Researchers led by engineers at The University of Texas at El Paso (UTEP) have proposed a low-cost, cactus-inspired nickel-based material to help split water more cheaply and efficiently. The material is described in a paper in the journal ACS Applied Materials & Interfaces.

Current electrolysis techniques to split water rely heavily on platinum as a catalyst, which is very expensive and just not feasible to use on a large scale because of its price, said UTEP Mechanical Engineering Professor Ramana Chintalapalle, Ph.D., who led the study.

Lead author Navid Attarzadeh first noticed the prickly pear cactus while walking to UTEP’s Center for Advanced Materials Research lab. The team had been exploring nickel as a catalytic replacement for platinum, a metal that is abundant on Earth and 1,000 times cheaper than platinum. Nickel, however, is not as quick and effective at breaking down water into hydrogen.

Every day, I passed this same plant. And I started connecting it to our catalyst problem. What caught my attention was how big the leaves and fruits were compared to other desert plants; the prickly pear has an extraordinary surface area.

—Navid Attarzadeh

Attarzadeh wondered what if they designed a 3D nickel-based catalyst in the shape of the prickly pear cactus? The larger surface area could accommodate more electrochemical reactions—creating more hydrogen than nickel typically can.

The team synthesized a 3D nanoarchitecture of aligned Ni₅P₄-Ni₂P/NiS (plate/nanosheets) using a phospho-sulfidation process.

The durability and unique design of prickly pear cactus in desert environments by adsorbing moisture through its extensive surface and ability to bear fruits at the edges of leaves inspire this study to adopt a similar 3D architecture and utilize it to design an efficient heterostructure catalyst for HER activity.

The catalyst comprises two compartments of the vertically aligned Ni₅P₄-Ni₂P plates and the NiS nanosheets, resembling the role of leaves and fruits in the prickly pear cactus. The Ni₅P₄-Ni₂P plates deliver charges to the interface areas, and the NiS nanosheets significantly influence Had and transfer electrons for the HER activity. Indeed, the synergistic presence of heterointerfaces and the epitaxial NiS nanosheets can substantially improve the catalytic activity compared to nickel phosphide catalysts.

Notably, the onset overpotential of the best-modified ternary catalysts exhibits (35 mV) half the potential required for nickel phosphide catalysts. This promising catalyst demonstrates 70 and 115 mV overpotentials to attain current densities of 10 and 100 mA cm^–2, respectively. The obtained Tafel slope is 50 mV dec^–1, and the measured double-layer capacitance from cyclic voltammetry (CV) for the best ternary electrocatalyst is 13.12 mF cm^–2, 3 times more than the nickel phosphide electrocatalyst.

Further, electrochemical impedance spectroscopy (EIS) at the cathodic potentials reveals that the lowest charge transfer resistance is linked to the best ternary electrocatalyst, ranging from 430 to 1.75 Ω cm^–2. This improvement can be attributed to the acceleration of the electron exchangeability at the interfaces. Our findings demonstrate that the epitaxial NiS nanosheets expand the active catalytic surface area and simultaneously elevate the intrinsic catalytic activity by introducing heterointerfaces, which leads to accommodating more H_ad at the interfaces.

—Attarzadeh et al.

The research project was supported by a grant from the National Science Foundation’s Partnerships for Research and Education in Materials (PREM) program.

Resources

• Navid Attarzadeh, Debabrata Das, Srija N. Chintalapalle, Susheng Tan, V. Shutthanandan, and C. V. Ramana (2023) “Nature-Inspired Design of Nano-Architecture-Aligned Ni5P4-Ni2P/NiS Arrays for Enhanced Electrocatalytic Activity of Hydrogen Evolution Reaction (HER)” ACS Applied Materials & Interfaces 15 (18), 22036-22050 doi: 10.1021/acsami.3c00781