Electrochemical conversion of CO₂ to hydrocarbons is limited by the low solubility and slow transport of CO₂ in aqueous systems. We demonstrate that we can reach partial current densities for CO₂-to-CO of 40 mA/cm² in fully aqueous systems, without the use of gas diffusion electrodes. We alleviate the mass transfer limitation by combining a suspension of catalytically active silver nanoparticles (Ag NPs) with a flow-through current collector. This extends the reactive area into the electrolyzer channel and improves the accessibility of dissolved CO₂ in a larger volume of electrolyte. The flow-through electrode system also outperforms a fully suspended electrode (based on carbon black particles), due to enhanced electric conductivity and smaller carbon area to minimize parasitic side-reactions. Additionally, we show that the distribution of the Ag NPs is pivotal for high CO₂ conversion rates, demonstrated by the highest CO current density obtained when a suspension of Ag NPs and SDS as surfactant is flowing through the 3D electrodes as pre-treatment. A stable CO current density can be sustained for more than 4 h. Although the conversion rate is still moderate compared to gas-fed CO₂ electrolzyers, the partial current density for flow-through electrodes is more than an order of magnitude larger than for planar flow systems. This work shows that CO₂ conversion in aqueous systems can be enhanced considerably by exploiting larger electrolyte volumes via smart electrode designs, such as a flow-through principle.

Electrochemical reactors, such as water electrolyzers, CO₂ electrolyzers, fuel cells, and flow batteries, will be essential in electrifying industry as part of the global transition towards a defossilized and sustainable economy. These technologies require further optimization to enhance efficiency and reduce costs for widespread adoption. Hydrodynamics and mass transfer at electrode–electrolyte interfaces significantly affect electrochemical conversion reactions by influencing the reactant availability and pH in the local reaction environment. 3D electrodes, such as flow-through foams and suspension electrodes, hold a great advantage over 2D electrodes as they moderate pH changes and reactant depletion by spreading the current over a larger electrode area and electrolyte volume. We study the diffusion boundary layer in operando around a single mm-sized particle, representing an element of a 3D electrode. We visualize the local and transient pH with Fluorescence Lifetime Imaging Microscopy (FLIM) during H₂O reduction at various current densities and electrolyte flow velocities at a resolution down to 9 μm and 2 Hz. In addition, we apply an intermittent current to investigate how long the capacitive electric double layer of a suspension electrode particle can maintain an electrochemical reaction during their time of non-contact with a current collector, mimicking applications with Faradaic charge transfer (i.e. flow batteries, microbial fuel cells, capacitance-based electrolyzers). We demonstrate that the diffusion boundary layer is not symmetrical, but depend on the direction of the electric field, the current density and the flow conditions. The substantial pH gradients and boundary layer formation at the scale of hundreds of micrometers underline the importance of controlling flow in or around electrodes, making 3D electrodes an important asset for creating suitable reaction conditions in mass transport-limited electrochemical conversions.

Electrochemical oxygen reduction is a promising and sustainable alternative to the current industrial production method for hydrogen peroxide (H₂O₂), which is a green oxidant in many (emerging) applications in the chemical industry, water treatment, and fuel cells. Low solubility of O₂ in water causes severe mass transfer limitations and loss of H₂O₂ selectivity at industrially relevant current densities, complicating the development of practical-scale electrochemical H₂O₂ synthesis systems. We tested a flow-by and flow-through configuration and suspension electrodes in an electrochemical flow cell to investigate the influence of electrode configuration and flow conditions on mass transfer and H₂O₂ production. We monitored the H₂O₂ production using Cu-tmpa (tmpa = tris(2-pyridylmethyl)amine) as a homogeneous copper-based catalyst in a pH-neutral phosphate buffer during 1 h of catalysis and estimated the limiting current density from CV scans. We achieve the highest H₂O₂ production and a 15-20 times higher geometrical limiting current density in the flow-through configuration compared to the flow-by configuration due to the increased surface area and foam structure that improved mass transfer. The activated carbon (AC) material in suspension electrodes, which have an even larger surface area, decomposes all produced H₂O₂ and proves unsuitable for H₂O₂ synthesis. Although the mass transfer limitations seem to be alleviated on the microscale in the flow-through system, the high O₂ consumption and H₂O₂ production cause challenges in maintaining the initially reached current density and Faradaic efficiency (FE). The decreasing ratio between the concentrations of the O₂ and H₂O₂ in the bulk electrolyte will likely pose a challenge when proceeding to larger systems with longer electrodes. Tuning the reactor design and operating conditions will be essential in maximizing the FE and current density.