Study On The Electrochemical Performance Of Solid Oxide Direct Carbon Fuel Cell

A solid oxide direct carbon fuel cell（SO-DCFC）converts the chemical energy stored in solid carbon fuels into electricity directly.It has the outstanding advantages of an all-solid-state structure,high energy efficiency,less pollution emissions and extended fuel feasibility.However,SO-DCFC technology is still far away from practical applications due to its poor cell performance.It is well known that the properties of carbon fuel itself,the single-cell structure and the electrode materials are all closely related to the cell performance.This work investigates the key factors affecting cell performance systematically and explores a series of methods to improve the performance of SO-DCFC from the aspects of development and treatment of carbon fuel,optimization of the cell structure and modification of electrode materials.1.Activated carbon（AC）and AC pretreated with NaOH and HNO₃ are investigated as the fuel of SO-DCFC to systematically study the effects of surface modification on the reactivity of AC in SO-DCFC.Both NaOH and HNO₃ treatments increase the oxygen content and decrease the graphitization degree of AC.The amount of hydroxyl groups on the surface of the three AC samples decreases remarkably,while the carbonyl and quinone groups on the surface of the AC remain stable during the heating process.The AC treated with HNO₃（AC-HNO₃）shows the highest reactivity towards oxidation and reverse Boudouard reactions owing to its highest oxygen content and lowest graphitization degree.The single cell with a 380μm-thick yttria-stabilized zirconia（YSZ）layer as the electrolyte and AC-HNO₃ as the fuel exhibits the lowest polarization and the highest maximum power density(P_max)of 128 mW cm^-2 at 800℃.2.A simple and effective spin coating method is developed to fabricate the anode supported SO-DCFC.The effects of the thicknesses of the anode functional layer（AFL）on the microstructure and output performance of the single cell are investigated.It is confirmed that the spin coating parameters affect not only the thickness of the YSZ electrolyte film,but also the YSZ grain size.The YSZ grain size increases during the film growth process.An optimal thickness of the YSZ electrolyte of 4μm is determined.Both the flatness of the YSZ film and the output performance of the corresponding cell are improved with the increase of AFL thickness.The single cell with a 14μm-thick AFL exhibits the highest P_max of 674 and 304 mW cm^-2 at 750°C with H₂ and AC as fuels,respectively.Both the ohmic and polarization resistances decrease with the increase of the thickness of the AFL,because the increase of the thickness of the AFL improves the contact between AFL and the electrolyte,and enlarges the three-phase boundary regions.3.The biochars derived from pine wood and birch wood,and commercial AC are used as the fuels of SO-DCFC,to systematically study the effects of the characteristics of carbon fuel on the output performance of the single cell.The birch char shows the highest reactivity towards oxidation and reverse Boudouard reactions owing to its lowest graphitization degree and highest content of biologically accumulated alkali metal and alkaline earth metal.The anode supported single cell with a Ni_0.9Fe_0.1O_δ-YSZ AFL demonstrates the highest P_max of 406.5 mW cm^-2 with birch char as fuel at 750°C,which also exhibits a discharge current density of 100 mA cm^-2 for about 4.9 h using0.24 g birch char as fuel.4.The surface properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF）perovskite electrode are modified with HNO₃ etching to improve its catalytic activity towards oxygen reduction reaction（ORR）between 500–650℃.The main perovskite structure is unchanged,the surface concentrations of A-site cations are reduced extremely and plenty of Fe O_x particles with a size of 5–7 nm are generated on the rough surface after LSCF is treated with the HNO₃ solution.After the calcination at 1150℃,the severe surface segregation of Sr is observed on the untreated LSCF powder,which is suppressed remarkably with HNO₃ pretreatment.Highly reactive oxygen and Fe⁴⁺species are formed on the surface of LSCF after the HNO₃ treatment,which is beneficial to ORR.The polarization resistance of the LSCF cathode is reduced remarkably by HNO₃ etching.The reduction of oxygen species adsorbed on the LSCF surface,the rate-determining step of the electrode process,is accelerated significantly with the acid etching.