Spinel-type Oxides For Fuel Electrodes Of Solid Oxide Cells

In recent years,the global economy has grown rapidly,but with it comes various climate problems and energy shortages.The massive use of carbon-containing fuels has led to a rapid increase in carbon emissions,thus leaving a large amount of CO₂ free in the air,causing the greenhouse effect.Fossil energy is not renewable,and today fossil energy is constantly being mined and facing depletion,while renewable energy is intermittent,territorial and not freely transportable.The search for new clean and efficient energy sources is imminent.Solid oxide cells（SOCs）,which include solid oxide fuel cells（SOFCs）and solid oxide electrolysis cells（SOECs）,can partially meet the current energy demand.SOFCs use a wide range of fuels,including low carbon hydrocarbon fuels and alcohol fuels.SOFCs are facing the problems of severe carbon deposition of traditional fuels,and need to find new fuel electrodes that are resistant to carbon deposition.SOECs mainly include electrolysis of water and CO₂,and because the reaction process is reversible with SOFCs,the choice of electrode materials is similar,but their stability performance is not as good as that of SOFCs.Exploring a fuel electrode material that can maintain high performance and long-term stability at high voltages is a hot topic of current research.SOFCs can be used for the high-efficiency conversion of chemical energy into electricity.The exploration of oxide anode will enhance the coking resistance and the oxidation-reduction（redox）stability comparing to the conventional Ni（O）cermet anode.An n-type semiconductor with electron charge carriers will be conducive to the electric conductivity in fuel conditions,but the research on n-type oxide electrodes is limited mostly to perovskite-type titanate that requires very high temperature and low oxygen partial pressure to provide a decent electric conductivity.Transparent conductive oxides（TCOs）with a superior electric conductivity even at room temperature are widely explored for electronic devices,but they have never been studied as the alternative oxide anode of an SOFC at a reduced temperature.An n-type TCO type material ZnGa₂O₄（ZGO）that could be reduced at a temperature below 700°C was used as the anode for the oxidation of H₂ and hydrocarbon（ethanol and propane）at≤650°C.ZGO provided a high electric conductivity of 1.5 and 0.33 S cm^-1 at 700°C and 600°C,respectively,and the cell with ZGO anode on Sc_0.18Ce_0.01Zr_0.81O_2-δelectrolyte showed a high redox stability.The performance of the cell with ZGO/GDC（Gd₂O₃ doped ceria）anode could be enhanced by the infiltration of 1%Ni,imparting a peak power of 574 m W cm^-2 at 650°C and a stable cell performance of 300 m W cm^-2 at 600°C for 300 hours.The cell was also found to be relatively stable under carbonaceous fuel,suppressing the carbon deposition at 600°C.This work provided a new avenue of designing an n-type oxide anode that could be reduced in situ in the fuel condition of a low-temperature SOFC.SOECs are devices that convert electrical energy into chemical energy and are commonly used for electrolysis of water to produce hydrogen,reduction of CO₂,or co-electrolysis of H₂O/CO₂ to produce H₂ and CO.Spinel-type oxides are commonly used as a metal interconnection coating between the cathode and electrolyte because of their high catalytic and electrical conductivity,and their performance as oxygen electrodes in solid oxide cells has been investigated in recent years,but their application as fuel electrodes is currently less studied.MnFeCr_1-xCo_xO₄（x=0,0.05,0.2）spinel oxides with a high electrical conductivity was used to explore their performance as an SOEC fuel electrode.The MnFeCr_0.95Co_0.05O₄（MFCC5）and MnFeCr_0.8Co_0.2O₄（MFCC20）with Co-doping on the Cr site decomposed during the electrolysis of water,producing Co⁰ and cobalt（hydro）-oxides,forming a metal/oxide interface that further enhanced the electrocatalytic performance and durability of the cathode.MFCC5 was able to achieve a current density of 1550 m A cm^-2 at 1.6 V and maintained a current density of about 600 m A cm^-2 after 150 h of operation at 1.3 V for the electrolysis of water.The in situ out-solution process makes the current change significantly during the operation of the cell,but still maintains the high performance.The mechanism of its action is studied in detail,and it is considered that the application of spinel-type oxides as fuel electrodes for SOECs through the formation of in situ out-solution phenomenon by doping metals is not a bad way.