Solid-oxide electrochemical cells (SOEC) that reduce steam to produce H₂ and generate oxygen ions for the oxidation of highly diluted CH₄ streams have been proposed [1] and are a potential approach to remove atmospheric methane (see Fig. 1). They have relatively straightforward verifiability but are currently estimated to be too expensive and difficult to scale to impact the atmospheric methane ppm on decadal timelines significantly [2].

The cost is driven by the energy required- (i) to generate the oxygen ions, (ii) to break the methane bond selectively, and (iii) to move the air through the reactor. This cost depends directly on the CH₄ conversion and energy efficiency, for which the state-of-the-art is 70% and 80%, respectively, for 1% CH₄ feed [1]. For cost-effective atmospheric CH₄ oxidation, the SOEC should demonstrate similar (>70%) conversion and efficiency when oxidizing 2 ppm methane. This requires a 5000 times higher turnover frequency (TOF) of the catalyst

The lower catalyst selectivity and TOF of CH₄ oxidation limit the conversion and efficiency of SOEC for atmospheric CH₄ removal. The dependence of selectivity and TOF on the structure of the catalyst and operating conditions is poorly understood; these include structure-activity relationships, air humidity, composition and flow rates, cell voltage, and current.

The primary goal by which success will be measured is the improvement of CH₄ TOF, leading to >70% conversion. To do so, design choices will be made based on the quantitative understanding of TOF’s dependence on the variables above.

Furthermore, the production of unintended products (such as CO/CH₃OH from partial oxidation [1,2a] and C₂H₄/C₂H₆ from oxidative coupling [2b]) should be assessed for different temperatures and voltages. Understanding the factors will be critical to the future acceptability of such a technology. Measuring these unintended products and optimizing SOEC to minimize their production will be a secondary key goal.

If successful, the understanding of atmospheric methane oxidation would be improved in the following ways:

• Improved understanding of thermo-electrochemical reactions (and kinetics) relevant to CH₄ and O₂- reactions and the byproducts produced.
• A rigorous quantification of the TOF that is required for thermos-electrolytic methane oxidizing reactors to be climate-beneficial and cost-effective for methane removal at atmospheric concentration (2 ppm).
• The single-atom catalyst design principles developed here can also advance other methane oxidizing reactors, such as hydroxyl radical-based reactors, photolytic reactors, and solid photocatalyst-based reactors that require CH₄-selective catalysts with higher TOF.
• Proposed SOEC technology enables H₂ production with atmospheric CH₄ removal.