Local Spin‐State Tuning of Iron Single‐Atom Electrocatalyst by S‐Coordinated Doping for Kinetics‐Boosted Ammonia Synthesis

Abstract

The development of highly active electrocatalysts for e-NRR is obviously the key and thus has attracted increasing attention in recent years. Theoretical calculation predicted that an ideal e-NRR electrocatalyst should have suitable adsorption ability, i.e., neither too strongly nor too weakly, toward the N atom on catalytic surface, which offers an approximate guideline for designing highly efficient catalysts. [3] Catalysts based on noble metals are demonstrated to show e-NRR with favorable catalytic performances, but the high cost and less abundance of these catalysts hinder their widespread applications. [4] Much attention has thus been paid to developing transition metal alternatives. As we know, the catalytic activities of transition metals for e-NRR usually correlate strongly with the e g and t 2g orbitals' occupancy of catalysts, which to some extent determines the binding strength between the metal and nitrogen species. [5] Recently, transition metal single-atom catalysts (SACs) have been proved to be promising electrocatalysts for NH 3 synthesis. [6] Among them, iron-nitrogen-carbon materials (Fe-N-Cs) with atomically dispersed FeN 4 sites have received increasing attention as efficient e-NRR catalysts. [7] However, the electronegativity of the symmetrical adjacent nitrogen atoms around the metal site is relatively large, which makes The electrochemical nitrogen reduction reaction (e-NRR) is envisaged as alternative technique to theHaber-Bosch process for NH 3 synthesis. However, how to develop highly active e-NRR catalysts faces daunting challenges. Herein, a viable strategy to manipulate local spin state of isolated iron sites through S-coordinated doping (Fe SA -NSC) is reported. Incorporation of S in the coordination of Fe SA -NSC can induce the transition of spin-polarization configuration with the formation of a medium-spin-state of Fe (t 2g 6 e g 1), which is beneficial for facilitating e g electrons to penetrate the antibonding π-orbital of nitrogen. As a consequence, a record-high current density up to 10 mA cm −2 can be achieved, together with a high NH 3 selectivity of ≈10% in a flow cell reactor. Both experimental and theoretical analyses indicate that the monovalent Fe(I) atomic center in the Fe SA -NSC after the S doping accelerates the N 2 activation and protonation in the rate-determining step of *N 2 to *NNH.

Highly dispersed Ru clusters toward an efficient and durable hydrogen oxidation reaction

“…Ru c /NHCS shows an ECSA-corrected j 0,s of 0.37 mA cm ECSA −2 , close to 0.39 mA cm ECSA −2 for Pt/C. To the best of our knowledge, the j 0,s achieved on Ru c /NHCS outperforms those of RuPd, 15 Rh, 19 IrNi@PdIr, 40 and Pd–CeO 2 catalysts. 41…”

“…Ru c / NHCS shows an ECSA-corrected j 0,s of 0.37 mA cm ECSA À2 , close to 0.39 mA cm ECSA À2 for Pt/C. To the best of our knowledge, the j 0,s achieved on Ru c /NHCS outperforms those of RuPd, 15 Rh, 19 IrNi@PdIr, 40 and Pd-CeO 2 catalysts. 41 In fuel cells, it is commonly accepted that anodic PGM catalysts are readily poisoned by CO that exists in the feeding H 2 gas.…”

“…Since then, various strategies have emerged to develop high-performance HOR catalysts, such as engineering the crystalline phase, 14 tuning the composition of alloy particles, 15,16 designing a core–shell structure, 17–22 constructing a hierarchical structure, 23 and so on. Accordingly, great advancements have been achieved, and most of the studies have focused on precious metals or their alloys.…”

Atomically dispersed metal catalysts for the electrochemical nitrogen reduction reaction

“…Apart from typical M–N–C catalysts, the doped heteroatoms (such as P and S) with different electronegativities into carbon supports can efficiently tune the electronic structure and coordination configuration of central active sites, and then optimize the adsorption process of intermediates, 117 further enhancing the eNRR performance. Hou et al 118 reported a N, S-coordinated single Fe atom catalyst immobilized on a carbon support derived from pyrolysis of a hydrogel to enhance eNRR activity (Fig. 7e).…”

Promoting ambient ammonia electrosynthesis on modulated Cuδ+catalysts by B-doping

“…5 As an alternative, the electrochemical N 2 -to-NH 3 conversion under ambient conditions has attracted significant interest with the aid of renewable electricity and the use of water as a hydrogen source. 6–8 Nonetheless, the high activation barrier of the NN triple bond and low solubility in electrolytes make it exhibit low selectivity and efficiency, 9–13 which hinders its practical use to some degree. To overcome these issues, other reactive N-containing compounds are emerging to have the possibility of achieving good performance in NH 3 electrosynthesis.…”