Promising Earth-abundant Electrocatalyst Research Articles

Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction.

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

Unveiling the geometric site dependent activity of spinel Co3O4 for electrocatalytic chlorine evolution reaction

Spinel cobalt oxide (Co3O4), consisting of tetrahedral Co2+ (CoTd) and octahedral Co3+ (CoOh), is considered as promising earth-abundant electrocatalyst for chlorine evolution reaction (CER). Identifying the catalytic contribution of geometric Co site in the electrocatalytic CER plays a pivotal role to precisely modulate electronic configuration of active Co sites to boost CER. Herein, combining density functional theory calculations and experiment results assisted with operando analysis, we found that the CoOh site acts as the main active site for CER in spinel Co3O4, which shows better Cl− adsorption and more moderate intermediate adsorption toward CER than CoTd site, and does not undergo redox transition under CER condition at applied potentials. Guided by above findings, the oxygen vacancies were further introduced into the Co3O4 to precisely manipulate the electronic configuration of CoOh to boost Cl− adsorption and optimize the reaction path of CER and thus to enhance the intrinsic CER activity significantly. Our work figures out the importance of geometric configuration dependent CER activity, shedding light on the rational design of advanced electrocatalysts from geometric configuration optimization at the atomic level.

Interface engineering of core-shell Ni0.85Se/NiTe electrocatalyst for enhanced oxygen evolution and urea oxidation reactions

The development of highly efficient oxygen evolution reaction (OER) and urea oxidation reaction (UOR) electrocatalysts with abundant resources is necessary for green hydrogen production. Ni-based compounds have received attention as the most promising earth-abundant electrocatalysts for OER and UOR, whereas some compounds in this main group, e.g., nickel selenides and tellurides, have received little attention. Herein, we demonstrate the interfacial engineered Ni0.85Se/NiTe array on Ni foam as a highly efficient catalyst for the OER, which exhibits an overpotential of 200 mV to obtain a current density of 10 mA cm−2 in alkaline solutions. Meanwhile, it exhibits a low potential of 1.301 V for the UOR at a current density of 100 mA cm−2. In particular, it even has the potential to be used in methanol oxidation reaction and ethanol oxidation reaction. The vertical NiTe array not only serves as the conductive substrate for highly improving the mass loading of Ni0.85Se, but also triggers the strong electron interaction between two components, leading to increased adsorption sites available for the intermediates formed in the OER and UOR on the Ni0.85Se surface. This study provides a broad avenue to construct hierarchical nanostructures as outstanding electrocatalysts for efficient OER and UOR.

Vanadium-phosphorus incorporation induced interfacial modification on cobalt catalyst and its super electrocatalysis for water splitting in alkaline media

Co-based compounds have been considered as promising earth-abundant electrocatalysts for water splitting, however, the catalytic performance need to be further enhanced. Herein, we report a nickel foam (NF) supported Co-VOx-P nanoflower electrocatalyst that shows excellent catalytic performances, achieving a small overpotential of 230 mV at 100 mA/cm2 for OER. Such performances surpass the recently reported best-performing Co-based catalysts in alkaline media. In addition, the present Co-VOx-P electrocatalyst can deliver 10 mA/cm2 with a small overpotential of 98 mV for HER. Experimental and DFT studies indicate that, due to the combination of P and V, the Co-VOx-P electrocatalyst shows improved intrinsic catalytic properties including enhanced conductivity, facile electron transfer and favorable surface adsorption strength. The strategy demonstrated here by incorporating metalloid with high valence states may be extended to design other cost-efficient electrocatalysts for more advanced water splitting performances.

Structural Dynamics of Ultrathin Cobalt Oxide Nanoislands under Potential Control

AbstractCobalt oxide is a promising earth abundant electrocatalyst and one of the most intensively studied oxides in electrocatalysis. In this study, the structural dynamics of well‐defined cobalt oxide nanoislands (NIs) on Au(111) are investigated in situ under potential control. The samples are prepared in ultra‐high vacuum and the system is characterized using scanning tunneling microscopy (STM). After transfer into the electrochemical environment, the structure, mobility, and dissolution is studied via in situ electrochemical (EC) STM, cyclic voltammetry, and EC on‐line inductively coupled plasma mass spectrometry. Cobalt oxide on Au(111) forms bilayer (BL) and double‐bilayer NIs (DL), which are stable at the open circuit potential (0.8 VRHE). In the cathodic scan, the cobalt oxide BL islands become mobile at potentials of 0.5 VRHE and start dissolving at potentials below. In sharp contrast to the BL islands, the DL islands retain their morphology up to much lower potential. The re‐deposition of Co aggregates is observed close to the reduction potential of Co2+ to Co3+. In the anodic scan, both the BL and DL islands retain their morphology up to 1.5 VRHE. Even under these conditions, the islands do not show dissolution during the oxygen evolution reaction (OER) while maintaining their high OER activity.

Utilizing spatial confinement effect of N atoms in micropores of coal-based metal-free material for efficiently electrochemical reduction of carbon dioxide

Nitrogen-doped carbon metal-free materials are widely used in electrochemical reduction of carbon dioxide due to high earth abundance, structural tunability, and excellent catalytic performance. Although specific surface area and nitrogen content are considered as the two most important factors in affecting the electrochemical performance of nitrogen-doped porous carbon catalysts, it does not always mean that the larger they are, the better the catalytic performance is. Tailoring pore structures and maximizing the catalytic effect of a limited number of nitrogen atoms are the keys to the improvement in the electrochemical performance. Herein, we develop a solvent evaporation induced self-assembly method to synthesize a coal-based nitrogen-doped porous carbon as an efficient metal-free electrocatalyst for CO2 electroreduction. Under the effect of pore shrinkage induced by high temperatures, nitrogen atoms derived from dicyandiamide are inserted into the carbon matrix of coal, and confined in the micropores produced by KOH activation. The stable nitrogen atoms restricted in the micropores can efficiently convert CO2 into CO due to the spatial confinement effect. The electrocatalyst shows a negligible onset overpotential (–0.16 V) for CO product, a maximum CO Faradaic efficiency of 95 % and a large CO production rate of 36.1 μmol h–1 cm–2 at a low potential of –0.67 V versus the reversible hydrogen electrode (vs. RHE). Moreover, its current density can remain stable after 10 h of potentiostatic electrolysis. This work not only provides a promising earth-abundant electrocatalyst for CO2 electroreduction, but would open up a new avenue for improving the electrochemical performance by coordinating the pore structures and active components.

Self-dissociation-assembly of ultrathin metal-organic framework nanosheet arrays for efficient oxygen evolution

Two-dimensional metal-organic frameworks (MOFs) are promising earth-abundant electrocatalysts, especially for the key process of oxygen evolution reaction (OER), but challenges remain in the lack of effective synthesis and stabilization strategies. Here we first develop a general self-dissociation-assembly (SDA) strategy for in situ synthesis of well-defined ultrathin CoNi-MOF nanosheet arrays (CoNi-MOFNA) as highly active OER electrode. Remarkably, CoNi-MOFNA exhibit excellent OER activity and long-term stability, which deliver the current density of 10 mA cm−2 at a low overpotential of 215 mV with negligible decay even after 300 h continuous electrolysis. The mass activity of CoNi-MOFNA is 14-times higher than commercial RuO2. In addition, the systematic electrochemistry and time-dependent XAS spectroscopy studies convincingly identify the OER active sites of coordinatively unsaturated metal nodes and uncover the structure evolution of CoNi-MOFNA. More importantly, we have demonstrated the universality of this significant SDA strategy for controllably fabricating other types of ultrathin MOF nanosheet arrays uniformly on diverse metal substrates.

Structure and Stability of Au-Supported Layered Cobalt Oxide Nanoislands in Ambient Conditions

Cobalt oxide is a promising earth-abundant electrocatalyst for water splitting; however, the structural complexity of oxides coupled with the difficulty of characterizing it in its operating environment means that fundamental understanding of its catalytic properties remains poor. In this study, we go beyond vacuum studies and investigate the morphological evolution of a CoOx/Au(111) model system from intermediate to high pressures of H2O vapor by means of scanning tunneling microscopy and near-ambient pressure and vacuum X-ray photoelectron spectroscopy. At elevated H2O pressure, we describe the formation of a well-defined Co(OH)2 nanoisland morphology with cobalt in the +2 oxidation state. In contrast, the presence of O2, in air and liquid water, results in only partially hydroxylated Co3+ phases comprising sheets of the CoO(OHx) trilayer, corresponding to a single sheet of cobalt(III)oxyhydroxide. We conclude that the oxyhydroxide structure, known to be the catalytically active phase for the oxygen evo...

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.

(Invited) Graphitic Intercalation Compounds: A Versatile Nano-Template for the Synthesis of Multi-Functional Electrocatalysts

Bifunctional catalysts, which can perform both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) at low overpotentials with high turnover efficiency (TON) and frequency (TOF), are highly sought after. Platinum group metals are among the best catalysts for both OER and ORR but their scarcity and high cost calls for alternatives. In this context non-precious transition metal and metal oxide are promising options despite their lower intrinsic catalytic activity and chemical stability.1 Carbon nano materials are promising materials for electro catalytic application thanks to their high surface areas, electrical conductivity and stability in acidic or basic aqueous solutions.2 Graphite can be intercalated by potassium forming a graphite intercalation compound (GIC) of defined stages. GICs can be readily exfoliated to monolayer graphene in organic solvents solely by stirring. These graphenide solutions are composed of charged graphene layers and are thermodynamically stable.3,4 In analogy to large flake size graphite, graphitic nano carbons with lateral sizes bellow 50 nm can also be intercalated successfully and can be dissolved in absolute organic solvents. Graphenide solutions have been used in a variety of different applications e.g. to generate transparent conductive films, quantum dots, and surfactant-free graphene in water.5 Additionally, their reducing character has been used to graft functional moieties covalently to the carbon framework.6 In this talk, the exploitation of the reduction potential of graphenide solutions for earth abundant metal nanoparticle generation will be discussed. The advantages of using graphenides as reduction reagent in nanoparticle synthesis are : i) graphenide solutions are strong reducing agents and no additional reducing agent is needed, ii) the redox reaction takes place in close proximity of the carbon lattice and the decoration of the carbon framework thus proceeds efficiently, iii) the amount of reduction agent can be controlled by the concentration of the respective graphenide solution used, iv) the by-product of the reaction is only the respective potassium salt, which can be removed easily. The resulting composite materials are interesting and promising earth abundant electrocatalysts, due to their structure, small size, morphology and conductive framework. These nanocarbon / metal nanoparticle composite materials exhibit high bifunctional electro catalytic activity towards ORR and OER as well as long term stability.7 Xia, W., et al., (2016) Angew. Chem. Int. Ed. 55, (8) 2650-2676.Mazzaro, R., et al., (2015) ChemistryOpen 4, (3), 268-273.Pénicaud, A., et al.,. (2013). Acc. Chem. Res. 46, (1), 129–137.Vallés, C., et al.. (2008) J. Am. Chem. Soc. 130, 15802–15804.Bepete, G., et al., (2016) Nat. Chem. 9, 347–352.Hof, F., et al., (2014) Chem. - A Eur. J. 20, 16644–16651.Hof, F., et al., (2017) Chem. - A Eur. J. 23, 15283–15288.

Crystallographic Facet Dependence of the Hydrogen Evolution Reaction on CoPS: Theory and Experiments

Cobalt phosphosulfide (CoPS) has recently emerged as a promising earth-abundant electrocatalyst for the hydrogen evolution reaction (HER). Nonetheless, the influence of crystallographic surface on the HER activity of CoPS and other nonmetallic electrocatalysts remains an important open question in the design of high-performance catalysts. Herein, the HER activities of the (100) and (111) facets of CoPS single crystals were studied using complementary experimental and computational approaches. Natural (111) and polished (100) facets of CoPS single crystals were selectively exposed to reveal that the HER behaviors on these two facets are quite different, with current density–potential curves crossing near 0.35 V vs RHE. Computational analysis can explain this phenomenon in terms of strongly differing H atom adsorption free energies and H–H recombination barriers on the facets, in conjunction with a simple kinetic model. At low potential (0–0.35 V), H adsorption (Volmer step) is rate limiting due to the ende...

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.

Synergistic Phase and Disorder Engineering in 1T-MoSe2 Nanosheets for Enhanced Hydrogen-Evolution Reaction.

MoSe2 is a promising earth-abundant electrocatalyst for the hydrogen-evolution reaction (HER), even though it has received much less attention among the layered dichalcogenide (MX2 ) materials than MoS2 so far. Here, a novel hydrothermal-synthesis strategy is presented to achieve simultaneous and synergistic modulation of crystal phase and disorder in partially crystallized 1T-MoSe2 nanosheets to dramatically enhance their HER catalytic activity. Careful structural characterization and defect characterization using positron annihilation lifetime spectroscopy correlated with electrochemical measurements show that the formation of the 1T phase under a large excess of the NaBH4 reductant during synthesis can effectively improve the intrinsic activity and conductivity, and the disordered structure from a lower reaction temperature can provide abundant unsaturated defects as active sites. Such synergistic effects lead to superior HER catalytic activity with an overpotential of 152 mV versus reversible hydrogen electrode (RHE) for the electrocatalytic current density of j = -10 mA cm-2 , and a Tafel slope of 52 mV dec-1 . This work paves a new pathway for improving the catalytic activity of MoSe2 and generally MX2 -based electrocatalysts via a synergistic modulation strategy.

Fabrication of MoS2/reduced graphene oxide hybrid as an earth-abundant hydrogen evolution electrocatalyst

MoS2/reduced graphene oxide (MoS2/rGO-NP) hybrid is fabricated through one-pot hydrothermal route with mediation of a water-soluble N-methylimidazole pillar[5]arene supramolecule. MoS2 sheets with more exposed active edge sites are well dispersed on the rGO surface. Electrochemical measurements demonstrate that MoS2/rGO-NP hybrid exhibits outstanding electrocatalytic performance for hydrogen evolution reaction with a low Tafel slope of 44.5mV/dec and significantly enhanced kinetics. Thus, the MoS2/rGO-NP hybrid holds high potential as a promising earth-abundant electrocatalyst for the cost-effective production of hydrogen.

Ultrasmall and phase-pure W2C nanoparticles for efficient electrocatalytic and photoelectrochemical hydrogen evolution.

Earlier research has been primarily focused on WC as one of the most promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER), whereas the other compound in this carbide family—W2C—has received far less attention. Our theoretical calculations suggest that such a focus is misplaced and W2C is potentially more HER-active than WC. Nevertheless, the preparation of phase pure and sintering-free W2C nanostructures represents a formidable challenge. Here we develop an improved carburization method and successfully prepare ultrasmall and phase-pure W2C nanoparticles. When evaluated for HER electrocatalysis, W2C nanoparticles exhibit a small onset overpotential of 50 mV, a Tafel slope of 45 mV dec−1 and outstanding long-term cycling stability, which are dramatically improved over all existing WC-based materials. In addition, the integration of W2C nanoparticles with p-type Si nanowires enables highly active and sustainable solar-driven hydrogen production. Our results highlight the great potential of this traditionally non-popular material in HER electrocatalysis.

Lamellar structured CoSe2 nanosheets directly arrayed on Ti plate as an efficient electrochemical catalyst for hydrogen evolution

Splitting water by electrochemical reactions is considered to be the most promising strategy to create hydrogen. Herein, an efficient hydrogen evolution cathode was developed by in-situ growth of lamellar structured cobalt diselenide (CoSe2) nanosheets on the surface of commercial titanium (Ti) plate (CoSe2NS@Ti) in the first time. Both SEM images and XRD data confirmed that the Ti plate is homogeneously coated by few stacking layers of lamellar structured CoSe2, which could provide more active sites for hydrogen evolution reaction (HER) with the thickness about 20nm. This catalytic cathode shows exceptionally high activity for hydrogen evolution with 152mV overpotential to afford a current density of 10mAcm−2 and a high exchange current density of 5.01×10−3mAcm−2, as well as high cyclic stability with negligible decrease of cathodic current after 500 cycles. Remarkably, the CoSe2NS@Ti catalyst exhibits fast hydrogen evolution kinetics with Tafel slope of 38.5mV per decade, which approaches the state-of-art Pt catalyst (Pt/C) in 0.5M H2SO4. This CoSe2NS@Ti material is easily prepared with excellent electrocatalytic performance, thus could be a promising earth-abundant electrocatalyst for the HER and other renewable energy applications.

General Strategy for the Synthesis of Transition Metal Phosphide Films for Electrocatalytic Hydrogen and Oxygen Evolution.

Transition metal phosphides recently have been identified as promising Earth-abundant electrocatalysts for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). Here, we present a general and scalable strategy for the synthesis of transition metal phosphide electrodes based on the reaction of commercially available metal foils (Fe, Co, Ni, Cu, and NiFe) with various organophosphine reagents. The resulting phosphide electrodes were found to exhibit excellent electrocatalytic HER and OER performance. The most active electrodes required overpotentials of only -128 mV for the HER in acid (Ni2P), -183 mV for the HER in base (Ni2P), and 277 mV for the OER in base (NiFeP) to produce operationally relevant current densities of 10 mA cm(-2). Such HER and OER performance compares favorably with samples prepared using significantly more elaborate and costly procedures. Furthermore, we demonstrate that the approach can also be utilized to obtain highly active and conformal metal phosphide coatings on photocathode materials, such as highly doped Si, that are relevant to solar fuels production.

Three-dimensional-networked Ni-Co-Se nanosheet/nanowire arrays on carbon cloth: A flexible electrode for efficient hydrogen evolution

Transition metal diselenides have been considered as one of the most promising earth-abundant electrocatalysts as alternatives to noble metal for the hydrogen evolution reaction (HER). Here, hierarchically three-dimensional nickel cobalt diselenides (Ni1/3Co2/3Se2) with networked nanowires and nanosheets characteristics have been successfully synthesized. Such morphological features may not only provide large surface area allowing for exposure of more active sites, but also benefit deep electrolyte penetration for electrons and electrolyte ions. The resulting binder-free flexible electrodes with large production yield exhibit superior electrochemical HER activity and excellent durability after potential sweep tests for 1000 cycles. Besides their catalytic activity remain stable for electrolysis over 8h at mixed overpotential. The outstanding electrocatalytic performance makes the present nickel cobalt diselenide as a promising earth-abundant electrocatalyst for HER application.

Bottom-up Electrosynthesis of Highly Active Tungsten Sulfide (WS3–x) Films for Hydrogen Evolution

Transition metal dichalcogenides have been extensively studied as promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER). However, despite the intention to achieve sustainable energy generation, conventional syntheses typically use environmentally damaging reagents and energy-demanding preparation conditions. Hence, we present electrochemical synthesis as a green and versatile alternative to traditional methods. In this fundamental study, we demonstrated the bottom-up synthesis of a mixed WS2/WS3 film-like material via cyclic voltammetry (CV). The film-like material can be directly electrosynthesized on any conductive substrates and renders the catalyst immobilization step redundant. Through stepwise analysis of deposition voltammograms facilitated by straightforward modification of CV conditions, and characterization using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), a two-step mechanism involving the initial WS3 deposition and subsequent partial reduction to WS2 was proposed. The WS2/WS3 material was determined to possess composition of WS2.64. Compared to non-electrosynthesized WSx materials, its predominantly basal orientation limited the heterogeneous electron transfer rate toward surface-sensitive redox couples. However, WS2.64 demonstrated excellent HER activity, with the lowest Tafel slope of 43.7 mV dec(-1) to date; this was attributed to different metal-chalcogen binding strengths within WS2.64. Fundamental understanding of the electrosynthesis process is crucial for green syntheses of inexpensive and highly electrocatalytically active materials for sustainable energy production. Albeit, the process may be different for a myriad of nanomaterials, this study can be exploited for its analyses from which the conclusions were made, to empower electrochemical synthesis as the prime fabrication approach for HER electrocatalyst development.