December 15, 2020. By Kolemann Lutz
Structure model of a) 3DPNi and b) Nickel Foam. c) Relative bubble migration time through 3DPNi and NF. d,e) Simulation with bubble shape during transport in (d) 3DPNi and e) NF
Researchers designed and tested a novel 3D-printed nickel electrode with carbon-doped nickel oxide (C-Ni-O) nanorods as a HER/OER bifunctional catalyst that significantly enhances bubble flow and release at large current densities. This output is substantially better than other metallic-based catalysts and opens up a new paradigm for ordered 3D-printed electrodes to decompose water into oxygen and hydrogen throughout the Solar system.
Alkaline water splitting (AWS) electrolysis offers many benefits over proton exchange membrane (PEM) water electrolyzers, including improved hydrogen gas purity, a less corrosive environment for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), as well as reduced-cost cell components. However, AWS electrolyzers operate at low current densities (<400mA cm-2) and voltage efficiencies (<80%) compared to PEM.
Passing higher current densities to increase the electrode activity to generate hydrogen gas is equally as important. However, high current densities generate significant gas bubbles that frequently collide and block large regions of the catalytic surface area, which decreases the hydrogen production rate and creates immense ohmic resistance that restricts voltage efficiency.
As one of the most widely used 3D porous electrodes, nickel foam (NF) has high electrical conductivity for HER and OER catalysts. However, the disordered porous skeleton structure of nickel foam easily blocks and traps gas bubbles, resulting in barriers between active sites and electrolytes, significant ohmic loss, and a decline in available active catalytic sites.
Researchers from Shanghai Jiao Tong University and the University of California hypothesized that when the orientation of electrode channels align with the direction of the gas bubbles, buoyancy will drive the gas bubbles upward, allowing them to flow easily through the electrode lattice before detachment from the surface.
To circumvent material misalignment with traditional conductive electrodes, researchers 3D-printed Ni (3DPNi) lattice-structured electrodes with extrusion-based 3D printing techniques such as direct ink writing (DIW) that is widely utilized to build highly ordered multiscale cellular materials from ceramics, polymers, metals, and carbons.
DWI procedure was prepared with a past-like ink composed of Ni particles, a polylactic-co-glycolic acid (PLGA), and graded volatility solvent including dichloromethane, and ethylene glycol butyl ether.
After printing and heat treatment, 3DPNi samples were coated with carbon-doped nickel oxide (C-Ni-O) nanorods as the HER/OER bifunctional catalyst to increase surface roughness, decrease contact area between gas bubbles and catalyst, and promote the rapid detachment of gas bubbles. The ink was then loaded into a syringe barrel to 3D-print the electrode structure, composed of five stacked layers with each layer orthogonal to the layer below, with center-to-center spacing (L) of 800 um and a pore size of 450 um.
By using a high-speed camera and air injection to monitor gas bubble release from disordered C-Ni-O/NF and ordered C-Ni-O/3DPNi, researchers noticed that the disordered nickel foam’s pore structure significantly influenced gas bubble movement. The researches noted that in both electrode lattices, significantly more gas bubbles were generated on HER electrodes compared to OER electrodes considering water electrolysis generates O gas and H gas in 1:2 ratio.
The C-Ni-O/NF electrodes had considerably larger gas bubbles dwelling on the surface compared to the C-Ni-O/3DPNi electrodes because the 3DPNi lattice’s bubble flow channels suppressed the in-plane drainage process and minimized the pressure increase inside, which reduced gas breakthrough time.
With computational modeling, they found a critical bubble diameter of 29 um and 20 um for the 3DPNi and NF structure, respectively, above which the bubble is trapped and incapable of passing through the electrode medium. The carbon-doped, 3D-printed lattice structure achieved significantly better catalytic performance and lower HER and OER overpotentials, or overvoltage in electricity loss, than NF or other Fe-based catalysts at similar current densities
In hindsight, the performance of a 3D printed electrode is largely dependent on three crucial variables: (i) size of pores, (ii) spatial distribution (ordered vs random structure), and (iii) amount and size of gas bubbles generated.
After developing and testing a novel water electrolyzer, the Chinese and U.S.-based researchers claim that current density achieved by this 3D-printed electrode is better than many industrial AWS results under similar conditions.
“We posit that this new ordered 3D electrode paradigm will deliver performance improvement not only for water electrolysis, but for any electrochemical reactions involving gas consumption or generation,” stated Tianyi Kou at the Department of Chemistry and Biochemistry at the University of California.
Early settlers can use ordered 3D-printed electrodes to mitigate gas bubble traffic in brine electrolysis, CO2 reduction, and the gas generation from the oxidation of metals.
This research brings the mass production and 3D printing of electrodes and water electrolyzers closer to reality to directly complement the green hydrogen revolution from limitless amount of water in Earth’s oceans.
Research Paper. Dr. T Kou, S. Wang, Prof Y. Li, et al. Periodic Porous 3D Electrodes Mitigate Gas Bubble Traffic during Alkaline Water Electrolysis at High Current Densities. Advanced Energy Materials. Oct 13, 2020.
