The interest received from the Bonds are subject to federal and state (where applicable) income tax.
As per current laws, any earned interest exceeding $10 in a year will be reported to the IRS. At the beginning of the following year, 1099-INT tax form will be made available for customers in the "Account Settings" page which you can use to file your taxes for interest earned.
Bond interest is taxed as interest income, not as capital gains. Capital gains tax rates apply only to assets that appreciate in value when sold, like stock shares or cryptocurrency.