A leak detection sensor for detecting a leakage of an electrolyte solution in a flow battery system is provided. The sensor includes a sensor housing, the sensor housing being at least partially surrounded by a fluid and having mounted therein at least one light source. The device also includes at least one light detector, wherein light emitted from at least one light source is incident on a portion of the housing in contact with a fluid and is at least partially refracted by an amount which is dependent on a refractive index of the refractor lens and the surrounding fluid, such that the amount of refraction at the refractor lens and the surrounding fluid causes a loss in a power of light detected by the at least one light detector, the light detector determines the intensity of the detected light, determines the composition of fluid surrounding the device housing based on the based on the difference between the measured light intensity and the intensity of light produced by the light source, and determines a leak if the composition of fluid is determined to be an electrolyte solution.