Is it unlawful to supplicate against one's husband regarding him marrying another woman, especially since he desires to have children, and the doctor informed me that there is no hope of me conceiving, while he loves me?

Do not lose hope in Allah, for Allah is capable of all things. He created Adam and Jesus by His power. Polygyny is permissible in Islam and has sublime wisdoms. If your husband is capable of upholding justice and providing, then he has the right to marry another. Supplicating against him for not succeeding in that is not appropriate, especially since he needs another wife for procreation.