For the long-term performance and stability test of the electrocatalytic CuOx-NLs, controlled-potential water electrolysis (CPE) and controlled-current electrolysis (CCE) are conducted in carbonate electrolyte. CPE at 1.81 V (η≈580 mV) shows an excellent and sustained oxygen evolution current density approaching 17 mA cm−2 (Figure 4a). The current density during 20 h remains highly stable with no noticeable decrease in catalytic performance. On the CuOx-NPs system, the observed O2 evolution current density is 5.2 mA cm−2 during the CPE run. The current density for CuOx-NLs layers is more than 3 times higher than for the CuOx-NPs film (Figure4a). For the CuOx-NLs sample, online oxygen measurements show 955 μmol of O2 generation in 10 h of CPE with a Faradaic efficiency of >98%, corresponding to an oxygen generation rate of ∼ 95.5 μmol per hour, and about 39 μmol of oxygen per hour for the as-prepared CuOx-NPs (Figure S10). During constant current electrolysis, the CuOx-NLs sample remains remarkably stable for water oxidation at current densities of 10 mA cm−2 and 20 mA cm−2. To achieve 10 mA cm−2, a very stable steady-state potential of∼1.75 V (vs RHE) is preserved for more than 20 h of water electrolysis (Figure4b). Similarly, a current density of 20 mA cm−2 is maintained at just ∼ 1.88 V (vs RHE) in a clean carbonate system. There is no observable catalytic degradation and change in the voltage during CCE, which is a direct indication of the stability and sustained catalytic activity of the CuOx-NLs