Membraneless electrolyzer based on angled mesh flow-through electrodes and one-component device body.(a) Two-dimensional schematic showing the basic operating principle of the membraneless electrolyzer. (b) Photograph of the two mesh electrodes in the flow channel with the separation angle (θ), electrode length (L), and product divider labeled. (c) Three-dimensional CAD blow-out rendering of the 3D printed body, glass viewing window, and two electrodes. Inset shows an SEM image of platinized titanium mesh electrode. O’Neil et al. Click to enlarge.

Electrolyzers, which use electricity and water to produce hydrogen and oxygen, are well-established commercially available technologies, but the cost of producing H₂ by water electrolysis is currently too expensive. Presently, much of the cost of producing H₂ by water electrolysis comes from the price of electricity, but as the price of electricity from wind and solar continues to decrease and time-of-use pricing schemes become more prevalent, decreasing the cost of electrolyzer technology will be of great importance to making a renewable hydrogen future a reality.

The majority of electrolyzers are based on a design in which the cathode and anode are separated by an ion-conducting membrane or diaphragm. … Within these devices, the membrane serves two key purposes, which are facilitating ion transport between the anode and cathode, and physically separating the product species produced at the anode and cathode to prevent crossover. Despite their importance to device operation, the membranes of these electrolyzers can be costly, prone to degradation or fouling, increase cell resistance, and entail the use of a membrane electrode assembly (MEA)-based design that requires at least 10 components. The high cost of electrolyzers arises from the high costs of individual components (e.g., membranes, bipolar plates, spacers, catalyst, etc.), as well as the cost of assembling the electrolyzer.

In this work, we seek to substantially decrease both materials and assembly costs by exploring novel membraneless electrolyzer designs. In addition to eliminating the material costs of membranes and associated components, a membraneless electrolyzer can significantly relax design constraints associated with an MEA-based electrolyzer, opening up the possibility for a substantially simplified overall device that is amenable to low-cost, high volume assembly and manufacturing.

—O’Neil et al.

According to the researchers, their significant advance is the design of cells with angled flow-through electrodes that can be integrated into a 1- or 2-component device body.

In the design reported in the paper, an aqueous electrolyte solution flows through two porous mesh electrodes placed at an angle in close proximity to each other. These electrolyzers do not require a membrane to achieve low product gas crossover because they employ flow-induced separation of product gases.

Once detached from the electrodes, gaseous products are immediately swept down one of two effluent channels that are separated by a thin divider that is part of the cell body.

The divider is completely insulating—ionic current must flow through the aqueous electrolyte solution.

Advantages of placing the flow-through electrodes at an angle relative to each other compared to parallel electrodes include:

• Fresh electrolyte constantly bathes the entire electrode;

• The electrodes may be more seamlessly integrated into a single-component device body; and

• Unlike circular parallel flow-through electrodes with annular inlet flow, scalability of individual electrolysis cells comprising angled flow-through electrodes may be easily achieved by increasing the height of the mesh electrodes.

As reported in the paper, prototype electrolyzers operating in acidic and alkaline solutions achieved electrolysis efficiencies of 61.9% and 72.5%, respectively, (based on the higher heating value of H₂) when operated at 100 mA cm⁻².

Gas chromatography (GC) analysis found that 2.8% of the H₂ crossed over from the cathode to the anode stream under electrolysis at 100 mA cm⁻² and fluid velocity of 26.5 cm s⁻¹.

In separate experiments, the team collected ≈90% of product gas, and a 3-cell stack was employed to demonstrate the modularity of the design.

Improvements in all performance metrics are expected with further optimization of cell geometries and operating conditions. Most importantly, the simple design furthermore enables easy fabrication and assembly, providing an opportunity to significantly reduce the capital cost of H₂ production from water electrolysis.

—O’Neil et al.

Resources

• Glen D. O’Neil, Corey D. Christian, David E. Brown, and Daniel V. Esposito (2016) “Hydrogen Production with a Simple and Scalable Membraneless Electrolyzer” J. Electrochem. Soc. 163(11): F3012-F3019; doi: 10.1149/2.0021611jes