Local Chemical Environment Governs Anode Processes in CO2 Electrolyzers

A major goal within the CO2 electrolysis community is to replace the generally used Ir anode catalyst with a more abundant material, which is stable and active for water oxidation under process conditions. Ni is widely applied in alkaline water electrolysis, and it has been considered as a potential anode catalyst in CO2 electrolysis. Here we compare the operation of electrolyzer cells with Ir and Ni anodes and demonstrate that, while Ir is stable under process conditions, the degradation of Ni leads to a rapid cell failure. This is caused by two parallel mechanisms: (i) a pH decrease of the anolyte to a near neutral value and (ii) the local chemical environment developing at the anode (i.e., high carbonate concentration). The latter is detrimental for zero-gap electrolyzer cells only, but the first mechanism is universal, occurring in any kind of CO2 electrolyzer after prolonged operation with recirculated anolyte.

the cathode temperature at 60°C during the electrolysis. In the case of the measurement with continuous fresh anolyte fed ~16.5 dm 3 0.1 M CsOH was pumped through the anode.
Humidified CO 2 (in case of CO 2 electrolysis, CO 2 RR) or Ar (in case of hydrogen evolution reaction, HER) was fed to the cathode with 12.5 cm 3 cm −2 min −1 flow rate.
The cell was operated in galvanostatic mode (I = 0.8 A, j = 100 mA cm -2 ) using a BioLogic VMP-300 potentiostat/galvanostat equipped with an impedance module and with a 5 A/10 V booster. The current densities were obtained by normalizing the current values to the geometric surface area of the electrodes (j = I/A geometric ).
The electrolysis products, CO and/or H 2 , were monitored during the electrolysis using a Shimadzu GC-2010 Plus gas-chromatograph, equipped with a barrier discharge ionization (BID) detector. The cathode gas outlet was sampled in every ~20 minutes during the measurements. Faradaic efficiency of the CO 2 electrolysis was calculated from the GC results and the measured gas glow rate (Agilent ADM flow meter). In some experiments the product stream composition was simultaneously monitored using an m/z analyzer (SRS UGA200) equipped with an atmospheric sampling capillary.
The pH of the anolyte was also measured with the same sampling rate. A Mettler Toledo FiveEasy Plus FP20 pH meter was applied in automatic temperature compensation mode. The anolyte temperature was 65-70°C in all cases. A Hg/HgO reference electrode (in 1 M KOH, E 0 =0.12 V vs. SHE) was integrated to the system connected to the cell at the anolyte inlet point. 2 To get information about the changes of the catalysts, interfaces and the membrane during the electrolysis, electrochemical impedance spectroscopy (EIS) measurements were performed. EIS spectra have been taken at I = 0.8 A (j = 100 mA cm -2 ) constant cell current with 10 mA perturbation amplitude, in the frequency range of 200 kHz to 1 Hz, with 10 points per frequency decade. The series resistance and the total charge transfer resistance (the sum of the width of the two semicircles) were calculated by semi-quantitatively fitting the Nyquist representation of the EIS spectra using BioLogic EC-Lab software Z fit. A serial combination of two simplified Randles circuits was used to describe the measured EIS spectra. These fittings only served to determine the high frequency intercept (R S ), and the total width of the two arcs observed in the Nyquist plots. This latter is considered here as the total charge transfer resistance of the two electrode reactions.
The composition of anode gas outlet was analyzed with a BGA-244 type Binary Gas Analyser (Stanford Research Systems), to monitor the CO 2 /O 2 ratio. 1 The anode gas outlet flow rate was measured using a bubble flowmeter. 3

Scanning electron microscopy -Energy dispersive X-ray analysis
A Hitachi S-4700 scanning electron microscope (SEM) coupled with a Röntec EDX detector was used to take cross-section images of the cut GDEs. The microscope was operated at 10 kV acceleration voltage.

X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (Al Kα) was carried out with a SPECS instrument equipped with a PHOIBOS 150 MCD 9 hemispherical analyzer. The analyzer was used in fixed analyzer transmission mode with 40 eV pass energy for the survey scans and 20 eV pass energy for the high-resolution scans. Charge referencing was done to the adventitious carbon (284.8 eV) on the surface of the sample. For spectrum evaluation, commercial CasaXPS software package was used.

X-ray micro-computed tomography analysis
A Bruker SkyScan 2211 Multiscale X-ray Nanotomograph (Bruker) instrument was used to record micro-CT images. The three-dimensional structure of the samples was scanned using an 11-megapixel CCD detector by applying a source voltage of 70 kV and current of 400 μA (in microfocus mode, with a resolution of 1 μm per pixel). NRecon reconstruction software (Skyscan, Bruker) was used to reconstruct the projected images, while the CTAn and CTVox software (Skyscan, Bruker) were applied to carry out the image segmentation and visualizing the 3D-rendered objects, respectively.

Stability of anode catalysts at different pH values studied by on-line ICP-MS
The setup consists of a three-electrode electrochemical scanning flow cell (SFC) directly connected to an inductively coupled plasma mass spectrometer (ICP-MS, Perkin Elmer, Nexion 350X). [4][5][6] The measurements were carried out in 0.1 M CsOH (pH ~ 13) and in CO 2  3 . 187 Re and 59 Co served as internal standards. Delays that arise from the transport of the electrolyte from the electrochemical flow cell to the ICP-MS were subtracted in order to directly correlate potential and dissolution data.

Reason behind the anolyte pH change
Assuming that the ion conduction between the cathode and the anode is maintained by carbonate ions (as shown in earlier studies), for every 2 e − , 1 CO 3 2− ion reaches the anolyte (equation (1) and (2)). This carbonate reacts with the H + ions formed during OER, and the forming H 2 CO 3 reacts with CsOH, lowering the pH of the solution (equations (3)(4)(5)). All these When working with Ir anode catalyst, the pH of the anolyte decreases in two steps, following the shape of a titration curve. The first step is related to the formation of Cs 2 CO 3 and a CO 3 2− /HCO 3 − buffer. Subsequently, HCO 3 − /CO 2 systems forms, and as a consequence, CO 2 liberates from the solution.
Using Ir as anode catalyst, the experimentally determined times necessary to completely neutralize the alkaline anolyte are shorter than the calculated values (Fig S8A). The reason behind this is that in addition to carbonate conduction, CO 2 gas diffusion also occurs from the cathode side to the anode side through the membrane, accelerating the process. When working with Ni anode catalyst the anolyte pH changes substantially slower, which is attributed to the loss of a large fraction of the total charge to a non-Faradaic process.

Charge associated with Ni dissolution
To identify whether Ni dissolution can consume a notable portion of the flown charge, we calculated the C charge necessary for the complete dissolution of the Ni catalyst: if all the charge is consumed in the dissolution.

pH of the anolyte
The shown pH values of the anolytes (Fig 1. C K w = 1.008 × 10 -14 , T = 75°C K w = 19.95 × 10 -14 . 8 (T=60°C K w = 9.55× 10 -14 ). The pH values of the anolytes for each electrolysis were recorded at room temperature before and after the electrolysis (before heating up and after cooling down the anolyte, respectively) (see Table S1, S2).   An explanation for the increase in series resistance is that the initial increase can be attributed to the formation of surface NiCO 3 upon dissolution of Ni. As the process continues, more and more Ni 2+ ion infuses into the membrane, forming NiCO 3 dendrites in it. After reaching a critical number (or length) of dendrites, percolation occurs, leading to decreased cell resistance.

X-ray photoelectron spectroscopy
As a reference commercially available NiCO 3 (98 % purity, Alfa Aesar) was also investigated (Fig. S9). Based on XPS measurements, the surface elemental composition of the two anode samples was determined (Fig. S10). The main difference is that the C 1s region of the sample after HER on the cathode did not have any contribution from surface carbonate species. In contrast, in the case of the anode where CO2RR was performed on the cathode side, significant carbonate contribution was observed (Fig. 3A).

a) Position b) Contribution
On the XPS survey scans of both the cathode GDE and the membrane (from the side facing the cathode) core lines related to Ni can be identified besides lines related to Ag (Fig. S11).
This suggests that the nickel released from the anode side passes through the membrane and subsequently deposits on the cathode GDE.  Figure S11. XPS survey scan of (A) the cathode side of the membrane and (B) the cathode GDE after CO 2 electrolysis with a Ni anode, as shown in Fig. 1 in the main text.

Energy dispersive X-ray analysis
After CO 2 electrolysis employing Ni anode, the cathode GDE was investigated by SEM-EDX to see if Ni only enters the membrane or even passes through it and deposits at the cathode. Cathode GDEs after CO2RR with Ir anode and after HER with Ni anode were also investigated as comparisons. Ni appears only on the EDX spectrum of cathode GDE after CO2RR with Ni anode, as shown on Fig S13. In contrast, no anode catalyst was observed on the cathode in the other two reference cases.