Study Of The Effects Of Supporting Electrode’s Pore Structure On The Electrochemical Performance Of Solid Oxide Cells

Solid oxide cells(SOCs)are clean and efficient energy conversion devices.Since SOCs have good compatibility with renewable energy sources,they are expected to play in an important role in constructing the carbon-neutral economy.The most studied SOC configuration is planar cells supported on a thick porous electrode.The pore structure of the supporting electrode shows great influences on the electrochemical performance of the cells.The porous electrode usually possesses tortuous pore structure,imposing a large resistance to the mass transport and hence resulting in severe concentration polarization.Preliminary studies by our laboratory have shown that the straight pore structure can be incorporated into the electrode,leading a significant increase of electrochemical performance over the electrode with tortuous pore structure.The present thesis was devoted to the study of the effects of the pore structure of the supporting electrode on the electrochemical performance.The experimental study was conducted with button cells,and simulation study was performed on single cells,providng guidance for the practical applications of SOCs.In Chapter 1,the working principle,key materials and current fabrication methods of SOCs are reviewed briefly,then the the scope of this thesis is described.SOCs can operate under solid oxide fuel cell(SOFC)mode,converting chemical energy to electrical energy.In Chapter 2,the effect of the pore structure of the supporting electrode on the cell performance was studied.Ni-8mol%yittra-stabilized zirconia(YSZ)was adopted as the fuel electrode,YSZ/Gd0.1Ce0.9O2-δ(GDC)as the bilayered electrolyte,and GDC-La0.6Sr0.4Co0.2Fe0.8O3-δ(LSCF)as the air electrode.Two different NiO-YSZ electrode substrates were prepared,one with the straight pore structure formed by the phase inversion tape casting method(denoted as electrode-1),and the other with tortuous pore structure by the conventional tape-casting method using the graphite as pore former(denoted as electrode-2).The cell supported on electrode-1(denoted as Cell-1)exhibited better electrochemical performance than the one supported on electrode-2(denoted as Cell-2).The difference in electrochemical performance between the two cells became pronounced at low H2 concentration.At 750 ℃,25%H2+75%H2O,Cell-1 attained a maximum power density of 941 mW·cm 2,while 470 mW·cm-2 for Cell-2.This was because that the straight pore structure in electrode-1 allowed fast mass transport and adsorption in it,thus mitigating the concentration polarization.With the expermental data on the relationship between the current density and the hydrogen concentration for the button cells,the one-dimensional plug flow reactor model was adoped to simulate the electrochemical performance of the single cells.The simulation results showed that the the fuel utilization and power density of the single cells with straight pore structure in the supporting electrode are higher than those of the single cells with tortuous pore structure in the supporting electrode.At 750℃,0.7 V,and feedstock(95%H2+5%H2O)feeding rate of 10 NmL·min-1·cm-2,the fuel utilization and power density for the former was 91%and 857 mW·cm-2,respectively,while only 61%and 591 mW·cm-2 for the latter,respectively.SOCs can also operate under solid oxide electrolysis cell(SOEC)mode,converting electrical energy to chemical energy.In Chapter 3,the effect of the pore structure of the supporting electrode on the steam electrolysis performance was investigated with the same cells as in Chapter 2.The cell with straight pore structure in the supporting electrode(Cell-1)was found to exhibit better electrolysis performance than the one with the tortuous pore structure in the supporting electrode(Cell-2),especially in steam-lean atmosphere.At 750℃,1.3 V,and steam concentration of 10%,the current density for Cell-1 was 1.19 A·cm-2,while only 0.69 A·cm-2 for Cell-2.This was because that the straight pore structure allowed for fast steam transport,thus effectively reducing the concentration polarization and thereby improving the cell performance.The simulation study was also conducted on single cells using the method similar to that shown in Chapter 2.The simulation results showed that the single cell with straight pore structure in the supporting electrode could achieve higher steam throughput conversion than the one with the tortuous pore structure.At 750℃,1.3 V,and steam(90%H2O +10%H2)feeding rate of 12 NmL·min-1·cm-2,the steam conversion was as high as 91%for the former,while 77%for the latter.In Chapter 4,the effect of the pore structure of the supporting electrode on the CO2 electrolysis performance was investigated,and the cells used was as same as in the previous chapters.The cell with the straight pore structure(Cell-1)demonstrated better electrolysis performance than the one with the tortuous pore structure(Cell-2),especially under CO2-lean conditions.At 750℃,1.5 V,and CO2 concentration of 30%,the current density for Cell-1 was 1.12 A·cm-2,while only 0.61 A·cm-2 for Cell-2.Moveover,Cell-1 demonstrated better coking resistance than Cell-2.The current density at which coking started to occur was found to be between 1.0 and 1.5 A·cm-2 for Cell-1,much higher than that of Cell-2(0.5-1.0 A·cm-2).Clearly,the electrodes with the straight pore structure allowed for facile gaseous transport,thus the electrolysis product CO could be rapidly removed,hence keeping its concentration below the coking threshold.Conventional fabrication method for porous Ni-YSZ supporting electrodes is through the reduction of the NiO-YSZ supporting electrodes in hydrogen.In Chapter 5,a new method for the in-situ electro-reduction of NiO-YSZ supporting electrodes in CO2 was explored.At 750℃,1.5 V,and 90%CO2+10%N2 feedstock,the NiO-YSZ electrodes with straight pore structure could be successfully reduced to Ni-YSZ with the electrolysis current density as high as 2.23 A·cm-2.However,electrodes with tortuous pore structure could not.The in-situ electro-reduction method was faster than the ordinary hydrogen reduction method.This was because that,in the electrodes with straight pore structure,the NiO paticles were uniformly distributed on the pore walls,easily to be electro-reduced.An unobstructed Ni conductive network was thus obtained,enabling smooth electron transfer and thereby promoting the reduction process.Moreover,cells with electrodes obtained from electro-reduction demonstrated better performance than those with electrodes obtained from hydrogen reduction.At 750℃,1.5 V,and 90%CO2+10%N2 feedstock,the electrolysis current density for the former was 2.23 A·cm-2,while only 1.75 A·cm-2 for the latter.The practical application of SOCs requires cells with adqueate mechanical strength.In Chapter 6,a new method for improving the mechanical strength of supporting electrodes was explored.It involved casting molten paraffin wax into the pores of green tapes followed by isostatic pressing.The mechanical strength of the astreated NiO-YSZ electrode was measured to be 80.4 Mpa,significantly higher than that of the untreated one(58.3 Mpa).It is worthwhile to note that for the as-treated electrode,its straight pore structure was well preserved.As a result,the cell comprising that electrode exhibited satisfactory electrochemical performance:a maximum power density as high as 1050 mW·cm-2 was obtained at 750℃ with hydrogen fuel.In Chapter 7,the research presented in this thesis is summarized,and future research needs are suggested.