Hydroxide Is Not a Promoter of C<sub>2+</sub> Product Formation in the Electrochemical Reduction of CO on Copper

Li, Jingchao; Wu, Donghuan; Malkani, Arnav S.; Chang, Xiaoxia; Cheng, Mu Jeng; Xu, Bingjun; Lü, Qi

doi:10.1002/anie.201912412

Cited by 90 publications

(84 citation statements)

References 53 publications

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“…These results are in contrast with a previous report using Cu foils, on which clear mass transport limitation of CO was observed at more negative potentials 32 , 47 . The physical characterizations of Cu MPs were previously reported by our group 42 . All C 2+ products exhibit a similar Tafel slope of ~118 mV dec −1 (Fig.…”

Section: Resultsmentioning

confidence: 99%

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

et al. 2021

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Rigorous electrokinetic results are key to understanding the reaction mechanisms in the electrochemical CO reduction reaction (CORR), however, most reported results are compromised by the CO mass transport limitation. In this work, we determined mass transport-free CORR kinetics by employing a gas-diffusion type electrode and identified dependence of catalyst surface speciation on the electrolyte pH using in-situ surface enhanced vibrational spectroscopies. Based on the measured Tafel slopes and reaction orders, we demonstrate that the formation rates of C2+ products are most likely limited by the dimerization of CO adsorbate. CH4 production is limited by the CO hydrogenation step via a proton coupled electron transfer and a chemical hydrogenation step of CO by adsorbed hydrogen atom in weakly (7 < pH < 11) and strongly (pH > 11) alkaline electrolytes, respectively. Further, CH4 and C2+ products are likely formed on distinct types of active sites.

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Section: Resultsmentioning

confidence: 99%

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

et al. 2021

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“…Note that the local [OH – ], which increases with increasing CO (2) R current, plays no direct role in promoting the rate of these two steps 23 , since they are driven by the field alone . However, the [OH – ] can alter the CO 2 concentration through the bicarbonate equilibria, suppress CH 4 formation 14 , and promote the activity towards acetate, even at a fixed U vs. SHE 24 .…”

Section: The Activity Towards Co and C 2 Products mentioning

confidence: 99%

A few basic concepts in electrochemical carbon dioxide reduction

Chan

2020

Nat Commun

103

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In this perspective, I discuss a few basic concepts in fundamental mechanistic studies of electrochemical carbon dioxide reduction. With the looming global environmental crisis, electrochemical CO 2 reduction (CO 2 R) is a hot topic. Excellent perspectives on mechanistic studies 1-3 , practical vapor-fed devices 4 , and technoeconomic and system-level analyses 5-7 have come out in the past few years, all with compelling visions for the future. We can also harken back to Hori's timeless review of his work over several decades 8 , which seeded many of the impressive advances today. But as the French maxim says: parfois, il faut reculer pour mieux sauter. Here, I showcase a few basic concepts in the fundamental mechanistic studies of CO 2 R. What computational electrocatalysis can and cannot do In heterogeneous catalysis, periodic density functional theory (DFT) simulations have really enabled us to computationally explore reaction mechanisms. For electrocatalysis, the "computational hydrogen electrode" model is our standard method to determine reaction thermodynamics 9. This method trivially translates simulations in vacuum to potential-dependent energetics, without requiring we simulate explicitly the ions or potential. Our models of the electrolyte and electrochemical reaction barriers, in contrast, are far from convergent. Our field abounds with different approaches towards the electrolyte: implicit continuum models, explicit ab initio ones, or a hybrid of the two (Fig. 1a) 10. We also have multiple ways to obtain the potential and the potential dependence of the reaction energetics 11. While continuum approximations give us huge reductions in computational cost, we see significant deviations in solvation energies determined with implicit vs. dynamic explicit water models 12. Furthermore, different ways to set up the applied potential result in differences in the computed reaction energetics 13. All these challenges could contribute to the wide range in the computed energetics and mechanisms towards the various C 2 products 1. Despite the difficulties in an ab initio treatment of electrochemical reaction barriers, we do need kinetics for mechanistic understanding. Case in point: our evolving understanding of CO (2) R to CH 4 on Cu 14. A thermodynamic analysis showed a proton-electron transfer to *CO to form *CHO to be the rate-limiting step. Considering the corresponding barriers across different materials, we suggested the transition state of this process to be the descriptor of activity. But when we simply consider the kinetics of electrochemical reactions with multiple proton-electron transfers, we see that this step cannot be rate limiting on Cu. The corresponding Tafel plots have defining features that depend on the symmetry factor β (0 < β < 1) for the rate-limiting step, as well as the number of proton-electron transfers preceding it, n. Figure 1b shows the Tafel slopes and the effect of pH on the overpotential for alkaline solutions, where H 2 O as the proton donor. Experiments show

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“…[25,26,40] We performed a control experiment where we progressively incorporated 18-crown-6 ether to a 0.5 M KHCO 3 solution to selectively chelate K + (denoted crown-K + , see illustration on Figure S8a, b). [41,42] The purpose of this experiment was to modulate the charge density at the OHP (by changing the effective cation size compared to solvated K + ), while preventing the polarization of water molecules contained in the hydration shell of the cation. We found that the FE CO values decreased by~15 % as the [18crown-6 ether] was increased from 0 M to 0.5 M (K + is chelated by equimolar amounts of 18-crown-6 ether) during electrolysis at a constant applied current density of 100 mA cm À 2 (Figure S8c).…”

Section: Effect Of Cation Identity On Bicarbonate Electrolysis and Co2rr Catalyst Activitymentioning

confidence: 99%

Impact of Alkali Cation Identity on the Conversion of HCO₃⁻ to CO in Bicarbonate Electrolyzers

et al. 2021

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The reduction of CO 2 to CO from a bicarbonate feedstock offers an opportunity to directly use aqueous carbon capture solutions, while bypassing ex-situ energy-intensive gaseous CO 2 regeneration. In this study, we resolved how the electrolyte cation identity (Li + , Na + , K + , Cs + ) affects the two reactions that make bicarbonate electrolysis possible: (i) the production of insitu CO 2 formed through reaction of HCO 3 À (from the catholyte) with H + (sourced from the membrane); and (ii) the electroreduction of CO 2 into CO. Our results show that cation identity does not change the rate of in-situ CO 2 formation, but it does enhance the rate of the CO 2 reduction reaction (CO2RR).Electrolysis experiments performed with a constant [HCO 3 À ] showed that CO selectivities progressively increased for the series Li + , Na + , K + , and Cs + , respectively. Optimization of the electrolyte composition yielded a CO selectivity of~80 % during electrolysis of 1.5 M CsHCO 3 solutions at 100 mA cm À 2 , while saturated LiHCO 3 solutions (0.84 M) yielded CO selectivities values of merely 30 % at the same current density. This study demonstrates a quantitative relationship between CO product selectivity and the cation radius, which provides a pathway to integrate bicarbonate electrolysis to carbon capture.

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Hydroxide Is Not a Promoter of C₂₊ Product Formation in the Electrochemical Reduction of CO on Copper

Cited by 90 publications

References 53 publications

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

A few basic concepts in electrochemical carbon dioxide reduction

Impact of Alkali Cation Identity on the Conversion of HCO₃⁻ to CO in Bicarbonate Electrolyzers

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Hydroxide Is Not a Promoter of C2+ Product Formation in the Electrochemical Reduction of CO on Copper

Cited by 90 publications

References 53 publications

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

Electrokinetic and in situ spectroscopic investigations of CO electrochemical reduction on copper

A few basic concepts in electrochemical carbon dioxide reduction

Impact of Alkali Cation Identity on the Conversion of HCO3− to CO in Bicarbonate Electrolyzers

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Hydroxide Is Not a Promoter of C₂₊ Product Formation in the Electrochemical Reduction of CO on Copper

Impact of Alkali Cation Identity on the Conversion of HCO₃⁻ to CO in Bicarbonate Electrolyzers