  The use of hydrogen as a clean and renewable alternative to fossil fuels
requires a suite of flammability mitigating technologies, particularly robust
sensors for hydrogen leak detection and concentration monitoring. To this end,
we have developed a class of lightweight optical hydrogen sensors based on a
metasurface of Pd nano-patchy particle arrays, which fulfills the increasing
requirements of a safe hydrogen fuel sensing system with no risk of sparking.
The structure of the optical sensor is readily nano-engineered to yield
extraordinarily rapid response to hydrogen gas (<3 s at 1 mbar H$_{2}$) with a
high degree of accuracy (<5%). By incorporating 20% Ag, Au or Co, the sensing
performances of the Pd-alloy sensor are significantly enhanced, especially for
the Pd$_{80}$Co$_{20}$ sensor whose optical response time at 1 mbar of H$_{2}$
is just ~0.85 s, while preserving the excellent accuracy (<2.5%), limit of
detection (2.5 ppm), and robustness against aging, temperature, and interfering
gases. The superior performance of our sensor places it among the fastest and
most sensitive optical hydrogen sensors.
