Stanford team reports new low-cost, non-precious metal catalyst for water splitting with performance close to platinum
|Structure of the NiO/Ni-CNT hybrid. Blue = nickel, green = nickel oxide. Credit: Gong et al. Click to enlarge.|
Researchers at Stanford University, with colleagues at Oak Ridge National Laboratory and other institutions, have developed a nickel-based electrocatalyst for low-cost water-splitting for hydrogen production with performance close to that of much more expensive commercial platinum electrocatalysts.
As described in their paper in Nature Communications, the catalyst comprises nanoscale nickel oxide/nickel heterostructures formed on carbon nanotube sidewalls (NiO/Ni-CNT nano-hybrids). The researchers were able to make the electrocatalysts active enough to split water at room temperature with a single 1.5-volt battery, said Hongjie Dai, a professor of chemistry at Stanford. This marked the first time anyone has used non-precious metal catalysts to split water at a voltage that low, he added.
Nickel (Ni) and stainless steel are typically used in industry for water reduction and oxidation catalysis respectively in basic solutions. However, Ni metal is not an ideal water reduction or hydrogen evolution reaction (HER) catalyst due to its high overpotential (~200 mV) and large Tafel slope. The state-of-art HER catalyst is platinum (Pt) and its alloys, but the scarcity and cost of Pt limit its large-scale application for electrolysis. Active and stable non-precious metal-based HER catalysts in alkaline solutions have been pursued, including Raney Ni and Nickel molybdenum (NiMo) alloy. It remains difficult to achieve both high activity and stability matching those of Pt.
Here, we report a nickel oxide/nickel (NiO/Ni) hetero-junction-like structure attached to mildly oxidized carbon nanotube (NiO/Ni-CNT) exhibiting high HER catalytic activity close to commercial Pt/C catalysts in several types of basic solutions (pH = 9.5–14). The NiO/Ni nano-hybrids is fabricated serendipitously in a low-pressure thermal annealing experiment, affording partial reduction of nickel hydroxide (Ni(OH)2) coated on oxidized CNTs that acts as an interacting substrate to impede complete reduction and aggregation of Ni.—Gong et al.
As reported in their paper, the team built an electrolyzer that achieves ~20 mA cm−2 at a voltage of 1.5 V at room temperature, and which may be operated by a single-cell AAA alkaline battery.
The mini electrolyzer used NiO/Ni-CNT as the water reduction catalyst and a high-performance NiFe-layered double hydroxide (NiFe LDH) water oxidation catalyst. The experiment was carried out in 1 M KOH at room temperature (~23 °C) and at ~60 °C.
The kinetics and thermodynamics were greatly improved at the higher temperature, showing lower voltage of ~1.42 V at 20 mA cm-2 and higher current increase, reaching 100 mA cm-2 at a voltage of ~1.45 V with good stability. The results suggest that the NiO/Ni-CNT catalyst could match the benchmark platinum catalyst for efficient electrolyzers with ultra-low overpotential, the team concluded.
The scientists do not fully understand the mechanics of the performance of the NiO/Ni-CNT catalyst; future work will tackle this and perform in situ spectroscopic techniques to elucidate the reaction mechanisms and pinpoint the HER active sites in the NiO/Ni material. However, they suggested that the high activity of NiO/Ni-CNT was due possibly to the nanoscopic NiO/Ni interfaces in the heterostructure.
The discovery was made by Stanford graduate student Ming Gong, co-lead author of the study. The nickel/nickel-oxide catalyst significantly lowers the voltage required to split water, which could eventually save hydrogen producers billions of dollars in electricity costs, according to Gong. His next goal is to improve the durability of the device.
The researchers also plan to develop a water splitter than runs on electricity produced by solar energy.
Other authors of the study are Wu Zhou, Oak Ridge National Laboratory (co-lead author); Mingyun Guan, Meng-Chang Lin, Bo Zhang, Di-Yan Wang and Jiang Yang, Stanford; Mon-Che Tsai and Bing-Joe Wang, National Taiwan University of Science and Technology; Jiang Zhou and Yongfeng Hu, Canadian Light Source Inc.; and Stephen J. Pennycook, University of Tennessee.
Principal funding was provided by the Global Climate and Energy Project (GCEP) and the Precourt Institute for Energy at Stanford and by the U.S. Department of Energy.
Ming Gong, Wu Zhou, Mon-Che Tsai, Jigang Zhou, Mingyun Guan, Meng-Chang Lin, Bo Zhang, Yongfeng Hu, Di-Yan Wang, Jiang Yang, Stephen J. Pennycook, Bing-Joe Hwang & Hongjie Dai (2014) “Nanoscale nickel oxide/nickel heterostructures for active hydrogen evolution electrocatalysis” Nature Communications 5, Article number: 4695 doi: 10.1038/ncomms5695