Pore Size Effects in Templated Porous Ag Electrodes for Electrochemical CO2 Reduction
Authors: van Ittersum Maaike , Keijzer Claudia , van den Akker Karen , Ngene Peter , de Jongh Petra E. 
Published: February 2024

Electrochemical CO2 reduction is a promising approach to overcome the intermittent nature of renewable energy sources and to close the carbon cycle. For the industrial application of this conversion, good catalysts converting CO2 at high current densities (> 10 mA/cm2) are required [1]. The use of porous metal electrodes is promising to achieve these high current densities, because of their high surface area. This high surface area leads to more active sides per geometric surface area. Apart from that, the bulk nature of porous electrodes limits the serial electrode resistance [2]. Unfortunately, porous structures also have disadvantages, such as potential diffusion limitations of reactants and products. Also, the often complex pore structures make it hard to disentangle the effect of different parameters. Hence, porous model catalysts are very useful for a fundamental understanding of the processes taking place in porous structures.

These porous model catalysts can be made by use of ordered templates, such as self-assembled polystyrene (PS) or poly(methyl methacrylate) (PMMA) spheres. In the case of CO2 reduction, it was shown that porous Ag electrodes made by Ag deposition in the voids of a PS template on gold coated glass-slides could be used as catalysts. For these templated Ag electrodes, it was shown that the mesostructure influenced the selectivity of the Ag catalysts [3]. To our knowledge, no one has tried to make these templates on commonly used substrates (such as carbon paper for gas diffusion electrodes). Next to that, the presence and effect of diffusion limitations in these well-defined systems have not been discussed in literature. Therefore, in this work, we discuss that template-based porous Ag can be prepared with PMMA sphere templates on different substrates and we discuss the effect of pore size.

First, PMMA template spheres were prepared as previously described in literature [4]. During the MMA polymerization reaction, the reaction conditions such as the monomer concentration, temperature and stirring rate were varied to obtain different particle diameters. Based on SEM images (e.g. Figure 1a) the average particle diameters of the different PMMA spheres were found to be 115, 123, 203, 308 and 372 nm. Thereafter, the PMMA sphere dispersions were dried on different substrates (carbon paper, Ag foil). By electrodeposition of fixed amount of Ag and removal of the template, inverse porous structures were obtained (e.g. Figure 1b). These inverse porous structures were successfully made on different substrates (carbon paper, Ag foil) as long as the substrate was conductive.

To investigate diffusion limitations in catalysis, electrochemical measurements were performed in a H-type cell and the gas products formed were analyzed with a GC. The obtained partial current densities for the porous Ag samples with different pore diameters were compared. Figure 1c shows that at more negative potentials, the pore diameter becomes more important for the partial current density towards CO. At a potential of -1.4V vs RHE, an optimum in CO current density was found around 200 nm pore diameter. At this pore size, the current density is more than 1.5 times higher. This is an indication that, especially at more cathodic potentials, diffusion in and out the pores becomes an important factor to take into account.

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Figure 1