**5. Conclusion**

The results show the influence of current collectors on gas access. Relevant control of material microstructures improves the diffusion of gaseous reactants and the current collecting. When diffusion is the limiting step, a relocation of the current sources within the volume of the electrodes is observed. On the contrary, if ionic ohmic drop becomes the rate determining step, current density sources are located close to the electrode/electrolyte interface. In some specific cases, the assumption that all electrochemical reactivity is located in the electrolyte interface vicinity cannot be made. That observation is emphasized when the competition between gas and ion transport is intentionally distorted, since a second reactive layer appears close to the cathodic gas channel.

It has been shown that it is possible to force the reaction to occur close to the electrolyte/electrode interfaces by layering the electrodes and introducing gradients of grain diameters. The obtained relocation is as high as 80% of the current being generated within the first 4 μm of the cathode thickness. The ohmic losses are reduced and gas access facilitated.

**Figure 9.**

**108**

*voltage = 1.3 V.*

*Electrodialysis*

*Current sources distribution through both electrodes underneath the middle of the gas channel (x = 0 mm) for case A and cases displaying the best performance in the grain size investigation (Vc and IIa) at cell*

## *Electrodialysis*

Contrary to the cathode, the changes of grain diameter gradient do not influence the electrochemical performance of the anode. However, its current production profile is consistently similar to the cathodic one. This means that specific attention should be given to the cathode microstructure.

*k* type of species

*DOI: http://dx.doi.org/10.5772/intechopen.90352*

*a=c* anode or cathode *ω* axis position

*S* electronic conductor *M* ionic conductor

*S=M* electronic or ionic conductor *YSZ* yttria-stabilized zirconia

*DH*2*O*\_*H*<sup>2</sup> <sup>¼</sup> <sup>3</sup>*:*<sup>16</sup> � <sup>10</sup>�<sup>8</sup> *<sup>T</sup>*<sup>1</sup>*:*<sup>75</sup> <sup>1000</sup>

*DO*2\_*N*<sup>2</sup> <sup>¼</sup> <sup>3</sup>*:*<sup>16</sup> � <sup>10</sup>�<sup>8</sup> *<sup>T</sup>*<sup>1</sup>*:*<sup>75</sup> <sup>1000</sup>

*LSM* strontium-doped lanthanum manganite La1-xSrxMnO3

*Solid Oxide Steam Electrolyzer: Gas Diffusion Steers the Design of Electrodes*

The diffusion coefficient for binary mixture of gases may be estimated from Fuller, Schettler, and Giddings relation with values coefficients for different mole-

> <sup>18</sup> <sup>þ</sup> <sup>1000</sup> 2 <sup>1</sup>*=*<sup>2</sup>

<sup>28</sup> <sup>þ</sup> <sup>1000</sup> <sup>32</sup> <sup>1</sup>*=*<sup>2</sup>

*<sup>P</sup>* <sup>7</sup>*:*<sup>06</sup> � <sup>10</sup>�<sup>6</sup> <sup>1</sup>*=*<sup>3</sup> <sup>þ</sup> <sup>12</sup>*:*<sup>7</sup> � <sup>10</sup>�<sup>6</sup> <sup>1</sup>*=*<sup>3</sup> <sup>2</sup> (26)

*<sup>P</sup>* <sup>17</sup>*:*<sup>9</sup> � <sup>10</sup>�<sup>6</sup> <sup>1</sup>*=*<sup>3</sup> <sup>þ</sup> <sup>16</sup>*:*<sup>6</sup> � <sup>10</sup>�<sup>6</sup> <sup>1</sup>*=*<sup>3</sup> <sup>2</sup> (27)

*a* Anode *c* Cathode

*Ni* nickel *eff* effective *bv* Butler-Volmer *ref* reference *eq* equilibrium *pol* polarization *K* Knudsen *tot* total

**Appendix**

**111**

cules tabulated in [34]:

Several aspects have been neglected in the present work and should be investigated to complete this approach and give global vision of the mechanisms that govern a SOEC response, e.g., the ohmic resistance due the dense ceramic membrane can be minimized using metal support technologies. In addition, the contact resistances shall be taken into consideration since they are a key parameter when optimizing the configuration of current collectors and will allow this model to be compared to experimental data of total SRU response.
