Betavoltaics is a way of getting ordinary, portable electricity out of the organic process of radioactive decay. Radioactive elements out in the world are always decaying and releasing electrons. Since electricity is just a stream of electrons traveling in the same direction, experts in energy technology have long looked to these elements to provide an efficient, safe, long-lasting, environmentally friendly, and tiny alternative to other battery systems that provide power.
We all understand the stable model of an atom, with the nucleus surrounded by a cloud of electrons that may come or go. But certain kinds of elements, called radioactive isotopes, behave quite differently over time. These isotopes, such as "heavy" hydrogen or molybdenum-100, go through a continuous process called decaying. The nucleus "decays," resulting in one less neutron and one more proton, thus releasing a single electron at a time. These high-energy, high-speed electrons are called beta particles.
Betavoltaic technology harnesses these stray electrons as a source of electrical energy. A unit functioning off of betavoltaics must contain an appropriate radioactive isotope as well as another component called a semiconductor that helps with routing the individual electrons into electricity. Although betavoltaics has been theorized for decades, it is now gaining in practical popularity as a way of producing very efficient, very long lasting batteries nicknamed "nuclear" batteries.
There are several challenges meeting the developers of reliable betavoltaics. For example, sometimes the isotope re-absorbs those floating electrons, so they are no longer free to become electricity. This might be solved by developing special semi-conductors out of silicon. Secondly, the isotope can react with the semi-conductor in an unhelpful way that makes it less efficient. Also, the decay rate may be too slow to power anything large, like a spaceship.
It is almost as important to explain what betavoltaics is not, as opposed to what it is. Betavoltaics has nothing to do with the way that nuclear reactors produce electricity in giant power plants. Nuclear reactors must force massive nuclear reactions, like fission and fusion, to produce energy out of very unstable elements. Betavoltaics, however, isn't a miniaturized version of a power plant that can fit inside a battery. It is an entirely different technology, and shouldn't be confused with the dangers or mystery of other nuclear energy. There is probably no radioactive waste produced and certainly no threat of nuclear meltdown.