Penn State, FSU team develops low-cost, efficient layered heterostructure catalyst for water-splitting
A team of scientists from Penn State and Florida State University have developed a lower cost and industrially scalable catalyst consisting of synthesized stacked graphene and WxMo1–xS2 alloy phases that produces pure hydrogen through a low-energy water-splitting process.
The results of their study, published in the journal ACS Nano, indicate that heterostructures formed by graphene and W0.4Mo0.6S2 alloys are far more efficient than WS2 and MoS2 by at least a factor of 2, and they are superior compared to other reported transition-metal dichalcogenide (TMD) systems. The researchers suggested that their strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER).
Currently, Pt is the most efficient HER catalyst due to its near-zero overpotential in acidic electrolytes. However, the high cost and scarcity of Pt prohibits its application to fulfil the energy demand. Thus, lowering the cost of HER catalysts is of paramount importance for clean, scalable and sustainable energy.
To lower the catalysts cost, a natural abundant alternative and low-cost scalable synthesis are required. During the past years, naturally abundant MoS2 coupled with other nanostructures started to gain more attention as HER catalyst, after being ignored for their poor catalytic activity in the bulk form. However, due to the semiconducting character of MoS2 and WS2 (2H phase), poor electrical conductivity limits the HER kinetics. In order to take advantage of the transition-metal dichalcogenide (TMD) catalytic activity, conducting pathways are essential to accelerate the electron transport.
… In this account, we report a facile, low energy consumption and scalable wet chemical approach able to prepare rGO/XS2 (X=W, Mo or W and Mo alloy) heterostructures consisting of few layers and monolayer TMDs with conducting rGO.—Lei et al.
Molybdenum disulfide has been predicted as a possible replacement for platinum, because the Gibbs free energy for hydrogen absorption is close to zero, explained Mauricio Terrones, professor of physics, materials science and engineering, and chemistry, Penn State and one of the corresponding authors of the paper. The lower the Gibbs free energy, the less external energy has to be applied to produce a chemical reaction.
However, experimentally, there are drawbacks to using molybdenum disulfide as a catalyst. In its stable phase, molybdenum disulfide is a semiconductor, which limits its ability to conduct electrons. To get around that problem, the team added reduced graphene oxide, a highly conducting form of carbon.
Then, to further decrease the free energy, they alloyed the molybdenum disulfide with tungsten to create a thin film with alternating graphene and tungsten-molybdenum disulfide layers. The addition of tungsten lowers the electrical voltage required to split water by half, from 200 millivolts with pure molybdenum disulfide, to 96 millivolts with the tungsten-molybdenum alloy.
The water splitting process uses a very small amount of electrical energy applied to an electrode immersed in water. Using this small potential, the protons in the solution can be absorbed onto the surface of the catalyst. Then two protons will migrate together to form a hydrogen bubble that rises to the surface and releases the hydrogen.
From the theoretical point of view, the electron orbitals play a crucial role. In the case of pure molybdenum disulfide, the electron orbitals from the metal do not overlap well with the electron orbital of hydrogen in the key reaction step. However, when the alloy is present these orbitals interact well and make the reaction more efficient.
This is similar to what platinum does, and the reason why platinum is so energy efficient when used in this chemical reaction. However, in this work, researchers showed that cheaper and more abundant elements can be used and reach an efficiency that outperforms all the best catalysts.
The resulting rGO/XS2 films exhibit enhanced HER catalytic activity due to three advantages: (i) the presence of exposed edges and curved regions from dendritic-like morphologies; (ii) additional surface area from mono- and few-layered XS2; and (iii) an interlayer electronic-coupling effect combining conducting rGO and catalytic XS2.
… DFT calculations indicate that the high reactivity observed in these TMD alloys (WxMo1-xS2), when compared to pure phases (MoS2 or WS2), is due to the lowering of the activation energy barrier when forming H2 molecules along the “inert” basal planes. This low activation barrier is due to the stabilization of the rate determinant transition state, where the electron density of H2 formation is favored by the overlap of the s-orbitals of the H atoms and the d-orbitals from the transition metals alloys from W and Mo. In other words, this electronic overlap stabilizes the transition state, which in consequence lower the Tafel slope, thus making the alloys responsible for a better catalytic activity regardless of the alloying concentration.—Lei et al.
The team concluded that such layered heterostructures thus have enormous potential in catalysis.
The US Army Research Office, the National Science Foundation, and national funding agencies in China supported this work.
Yu Lei, Srimanta Pakhira, Kazunori Fujisawa, Xuyang Wang, Oluwagbenga Oare Iyiola, Néstor Perea López, Ana Laura Elías, Lakshmy Pulickal Rajukumar, Chanjing Zhou, Bernd Kabius, Nasim Alem, Morinobu Endo, Ruitao Lv, Jose L. Mendoza-Cortes, and Mauricio Terrones (2017) “Low-temperature Synthesis of Heterostructures of Transition Metal Dichalcogenide Alloys (WMoS) and Graphene with Superior Catalytic Performance for Hydrogen Evolution” ACS Nano 11 (5), 5103-5112 doi: 10.1021/acsnano.7b02060