Summary of operando techniques applied to continuous-flow CO₂ electrolyzers.

Operando technique	Information content/main advantages	Main limitations	Time resolution	Spatial resolution
XASa 18,113,115,117,119–123	• Catalyst structure (oxidation state, coordination environment)	• Bulk sensitive	• Typically 20–30 minutes	N/A
	• Only minor cell modification needed	• Synchrotron irradiation	• Subseconds for QXAFS
Raman spectroscopy51,52,124,125	• Catalyst structure (oxidation state)	• SERS effect only on certain metal nanostructures	• Few tenths of seconds	• μm-scale (xb)
	• Adsorbed intermediates/products	• Less suitable for zero-gap design
	• Local pH with spatial resolution
	• Widely available at lab-scale
Fluorescent microscopy50	• Local pH measurement with spatial resolution in the x, y, z direction	• Fluorescent probe added to the electrolyte might change the local reaction environment	N/A	• μm- scale (x, y, zb)
	• pH measurement within catalyst trenches
UV-Vis spectroscopy30	• pH measurement at the catalyst/membrane interface (MEA-design)	• Significantly different cell design compared to the real device (in situ)	N/A	• nm-scale (z)
SECMa	• Local reactivity and local pH	• Low achievable current densities	N/A	• μm-scale (x, y)
	• Spatially resolved activity maps
AFM126 a	• High resolution topography and mechanical property maps	• Small scan size	• Few minutes	• Few tenths/hundreds of nm
		• Rapid flooding at high currents
MS81,127 a	• Real-time detection of products/intermediates with low detection limit	• Deconvolution of signals (e.g., CO, CO2) can be challenging	• Seconds/tenths of seconds	N/A
	• Mechanistic information
	• Isotopic information
Neutron/X-ray radiography78,91,100,128,129	• Water management	• Neutron/synchrotron facility	• Few seconds for X-ray few tenths of seconds for neutron	• μm – scale (x, y)
	• Gas evolution	• Limited elemental information
	• Precipitate formation
	• Operando cell failure diagnostics
	• Only minor cell modification needed
Thermography130	• Local activity mapping	• One-to-one correlation between temperature change and activity has to be proved	• Depends on catalyst substrate, layer thickness, etc.
	• Noninvasive probing to assess activity distribution

^aXAS: X-ray absorption spectroscopy, SECM: scanning electrochemical microscopy, AFM: atomic force microscopy, MS: mass spectrometry.

^b x and y are directions in the plane of the GDE, while z is perpendicular to that.