Surface-Immobilized Cross-linked Cationic Polyelectrolyte Enables CO2 Reduction with Metal Cation-free Acidic Electrolyte

Abstract

Conducting electrochemical CO2 reduction with acidic electrolyte is a promising strategy to achieve high utilization efficiency of CO2, which is an essential prerequisite for industrializable CO2 electroreduction technique. Recent progress of CO2 electroreduction in acidic electrolyte has validated that alkali cations in the electrolyte play a vital role to suppress hydrogen evolution and promote CO2 reduction. However, the addition of alkali cations causes precipitation of bicarbonate on gas diffusion electrode (GDE), flooding of electrolyte through GDE, and drifting of the pH of the electrolyte during electrolysis. In this work, we realized the electroreduction of CO2 in metal cation-free acidic electrolyte by covering the catalyst with cross-linked poly-diallyldimethylammonium chloride. This polyelectrolyte provides high density of cationic sites immobilized on the surface of catalyst, which suppresses the mass transport of H+ and modulates the interfacial field strength. By adopting this strategy, the Faradaic efficiency (FE) of CO reached 92% with Ag catalyst and the FE of formic acid reached 74% with In catalyst. More importantly, with metal cation-free acidic electrolyte, the amount of electrolyte flooding through the GDE decreased to 1% of that with alkali cation-containing acidic electrolyte, and the pH values of both catholyte and anolyte kept constant. Thanks to these features, the stability of CO2 reduction performance was greatly improved.

Stability Issues in Electrochemical CO2Reduction: Recent Advances in Fundamental Understanding and Design Strategies

“…The surface functionalization of catalyst with immobilized cross-linked polydiallyldimethylammonium cations also enabled the CO 2 electroreduction in metal-cation-free acidic electrolyte to produce CO and formate. [148] Importantly, the carbonate precipitation was avoided in metal-cation-free acidic electrolyte, and the amount of electrolyte flooding decreased to 1% of that in metal-cationinvolved system. This led to the enhanced stability of metalcation-free acidic CO 2 RR in compared to CO 2 RR in metal-cationcontaining acidic electrolyte (36 h vs 10 h for CO production, at 200 mA cm −2 , in flow cell).…”

Weakly Coordinating Organic Cations Are Intrinsically Capable of Supporting CO2 Reduction Catalysis

“…31 However, in all these studies, the organic cation is present as a minor constituent of the medium alongside either a superstoichiometric excess of alkali metal cations for studies in neutral pH electrolytes (Figure 1b, top) or in the presence of a super-stoichiometric excess of hydronium ions for studies in bulk acidic electrolytes. 32 Because these additives are only a minor constituent in the overall electrolyte, even if they are concentrated at the surface, it is challenging to definitively attribute CO 2 activation to these organic species versus alkali metal cations in the medium that may also be present at the surface (Figure 1b, bottom). Indeed, these systems often still display a strong sensitivity to the identity of the bulk alkali metal cation, 24 suggesting that alkali metal cations are still significantly influencing catalysis despite the presence of the organic additive.…”

“…Experimentally, CO 2 reduction has been extensively investigated in the presence of organic cations of various types, including surfactants, , ionomer films, − and cationic conjugate bases of superacids . However, in all these studies, the organic cation is present as a minor constituent of the medium alongside either a super-stoichiometric excess of alkali metal cations for studies in neutral pH electrolytes (Figure b, top) or in the presence of a super-stoichiometric excess of hydronium ions for studies in bulk acidic electrolytes . Because these additives are only a minor constituent in the overall electrolyte, even if they are concentrated at the surface, it is challenging to definitively attribute CO 2 activation to these organic species versus alkali metal cations in the medium that may also be present at the surface (Figure b, bottom).…”

Weakly Coordinating Organic Cations are Intrinsically Capable of Supporting CO2 Reduction Catalysis

“…Experimentally, CO2 reduction has been extensively investigated in the presence of organic cations of various types, including surfactants 22,23 , ionomer films [24][25][26][27][28][29][30] , and cationic conjugate bases of superacids 31 . However, in all these studies, the organic cation is present as a minority constituent of the medium alongside either a super-stoichiometric excess of alkali metal cations for studies in neutral pH electrolytes (Figure 1b, top) or in the presence of super-stoichiometric excess of hydronium ions for studies in bulk acidic electrolyte 32 . Because these additives are only a minority constituent in the overall electrolyte, even if they are concentrated at the surface, it is challenging to definitively attribute CO2 activation to these organic species versus alkali metal cations in the medium that may also be present at the surface (Figure 1b, bottom).…”