Park Cheolmin, Seo Min Young, Kwon Taesung, Kim Jiyoon, Nam Ki Min, Kim YongJoo, Chang Jinho
Department of Chemistry, Hanyang University, Seoul 04763, Republic of Korea.
Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea.
J Am Chem Soc. 2025 Jan 8;147(1):687-700. doi: 10.1021/jacs.4c13117. Epub 2024 Dec 24.
The electrochemical nitrate reduction reaction (NORR) involves multiple hydrogenation and deoxygenation steps, which compete with the hydrogen evolution reaction (HER). Therefore, NORR driven in acidic media is challenging in spite of advantageous fast hydrogen transfers in its elementary steps. The findings presented in this article first demonstrate that the NORR is significantly activated even in acidic lithium nitrate solutions at LiNO concentrations exceeding 6 m on a Pt electrode (the highly effective catalyst for HER) by the formation of a "hydronium-in-salt" electrolyte (HISE), a new type of aqueous high concentration salt electrolyte. The observed enhancement of NORR while the suppression of HER-activity in the LiNO based HISE was verified by scanning electrochemical microscopy, electrochemical impedance spectroscopy, UV-vis/IR spectroscopy, and molecular dynamics simulations. The formation of a HISE in acidic LiNO solutions contrasts with that of a "water-in-salt" electrolyte in LiTFSI with the same concentration. The mechanism of NORR activation in a HISE suggests facilitated proton-coupled electron transfers (PCETs) from HO to NO and subsequent reactive intermediates owing to the proximity between the two ions induced by the unique solvation structure blended with all ions together (Li + NO + HO). In contrast, all the ions are separately hydrated at low concentrations of LiNO electrolytes. On the other hand, PCET from HO to NO in a high concentration LiNO electrolyte (e.g., 9 m) is not kinetically preferred, probably owing to the slow dissociation kinetics of HO, and therefore, HO reduction is not suppressed by the NORR.
电化学硝酸盐还原反应（NORR）涉及多个氢化和脱氧步骤，这些步骤与析氢反应（HER）相互竞争。因此，尽管在其基本步骤中具有快速氢转移的优势，但在酸性介质中驱动NORR仍具有挑战性。本文提出的研究结果首次表明，通过形成一种新型的水性高浓度盐电解质“盐包水合氢离子”电解质（HISE），即使在硝酸锂浓度超过6 m的酸性硝酸锂溶液中，在铂电极（HER的高效催化剂）上，NORR也会被显著激活。通过扫描电化学显微镜、电化学阻抗谱、紫外-可见/红外光谱和分子动力学模拟，验证了在基于LiNO的HISE中NORR增强而HER活性受到抑制的现象。酸性LiNO溶液中HISE的形成与相同浓度的双三氟甲烷磺酰亚胺锂（LiTFSI）中的“盐包水”电解质形成形成对比。HISE中NORR活化的机制表明，由于独特的溶剂化结构使所有离子（Li⁺ + NO₃⁻ + H₂O⁺）混合在一起，两种离子之间距离较近，促进了质子耦合电子转移（PCET）从H₂O⁺到NO₃⁻以及随后的反应中间体。相比之下，在低浓度LiNO电解质中，所有离子都是单独水合的。另一方面，在高浓度LiNO电解质（例如9 m）中，从H₂O⁺到NO₃⁻的PCET在动力学上并不占优势，这可能是由于H₂O⁺的解离动力学较慢，因此，NORR不会抑制H₂O⁺的还原。
