Tuning electrochemical reactions with ratchet-based ion pumps

TL;DR

This work introduces ratchet-based ion pumps (RBIPs) as active, membrane-like devices that drive net ionic flux without redox chemistry by applying temporally modulated potentials across nanoporous AAO surfaces coated with metals. When used as a separating membrane in a two-compartment electrochemical cell, RBIPs generate a ratchet-induced voltage that can either accelerate or inhibit electrochemical reactions at a Pt cathode by directing proton flux toward or away from the electrode, thereby modulating the hydrogen evolution reaction (HER) and the local pH. Key findings include proton pumping toward the cathode mitigating proton depletion during HER and high duty-cycle pumping away from the cathode increasing proton depletion, along with voltammogram shifts of up to ~$142$ mV and HER onset shifts depending on the duty cycle $d_C$. This work demonstrates a transistor-like control modality for electrochemical systems, offering a new degree of freedom for tuning reaction overpotentials and selectivity in applications ranging from renewable fuels to chemical sensing.

Abstract

Electrochemical reactions are highly sensitive to the physical and chemical environment near the electrodes. Thus, controlling the electrolyte ionic composition and the electrochemical potential of specific ions can modify the overpotential of electrochemical reactions and enhance their selectivity toward the desired products. Ratchet-based ion pumps (RBIPs) are membrane-like devices that utilize temporal potential modulation to drive a net ionic flux with no associated electrochemical reactions. RBIPs were fabricated by coating the surfaces of nanoporous alumina wafers with metals, forming nanoporous capacitors. Placing the RBIP between two electrolyte compartments and applying an alternating signal between the metal layers resulted in a voltage buildup across the membrane, leading to ion pumping. Here, we demonstrate that by modifying the electrochemical potential of ions, RBIPs can accelerate or inhibit electrochemical reactions on the surface of adjacent water-splitting electrodes according to the RBIP input signal. Proton pumping towards a water-splitting cathode prevented proton depletion due to the hydrogen evolution reaction and maintained the pH in the cathode compartment. The combination of ion pumping and ion selectivity can enable the electrolyte composition to be tuned near the electrodes, providing greater control over the electrochemical process.

Figures (5)

• Figure 1: (a) A photograph of the electrochemical cell used for RBIP characterization. The RBIP is placed between the two compartments, and the copper tape is used to contact the RBIP. (b) A schematic illustration of the experimental setup. The RBIP is placed as a membrane separating the two compartments of an electrochemical cell. A signal generator connected to the RBIP provides the ratchet input signal $V_{in}$. The working, reference, and counter electrodes (WE, REF, and CE, respectively) are connected to a potentiostat.
• Figure 2: Cyclic voltammetry measurements of the platinum working electrode. The scan rate is 50 mVs$^{-1}$, and the solutions are aqueous HCl (pH=4.2 ,0.2 mm and pH=2.56, 2.75 mm ), KCl (pH=6.2, 0.2 mm), and H2SO4 (pH=2.5, 1.6 mm). The arrows point to the HUPD peaks in each solution.
• Figure 3: (a, c) The measured current for a system in configuration A and B, respectively. The shaded areas in a and c indicate the times when the RBIP is OFF ($V_{in}$ = 0 V), and the bright areas indicate the times when it is on. The colorbar indicates the duty cycle of the input signal when the RBIP is ON. The duration of each ON and OFF cycle was 30 seconds. (b, d) The temporally averaged RBIP-induced current as a function of the duty cycle obtained from (a, c). In all measurements, the WE potential is 0 V vs. RHE. The input signal frequency is 100 Hz, and the amplitude is $V_{p-p}= 1.4\space V$. The sample was fabricated as described in the experimental section with a TiO2 ALD coating. The compartments are filled with a 0.2 mm HCl aqueous solution. The insets in b and d are illustrations of configurations A and B, respectively.
• Figure 4: The pH of the anode and cathode compartments during 2.5 hours of operation with the RBIP operating and when it is disconnected. The cathode current is $-3\space\mu A$, and the system is in configuration A. The sample was fabricated as described in the experimental section with a TiO2 ALD coating. The compartments are filled with a 0.2mmHCl aqueous solution, the input signal frequency is $100 Hz$, and the amplitude is $V_{p-p}= 1.4\space V$.
• Figure 5: (a) Three-electrode cyclic voltammetry measurements with the RBIP OFF ($V_{in}=0$, dashed black curves) and with the RBIP driven with several input signal duty cycles. The scan rate is 50 mVs$^{-1}$. The sample was fabricated as described in the experimental section with an alumina ALD coating. The electrolyte is 2.75 mmHCl aqueous solution. The input signal frequency is 15 kHz, and the amplitude is $V_{p-p}= 1.4\space V$. (b) The potential of the HER onset and proton desorption peak extracted from (a) as a function of the input signal duty cycle. (c) The extracted RBIP-induced HER and proton desorption currents as a function of the input signal duty cycle.