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concentration has increased from ≈277 ppm in 1750, prior to Industrial Revolution, to ≈410 ppm in 2019 and the average annual growth of carbon emission in 2018 and 2019 is greater than its 10-year average. [1] Thus, it is urgent to search for renewable and zero-carbon energy sources as alternatives to replace fossil fuels. [2] Although solar, wind and tidal energy are abundant and sustainable, their intermittent and weather-dependent limitations require development and deployment of highly efficient energy conversion and storage systems at large scale to bridge the time gap between supply and demand. [3] An attractive solution is to convert the electrical energy derived from the aforementioned renewable energy sources into chemical energy in the form of hydrogen. [4,5] Pure or mixed hydrogen is playing important roles in transportation, industrial sectors, and chemical transformation processes. [6] However, at present, 95% of hydrogen is still produced by reforming of fossil fuels that emits significant amount of CO 2 and only 4% is through water electrolysis, mainly owing to the much higher production cost of the latter. [7] Therefore, crucial to enable this sustainable vision is to improve the efficiency and scalability of water electrolysis technology.