Due to the demand for renewable energies, gasses likeH2 need to be detected with sensitive, accurate and fast detectors. The US Department of Energy has made a list of requirements that H2 sensors need to fullfill of which the minimum detectable concentration (0.1%) and the reaction time (one second) are challenging for current hydrogen sensors, particularly for low-power and low-cost mass-produced sensors. This thesis covers the design and simulation of a photonic crystal nanobeam cavity sensor, using the Pt-catalyzed WO3 as a H2 sensitive material. The thicknesses of both layers show a trade-off between the minimally detectable concentration and the reaction times, resulting in a cavity with sensitivity of almost 40 nm/RIU and a Q factor varying between 5e5 and 5e4 depending on how much catalyst is required to meet the one second reaction time performance target. Based on the conservative assumptions regarding the reaction times of the sensor, the 0.1% performance target is almost achieved. This means that if in practice, any of the layer thicknesses show to be more favourable than assumed in this thesis, together with the fact that for optical sensors there is no risk of sparks, the photonic crystal nanobeam cavity as a H2 sensor shows to be a good fit for a high performance, low-cost, mass-produced H2 sensor that meets the DoE performance targets.