Hexagonal Arrays of Cylindrical Nickel Microstructures for Improved Oxygen Evolution Reaction

Three-Dimensional Metallic Surface Micropatterning through Tailored Photolithography-Transfer-Plating

Precise micropatterning on three-dimensional (3D) surfaces is desired for a variety of applications, from microelectronics to metamaterials, which can be realized by transfer printing techniques. However, a nontrivial deficiency of this approach is that the transferred microstructures are adsorbed on the target surface with weak adhesion, limiting the applications to external force-free conditions. We propose a scalable "photolithography-transfer-plating" method to pattern stable and durable microstructures on 3D metallic surfaces with precise dimension and location control of the micropatterns. Surface patterning on metallic parts with different metals and isotropic and anisotropic curvatures is showcased. This method can also fabricate hierarchical structures with nanoscale vertical and microscale horizontal dimensions. The plated patterns are stable enough to mold soft materials, and the structure durability is validated by 24 h thermofluidic tests. We demonstrate micropatterned nickel electrodes for oxygen evolution reaction acceleration in hydrogen production, showing the potential of micropatterned 3D metallic surfaces for energy applications.

Tailoring 3D-Printed Electrodes for Enhanced Water Splitting

Alkaline water electrolysis, a promising technology for clean energy storage, is constrained by extrinsic factors in addition to intrinsic electrocatalytic activity. To begin to compare between catalytic materials for electrolysis applications, these extrinsic factors must first be understood and controlled. Here, we modify extrinsic electrode properties and study the effects of bubble release to examine how the electrode and surface design impact the performance of water electrolysis. We fabricate robust and cost-effective electrodes through a sequential three-dimensional (3D) printing and metal deposition procedure. Through a systematic assessment of the deposition procedure, we confirm the close relationship between extrinsic electrode properties (i.e., wettability, surface roughness, and electrochemically active surface area) and electrochemical performance. Modifying the electrode geometry, size, and electrolyte flow rate results in an overpotential decrease and different bubble diameters and lifetimes for the hydrogen (HER) and oxygen evolution reactions (OER). Hence, we demonstrate the essential role of the electrode architecture and forced electrolyte convection on bubble release. Additionally, we confirm the suitability of ordered, Ni-coated 3D porous structures by evaluating the HER/OER performance, bubble dissipation, and long-term stability. Finally, we utilize the 3D porous electrode as a support for studying a benchmark NiFe electrocatalyst, confirming the robustness and effectiveness of 3D-printed electrodes for testing electrocatalytic materials while extrinsic properties are precisely controlled. Overall, we demonstrate that tailoring electrode architectures and surface properties result in precise tuning of extrinsic electrode properties, providing more reproducible and comparable conditions for testing the efficiency of electrode materials for water electrolysis.

Arrays of Microscale Linear Ridges with Self-Cleaning Functionality for the Oxygen Evolution Reaction

Gas management during electrocatalytic water splitting is vital for improving the efficiency of clean hydrogen production. The accumulation of gas bubbles on electrode surfaces prevents electrolyte access and passivates the electrochemically active surface area. Electrode morphologies are sought to assist in the removal of gas from surfaces to achieve higher reaction rates at operational voltages. Herein, regular arrays of linear ridges with specific microscale separations were systematically studied and correlated to the performance of the oxygen evolution reaction (OER). The dimensions of the linear ridges were proportional to the size of the oxygen bubbles, and the mass transfer processes associated with gas evolution at these ridges were monitored using a high-speed camera. Characterization of the adhered bubbles prior to detachment enabled the use of empirical methods to determine the volumetric flux of product gas and the bubble residence times. The linear ridges promoted a self-cleaning effect as one bubble would induce neighboring bubbles to simultaneously release from the electrode surfaces. The linear ridges also provided preferential bubble growth sites, which expedited the detachment of bubbles with similar diameters and shorter residence times. The linear ridges enhanced the OER in comparison to planar electrodes prepared by electrodeposition from the same high-purity nickel (Ni). Linear ridges with a separation distance of 200 μm achieved nearly a 2-fold increase in current density relative to the planar electrode at an operating voltage of 1.8 V (vs Hg/HgO). The electrodes with linear ridges having a separation distance of 200 μm also had the highest sustained current densities over a range of operating conditions for the OER. Self-cleaning surface morphologies could benefit a variety of electrocatalytic gas evolving reactions by improving the efficiency of these processes.

General Strategy to Optimize Gas Evolution Reaction via Assembled Striped-Pattern Superlattices

Redesigning heterogeneous catalysts so that they can simultaneously integrate the efficiency and durability under reaction environments with respect to gas fuel production, such as hydrogen (H₂), oxygen (O₂), or carbon monoxide (CO), has proven challenging. In this work, we report the successful template-assisted printing-based assembly of platinum (Pt) nanoparticles (NPs) into striped-pattern (SP) superlattices to produce H₂. In comparison to drop-casting flat Pt NPs films, SP superlattices lead to higher mass transference and smaller bubble stretch force, representing a general strategy to improve the efficiency and durability of pre-existed Pt catalysts for the hydrogen evolution reaction (HER), as well as higher current densities than commercial Pt/C, Pt NP films, and many of the other Pt-based or non-Pt-based HER catalysts reported in the literature. The generic nature of template-assisted printing leads to flexibility in the composition, size, and shape of the constituent NPs or molecules, and thus extends such an accelerated technique for producing the oxygen evolution reaction and electrochemical reduction of CO₂ to CO.

Nitrogen-Doped Sponge Ni Fibers as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction

Controllable synthesis of highly active micro/nanostructured metal electrocatalysts for oxygen evolution reaction (OER) is a particularly significant and challenging target. Herein, we report a 3D porous sponge-like Ni material, prepared by a facile hydrothermal method and consisting of cross-linked micro/nanofibers, as an integrated binder-free OER electrocatalyst. To further enhance the electrocatalytic performance, an N-doping strategy is applied to obtain N-doped sponge Ni (N-SN) for the first time, via NH₃ annealing. Due to the combination of the unique conductive sponge structure and N doping, the as-obtained N-SN material shows improved conductivity and a higher number of active sites, resulting in enhanced OER performance and excellent stability. Remarkably, N-SN exhibits a low overpotential of 365 mV at 100 mA cm^-2 and an extremely small Tafel slope of 33 mV dec^-1, as well as superior long-term stability, outperforming unmodified sponge Ni. Importantly, the combination of X-ray photoelectron spectroscopy and near-edge X-ray adsorption fine structure analyses shows that γ-NiOOH is the surface-active phase for OER. Therefore, the combination of conductive sponge structure and N-doping modification opens a new avenue for fabricating new types of high-performance electrodes with application in electrochemical energy conversion devices.

Three-Dimensional Fe,N-Decorated Carbon-Supported NiFeP Nanoparticles as an Efficient Bifunctional Catalyst for Rechargeable Zinc-O2 Batteries

The electro-catalyzed oxygen reduction and evolution reactions (ORR/OER) are the key elements of many energy conversion systems, such as fuel cells, water electrolyzers, and rechargeable metal-air batteries. Structural design of durable non-noble nanomaterials as bifunctional OER/ORR catalysts is a major drawback to commercial applications. Herein, we exposed a strongly coupled hybrid material comprising of NiFeP-cubes nanoparticles supported on three-dimensional interconnected Fe,N-decorated carbon (3D-FeNC) as a robust bifunctional ORR/OER catalyst. The strongly coupled NiFeP@3D-FeNC catalyst shows better electron and mass transfer capability, exposure of abundant ORR/OER active sites on the surface, and strongly coupled effects. Accordingly, the as-prepared NiFeP@3D-FeNC catalyst exhibits robust ORR activity (half-wave potential of 0.84 V vs reversible hydrogen electrode) and OER performance (over-potential 0.25 V@10 mA cm^-2) in alkaline media. Significantly, the oxygen electrode prepared from the NiFeP@3D-FeNC catalyst further demonstrated superior charge/discharge behavior and long-lasting rechargeability than the benchmark Pt/C + IrO₂ catalyst in rechargeable zinc-O₂ batteries. This approach opens up a new avenue for the synthesis and advanced the hybrid nanomaterials for various applications.