Operando pH Imaging to Investigate the Influence of the Micrometer-Scale Morphology on CO₂ Gas Diffusion Electrode Performance

We developed an experimental technique to map the local in situ pH value around operating CO₂ reduction gas diffusion electrodes with confocal laser scanning microscopy and two fluorescent ratiometric dyes. This allowed us to demonstrate that microcavities on the order of 5 µm on the surface of copper CO₂ reduction gas diffusion electrodes serve as local hotspots with enhanced activity for CO2 reduction, including multicarbon products.<br/>Fluorescent confocal laser scanning microscopy is a very powerful tool because it allows to map an entire macroscopic sample in three spatial dimensions with sub-micrometer resolution. In combination with pH-sensitive fluorescent dyes, it offers the unique opportunity to probe the pH value within microstructures on the surface of a sample of interest such as an active gas diffusion electrode. As pH probes, we use two different commercially available ratiometric fluorescent photoacids with different sensing mechanisms. The first dye we use is 6,8-dihydroxypyrene-1,3-disulfonic acid disodium salt (DHPDS). Its pH sensing capacity is connected to proton-transfer reactions in its electronic ground state to perturb its absorption spectrum. We excite it with lasers at 458 nm and at 488 nm and collect the signal separately for both excitations between 405 nm and 754 nm. The ratio between the two signals is a measure for the local pH value and is independent of the local dye concentration. It is sensitive to pH values between 6 and 10. To extend the accessible pH range, we implemented a measuring protocol for a second novel pH probe, 8-aminopyrene-1,3,6-trisulfonic acid trisodium salt (APTS). Its sensing mechanism relies on quenching of its thermally equilibrated electronic excited state by direct proton transfer to aqueous OH^—. We excite this probe at 458 nm and collect the emission signal separately in the range 480-550 nm and 551-754 nm. The ratio between the two signals is sensitive to pH values between 11.2 and 14. This work presents the demonstration of APTS as a probe for the local pH value for the first time. By combining both probes DHPDS and APTS, we can cover pH values between 6 and 14 with a gap between 10 and 11.2.<br/>We developed an experimental setup that uses these two dyes to probe the local pH value around an operating CO₂ reduction gas diffusion electrode made of carbon paper with a 300 nm copper catalyst layer. The pH-sensitive dyes are diluted in 100 mM KHCO3 electrolyte which is pumped through an electrochemical cell optimized for use with confocal microscopy. The pH is monitored under operation of the CO₂ reduction gas diffusion electrode with current densities as high in magnitude as 200 mA/cm². We can create three-dimensional maps of the pH value by collecting of the fluorescent emission from the focal point of the laser and scanning the laser beam over the sample. Using this technique, we could gain valuable insight into the influence of microstructures on the CO₂ reduction performance of a gas diffusion electrode. We demonstrated that narrow trenches in the electrode surface are local hotpots with enhanced CO₂ reduction activity and selectivity.<br/>We expect that these insights will help inform the design of advanced gas diffusion electrodes for CO₂ reduction with enhanced efficiency and multicarbon product selectivity. Furthermore, the developed technique can potentially be used in other electrochemical applications where the pOH value plays an important role and could be extended to additional fluorescent dyes.