Synthesis of g-C3N4/TiO2-based photocatalysts for hydrogen production from organic substrates.

MoS2 is a promising candidate for hydrogen evolution reaction (HER), while its active sites are mainly distributed on the edge sites rather than the basal plane sites. Herein, a strategy to overcome the inertness of the MoS2 basal surface and achieve high HER activity by combining single-boron catalyst and compressive strain was reported through density functional theory (DFT) computations. The ab initio molecular dynamics (AIMD) simulation on B@MoS2 suggests high thermodynamic and kinetic stability. We found that the rather strong adsorption of hydrogen by B@MoS2 can be alleviated by stress engineering. The optimal stress of −7% can achieve a nearly zero value of ΔGH (~ −0.084 eV), which is close to that of the ideal Pt-SACs for HER. The novel HER activity is attributed to (i) the B-doping brings the active site to the basal plane of MoS2 and reduces the band-gap, thereby increasing the conductivity; (ii) the compressive stress regulates the number of charge transfer between (H)-(B)-(MoS2), weakening the adsorption energy of hydrogen on B@MoS2. Moreover, we constructed a SiN/B@MoS2 heterojunction, which introduces an 8.6% compressive stress for B@MoS2 and yields an ideal ΔGH. This work provides an effective means to achieve high intrinsic HER activity for MoS2.

Hydrogen (H2) is a promising energy source to replace fossil fuels and the key to solving current energy and environment problems. Hydrogen production from water splitting via photocatalysis or electrolysis is considered to be an economical and environmentally friendly approach to convert clean energy into chemical fuels. The main difficulty of water splitting is the lack of low-cost, stable and efficient catalysts. The works presented in this thesis are focused on the development of highly efficient noble metal free catalysts for both hydrogen evolution and oxygen evolution reactions via photocatalytic and electrocatalytic water splitting. In these multi-step processes, more than one component are generally required to accomplish light absorption and charge separation (for photocatalytic reaction), charge carrier transportation and surface redox reaction. The overall efficiency of the process is strongly affected by the interplay among the components as well as the interfacial properties. Therefore, the overall aim of this thesis works is to assemble appropriate functional components with engineered interfaces to achieve remarkably enhanced photocatalytic and electrocatalytic performances for water splitting. In the first part of the research works, MoP particulates were synthesized and dispersed into nanosized particles using probe sonicator. The MoP nanoparticle as a co-catalyst exhibits 4 times of improved photocatalytic hydrogen evolution reaction (HER) activity compared to the bulk form due to the small particle size with increased surface area and better integration with the semiconductor light absorber, CdS quantum dots (QDs). The nanosized dimension of CdS QDs facilitate the charge migration from bulk to surface where holes are consumed by lactic acid. More importantly, the good dispersion of CdS QDs in solution allows them to be trapped in the cluster of MoP nanoparticles. An intimate interface between CdS QDs and MoP is thus formed, which is favorable for the efficient charge transfer from CdS QDs to MoP. Besides, the metallic property and good HER activity of MoP lead to efficient and stable H2 evolution. Next, to further reduce the particle size of metal phosphides and improve the interface between light absorber and co-catalysts, metal oxide (ZnO) was introduced as a low-cost metal oxide support to disperse and stabilize CoP on its surface. The interface between metal oxides and CdS QDs is formed via electrostatic interactions since ZnO is positively charged whereas CdS QDs is negatively charged. Besides, the band structure alignment between ZnO and CdS QDs facilitate the charge transfer from CdS QDs to ZnO, which was further transferred to CoP. The excellent HER activity of CoP and the engineered interface result in the highly efficient and stable H2 production under visible light irradiation. Apparent quantum efficiency of this system can reach as high as 66% at 420 nm and no activity loss is observed for this system after 144 h photocatalytic reaction. The third part of the research work is focused on the development of a noble metal free HER electrocatalyst that has an activity close to that of Pt. CoNA/PDA (NA: Nitrilotriacetic acid; PDA: Polydopamine) core/shell nanowires were first synthesized by coating PDA on the surface of CoNA nanowires (NWs). N, P co-doped carbon nanotube is obtained through phosphidation of CoNA/PDA NWs with subsequent pyrolysis in N2 atmosphere. CoNA NWs decomposed to Co nanoparticles wrapped by several graphene layers. CoP is formed at the cobalt/carbon interface. After activation, the wrapped nanoparticles become accessible and less stable Co nanoparticles are removed by acidic solution. The CoP nanoparticles stabilized by NCoP bonding are exposed which exhibit a high HER activity and stability. Lastly, research efforts of this thesis work were also spent to tackle the other half of the water splitting reaction, oxygen evolution reaction (OER), since the sluggish kinetics of OER is the bottleneck of the overall performance of water splitting. In this part of the work, a promising OER catalyst, Ni-Fe layered double hydroxide (LDH) was chosen and its intrinsic high OER activity was harnessed by blending ultra-fine NiFe-LDH nanocrystals with conductive carbon. The NiFe-LDH/C hybrid was fabricated by a novel one-pot solvothermal method using molecule precursors of metal cations and organic ligand. The resultant NiFe-LDH/C nanosheet consists of poorly crystalized NiFe-LDH (< 5 nm) interconnected with N doped carbon nanodomains. The in situ formation of both components leads to a self-confined growth and fine blending of NiFe-LDH nanocrystals and carbon domains. Such a unique structure results in improved electrical conductivity, increased active sites and enhanced electrochemical active surface area. In addition, the strong interaction between metal centers and carbon leads to the local electronic structure modification of metal centers. These factors contribute together to the development of a highly efficient and stable NiFe-LDH based OER catalyst. In summary, the research efforts in this thesis were spent on designing efficient and noble metal free photocatalysts and electrocatalysts for water splitting reactions. In particular, engineering suitable interfaces is a key focus. Detailed materials characterization and structural analyses were carried out to understand the key factors contributing to the high performances of the catalysts. Through such efforts, several promising transition-metal based catalysts have been developed with high efficiency for HER and OER reactions. It is believed that the findings from this work would contribute to the advancement of the energy research field and the development of practical catalysts for water splitting utilizing solar energy directly or electricity from clean energy.

A study on pulse plating amorphous Ni–Mo alloy coating used as HER cathode in alkaline medium

Polyoxothiometalate ions (ThioPOM) are active hydrogen-evolution reaction (HER) catalysts based on modular assembly built from electrophilic clusters {MoSx } and vacant polyoxotungstates. Herein, the dumbbell-like anion [{(PW11 O39 )Mo3 S4 (H2 O)3 (OH)}2 ]8- exhibits very high light-driven HER activity, while the active cores {Mo3 S4 } do not contain any exposed disulfido ligands, which were suspected to be the origin of the HER activity. Moreover, in the catalyst architecture, the two central {Mo3 S4 } cores are sandwiched by two {PW11 O39 }7- subunits that act as oxidant-resistant protecting groups and behave as electron-collecting units. A detailed photophysical study was carried out confirming the reductive quenching mechanism of the photosensitizer [Ir(ppy)2 (dtbbpy)]+ by the sacrificial donor triethanolamine (TEOA) and highlighting the very high rate constant of the electron transfer from the reduced photosensitizer to the ThioPOM catalyst. Such results provide new insights into the field of molecular catalytic systems able to promote high HER activity.

Nickel phosphides (NixPy) are a class of materials that are made out of earth abundant elements and have shown relatively high hydrogen evolution reaction (HER) activity. Here, we perform first-principles density functional theory (DFT) calculations to systematically investigate the stoichiometric and nonstoichiometric surface reconstructions of six different NixPy, i.e., Ni3P, Ni12P5, Ni2P, Ni5P4, NiP2, and NiP3, under electrochemical conditions and to illustrate the implications of such reconstructions for the catalytic activity toward HER. Our results can explain a broad range of experimental observations on the HER activity of NixPy in a unified framework. For the majority of cases, our protocol can closely reproduce the experimentally measured overpotential trends in the literature, which validates its usefulness in generating physical insight into the surface phenomena responsible for HER activity. We find that, among the NixPy studied here, Ni3P and Ni5P4 are the most active catalysts toward HER in acid, whereas Ni5P4 performs the best compared to other NixPy in base, in agreement with previous experimental reports. We show that P-vacancy formation in base renders the Ni-rich NixPy (Ni3P, Ni12P5, Ni2P, and Ni5P4) worse performers in base when compared to their activity in acid and hence propose that introducing nonmetals, which are less prone to dissolution, can improve their catalytic performance. In terms of active site design, we find Ni3 hollow sites bind H too strongly and surface P sites with P–Ni bonds bind H too weakly. On the other hand, we identify that surface P sites with P–P bonds offer the best catalytic performances, and therefore, we predict that active site engineering to maximize the abundance of such surface motifs can further improve the HER activity. Moreover, we unravel the nature of H binding across the material class for different binding motifs via electronic structure theory analysis. The chemical insight we provide in this work can help rationalize the search for materials composed of inexpensive earth abundant elements that provide improved HER catalytic activity.

Molybdenum carbides show great potential to replace platinum for electrocatalytic hydrogen evolution reaction (HER) to resolve the problem of hydrogen production, due to their high reserves, stability, low cost, and structural diversity. However, the effect of atomic configurations of different surfaces on HER is still lacking theoretical insights. In this work, the HER activity on 29 surfaces of seven phases is systematically explored by density functional theory, taking into account water effect. The exchange current for each surface is also given. Totally, there are nine surfaces which own high exchange current (>0.1 mA/cm2), especially the hydroxylated (014)-C and (010) of TiP-MoC, (110) of β-Mo2C, and (100)-C of α-Mo2C (1.410, 0.835, 0.687, and 0.464 mA/cm2, respectively). Combining with the stabilities of the surfaces for each phase, the phases with high HER activity could be also screened out. The electronic properties, including electron transfer to adsorbed hydrogen and the shift of the electronic states coupled by oxygen and adsorbed hydrogen orbitals, are applied to uncover the termination of surface and water effect on HER. Our results are expected to contribute to the understanding of the HER on different surfaces of molybdenum carbides and give some evidence for control synthesis of high HER activity surfaces.

Electrocatalytic properties of scandium metallofullerenes for the hydrogen evolution reaction

Hydrogen (H2) production have received considerable attention resulting from the urgent need of clean and renewable energy. Water splitting is a typical and eco-friendly strategy for H2 production method through efficient acidic electrolyzers with expensive platinum group metal catalysts (Pt). Therefore, researchers have developed alternative, cheap and robust catalysts made from earth-abundant elements. Very recently, nickel phosphides (NixPy) have been proven to be promising catalysts with high activity and durability in strong acidic media (0.5M or 1M H2SO4) for hydrogen evolution reaction (HER). The crystalline phase of NixPy is an important factor for determining the activity for HER (better HER activity as higher P contents in NixPy). Herein, highly active and durable NixPy catalysts for HER are prepared using modified thermal decomposition method. In this process, nickel oxide (NiO) powder reduced to NixPy by phosphine gas (PH3) produced from sodium hypophosphite (NaH2PO2). Unlike conventional method, NiO is phosphidized from bi-located P source (NaH2PO2). One is placed upstream of furnace (traditional method) and the other is mechanically mixed with NiO powder. Based on X-ray diffraction analysis, synthesized NixPy catalysts had multi-phase crystalline structure (Ni2P, Ni5P4 and NiP2) and the NixPy with higher P content exhibited the smaller crystallite size. (Ni2P < 50 nm, Ni5P4 ~ 10 nm, NiP2 ~ 2nm). Consequently, Ni5P4 and NiP2 nanoparticles with higher HER activity are supported on micro-scaled Ni2P with better conductivity (self-supported NixPy nanoparticles), which result in improvement of electrochemical properties for HER. The NixPy catalysts showed exceptionally higher HER activity and durability in acidic media than single-phased Ni2P or Ni5P4 and this performance of the NixPy is comparable with that of Pt. Therefore, modified phosphidation method and self-supported NixPy catalyst provide new instruction for designing excellent HER catalysts.

Pd coated MoS2 nanoflowers for highly efficient hydrogen evolution reaction under irradiation

Anion electrolyte membrane water electrolysis (AEM WE) using non-precious metal catalyst is one of the most prospective systems for pure hydrogen generation. The Ni based oxide shows relatively lower overpotential of oxygen evolution reaction (OER) by adding the transition metals of Co, Fe and Mn, and approaches that of precious metal of Ir based oxide. Our previous study reported that the Ni-Co based catalyst was highest OER activity at operating temperature. The crystallized Ni-Co metal-based core particles with amorphous Ni oxyhydrates of top surface (shell) was confirmed to be preferable to obtain the higher OER activity by rotating disk electrode method, transmission electron microscopy and DFT calculation. Moreover, the fused aggregated network microstructure of Ni-Co based catalyst assisted to show a metallically electronic conductivity. In this study, we evaluated both OER and hydrogen evolution reaction (HER) activity of Ni based catalysts to confirm the prospective non-precious metal catalysts for AEM WE.Each Ni based catalysts with additive of transition metals were synthesized by the flame oxide-synthesis method.1 The OER and HER activities of these catalysts were evaluated in 1 M KOH at 20 to 80 oC by use of the RDE. The Ni-Co based catalyst was highest OER activity in these Ni based catalysts obtained above. The amorphous top surface layer was correlated with a negative shift in the oxyhydroxide formation peak potential from the results of DFT calculations. 2 The HER activity of the Ni-Fe based catalyst also showed the higher OER activity. Especially, the Ni-Fe based catalyst had highest HER activity in these Ni cased catalysts above. The HER activity enhanced by the construction of well crystallized top surface in comparison with its amorphous or poorly crystalline ones. DFT calculation indicated that the defective or disordered surface was not as active for the HER. These results will provide a strategy of Ni based catalysts optimization for AEM. Acknowledgement This work was partially supported by funds for the JSPS KAKENHI (20H02839), and the project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References Kakinuma, M. Uchida, T. Kamino, H. Uchida, and M. Watanabe, Electrochim. Acta, 56, 2881 (2011).Shi, T. Tano, D. A. Tryk, A. Iiyama, M. Uchida, and K. Kakinuma, ACS Catal., 11, 5222 (2021). Figure 1

Exploring low-cost, efficient, and stable nonprecious alternatives for Pt-based catalysts is of significance in the hydrogen evolution reaction (HER) in acidic environments. Previous experiments have found that 3d transition metals Fe, Co, and Ni incorporated with inert carbon templates or carbon–nitrogen materials exhibit long-term durability and high HER activity in acidic electrolytes. To clarify the underlying mechanism determining the HER activity, here we report a theoretical investigation of the HER on a series of defective carbon nanotubes (CNTs), doped with atomic Co (CoCNT(n,n), n = 3, 5, 7, and 9) and codoped with Co and double N (CoN2CNT(5,5)), based on the first-principle density functional calculations. Our calculations indicate that the HER on these Co- and Co, N-(co)doped CNTs occurs via the Volmer–Heyrovsky mechanism, and the primary active sites are the C atoms adjacent to the metal center. The enhancement of the HER activity is due to uplifting of the p-band center (εp) of the active C atoms induced by using a CNT with appropriate curvature, Co doping, and Co and N codoping. The HER activity of CoCNT(n,n)s follows a volcano dependence with surface curvature, showing nearly six orders of magnitude difference in exchange currents, peaked at CoCNT(5,5), with the activity comparable with Pt-catalysts. Doped with double N atoms in CoCNT(5,5), the exchange current could be further substantially enhanced (by 30 times), even one order of magnitude higher than that of Pt(111). The fact that CoN2CNT(5,5) has an εp (−4.16 eV) very close to the optimum value for the maximum exchange current (−4.14 eV) justifies the advance in improving the HER activity of CNTs.

Evaluation of hydrogen evolution reaction activity of molybdenum nitride thin films on their nitrogen content

Screened (Co, Rh, Ir)–N4 graphene single atom catalysts for hydrogen production via tunable surrounding coordination shell: Computation and machine learning

Asymmetrical radial strain energy strategy of M–N–SWCNT single atom catalysts for highly efficient hydrogen evolution: A high-throughput DFT study

Water electrolysis shows great promise for the low-cost mass production of high-purity hydrogen. The relatively high dissociation energy of water, however, often results in rather sluggish kinetics of the hydrogen evolution reaction (HER) in alkaline conditions, even for the case of state-of-the-art Pt-based electrocatalysts. Here, we show the high efficiency of the hybrids of PtRu nanoclusters (NCs) and black phosphorus (BP) nanosheets in HER. Our PtRu NCs/BP electrocatalysts demonstrate a HER activity of 88.5 mA cm–2 at −70 mV in 1 M KOH, which is higher than that of commercial Pt/C by 1 order of magnitude. The observed extraordinarily high HER activity of the PtRu NCs/BP hybrids is interpreted in the framework of density functional theory. Theoretical modeling indicates that the electronic interaction between BP and PtRu NCs speeds up the dissociation of water and optimizes the adsorption strength for H* species, giving rise to the remarkably high HER activity of the PtRu NCs/BP hybrids.