Highly Conductive and Mechanically Robust NiFe Alloy Aerogels: An Exceptionally Active and Durable Water Oxidation Catalyst

Abstract

Poor stability of nanostructured electrocatalysts at rigorous industrial conditions significantly inhibits their performances in practical electrolyzers. Although many substrate‐supported nanostructured electrocatalysts present attractive performance at small currents, they cannot sustain industry‐level high current densities for long‐term operation. Herein, by chemically organizing nanoscale electrocatalysts into a macroscopic substrate‐free metallic alloy aerogel, this NiFe‐based nano‐catalyst achieves 1000‐h durability at industrial‐level current densities, with exceptionally high activities of 500 mA at the overpotential of only 281 mV. This NiFe alloy aerogel is constructed by a magnetic‐field assisted growth and assembly of ferromagnetic NiFe nanoparticles, in which nanowires are loosely crosslinked by metallic joints. This alloy aerogel shows a high electric conductivity of 500 S m−1, structural stability for more than 1.5 years in alkaline electrolyte, and almost complete recovery after compression exceeding 50% strain for 1000 cycles. The excellent mechanical stability of this metallic aerogel behaves as the key contributor to the superior electrocatalytic stability under industrially relevant conditions. This work offers a paradigm for electrode design for the practical application of nano‐catalysts in industrial alkaline water electrolysis.

Grain Boundary Effects of Hierarchical Ni‐Fe (Oxy)hydroxide Nanosheets in Water Oxidation

“…[ 11 ] Informed by this theory and guided by TEM analysis (Figure 1f; Figure S5, Supporting Information) and previous studies on the effects of grain boundaries in catalysts, we propose that the introduction of grain boundaries in the Ni‐Fe catalysts lead to the enhanced OER activity. [ 9b,c,10a ] Table S1 (Supporting Information) summarizes the overpotentials to yield a high current density of 500 mA cm −2 of recently reported state‐of‐the‐art OER catalysts including ultrathin amorphous NiFe oxyhydroxide@NiFe alloy (248 mV), [ 7d ] Ni(OH) 2 /Fe 2 O 3 (257 mV), [ 20 ] Ni‐Co‐Se (279 mV), [ 21 ] FeCoNiP/NF (281 mV), [ 22 ] Ni 0.80 Fe 0.20 aerogel (281 mV), [ 23 ] K 2 Fe 4 O 7 nanocrystals (340 mV) [ 24 ] etc. The comparison also indicates that the Ni‐Fe electrode is comparable or even outperforms most of the previous advanced Fe‐based OER electrocatalysts, in terms of the performance of high current densities such as overpotentials at 500 and 1000 mA cm −2 , and Tafel slope.…”

Dynamically Adaptive Bubbling for Upgrading Oxygen Evolution Reaction Using Lamellar Fern‐Like Alloy Aerogel Self‐Standing Electrodes

“…The as-synthesized LFA shows a hierarchically ordered structure, and their fabrication condition is analogous to the authors' previously reported Ni-based nanowires (see Methods for details). [15,20,21] Ferns typically have a robust and rigid shaft and rich and relatively soft pinnules, which can provide directional stress release through elastic deformation to avoid stress concentration (Figure 1a). Figure 1b illustrates the synthesis process of LFA, which involves the initial formation of parallel "shafts" (Ni nanowires) and the subsequent growth of the "pinnules" structure (short NiFe nanowires) perpendicular to the "shafts".…”

“…At the same time, the thinner and shorter NiFe alloy NWs show an optimum atomic Ni: Fe ratio of 4: 1 for OER (Table S1, Supporting Information). [15,20,21,28] These "pinnules" provide enormous active sites in LFA due to the superior OER catalytic performance of the NiFe layered double hydroxides (NiFe LDH), an active phase that naturally evolves from NiFe alloy. Scanning electron microscopy (SEM) images show the successful fabrication of LFA with designed "shaft" and "pinnules" structures by using the magnetic field-induced synthesis method (Figure 1c-e).…”

“…So the immediate challenge is how to timely evacuate the bubbles along the most optimum path and effectively release stress generated by the bubble accumulation. [20,21] Developing asymmetric superaerophilic microstructures can induce directional transport of gas bubbles in a liquid fluid, [22][23][24] yet for those long-distance gas transport on aerophobic surfaces, the design rules and the related methodologies have been barely explored.…”

“…Metallic aerogels with self-supporting and highly porous structures are prospective candidates for electrocatalytic applications. [20,[25][26][27] However, most reported metallic aerogels are composed of filamentous building blocks with meandering channels in their bulk. Thus, they are often mixed with polymer binders and inevitably exhibit depreciated catalytic properties.…”