Impedance-Based Detection of NO₂ Using Ni-MOF-74: Influence of Competitive Gas Adsorption

Chemically robust, low-power sensors are needed for the direct electrical detection of toxic gases. Metal-organic frameworks (MOFs) offer exceptional chemical and structural tunability to meet this challenge, though further understanding is needed regarding how coadsorbed gases influence or interfere with the electrical response. To probe the influence of competitive gases on trace NO₂ detection in a simulated flue gas stream, a combined structure-property study integrating synchrotron powder diffraction and pair distribution function analyses was undertaken, to elucidate how structural changes associated with gas binding inside Ni-MOF-74 pores correlate with the electrical response from Ni-MOF-74-based sensors. Data were evaluated for 16 gas combinations of N₂, NO₂, SO₂, CO₂, and H₂O at 50 °C. Fourier difference maps from a rigid-body Rietveld analysis showed that additional electron density localized around the Ni-MOF-74 lattice correlated with large decreases in Ni-MOF-74 film resistance of up to a factor of 6 × 10³, observed only when NO₂ was present. These changes in resistance were significantly amplified by the presence of competing gases, except for CO₂. Without NO₂, H₂O rapidly (<120 s) produced small (1-3×) decreases in resistance, though this effect could be differentiated from the slower adsorption of NO₂ by the evaluation of the MOF's capacitance. Furthermore, samples exposed to H₂O displayed a significant shift in lattice parameters toward a larger lattice and more diffuse charge density in the MOF pore. Evaluating the Ni-MOF-74 impedance in real time, NO₂ adsorption was associated with two electrically distinct processes, the faster of which was inhibited by competitive adsorption of CO₂. Together, this work points to the unique interaction of NO₂ and other specific gases (e.g., H₂O, SO₂) with the MOF's surface, leading to orders of magnitude decrease in MOF resistance and enhanced NO₂ detection. Understanding and leveraging these coadsorbed gases will further improve the gas detection properties of MOF materials.