In the context of carbon reutilization, the direct electroreduction of captured CO₂ (c-CO₂RR) appears as an appealing approach since it avoids the energetically costly separation of CO₂ from the capture agent. In this process, CO₂ is directly reduced from its captured form. Here, we investigate the influence of the capture agent and proton source on that reaction from a combination of theory and experiment. Specifically, we consider methoxide-captured CO₂, NH₃-captured CO₂, and bicarbonate on silver electrocatalysts. We show that the proton source plays a key role in the interplay of the chemistries for the electroreduction of protons, free CO₂, and captured CO₂. Our density functional theory calculations, including the influence of the potential, demonstrate that a proton source with smaller pK_a improves the reactivity for c-CO₂RR, but also increases the selectivity toward the hydrogen evolution reaction (HER) on silver surfaces. Since c-CO₂RR requires an additional chemical protonation step, the influence of the proton source is stronger than that of the HER. However, c-CO₂RR cannot compete with the HER on Ag, Experimentally, the dominant product observed is H₂ with low amounts of CO being produced. Through a rotating cylinder electrode cell of well-defined mass-transport properties, we conclude that although methanol solvent exhibits a lower HER activity, HER remains dominant over c-CO₂RR. Our work suggests that methoxide is a potential alternative capture agent to NH₃ for direct reduction of captured CO₂, though challenges in catalyst design, particularly in reducing the onset potential of c-CO₂RR to surpass the HER, remain to be addressed.

中文翻译：

---

捕获的 CO2 在银催化剂上的电还原：捕获剂和质子源的影响


在碳再利用的背景下，捕获的 CO ₂ (c-CO ₂ RR) 的直接电还原似乎是一种有吸引力的方法，因为它避免了能源成本高昂的 CO ₂ 来自捕获代理。在此过程中，CO ₂ 从其捕获形式直接还原。在这里，我们结合理论和实验研究了捕获剂和质子源对该反应的影响。具体来说，我们考虑银电催化剂上的甲醇盐捕获的 CO ₂ 、NH ₃ 捕获的 CO ₂ 和碳酸氢盐。我们表明，质子源在质子电还原、游离 CO ₂ 和捕获的 CO ₂ 化学反应的相互作用中起着关键作用。我们的密度泛函理论计算（包括电势的影响）表明，具有较小 pK _a 的质子源提高了 c-CO ₂ RR 的反应性，同时也提高了对银表面的析氢反应（HER）。由于c-CO ₂ RR需要额外的化学质子化步骤，因此质子源的影响比HER的影响更强。然而，c-CO ₂ RR 无法与 Ag 上的 HER 竞争。实验上，观察到的主要产物是 H ₂ ，并产生少量 CO。通过具有明确传质特性的旋转圆柱电极电池，我们得出结论，尽管甲醇溶剂表现出较低的 HER 活性，但 HER 仍然优于 c-CO ₂ RR。 我们的工作表明，甲醇盐是 NH ₃ 的潜在替代捕获剂，用于直接还原捕获的 CO ₂ ，尽管催化剂设计方面存在挑战，特别是在降低 c-CO 的起始潜力方面 ₂ RR能否超越HER，还有待解决。