One useful thing, not currently used due to space, weight, and power issues, most of which would be minimal issues on the moon, is the Bosch reaction. Basicall

One useful thing, not currently used due to space, weight, and power issues, most of which would be minimal issues on the moon, is the Bosch reaction. Basically, CO2 + H2 => C + H2O. Split the H2O(water), and you get back the H2 to reuse in the Bosch reaction, and breathable O2.