Is it permissible for a wife to give out charity (Sadaqa) and Zakat from a joint bank account with her husband without his knowledge, intending that what she gives is from her own money, given her husband's refusal of that?

There is no blame upon you for what you are doing. You must know the amount of your balance to give the obligatory zakat (definition), and you do not need your husband's permission for that. You may give charity from your own money, precise to what you have spent. If your husband permits you to give out his zakat, then that suffices for him. Otherwise, it is better to separate your money and open a separate account, because the husband's wealth from which zakat has not been paid has no blessing in it. You should advise him by explaining the gravity of the sin of one who withholds zakat.