Environment Matters: CO₂RR Electrocatalyst Performance Testing in a Gas-Fed Zero-Gap Electrolyzer

Abstract

Among the electrolyzers under development for CO2 electroreduction at practical reaction rates, gas-fed approaches that use gas diffusion electrodes (GDEs) as cathodes are the most promising. However, the insufficient long-term stability of these technologies precludes their commercial deployment. The structural deterioration of the catalyst material is one possible source of device durability issues. Unfortunately, this issue has been insufficiently studied in systems using actual technical electrodes. Herein, we make use of a morphologically tailored Ag-based model nanocatalyst [Ag nanocubes (NCs)] assembled on a zero-gap GDE electrolyzer to establish correlations between catalyst structures, experimental environments, electrocatalytic performances, and morphological degradation mechanisms in highly alkaline media. The morphological evolution of the Ag–NCs on the GDEs induced by the CO2 electrochemical reduction reaction (CO2RR), as well as the direct mechanical contact between the catalyst layer and anion-exchange membrane, is analyzed by identical location and post-electrolysis scanning electron microscopy investigations. We find that at low and mild potentials positive of −1.8 V versus Ag/AgCl, the Ag–NCs undergo no apparent morphological alteration induced by the CO2RR, and the device performance remains stable. At more stringent cathodic conditions, device failure commences within minutes, and catalyst corrosion leads to slightly truncated cube morphologies and the appearance of smaller Ag nanoparticles. However, comparison with complementary CO2RR experiments performed in H-cell configurations in a neutral environment clearly proves that the system failure typically encountered in the gas-fed approaches does not stem solely from the catalyst morphological degradation. Instead, the observed CO2RR performance deterioration is mainly due to the local high alkalinity that inevitably develops at high current densities in the zero-gap approach and leads to the massive precipitation of carbonates which is not observed in the aqueous environment (H-cell configuration).

Electrochemical Reduction of CO2 on Au Electrocatalysts in a Zero-Gap, Half-Cell Gas Diffusion Electrode Setup: a Systematic Performance Evaluation and Comparison to a H-cell Setup

“…We chose a potential window for testing the CO2RR from Commercially more relevant current densities can be obtained in GDE setups, where the reactant gas does not need to be first dissolved in liquid electrolyte. To demonstrate this influence, we performed the same CO2RR studies in our recently introduced zero-gap, half-cell GDE setup 8,36 setup instead of the H-cell setup. The obtained FECO and current densities are shown in Figure 4 and Figure S7.…”

“…Our recently introduced GDE setup was employed in this study 8,36,37 the RHE conversions. Moreover, the resistance between the WE and RE and the applied electrode potentials was monitored online using an AC signal (5 kHz, 5 mV).…”

“…The origin of the low current densities reported cannot solely be attributed to the low performance of the used CO2 electroreduction catalysts but also can be related to the employed screening method used for the catalytic testing itself. The most common catalyst screening method for CO2RR is based on H-cell experiments in which the CO2 reactant gas is dissolved in an aqueous bicarbonate-based electrolyte 8 . In such a setup, the low solubility of the CO2 reactant gas in aqueous solutions (∼35 mM at 298 K and 1 atm pressure) 7 causes mass transport limitations for the preferred CO product, whereas the reactant for the unfavored evolution of H2 gas (i.e.…”

Electrochemical Reduction of CO2 on Au Electrocatalysts in a Zero-Gap, Half-Cell Gas Diffusion Electrode Setup: a Systematic Performance Evaluation and Comparison to a H-cell Setup

“…Commercially more relevant current densities can be obtained in GDE setups, where the reactant gas does not need to be first dissolved in liquid electrolyte. To demonstrate this influence, we performed the same CO2RR studies in our recently introduced zero-gap, half-cell GDE setup 8,36 setup instead of the H-cell setup. The obtained FECO and current densities are shown in Figure 4 and Figure S9.…”

“…So doing, we can have a fairer comparison between the results of the GDE setup and the H-cell setup. Preparation of the GDE setup Our recently introduced GDE setup was employed in this study8,36,37 . As described above, the WE in the form of a GDE was placed on top of the flow field in the stainless-steel lower cell body and an activated Anion exchange membrane was placed on top of the GDE to separate the liquid electrolyte from the catalyst layer.…”