>>64101938
Nah, most of the current LIB has an energy density of about 150 Wh/kg at best, for a total 120 Mwh.
going at 15 knots alone would require at least 3 Mw, so that not even 2 days.
A brazillian study on the u216 concept also stated the endurance of said system could be roughly about 75 days on fuel cells alone, and 9 knots for about 30 days
And with LIB, pushing for 80+ days fully submerged wouldn't be much of a problem.

> Nossa escolha pelo sistema AIP recai sobre o processo Siemens PEM-FC33
> BZM120 (ver Fig. 1). No U-6800 preve mos a instalação com quatro células de 120 kW, total de 480 kW. Esta potência e os estoques de LOX e metanol calculados permitem um regime de patrulha por 75 dias de 4 a 5 nós ou 35 dias com veloci dade de 8 a 9 nós.
> Os módulos PEM-FC mais dois re formadores de metanol de 240 kW cada
teriam o mesmo layout do U-216 propos to pela TKMS para a RAN (vide Fig. 2).
> Para 2023, o grupo Siemens já anunciou o lançamento da quarta geração de PEM, as BZM-Evo, gerando mais energia com maior eficiência.

And the system can be scaled up for extra performance
TLDR: can the PEMFC be phased out with better battery tech ? Sure, but the current LIBs isn't an alternative over it