Edited by: Nosang Vincent Myung, University of California, Riverside, United States
Reviewed by: Syed Mubeen Jawahar Hussaini, University of Iowa, United States; Federica Valentini, Università di Roma Tor Vergata, Italy
This article was submitted to Green and Sustainable Chemistry, a section of the journal Frontiers in Chemistry
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In this manuscript, an electrochemical architecture is designed that controls the kinetics of proton transfer to metal triazole complexes for electrocatalytic O2 and CO2 reduction. Self-assembled monolayers of these catalysts are attached to a glassy carbon electrode and covered with a lipid monolayer containing proton carriers, which acts as a proton-permeable membrane. The O2 reduction voltammograms on carbon are similar to those obtained on membrane-modified Au electrodes, which through the control of proton transfer rates, can be used to improve the selectivity of O2 reduction. The improved voltage stability of the carbon platforms allows for the investigation of a CO2 reduction catalyst inside a membrane. By controlling proton transfer kinetics across the lipid membrane, it is found that the relative rates of H2, CO, and HCOOH production can be modulated. It is envisioned that the use of these membrane-modified carbon electrodes will aid in understanding catalytic reactions involving the transfer of multiple protons and electrons.
The electrocatalysis of small molecules is important in a wide range of renewable energy devices (Duan et al.,
In both CO2 reduction and the O2 reduction reaction (ORR), the dynamics of proton transfer to catalytic sites are instrumental in dictating catalyst selectivity and performance (Hammes-Schiffer and Soudackov,
Previous results demonstrate that the incorporation of alkyl proton carriers inside the lipid layer of these electrodes can be used to control the kinetics of proton transfer to catalysts (Tse et al.,
In this manuscript, membrane-modified carbon electrodes are designed that allow for proton transfer dynamics to electrocatalysts to be controlled and that exhibit greater electrochemical stability than their Au counterparts. The architecture developed here enables the interrogation of the membrane-modified ORR, and also the study of reactions such as the CO2 reduction reaction, which occurs at high overpotentials.
Chemicals were obtained from commercial sources and were not subjected to additional purification. 1,2-dimyristoyl-
For experiments with carbon, a 5 mm diameter glassy carbon electrode was used as the working electrode. The working electrode was cleaned before each experiment by rinsing the electrode surface using de-ionized water, followed by manual polishing with 0.3 μm alumina particles suspended in de-ionized water on a polishing pad for approximately 6 min. After polishing, the electrode was sonicated in de-ionized water, followed by sonication in acetone, and finally sonication in isopropyl alcohol for 3 min each before being dried under a stream of air. For the electrochemical attachment of the amino-terminated triazole onto the glassy carbon electrode, cyclic voltammograms were conducted with the cleaned electrode in a 10 mL ethanolic solution containing 5 mM amino-terminated triazole and 100 mM LiClO4 from a potential of 2 V to −0.01 V at a scan rate of 10 mV/s for 10 cycles. Following the attachment, the electrode was sonicated in pH 7 potassium phosphate buffer for 10 min to remove excess, unattached triazole molecules. After sonication, the amino-terminated triazole surface was immersed in a 10 mL de-ionized water solution containing 10 mM CuSO4 or 10 mM AgNO3 for 1 hr to form the Cu-triazole or Ag-triazole complex, respectively. For the attachment of the lipid membrane containing a proton carrier, the glassy carbon electrode modified with the Cu-triazole complex was immersed in a 1 mL CHCl3 solution containing 7.4 mM DMPC and 5.6 mM proton carrier for 20 s followed by a brief submersion into 3 mL de-ionized water containing 100 mM KCl until excess CHCl3 solution separated away from the electrode surface. Finally, the membrane-modified electrode was rinsed with pH 7 phosphate buffer before electrochemical analyses were performed.
To test the catalytic activity of the membrane-modified glassy carbon electrode, O2 reduction and CO2 reduction reactions were performed. A pH 7 phosphate buffer solution was sparged with air or CO2 for a minimum of 20 min to ensure the solution was saturated with the specific gas. Electrocatalytic activity was evaluated using linear sweep voltammetry from 0.3 V to −0.7 V for O2 reduction or 0.3 V to −2.0 V for CO2 reduction at a scan rate of 10 mV/s. A blocking test to assess the integrity of the membrane-covered electrode was performed after each reduction reaction using a CV from 0.5 V to −0.5 V at a scan rate of 50 mV/s in a de-ionized water solution containing 1.5 mM K3Fe(CN)6 and 100 mM NaCl. The products of the CO2 reduction reaction were identified using protocols modified from the literature (Tornow et al.,
To construct membrane-modified electrodes for electrocatalysis, two triazole molecules were first synthesized following literature protocols (Li et al.,
Functional and structural features of two triazoles used in electrocatalysis.
After synthesizing these triazole ligands, the electrocatalytic activity of the Cu triazole complex on Au electrodes was first analyzed with and without lipid membranes. First, the Cu catalyst was attached to Au electrodes by forming a SAM of the thiol-modified triazole and subsequently immersing the SAM in a solution of CuSO4 (Figure
Schematic of membrane-modified electrode consisting of a metal triazole catalyst (green) and a lipid monolayer (blue) with proton carrier (red). M = Cu2+ or Ag+ for catalysts that reduce O2 or CO2, respectively.
Linear sweep voltammograms of O2 reduction by a Au electrode modified with the Cu complex of the thiol-terminated triazole (black line) covered by a lipid membrane (blue line) with DBA proton carrier (red line) in pH 7 phosphate buffer at a scan rate of 10 mV/s.
The enhancement of current elicited by the proton carrier is due to a change in the ORR mechanism as demonstrated in previous work (Tse et al.,
Next, the ORR catalytic activity on carbon electrodes was analyzed since carbon is more durable and inexpensive than Au, making it the electrode of choice for commercial fuel cells. Toward this end, membrane-modified glassy carbon electrodes were designed. First, the amino-terminated triazole was covalently attached to the electrode surface through the oxidation of the primary amine group using cyclic voltammetry (CV). The CVs recorded during the attachment process display anodic peaks at around 0.8–0.9 V, which indicate that the amine is oxidized at the carbon surface (Figure
Cyclic voltammogram cycles 1 (red line), 5 (blue line), and 10 (black line) of a glassy carbon electrode in an ethanolic solution of 5 mM amino-terminated triazole and 100 mM LiClO4 at a scan rate of 10 mV/s.
After electrochemical attachment of the amino-terminated triazole to the carbon electrode, the Cu-triazole complex was formed by soaking the electrode in a solution of CuSO4 (Figure
Linear sweep voltammograms of O2 reduction by a glassy carbon electrode (blue line) modified with the amino-terminated triazole (red line) and the Cu complex of the amino-terminated triazole (black line) in pH 7 phosphate buffer at a scan rate of 10 mV/s.
Having established the electrocatalytic activity of the Cu triazole complex on a carbon electrode, the surface was next modified with a lipid membrane to control proton transfer to the catalyst. The lipid membrane was formed by soaking the electrode in a solution containing DMPC using a method adapted from a previously reported procedure (Han et al.,
Linear sweep voltammograms of O2 reduction by a glassy carbon electrode modified with the Cu complex of the amino-terminated triazole (black line) covered by a lipid membrane (blue line) with DBA proton carrier (red line) in pH 7 phosphate buffer at a scan rate of 10 mV/s.
To assess the integrity of the lipid layer during the ORR, blocking experiments were performed using K3Fe(CN)6 in bulk solution after the ORR as described in other systems (Barile et al.,
Electrocatalytic CO2 reduction typically occurs at high overpotentials (Qiao et al.,
In contrast to a multilayer architecture, we electrochemically attach a monolayer of catalyst to glassy carbon electrodes that do not require the use of a binder. This method of surface modification allows for a more direct assessment of the activity of molecular CO2 catalysts. Moreover, binders such as Nafion dramatically alter the proton transfer rates to embedded catalysts. The binder-free system devised here enables us to systematically analyze the effect of proton transfer on catalyst performance. In a manner similar to the previously described Cu triazole ORR catalyst, the kinetics of proton transfer to a CO2 reduction catalyst can be tuned by covering the catalyst with a lipid monolayer.
Ag complexes containing N-based heterocycles form one class of molecular CO2 reduction catalysts (Tornow et al.,
Linear sweep voltammograms of CO2 reduction by a glassy carbon electrode (blue line) modified with the amino-terminated triazole (red line) and the Ag complex of the amino-terminated triazole (black line) in pH 7 phosphate buffer at a scan rate of 10 mV/s. CO2 reduction by a glassy carbon electrode immersed only in AgNO3(aq) and subsequently rinsed with water was also evaluated as a control experiment (green line).
A further control experiment of a LSV of the Ag triazole complex conducted in a N2 environment shows a similar onset potential of about −1.25 V and also does not exhibit a peak at −1.5 V (Figure
Linear sweep voltammograms by a glassy carbon electrode modified with the Ag complex of the amino-terminated triazole in CO2-saturated (black line) and N2-saturated (red line) pH 7 phosphate buffer at a scan rate of 10 mV/s.
Having established that the Ag triazole complex catalyzes CO2 reduction, its catalytic performance was next measured using a membrane-modified electrode. When the catalyst is covered by a lipid monolayer (Figure
Linear sweep voltammograms of CO2 reduction by a glassy carbon electrode modified with the Ag complex of the amino-terminated triazole (black line) covered by a lipid membrane (blue line) with MDP proton carrier (red line) in pH 7 phosphate buffer at a scan rate of 10 mV/s.
The addition of a proton carrier, either an alkyl phosphate, MDP, or an alkyl boronic acid, DBA, into the lipid layer (Figure
The CO2 reduction products obtained using the Ag triazole catalyst in different electrode environments at −1.75 V were quantified (Figure
Faradaic efficiencies for CO (gray), HCOOH (red), and H2 (blue) production from the Ag triazole complex (left) with lipid (middle) and DBA proton carrier (right) obtained from chronoamperometry experiments at −1.75 V vs. Ag/AgCl.
Covering the Ag triazole catalyst with a lipid layer decreases the Faradaic efficiency of H2 production from ~71 to ~56%. The decreased quantity of H2 produced is attributed to the hydrophobic nature of the lipid environment, which decreases the rate of proton transfer to the catalyst. With an impeded proton transfer rate, the catalyst has more time to bind and reduce CO2 to either CO or HCOOH. This alteration in mechanism with a change in proton transfer rate is displayed schematically in Figure
The product selectively is further altered when a proton carrier is incorporated in the lipid layer. Specifically, the Faradaic efficiency for H2 increases from ~56 to ~77% upon adding the proton carrier. The proton carrier increases proton transfer kinetics to the catalyst, which favors the production of H2. Interestingly, the proton carrier drastically increases the ratio of CO to HCOOH generated and almost completely eliminates HCOOH production (~0.03% Faradaic efficiency). The exact origin of this change in product selectivity is unknown, but possibly originates from interactions between the proton carrier and CO2 reduction intermediates. The CO2 reduction products of this system were also quantified as a function of temperature (Figure
Lastly, the effect of voltage on the CO2 product speciation was tested. The CO2 products generated at −2 V are displayed in Figure
We designed membrane-modified electrodes containing metal triazole complexes that electrocatalyze the reduction of O2 and CO2. For the O2 reduction reaction, the complexes were anchored using SAMs on both Au and glassy carbon electrodes. By covering the catalysts in a lipid layer containing proton carriers, the kinetics of proton transfer to the complexes can be controlled on both substrates. The membrane-modified electrocatalytic systems developed on glassy carbon electrodes have a wider electrochemical window than those using Au, which enable the study of CO2 reduction by lipid-covered catalysts. The results suggest that the relative rates of H2, CO, and HCOOH production can be altered through the use of membranes.
SS performed experiments. Both SS and CB designed experiments, interpreted the data, and wrote the paper.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
We acknowledge the Shared Instrumentation Laboratory in the Department of Chemistry at the University of Nevada, Reno. We thank Dr. Edmund Tse for useful discussions.
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