Transformative valorization of semi-coking wastewater: Design of rare earth/nitrogen-codoped porous carbon for efficient water splitting

Converting pollutants into high-value-added chemical and energy products has the potential to optimize the fossil fuel-dominated energy structure. Herein, we utilize phenolic resin derived from semi-coking wastewater as a carbon precursor and successfully prepare La/N-, Ce/N-, and Er/N-codoped porous carbon (PC) materials by introducing rare-earth elements. The obtained materials are used as highly effective electrocatalysts with dual functionality, facilitating hydrogen and oxygen production in an alkaline condition. In overall water splitting tests with symmetrical electrodes, Er/N-codoped PC demands only 1.50 V to reach 10 mA cm⁻², outperforming or matching current metal/N-codoped PC catalysts. Theoretical calculations indicate that the Er metal site functions as the catalytic center, exhibiting a lower energy barrier of 0.36 eV in comparison to Ce/N-(0.60 eV) and La/N-(1.22 eV) co-doped PC. This lower free energy barrier facilitates the charge transfer during the H∗ intermediate and H₂ formation processes. Our developed environmentally friendly and cost-effective PC catalyst preparation strategy provides new insights for the high-value transformation of other wastewater pollutants.