Earth-abundant Electrocatalysts For Hydrogen Evolution Reaction Research Articles

Transition Metal Sulfide-Based Electrocatalysts for Hydrogen Evolution Reaction in Alkaline Environment

Hydrogen already plays a key role in many industrial processes and the interest in this compound has recently increased since it is pointed-out as a promising future energy carrier. Besides to present a high energy yield, no greenhouse gas is produced during its oxidation. However, most of current hydrogen production still comes from fossil feedstock and, due to environmental concerns, methods aiming the renewable, cleaner and economically suitable production of hydrogen are being developed.Hydrogen production from water, in a process called “water electrolysis”, has been receiving great attention because if it is coupled to a renewable source of energy, it results in zero carbon emission. In this process, water is splitted to originate two gases, hydrogen at the cathode and oxygen at the anode, both containing high purity. The reactions responsible for their production in each half-cell are called hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively.The use of catalysts is compulsory in order to minimize the overpotential of aforementioned reactions and, consequently, increase process efficiency. Noble metal-based materials, mainly Pt, Ru and Ir, remain as benchmark to catalyze these reactions, but their high-cost and scarcity are responsible for limiting their application in large scale. In order to overcome these barriers, new candidate catalysts must be searched and they need to be active, stable, cheap and sustainable.Nørskov et al. [1] presented one important, but not unique, indicator for highly active HER catalysts, the free energy of H adsorption (∆GH 0) on the sites; the closer to zero, the better. They also brought attention to the similar sulfur coordination to metals in both MoS2 edges and active sites of nitrogenase, an efficient hydrogen evolution enzyme. Thus, authors stated that edge sites of MoS2 materials are active since they attend thermoneutrality criterion, although their basal planes are inert. Later, it was experimentally confirmed by Jaramillo et al. [2]. As consequence, further works have also shown good activity of other transition metal sulfides (TMS) towards HER and, therefore, it has become a promising class of materials to replace the expensive Pt-based electrocatalysts.Noteworthy, Nørskov et al. [1] performed calculations that take into account an acidic environment and, for this reason, only ∆GH 0 descriptor was used to estimate HER activity. However, HER in alkaline medium brings an additional step of water dissociation, responsible to generate the proton which is further reduced to adsorbed hydrogen specie; such step may increase energy consumption of reaction. Furthermore, lower binding energy of water compared to hydronium and the interaction of hydroxyl groups with the catalytic sites, abundant in high pH’s, may play a key role in catalysts’ activity. As result, the mechanism of HER in alkaline electrolyte is more complex. Despite that, the alkaline environment allows the application of wider range of materials, including non-noble metal-based ones, in HER and OER. This, summed up to new developments in alkaline exchange membranes, make it worth to search for materials which may be applied in alkaline exchange membrane water electrolyzers (AEMWE).In the present work, by applying the synthesis reported by Li et al. [3] with modifications, several TMS were produced by hydrothermal method employing sodium diethyldithiocarbamate (C5H10NS2Na), non-conventional precursor, as sulfur source. Adjusts in synthesis parameters were performed aiming the development of earth-abundant HER electrocatalysts to be applied in AEMWE devices. As preliminary results, 225 and 50 mV overpotentials were measured at -10 and -100 mA cm-2, respectively on HER polarization curves obtained in 1 M KOH electrolyte on MoSx-based cathodes, compared to a commercial 40% Pt/C material as reference. Doping strategies with M-M’ TMS synthesis are being implemented for improving HER efficiency. Acknowledgements: This work was mainly conducted within the framework of a PhD thesis financially supported by the European Union (ERDF), GrandAngoulême and Région Nouvelle-Aquitaine. References Hinnemann, P.G. Moses, J. Bonde, K.P. Jørgensen, J.H. Nielsen, S. Horch, I. Chorkendorff, J.K. Nørskov, J. Am. Chem. Soc. 122 (2005) 5308-5309.F. Jaramillo, K.P. Jørgensen, J. Bonde, J.H. Nielsen, S. Horch, I. Chorkendorff, Science 317 (2007) 100-102. Li, M. Xiao, Y. Zhou, D. Zhang, H. Wang, X. Liu, D. Wang, W. Wang, Dalton Trans. 47 (2018) 14917-14923.

Rational design of phosphorus-doped cobalt sulfides electrocatalysts for hydrogen evolution

Moderate anionic doping is an effective approach to improve the performance of electrocatalysts toward hydrogen evolution reaction (HER) since it can adjust the electronic structure, active site, and phase. The reported studies mainly focus on designing P doped CoS2, while other phases of cobalt sulfide, such as Co1−xS and Co9S8 doped with P are rarely investigated for HER electrocatalysts. Herein, various cobalt sulfides (including CoS2/Co1−xS, Co1−xS, and Co9S8) doped with P are anchored on carbon cloth through a facile hydrothermal method. Among tested electrocatalysts, P doped Co1−xS exhibits excellent electrocatalytic performance with an overpotential of 110 and 165 mV (vs. RHE) for current densities of 10 and 100 mA·cm−2, respectively. Density functional theory calculations reveal that P doped Co1−xS possesses a smaller bandgap and more optimal hydrogen adsorption sites than pristine Co1−xS. This work initially investigates various cobalt sulfides doped with P and further gets insight into the activity improvement mechanism of P doping, which could guide the design of earth-abundant HER electrocatalysts for the “hydrogen economy”.

Ultrahigh length-to-diameter ratio nickel phosphide nanowires as pH-wide electrocatalyst for efficient hydrogen evolution

The hydrogen evolution reaction (HER) represents one promising solution for energy and environmental sustainability. Nickel phosphide with various morphology has been recently investigated as promising earth abundant electrocatalysts for HER. Compared with three or two dimensional morphology, one dimensional (1D) nickel phosphide could provide larger specific surface area and hence improves the HER electrocatalytic performance. However, most reported 1D nickel phosphides show a small length-to-diameter ratio (less than 50). Herein, ultrahigh length-to-diameter ratio Ni2P nanowires synthesized by one-step phosphorization-sulfuration treatment are reported for the first time. The as-prepared nanowires have extremely high length to diameter ratio which ranges from 500 to 1000. The formation mechanism of such morphology is illustrated by a systematic study of the morphology, microstructure and elementary distribution of the nanowire array cathode. The ultralong nanowire morphology clearly helps improve the HER performances, since the self-supported Ni2P nanowire array cathode exhibits outstanding HER electrocatalytic activity and long-term durability in all of the acidic, alkaline, and neutral conditions.

Dominating Role of Aligned MoS2/Ni3S2 Nanoarrays Supported on Three-Dimensional Ni Foam with Hydrophilic Interface for Highly Enhanced Hydrogen Evolution Reaction.

When using water splitting to achieve sustainable hydrogen production, low-cost, stable, and naturally abundant electrocatalysts are required to replace Pt-based ones for the hydrogen evolution reaction (HER). Herein, for the first time, a novel nanostructure with one-dimensional (1D) MoS2/Ni3S2 nanoarrays directly grow on a three-dimensional (3D) Ni foam is developed for this purpose, showing excellent catalytic activity and stability. The as-prepared 3D MoS2/Ni3S2/Ni composite has an onset overpotential as low as 13 mV in 1 M KOH, which is comparable to Pt-based electrocatalyst for HER. According to the classical theory, the Tafel slope of the new composite is relatively low, as it goes through a combined Volmer-Heyrovsky mechanism during hydrogen evolution. All the results attribute the excellent electrocatalytic activity of the nanostructure to the electrical coupling among Ni, Ni3S2, and MoS2, the super hydrophilic interface, the synergistic catalytic effects produced by the MoS2/Ni3S2 nanoarrays, and abundant exposed active edge sites. These unique and previously undeveloped characteristics of the 3D MoS2/Ni3S2/Ni composite make it a very promising earth-abundant electrocatalyst for HER.

Electrochemical fabrication of porous Ni-Cu alloy nanosheets with high catalytic activity for hydrogen evolution

Porous Ni-Cu alloy films composed of nanosheets are directly fabricated on Cu substrate using a facile potentiostatic electrodeposition approach from a 1:2 choline chloride-ethylene glycol eutectic solvent (ethaline) containing nickel chloride and copper chloride. Electrochemical co-deposition of Ni-Cu alloy in ethaline is found to be thermodynamically favorable in comparison to conventional aqueous solutions. The deposition process occurs via an instantaneous nucleation and growth of nanoparticles at the initial stage and develops vertically to form nanoporous structures arrayed with nanosheets over deposition time. The as-prepared porous Ni-Cu alloy nanosheet arrays exhibit enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline media, which displays a small Tafel slope of 57.2mVdec−1 with a low onset potential of −48mV vs. RHE at an optimal condition. It requires small overpotentials of −128 and −150mV to drive catalytic current densities of 10 and 20mAcm−2, respectively. This facile electrodeposition method performed in ethaline provides an efficient way to fabricate porous Ni-based alloy nanosheets directly on a current collector, which is a promising technology for the synthesis of earth-abundant HER electrocatalysts.

Co-Te-Se Nano-Compounds as Electrocatalysts for Hydrogen Evolution Reaction

Developing non-noble-metal catalysts with high activity and stability is critical for hydrogen production through water splitting. In this study, Co-Te-Se nano-compounds comprised of earth-abundant and cheap elements were synthesized via a facile hydrothermal method. The morphological study of transmission electron microscope (TEM) showed the nanoparticles of Co-Te-Se with a diameter about 25 nm. The composition of Co-Te-Se nano-compounds was confirmed by X-ray photoelectron spectroscopy (XPS). The nano-compounds of Co-Te-Se loaded at multi-wall carbon nanotubes (MWCNTs) were evaluated as electrocatalysts toward hydrogen evolution reaction (HER) in H2SO4. It showed the onset potentials of only −169 mV vs. RHE. The overpotential required to drive the HER at a current density of 10 mA/cm2 is about 217 mV and that of 50 mA/cm2 is about 267 mV with a small Tafel slope of 42 mV/dec for Co-Te-Se/MWCNT. Galvanostatic measurements and continuous potential cycles indicated the stability of Co-Te-Se/MWCNT. This development is potentially attractive earth-abundant electrocatalysts for HER and provides new opportunities for their application in water-splitting.

Highly active electrocatalysis of the hydrogen evolution reaction by cobalt phosphide nanoparticles.

Nanoparticles of cobalt phosphide, CoP, have been prepared and evaluated as electrocatalysts for the hydrogen evolution reaction (HER) under strongly acidic conditions (0.50 M H2SO4, pH 0.3). Uniform, multi-faceted CoP nanoparticles were synthesized by reacting Co nanoparticles with trioctylphosphine. Electrodes comprised of CoP nanoparticles on a Ti support (2 mg cm(-2) mass loading) produced a cathodic current density of 20 mA cm(-2) at an overpotential of -85 mV. The CoP/Ti electrodes were stable over 24 h of sustained hydrogen production in 0.50 M H2SO4. The activity was essentially unchanged after 400 cyclic voltammetric sweeps, suggesting long-term viability under operating conditions. CoP is therefore amongst the most active, acid-stable, earth-abundant HER electrocatalysts reported to date.