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The electrochemical reduction of CO 2 is promising for mitigating anthropogenic greenhouse gas emissions; however, voltage instabilities currently inhibit reaching high current densities that are prerequisite for commercialization. Here, for the first time, we elucidate that product gaseous bubble accumulation on the electrode/electrolyte interface is the direct cause of the voltage instability in CO 2 electrolyzers. Although bubble formation in water electrolyzers has been extensively studied, we identified that voltage instability caused by bubble formation is unique to CO 2 electrolyzers. The appearance of syngas bubbles within the electrolyte at the gas diffusion electrode (GDE)-electrolyte chamber interface (i.e. $10% bubble coverage of the GDE surface) was accompanied by voltage oscillations of 60 mV. The presence of syngas in the electrolyte chamber physically inhibited two-phase reaction interfaces, thereby resulting in unstable cell performance. The strategic incorporation of our insights on bubble growth behavior and voltage instability is vital for designing commercially relevant CO 2 electrolyzers.

We present electrospinning as a versatile technique to design and fabricate tailored polymer electrolyte membrane (PEM) fuel cell gas diffusion layers (GDLs) with a pore-size gradient (increasing from catalyst layer to flow field) to enhance the high current density performance and water management behavior of a PEM fuel cell. The novel graded electrospun GDL exhibits highly robust performance over a range of inlet gas relative humidities (RH). At relatively dry (50% RH) inlet conditions that exacerbate ohmic losses, the graded GDL lowers ohmic resistance and improves high current density performance compared to a uniform GDL with larger pores and fiber diameters. Specifically, the graded GDL facilitates a beneficial degree of liquid water retention at the catalyst layer/GDL interface due to the high capillary pressure inherent in its microstructure, thereby improving membrane hydration. Additionally, enhanced graphitization and connectivity of the graded electrospun fibers improves heat dissipation from the catalyst layer interface compared to the GDL with larger fiber diameters, thereby reducing membrane dehydration. When the inlet RH is raised to fully humid (100% RH) conditions, the graded GDL mitigates liquid water accumulation and lowers mass transport resistance. Specifically, the pore size gradient directs the removal of liquid water from the GDL, resulting in superior performance at high current densities.

Mass transport losses ultimately suppress gas evolving electrochemical energy conversion technologies, such as fuel cells and carbon dioxide electrolyzers, from reaching the high current densities needed to realize commercial success. In this work, we reach ultrahigh current densities up to 9 A/cm 2 in a polymer electrolyte membrane (PEM) water electrolyzer with the application of custom porous transport layers (PTLs) with patterned through-pores (PTPs), and we reduce the mass transport overpotentials of the electrolyzer by up to 76.7 %. This dramatic performance improvement stems from the 43.5 % reduction in gas saturation at the catalyst layer-PTL interface region. Moreover, the presence of PTPs leads to more rapid bubble coalescence and subsequently more frequent bubble snap-off (∼3.3 Hz), thereby enhancing the rate of gas removal and liquid water reactant delivery to the reaction sites. This work is highly informative for designing PTLs for optimal gas removal for a wide range of gas evolving electrochemical energy conversion technologies.

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