Fig. 4: In-situ stability of HEMs for water electrolysis.

a Temperature profiles of the cathode catalyst layer near the HEM (T_b). b T_b of the HEMWE with QCS/BN composite HEMs with different thermal conductivities measured at different current densities. c Voltage evolution of HEMs with different thermal conductivities at 1 A cm⁻². d Relationship between in-situ voltage degradation rates and temperature vs. thermal conductivity of HEMs. e Possible degradation pathways and (f) the accordingly energy barriers calculated by DFT. g ¹H NMR spectrum and (h) the accordingly QA content retention of the HEMs after the in-situ stability test. Peaks of GLU and QA are indicated by the purple shadows in panel (g). The peak at 3.8–4.2 ppm ascribed to the glucosamine (GLU) unit of chitosan is considered a reference peak. The HEM after the stability test was treated by dissolving it in acetic acid solution and centrifugal filtration to remove the catalyst and boron nitride for the ¹H NMR test. i Long-term in-situ stability of FAA-3, QCS/10.6%BN, and QCS for water electrolysis. Unit of k is W m⁻¹ K⁻¹.