A Nose for Hydrogen Gas: Fast, Sensitive H₂ Sensors Using Electrodeposited Nanomaterials

Hydrogen gas (H₂) is odorless and flammable at concentrations above 4% (v/v) in air. Sensors capable of detecting it rapidly at lower concentrations are needed to "sniff" for leaked H₂ wherever it is used. Electrical H₂ sensors are attractive because of their simplicity and low cost: Such sensors consist of a metal (usually palladium, Pd) resistor. Exposure to H₂ causes a resistance increase, as Pd metal is converted into more resistive palladium hydride (PdH_x). Sensors based upon Pd alloy films, developed in the early 1990s, were both too slow and too insensitive to meet the requirements of H₂ safety sensing. In this Account, we describe the development of H₂ sensors that are based upon electrodeposited nanomaterials. This story begins with the rise to prominence of nanowire-based sensors in 2001 and our demonstration that year of the first nanowire-based H₂ sensor. The Pd nanowires used in these experiments were prepared by electrodepositing Pd at linear step-edge defects on a graphite electrode surface. In 2005, lithographically patterned nanowire electrodeposition (LPNE) provided the capability to pattern single Pd nanowires on dielectrics using electrodeposition. LPNE also provided control over the nanowire thickness (±1 nm) and width (±10-15%). Using single Pd nanowires, it was demonstrated in 2010 that smaller nanowires responded more rapidly to H₂ exposure. Heating the nanowire using Joule self-heating (2010) also dramatically accelerated sensor response and recovery, leading to the conclusion that thermally activated H₂ chemisorption and desorption of H₂ were rate-limiting steps in sensor response to and recovery from H₂ exposure. Platinum (Pt) nanowires, studied in 2012, showed an inverted resistance response to H₂ exposure, that is, the resistance of Pt nanowires decreased instead of increased upon H₂ exposure. H₂ dissociatively chemisorbs at a Pt surface to form Pt-H, but in contrast to Pd, it stays on the Pt surface. Pt nanowires showed a faster response to H₂ exposure than Pd nanowires operating at the same elevated temperature, but they had a surprising disadvantage: The resistance change observed for Pt nanowires was exactly the same for all H₂ concentrations. Electron surface scattering was implicated in the mechanism for these sensors. Work on Pt nanowires lead in 2015 to the preparation of Pd nanowires that were electrochemically modified with thin Pt layers (Pd@Pt nanowires). Relative to Pd nanowires, Pt@Pd nanowires showed accelerated response and recovery to H₂ while retaining the same high sensitivity to H₂ concentration seen for sensors based upon pure Pd nanowires. A new chapter in H₂ sensing (2017) involves the replacement of metal nanowires with carbon nanotube ropes decorated with electrodeposited Pd nanoparticles (NPs). Even higher sensitivity and faster sensor response and recovery are enabled by this sensor architecture. Sensor properties are strongly dependent on the size and size monodispersity of the Pd NPs, with smaller NPs yielding higher sensitivity and more rapid response/recovery. We hope the lessons learned from this science over 15 years will catalyze the development of sensors based upon electrodeposited nanomaterials for gases other than H₂.