Oxygen evolution reaction dynamics monitored by an individual nanosheet-based electronic circuit

The oxygen evolution reaction involves complex interplay among electrolyte, solid catalyst, and gas-phase and liquid-phase reactants and products. Monitoring catalysis interfaces between catalyst and electrolyte can provide valuable insights into catalytic ability. But it is a challenging task due to the additive solid supports in traditional measurement. Here we design a nanodevice platform and combine on-chip electrochemical impedance spectroscopy measurement, temporary I-V measurement of an individual nanosheet, and molecular dynamic calculations to provide a direct way for nanoscale catalytic diagnosis. By removing O2 in electrolyte, a dramatic decrease in Tafel slope of over 20% and early onset potential of 1.344 V vs. reversible hydrogen electrode are achieved. Our studies reveal that O2 reduces hydroxyl ion density at catalyst interface, resulting in poor kinetics and negative catalytic performance. The obtained in-depth understanding could provide valuable clues for catalysis system design. Our method could also be useful to analyze other catalytic processes.

At a scan rate of 5 mV/s, for Ni catalyst ( Supplementary Fig. 7a), a current density of 10 mA/cm 2 was achieved at a potential of 0.71 V vs. SCE under oxygen-absence condition, which is lower than that under the oxygen-presence condition, i.e., 0.742 V vs. SCE. In the case of Ni-graphene ( Supplementary Fig. 7b), a current density of 10 mA/cm 2 occurs at 0.593 V vs. SCE under oxygen-absence condition, whereas a higher potential of 0.627 V vs. SCE can be seen under oxygen-presence condition. A similar trend is also observed in the NiOgraphene catalysts ( Supplementary Fig. 7c). Results are consistent to our new setup. The experimental impedance data were fitted to the equivalent circuit model with a hundred of iterations in NOVA 1.10, an electrochemical work station software. The EIS data in the DC potential range between 1.2 V and 1.6 V vs. RHE together with the corresponding fitted curve are displayed in and the equivalent circuit model is seen to the EIS data very well.

Supplementary Note 5. MD simulation
To explore the influence of oxygen molecules on the ion concentration near the surface of active materials (Ni), we carry out molecular dynamics (MD) simulations of sodium hydroxide (KOH) solution on Ni surface. Specifically, water molecules were filled in the supercell to generate a pressure close to 1 bar at 300 K. The numbers of the electrolyte ions were determined with two conditions: the solution concentration (set as 1 mol/L) and charge neutrality of the whole system. In our simulations, different charge conditions for active materials (Ni) were investigated. Given the surface capacitance value of Ni was 20 uF/cm 2 and electric potential window as 1 V 1 , the maximum charge per target Ni atom (upper Ni atoms which are exposed to electrolyte) is calculated to be 0.033e. Accordingly, 0, +0.0083, +0.0167, +0.025 and +0.033e was imposed on each target Ni atom respectively to consider different charging conditions. Supplementary Table 1 summarizes the numbers of water molecules and ions in the simulation systems.
The crystal structure of Ni is the face-centered cubic lattice with lattice constant of 3.524 Å, and the (1, 1, 1) surface of the crystal lattice is exposed to the electrolyte by defining the x-y-z orientations to be (1, 1, 1) (1, -1, 0) (1 1 -2) respectively. The TIP/3P model was used for water. The interactions among Ni, water, KOH, and oxygen molecules (O 2 ) were described by Lennard-Jones (LJ) pairwise potential 2 . The potential function has the form of: where, r ij denotes the distance between atoms, ε characterizes the strength of the interaction and σ determines the distance at which the two atoms are at equilibrium. The substituted i and j denote the component atoms and ions of the whole model. Parameters of LJ potentials of K + ions, O 2 molecules and Ni atoms were taken from CHARMM force field 3 , expect that of OH -29 ions taken from reference 4. Supplementary Table 2 summarizes the parameters and the charges of each type of ions.
All MD simulations were conducted using LAMMPS package. Here, the positions of Ni atoms were frozen. The Berendsen thermostat was adopted to control system temperature.
The NVT ensemble simulations at 298 K were carried out for 2.5 ns to ensure that the systems indeed reached thermal equilibrium state. Results of last 1 ns were taken for the concentration profile analyze.
The Ag/AgCl/saturated KCl reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). The calibration was performed in the high purity hydrogen saturated electrolyte with a Pt foil as the working electrode. CVs were run at a scan rate of 1 mV s -1 , and the average of the two potentials at which the current crossed zero was taken to be the thermodynamic potential for the hydrogen electrode reactions. This calibration resulted in a shift of +0.95 V versus the RHE.