Fiber-based distributed hydrogen measurement at high temperatures

A wide range of high-value applications, including power generation and chemical manufacturing, involve harsh chemical conditions and extreme temperatures. Options for in-situ monitoring of these processes are very limited, as traditional electronic sensing materials, packaging, and metallic interconnects rapidly degrade under such conditions. Even high-temperature stable electrochemical sensors require electrical feedthrough which may limit applicability. Silica optical fiber-based sensors provide a low cost and extremely rugged platform for applications up to 800 °C but tend to degrade over long time scales at higher temperatures. For higher temperature applications, single crystal optical fiber may be employed, limited only by the melting point of the material (e.g., approximately 2000 °C for sapphire). In this work, we discuss the implementation of evanescent field optical fiber sensors for distributed gas sensing of H₂, focusing on results using a Ni/Gd-doped CeO₂ nanocomposite sensing material for detection of low levels of H₂ (0-4%) at 700 °C. This approach utilizes sensors prepared using a low-cost, wet chemical deposition process, in conjunction with a custom-built interrogator system leveraging optical time domain reflectometry (OTDR). Using a specially designed dual-gas flow reactor system, the sensor is tested by establishing a controlled equilibrium gradient of gas concentration. Initial results shown using silica fiber provide a pathway for utilization with high-temperature stable single crystal optical fiber for operation at higher temperatures and higher levels of H₂ relevant for solid oxide fuel cell (SOFC) operating conditions.