Carbon nitrides have emerged as a new class of functional materials for a wide range of energy and environmental applications due to their chemical, photophysical and catalytic properties as well as their low-price, facile synthesis and stability.

A general synthesis of carbon nitride (CN) films with extended optical absorption, excellent charge separation under illumination, and outstanding performance as a photoanode in water-splitting photoelectrochemical cells is reported. To this end, we introduced a universal method to rapidly grow CN monomers directly from a hot saturated solution on various substrates. Upon calcination, a highly uniform carbon nitride layer with tuned structural and photophysical properties and in intimate contact with the substrate is obtained. Detailed photoelectrochemical and structural studies reveal good photoresponse up to 600 nm, excellent hole extraction efficiency (up to 62%) and strong adhesion of the CN layer to the substrate. The best CN photoanode demonstrates a benchmark-setting photocurrent density of 353 µA cm⁻² (51% faradaic efficiency for oxygen), and external quantum yield value above 12% at 450 nm at 1.23 V versus RHE in an alkaline solution, as well as low onset potential and good stability.

Atomic Fe in N‐doped carbon (FeNC) electrocatalysts for oxygen (O ₂ ) reduction at the cathode of proton exchange membrane fuel cells are the most promising alternative to platinum‐group‐metal catalysts. Despite recent progress on atomic FeNC O ₂ reduction, their controlled synthesis and stability for practical applications remain challenging. A two‐step synthesis approach has recently led to significant advances in terms of Fe‐loading and mass activity; however, the Fe utilization remains low owing to the difficulty of building scaffolds with sufficient porosity that electrochemically exposes the active sites. Herein, this issue is addressed by coordinating Fe in a highly porous nitrogen‐doped carbon support (≈3295 m ² g ⁻¹ ), prepared by pyrolysis of inexpensive 2,4,6‐triaminopyrimidine and a Mg ²⁺ salt active site template and porogen. Upon Fe coordination, a high electrochemical active site density of 2.54 × 10 ¹⁹ sites g _FeNC ⁻¹ and a record 52% FeN _x electrochemical utilization based on in situ nitrite stripping are achieved. The Fe single atoms are characterized pre‐ and post‐electrochemical accelerated stress testing by aberration‐corrected high‐angle annular dark field scanning transmission electron microscopy, showing no Fe clustering. Moreover, ex situ X‐ray absorption spectroscopy and low‐temperature Mössbauer spectroscopy suggest the presence of penta‐coordinated Fe sites, which are further studied by density functional theory calculations.