Producing hydrogen without carbon dioxide emissions looks simple: pass a current through water and split it into oxygen and hydrogen. The main difficulty lies in electrocatalysts, substances that speed up the reactions at the electrodes. The best of them are made from platinum, ruthenium and iridium, rare and expensive metals. A group led by Shiv Singh of CSIR-AMPRI in Bhopal tested whether iron and nickel oxides on carbon nanofibers can handle the task. The results were published in ACS Applied Nano Materials.
Today about 96% of the world's hydrogen is produced from fossil fuels. Producing one kilogram of such hydrogen releases roughly 10 kg of carbon dioxide into the atmosphere, and producing so-called blue hydrogen releases about 6 kg. Green hydrogen, obtained by water electrolysis, produces none of these emissions. Electrolysis involves two key reactions: oxygen evolution at the anode and hydrogen evolution at the cathode. Both need a catalyst to proceed at an acceptable voltage.
The base of the new material was MXene, a two-dimensional conductive material with the composition Ti3C2Tx, thin layers that conduct current well. Such layers oxidize and stick together easily, which increases resistance. To get around this, the researchers grew carbon nanofibers on them by chemical vapor deposition: they mixed MXene with an iron or nickel salt, dried it and heated it in nitrogen to 400 degrees, then raised the temperature to 720 degrees and supplied acetylene as a carbon source for two hours. Particles of metal oxide ended up at the tip of each fiber, a process called tip growth. The resulting samples were designated FeO-CNF@MX and NiO-CNF@MX.
Electron microscopy showed fibers growing across the MXene layers, with an oxide particle at the end. X-ray diffraction and Raman spectroscopy confirmed that the layered structure of MXene was preserved. In the nickel composite the carbon turned out to be more graphitized, that is, less defective: the defect ratio from the spectra was 0.40 versus 0.68 for the iron one.
Both materials were tested in a 1 molar potassium hydroxide solution. Applying the fibers to MXene improved performance compared with the fibers alone. For hydrogen evolution, the nickel composite required an overpotential of 490 mV to reach a current density of 10 mA/cm2, the iron one 520 mV, and the fibers without MXene 570 and 550 mV respectively. Overpotential is the extra voltage beyond the theoretical minimum needed for the reaction.
For oxygen evolution, the nickel sample needed only 300 mV, the iron one 360 mV. The nickel composite lagged just 50 mV behind the reference ruthenium oxide, which needs 250 mV. The charge transfer resistance of the nickel material was also slightly lower, 15.5 ohms versus 16.5 ohms for the iron one.
When the same material was placed on both electrodes of a small electrolyzer prototype, the picture changed. The iron composite reached 10 mA/cm2 at 2.2 V, while the nickel one required 2.7 V. According to the authors, iron centers help stabilize reactive oxygen intermediates, which speeds up oxygen evolution in the full cell. Over 24 hours of testing, the cell voltage shifted by only 0.2-0.3 V for both materials, and slightly less for the nickel one: from 2.38 to about 2.6 V versus 2.4-2.7 V for the iron one.
The new catalysts are still far from platinum: it reaches 10 mA/cm2 at an overpotential of just 70 mV. Nevertheless, the work shows that a simple and scalable method can turn cheap metal oxides into working catalysts for both reactions. The iron composite was more active in the full cell at the start, while the nickel one was more stable during prolonged operation. These are laboratory test results, and industrial application is not yet on the agenda.