Insights into pressure tunable reaction rates for electrochemical reduction of CO2 in organic electrolytes,Green Chemistry

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Insights into pressure tunable reaction rates for electrochemical reduction of CO2 in organic electrolytes
Green Chemistry ( IF 9.3 ) Pub Date : 2020-02-13 , DOI: 10.1039/d0gc00013b
Charles I. Shaughnessy _{1,

2,

3,

4,

5} , David J. Sconyers _{1,

2,

3,

4,

6} , Hyun-Jin Lee _{1,

2,

3,

4} , Bala Subramaniam _{1,

2,

3,

4,

5} , James D. Blakemore _{1,

2,

3,

4,

6} , Kevin C. Leonard _{1,

2,

3,

4,

5}

Affiliation

Electrochemical CO₂ reduction rates are limited in aqueous-based electrolytes by the low solubility of CO₂. We recently demonstrated that organic solvent-based CO₂ Expanded Electrolytes (CXEs) can solubilize multi-molar amounts of CO₂ at moderate pressures while retaining sufficient supporting electrolyte to facilitate electrochemistry. Up to an order of magnitude enhancement in CO₂ reduction rates to CO was achieved on polycrystalline Au electrodes at faradaic efficiencies approaching 80%. Herein, we show that similar enhancements are observed on Cu catalysts as well, on the basis of enhanced current flow. On both systems, a maximum in CO₂ reduction rate was observed at ca. 5 M CO₂ concentration (i.e., 3.1 MPa head-space pressure) beyond which the rate decreases. To explain this counterintuitive phenomenon, we developed a detailed COMSOL-based mechanistic model of CO₂ reduction in organic electrolytes under elevated pressures on Au electrodes. This model incorporates the dissimilar variations of the key physicochemical properties (viz., CO₂ concentration, CO₂ diffusion rate, and solution polarity) with CO₂ pressure. We thereby demonstrate that the overall rate is limited by CO₂ concentrations at lower than optimum CO₂ pressures. At pressures higher than the optimum the rate is limited by both an attenuation of the first electron-transfer step and an increase in the ohmic resistance of the system. Excellent quantitative match between experimental and model-predicted rates lend credence to the proposed underlying mechanism of electrocatalytic CO₂ reduction in CXEs. These fundamental insights provide guidance for the rational design and scaleup of highly efficient CXE-based CO₂ reduction systems.

中文翻译：

洞察压力可调反应速率以电化学还原有机电解质中的CO2

在水基电解质中，由于CO _2的溶解度低，电化学的CO ₂还原速率受到限制。最近，我们证明了基于有机溶剂的CO ₂膨胀电解质（CXE）可以在中等压力下溶解多摩尔量的CO ₂，同时保留足够的支持电解质以促进电化学。在法拉第效率接近80％的多晶Au电极上，CO ₂还原成CO的速率最多提高了一个数量级。在本文中，我们表明，基于增强的电流，在铜催化剂上也观察到了类似的增强。在两个系统上，均观察到最大的CO ₂还原速率为ca. 5 M CO ₂浓度（即3.1 MPa顶空压力），超过该浓度速率会降低。为了解释这种违反直觉的现象，我们开发了基于COMSOL的详细模型，该模型在Au电极上在高压下有机电解质中CO ₂还原的机理。该模型结合了关键物理化学性质（即，CO ₂浓度，CO ₂扩散速率和溶液极性）随CO ₂压力的不同变化。因此，我们证明了总速率受低于最佳CO _2的CO ₂浓度的限制压力。在高于最佳压力的压力下，速率受到第一电子转移步骤的衰减和系统欧姆电阻的增加的限制。实验速率与模型预测速率之间的出色定量匹配为所提出的CXE中电催化还原CO _2的潜在机理提供了依据。这些基本见解为高效设计基于CXE的CO ₂还原系统的合理设计和规模扩展提供了指导。

更新日期：2020-02-13

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