The advancement of microbial electrosynthesis systems (MES) towards industrialization is currently hindered by a limited understanding of the fundamental constraints affecting selective production of high-value chemicals. To address this challenge, we develop a comprehensive computational model that integrates microbial, electrochemical, and acid–base reactions with pore-scale transport processes within a three-dimensionally resolved biofilm. This study investigates the H₂-mediated CO₂ fixation pathway to acetate, butyrate, and caproate. The effect of applied cathode potential and biofilm thickness on macroscopic parameters, such as efficiency and selectivity, is analyzed based on local concentrations and electrochemical and biochemical fluxes. Among the limiting factors, the availability of CO₂ emerges as the main limitation for biochemical reactions due to its low solubility and high half-saturation constant. Additionally, hydrogen – serving as the electron mediator – limits the reaction rate at low current densities and reduces electron transfer efficiency at higher current densities. A key insight from our study is the identification of an optimal electrode potential for each biofilm thickness, balancing both H₂ transfer and CO₂ consumption efficiencies. Furthermore, carbon selectivity shifts with increasing biofilm thickness: net acetate production declines while caproate production increases. This trend is attributed to the prolonged residence time of metabolic intermediates within thicker biofilms, promoting chain elongation pathways. Thus, our work takes an important step towards a fundamental understanding of caproate selectivity across different biofilms, which can be used to optimize the electrode structure and operating conditions to control the local biofilm thickness.

The presence of quasiparticles typically degrades the performance of superconducting microwave circuits. The readout signal can generate nonequilibrium quasiparticles, which lead to excess microwave loss and decoherence. To understand this effect quantitatively, we measure quasiparticle fluctuations and extract the quasiparticle density across different temperatures, readout powers, and resonator volumes. We find that microwave power generates a higher quasiparticle density as the active resonator volume is reduced and show that this effect sets a sensitivity limit on kinetic inductance detectors. We compare our results with theoretical models of direct microwave photon absorption by quasiparticles and conclude that an unknown, indirect mechanism plays a dominant role in quasiparticle generation. These results provide a route to mitigate quasiparticle generation due to readout power in superconducting devices.