Long-term and high-density energy sources are described that utilize radioisotope batteries that provide a tritium (3H)-based direct energy conversion system. The beta radiation source is a nitroxide compound with a synthetic yield near 100%. The nitroxide compound can be molded into flexible films to cover multiple structures, and has a specific activity and gravimetric density (wt % 3H) comparable to that of tritiated hydrides. A carrier system based on such tritiated nitroxides can be scaled up without significant beta self-absorption, permitting the energy source to have higher specific volume power densities than current radioisotope batteries. Such a carrier can be applied directly to a semiconductor array (such as an integrated circuit) in order to provide electrical power or, alternatively, be prepared with a phosphor and applied to a photovoltaic cell.