MnOx/IrOx as Selective Oxygen Evolution Electrocatalyst in Acidic Chloride Solution

The oxygen evolution reaction (OER) and chlorine evolution reaction (CER) are electrochemical processes with high relevance to water splitting for (solar) energy conversion and industrial production of commodity chemicals, respectively. Carrying out the two reactions separately is challenging, since the catalytic intermediates are linked by scaling relations. Optimizing the efficiency of OER over CER in acidic media has proven especially difficult. In this regard, we have investigated the OER versus CER selectivity of manganese oxide (MnOx), a known OER catalyst. Thin films (∼5–20 nm) of MnOx were electrodeposited on glassy carbon-supported hydrous iridium oxide (IrOx/GC) in aqueous chloride solutions of pH ∼0.9. Using rotating ring–disk electrode voltammetry and online electrochemical mass spectrometry, it was found that deposition of MnOx onto IrOx decreases the CER selectivity of the system in the presence of 30 mM Cl– from 86% to less than 7%, making it a highly OER-selective catalyst. Detailed studies of the CER mechanism and ex-situ structure studies using SEM, TEM, and XPS suggest that the MnOx film is in fact not a catalytically active phase, but functions as a permeable overlayer that disfavors the transport of chloride ions.


S2
To prepare the IrOx/GC electrode, the IrOx particles were electroflocculated on the GC surface under rotation. A 4 mL aliquot of the bulk alkaline solution was re-acidified with 60% HClO4 down to pH ≈ 1.5. The removal of OHinduced the deprotection of the IrOx colloids, which manifested as a solution color change from purple to greyish violet. Care must be taken not to make the IrO x solution too acidic, since competing CER at low pH may interfere with the reproducibility of the deposition. CER originates from the Clwhich is present as byproduct from the Na 2 IrCl 6 hydrolysis. Deposition was then performed at a rotation rate of 600 RPM on a freshly prepared GC surface. The disk electrode was first scanned at 250 mV s -1 in range of 0.16 -1.36 V vs. Ag/AgCl, to observe initial IrO x deposition coupled to increasing OER and CER catalysis. This was followed by amperometry for 300 s at 1.255 V vs. Ag/AgCl at a potential 10 mV positive of the onset of mixed OER and CER, currents rising over time indicated further IrOx deposition. After deposition, a reflective blue film was visible on the GC surface.

Rotating ring-disk studies (RRDE) of chlorine evolution vs. MnOx deposition
RRDE measurements were done with a MSR rotator and E6 ChangeDisk RRDE tips in a PEEK shroud (Pine Research). The Luggin tip connected to the reference electrode was aligned to the center of the RRDE electrode to minimize electrical cross-talk. 5,6 The liquid phase collection factor of the ring-disk system, , was determined by studying the Fe[CN] /Fe[CN] redox couple in a solution of 10 mM K3Fe[CN]6 and 0.1 M KNO3, using the Pt ring with both a freshly prepared blank GC electrode and the IrOx/GC electrode. The value was 0.244 within 5% accuracy. The collection factor for dissolved Cl2 was also measured in the same setup, by evolving chlorine selectively on a Pt disk electrode at 1.6 V vs. RHE, well before the kinetic onset of OER. Provided that the solution pH was lower than 1, 7 the collection factor for dissolved chlorine was found to be 0.240, almost identical to .
Before RRDE experiments, the Pt ring was electropolished by scanning from -0.1 V to 1.7 V at 500 mV s -1 for 40 scans at 1500 RPM, after which the individual scans did not change. This step was vital to remove traces of IrOx that deposited on the ring during IrOx electroflocculation under rotation. 8 Hydrodynamic experiments were done at 1500 RPM by scanning the disk electrode in the range of 1.1 -1.55 V at 10 mV s -1 . The ring was kept at 0.95 V during measurement of CER rates. Presence of Mn 2+ is not expected to interfere with the Pt ring during experiments. 9,10 In between experiments, the IrOx/GC electrode was kept at 1.1 V. Ring currents were corrected for constant background S5 currents and product collection delay. The latter arises from the time needed for products formed on the disk to reach the ring, and was approximately 200 ms at 1500 RPM. Before initiating quantitative measurements, the IrOx/GC electrode was scanned 40 times in a chloride-free electrolyte between 1.3 -1.55 V (into the OER region) at 1500 RPM. This was done to ensure stable IrO x film behavior during experiments. All currents were reported without normalization to catalyst surface area, since we were purely interested in selectivity trends (which are ratios of currents) and kinetic parameters such as Tafel slopes, none of which are affected by normalization.

On-line electrochemical mass spectroscopy (OLEMS)
During OLEMS experiments, volatile reaction products were collected in situ using a 0.5 mm diameter tip with a porous PTFE membrane at a close distance (~10 μm) to the electrode surface, positioned with a micrometric screw system. 11 Mass signals were measured using an EvoLution mass spetrometer setup (ESS Ltd.) equipped with a Balzers QMS200 quadrupole (Pfeiffer). Pressure in the mass chamber was maintained lower than 10 -6 mbar using a rotary vane pump (Edwards nXDS6i) and a TMH-071P turbo molecular pump (Pfeiffer  12 In all cases, when a satisfactory MnOx coverage was reached, the experiment was discontinued by raising the working electrode while rotating, breaking electrical contact and spin-drying the surface. This procedure was employed to minimize transient MnOx dissolution in the acidic environment. Immediately after spin-drying, the electrode was rinsed with ample water and dried with compressed filtered air.

Scanning electron microscopy (SEM)
GC disks were carefully removed from the RRDE tip after deposition and glued to a SEM specimen mount using conductive silver epoxy. Scanning electron micrographs were made using a FEI Nova NanoSEM 200 equipped with a field emission electron source, operating at 10 kV beam accelerating voltage and 10 pA probe current. Images were recorded in immersion mode using a through-thelens detector, at a working distance of ~4.5 mm. EDS measurements were performed in a JSM 6010LA setup (JEOL). After recording overview images at 10 kV, the beam accelerating voltage was increased to 15 kV and elemental data were recorded from a wide variety of locations on the electrode.

Transmission electron microscopy (TEM)
GC disks were carefully removed from the RRDE tip after deposition, and the IrO x /MnO x films were then carefully scraped off the electrode surface. The films were sonicated for 30 minutes in absolute ethanol, and the suspension was dropcasted onto a Cu TEM grid. Bright-field transmission micrographs were recorded using a JEM-1400Plus apparatus (JEOL) equipped with a LaB6 filament and a 2kx2k Orius camera (Gatan), operating at 120 kV accelerating voltage. For measurements at higher resolution, a model JEM-2010 (JEOL) equipped with a LaB6 filament and a Orius 831 camera (Gatan) was used, operating at 200 kV. Selected-area diffraction patterns were analyzed using the CrysTBox software suite. 13

X-ray diffraction measurements (XRD)
GC disks were carefully removed from the RRDE tip after deposition and mounted directly in the diffractometer using a height-adjustable mount to correct for disk protrusion. X-ray diffraction spectra were recorded using a D8-Advance diffractometer (Bruker) operated in Bragg-Brentano geometry, equipped with a Co-Kα anode (λ = 1.78897 Å) and a Lynxeye position sensitive detector. Diffraction data were collected over angles ranging between 10° -100° with a step size of 0.02° and scan speed of 0.2° s -1 .

X-ray photoelectron spectroscopy (XPS)
GC disks were carefully removed from the RRDE tip after deposition and mounted directly in the spectrometer. X-ray photoelectron spectroscopy measurements were performed in a K-alpha spectrometer (Thermo Fisher Scientic) using a monochromatic Al-Kα X-ray source. The measurements were carried out at a chamber pressure of about 10 -8 mbar, a flood gun was used for charge compensation. The energy analyzer was operated with a pass energy of 200 eV and 0.25 eV energy spacing for the survey spectrum, and a pass energy of 50 eV and 0.1 eV energy spacing for the high-resolution spectrum. All binding energies were referenced to the C1s peak (284.8 ± 0.025 eV). Each spectrum reported is the statistical average of 10 measured scans. Spectral peaks were analyzed and processed using Thermo Avantage v5.903 software (Thermo Fisher Scientic). The Shirley algorithm was used to calculate background contributions. Relative atomic contributions on the surface were calculated using full width integration over the core-level signals along with tabulated atomic sensitivity factors (ASF). For the core-level Ir 4f peak deconvolution, we used a model from existing literature, excluding contributions from 4f satellite peaks. 14

Estimation of iL of MnOx deposition
The diffusion limited current of MnOx deposition at 1500 RPM was determined by means of the equation for the diffusion-limited current: where = 2 is the amount of electrons transferred in the reaction, F is Faraday's constant (96485 C/mol), is the diffusion constant of Mn 2+ in water (7.12•10 -10 m 2 s -1 ), is the species concentration [Mn 2+ ] = 0.6 mol m -3 ) and is the diffusion layer thickness, which is dependent on the rotation rate (157.08 rad/s) and can be determined via the Levich equation:

Eq. (SI 2)
The value of , the kinematic viscosity of the solution, can be estimated at 1.0•10 -6 m 2 s -1 for dilute ( < 1 M) aqueous salt solutions, yielding ≈ 140 μA.       45 V (B, black), coinciding with a rise in ring current as BER rates increase. Due to the backward scanning potential, BER rates on the ring decrease quickly after their rise. Resuming the forward direction after crossing the 1.10 V lower potential limit, the IrOx/GC electrode is MnOx-free, which is apparent from a rise in current near 1.20 V (C, black), corresponding to onset of BER. As the potential is scanned higher than 1.45 V, crossing into the second scan (A, blue), the disk current is composed of bromine evolution and the onset of OER. The ring shows significant bromine reduction current, approaching near diffusion limited BER rates on the disk.

BER (Bromine Evolution Reaction) measurement on MnOx/IrOx
There is also minor hysteresis in the ring current during scan reversal, currents decrease over time due to transient MnOx growth. In absence of an integral MnOx film, the remainder of the second scan (B and C, blue) shows simply the decrease in OER and BER. Scan 3 (orange trace) overlapped strongly with scan 2.
A more elaborate study of the effect of MnOx on BER selectivity was not pursued, since it is outside the scope of this article. The nature of bromine evolution is substantially different from CER, 15   (4) Fine, D. A. Inorg. Chem. 1964Chem. , 32, 1964Chem. -1966