtained for GdBa_1-xSr_xCo₂O_5+? sintered at 1200°C(GdBa_1-xSr_xCo₂O_5+?-2).Interestingly,the HER activity of GdBa_1-xSr_x Co₂O_5+?-1 is far superior to GdBa_1-xSr_xCo₂O_5+?-2 because the presence of GdCoO_3-? could increase the electrochemical active surface area and reduce the charge transfer resistance.In addition,the intrinsic properties such as electronic conductivity,valence state of the B-site Co cation,oxygen vacancy concentration,and oxygen species could be further optimized by adjusting Ba/Sr ratio.After the optimization,GdBa_0.2Sr_0.8Co₂O_5+?-1?GB2S8CO-1? exhibits the best HER activity with an onset potential of 71.6 mV and a corresponding Tafel slope of 107.5 mV dec^-1.Furthermore,it shows excellent operational stability by evaluating the cyclic voltammetry for 1000 cycles and the chronoamperometry for 10 h.The increased HER activity and stability could be attributed to factors such as a large electrochemical active surface area,a small charge transfer resistance,the formation of amorphous region,and a high valence state of the Co cation.