Design of bifunctional oxygen evolution/reduction electrocatalysts on g-C3N3 monolayer by a defect physics method

Designing efficient and stable bifunctional oxygen evolution/reduction reaction (OER/ORR) electrocatalysts is important to developing renewable energy technologies. However, integrating OER/ORR activity in a single electrocatalyst remains challenging due to the difficulty in balancing the adsorption strengths of reaction intermediates. Herein, density functional theory (DFT) calculations were conducted to investigate 4d-transition metal (TM) doped graphitic carbon nitride (4d-TM@C₃N₃) systems as potential bifunctional OER/ORR electrocatalysts by considering the charge states through a defect physics method. Our results identified 30 stable 4d-TM_N@C₃N₃ systems, including TM doped at nitrogen sites (4d-TM_N@C₃N₃) and those occupying interstitial sites (4d-TM_int@C₃N₃). Machine learning analysis showed that the the bond lengths between TM and O (d_TM-O) and OH (d_TM-OH), and the charge transfer of TM atoms (Q_e) are the three primary descriptors characterizing the adsorption behavior. Among these, Pd_int^×@C₃N₃ (Pd interstitial site, neutral charge state), Pd_int^•@C₃N₃ (Pd interstitial site, +1 charge state), and Rh_int^•@C₃N₃ (Rh interstitial site, +1 charge state) exhibit outstanding OER/ORR catalytic activity with η^OER/η^ORR of 0.56/0.37 V, 0.66/0.37 V, and 0.59/0.44 V, respectively, which are comparable to benchmark electrocatalysts such as RuO₂ for OER (0.69 V) and Pt for ORR (0.61 V). The enhanced performance arises from charged defects adjustment that optimizes the bifunctional OER/ORR activity, offering potential electrocatalysts for energy conversion applications.