Ferrying electrons with ferrocene to enhance nickel electrochemistry

Redox-active transition metal complexes, those that can undergo multiple oxidation and reduction events, are ideal candidates for electrochemical energy storage and fuel technologies. A significant caveat to employing these complexes for electrochemical processes is that their solubilities can drastically change across redox states, creating insoluble oxidation or reduction products that precipitate out of solution. Even when the insoluble redox product is still chemically-intact (as in a reversible electrochemical reaction), it can often be difficult to electrochemically convert it back to its original, soluble redox state. Researchers in the US have now come up with a new technique to overcome this, using ferrocene as a redox mediator to assist with electron-transfer between the insoluble materials and electrodes.

The researchers studied the redox-active nickel complex, [Ni(PPh2NPh2)2(CH3CN)]2+, which is often used as a catalyst for electrochemical hydrogen evolution. The electrochemistry of the nickel complex was explored under non-catalytic conditions, where the absence of a proton source was previously unexplored. Cyclic voltammetry experiments of [Ni(PPh2NPh2)2]2+ in acetonitrile indicated two, one-electron reduction events that correspond to the NiII/Iand NiI/0 redox couples, both of which were electrochemically and chemically reversible at low concentrations (Figure 1). The researchers noted a concentration dependence, where the reversibility is increasingly lost at higher concentrations of the complex (blue scan, Figure 1A). This was attributed to the formation of the two-electron reduced product, [Ni(PPh2NPh2)2], which proved insoluble in acetonitrile, precipitating out of solution upon its electrochemical formation and depositing on the electrode surface.

Cyclic voltammograms of [Ni(PPh2NPh2)2]2+ in acetonitrile

Figure 1. Cyclic voltammograms of [Ni(PPh2NPh2)2]2+, showing the two redox events with the two, separate peaks. A) Concentration dependence, whereby reversibility decreases upon increasing concentration and B) Scan-rate dependence, whereby reversibility is regained at higher scan-rates.

Once the researchers established the electrochemically-driven solubility changes for the nickel complex, they looked at enhancing the overall reversibility of this reaction. Whilst the two-electron reduction to form insoluble [Ni(PPh2NPh2)2] proceeded smoothly, regenerating [Ni(PPh2NPh2)2]2+ by oxidation of this insoluble product was slow and inefficient, due to poor electron transfer between the deposited material and the electrode. The researchers therefore added ferrocene as a freely diffusing redox mediator to the electrochemical reaction, to essentially shuttle electrons from the insoluble reduction product to the electrode. This proved successful, with subsequent electrochemical experiments of [Ni(PPh2NPh2)2]2+ in the presence of ferrocene showing faster and catalytic regeneration of the original nickel complex.

Redox cycle scheme for [Ni(PPh2NPh2)2]2+

Figure 2. A scheme showing the redox cycle of [Ni(PPh2NPh2)2]2+, with annotations to describe the experimental kinetics observed.

In addition to the experimental studies, the researchers also turned to mathematical modelling to gain more understanding of electrochemically-driven solubility cycling in electrochemical reactions. Two models were presented showing the effect of the deposited materials on the electrochemical response, either with or without possible electrode inhibition effects. Overall, the researchers have presented a unique strategy for improving the reversibility of redox reactions that are limited by insoluble redox products, which is beneficial for systems where both materials deposit on electrodes or are suspended in solution.

 

To find out more, please read:

Redox mediators accelerate electrochemically-driven solubility cycling of molecular transition metal complexes

Katherine J. Lee, Kunal M. Lodaya, Cole T. Gruninger, Eric S. Rountree and Jillian L. Dempsey

Chem. Sci., 2020, 11, 9836-9851

 

About the blogger:

Dr. Samantha Apps just finished her post as a Postdoctoral Research Associate in the Lu Lab at the University of Minnesota, USA, and obtained her PhD in 2019 from Imperial College London, UK. She has spent the last few years, both in her PhD and postdoc, researching synthetic nitrogen fixation and transition metal complexes that can activate and functionalise dinitrogen. Outside of the lab, you’ll likely find her baking at home, where her years of synthetic lab training has sparked a passion in kitchen chemistry too.

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