Amorphous diamond collects energy from carbon-14 reactor waste. Diamond is a semiconductor, and like the operation of a silicon solar panel cell, electric current flows when valence electrons are given enough energy to be promoted into the conduction band. post 
Average fast electron energy is 50keV, and the β particles create successive electron hole pairs due to inelastic impacts with other carbon atoms, generating a cascade of lower energy electrons that are collected at a metal contact on the diamond.
.
Expected output of 15 joules/day would decay with the carbon-14 half-life of 5000 years. Although delivered slowly, this is many orders of magnitude higher energy density than chemical batteries.
This supply of weak but continuous power would be a good match with digital sensors which can report their measurements by similarly low power variable radio reflectivity. See Low Power WiFi