As an agent appointed under your husband's power of attorney, you have authority to make either medical or financial decisions for him, depending on the scope of the POA document. If you hold a durable power of attorney and your husband is incompetent, you may be permitted to renew his passport.
Acting on Behalf
A power of attorney, or POA, has nothing to do with lawyers. It is a legal document drawn up by a competent adult, the principal, appointing an agent to make decisions on his behalf about medical or financial issues. Forms are available from state courts and online legal service providers. The principal can limit the authority of the POA agent, but a general POA gives the agent the right to take any action the person could have taken himself. A durable POA is valid even if the principal becomes incompetent.
Renewing Travel Documents
Passports are renewed by submitting application forms issued by the U.S. State Department. If your husband is competent, he can sign his own passport renewal form. If he is incompetent, his guardian or an agent authorized under a power of attorney can execute the form for him. Be certain, however, that the terms of your POA don't limit your authority to make this decision.