Resistojets heat a compressed gas or vaporize a liquid to expand it through a nozzle for thrust. They improve a bit on the performance of cold gas thrusters. This experimental water resistojet managed an Isp of 140 s.
I'm thinking about what sort of simple rocket engines might be feasible in the initial stages of a permanent lunar base, made on-site mostly from local materials. Oxygen is the only volatile that can be extracted in volume anywhere. Obviously the example shows water is a fuel option, but the paper cited above answers most of my questions regarding that, and also I wonder if oxygen might have advantages.
What materials that might be available early on for such complex fabrication is unclear, however I'm going to suggest that something can be come up with, that can handle storing liquid oxygen and gaseous elemental oxygen at the nozzle up to, say, 500 °C. Let's say iron combined with metal oxides or basalt. The resistance heater and other complex bits come from Earth. The LOX flows to the resistance heater and then out the nozzle.
Could such a system work? What sort of Isp and thrust might it have?
For what its worth, the idea is that these engines only need to circularize an orbit around the Moon, and do other orbital maneuvers or a burn to enter orbit around EML1. Launch is by a sling launcher using basalt fiber cable, that imparts orbital or escape speed. So low Isp isn't a problem in itself.