*3.3. Maximum Output Performance at a Given pH*<sup>2</sup>

Figure 5 reflects the effect of hydrogen inlet pressure on *Pmax* and *ηmax* of HT-PEMFC with. It can be seen from several figures that *Pmax* and *ηmax* increase with the raise of *pH*<sup>2</sup> , but from a numerical point of view the increase is not large. The rise of pressure not only increases the diffusivity of the bipolar gas, but also improves the concentration of the bipolar gas and boosts the mass transfer of the reaction gas, which will strength the mass transfer of the reaction gas and reduce the influence of concentration polarization on the reversible potential.

**Figure 5.** Effect of operating temperature on *Pmax* and *ηmax* of HT-PEMFC under different parameters. (**a**) Different *lm*; (**b**) different *X*; (**c**) different T; (**d**) different *pO*<sup>2</sup> .

Figure 5a shows the impact of *pH*<sup>2</sup> on *Pmax* and *ηmax* of HT-PEMFC under different *lm*. It can be seen that as *lm* decreases, *Pmax* and *ηmax* will increase. The thinner the proton membrane is, the smaller the barrier for ions to pass through the proton membrane, that is, the ohmic overpotential decreases.

Figure 5b reveals the influence of *pH*<sup>2</sup> in *Pmax* and *ηmax* of HT-PEMFC under different *X*. It is obvious that as the doping level of phosphoric acid increases, *Pmax* and

*ηmax* will increase. The improvement of doping level increases the proton conductivity of the membrane.

Figure 5c displays the impact of *pH*<sup>2</sup> on *Pmax* and *ηmax* of HT-PEMFC under different *T*. It is obvious that as the temperature rises, *Pmax* and *ηmax* will increase, and the increase is relatively large. When the operating temperature increases, the proton conductivity of the membrane improves and electrochemical polarization decreases; at the same time, the increase of operating temperature can boost the exchange current density, which reduces the activation overpotential.

Figure 5d describes the effect of *pH*<sup>2</sup> on *Pmax* and *ηmax* of HT-PEMFC under different *pO*<sup>2</sup> . As can be seen from Figure 5d, with the increase of *pO*<sup>2</sup> , *Pmax* and *ηmax* will strength. Obviously, *pO*<sup>2</sup> has little effect on the maximum output of the cell.
