ZnS-stabilized single atoms for highly-efficient water electrolysis

The effective utilization of clean energy and finding alternatives to fossil resources are highly important to ensure the sustainability of human society and are always among the major goals of both chemistry and material science research. Advanced electrochemical devices, such as fuel cells, water electrolysers and metal-air batteries, represent the most promising strategies for clean-energy utilization. In an electrochemical device, the redox reactions are spatially separated by a membrane, allowing direct extraction/transfer of electrons at an electrode-electrolyte interface, which leads to higher intrinsic energy conversion efficiencies, milder process conditions, easy product separation and excellent design features for coupling to renewable energy infrastructure. The performance of such electrochemical processes is fundamentally determined by the physicochemical properties of the electrochemical interfaces, encompassing both the electrocatalyst and the structure of the adjacent electrochemical double layer. Specifically, electrocatalysts play key roles in electrochemical reactions and often limit the performance of entire systems due to their insufficient activity, low durability or high cost. Ideally, the rate, efficiency, and selectivity of the above electrochemical reactions can be substantially improved by developing high-performance electrocatalyst. One of the central tasks for chemists and material scientists is to design and fabricate the high-efficient efficiency but low-cost electrocatalysts systems. The current promising electrochemical reactions mainly focus on the realization of the reversible conversion between chemical and electricity energy, e.g., the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen oxidation reaction (HOR), and hydrogen evolution reaction (HER). Coupling of the above electrochemical reactions provide a solid foundation for various essential electrochemical devices, such as direct hydrogen fuel cells (HOR + ORR); electrolysers (OER + HER); rechargeable zinc (Zn)-air battery (ORR + OER). Therefore, this thesis aims to design and synthesize high-performance electrocatalysts for HER, ORR and OER based on earth-abundant materials with proper hierarchical 2D or 3D nanostructures. Combined with the advanced characterization techniques and density functional theory (DFT) calculations, the relationship between the electrochemical activity and active sites of these earth-abundant electrocatalysts were detailedly explored and confirmed. Furthermore, to emphasize the hierarchical 2D or 3D nanostructures, the actual performance of these electrocatalysts was all evaluated in practical devices including Zn-air battery and proton exchange membrane fuel cell (PEMFC), specifically as follows: (1) The vast majority of the reported HER electrocatalysts performs poorly under alkaline conditions due to the sluggish water dissociation kinetics. In the first work, a hybridization catalyst construction concept is presented to dramatically enhance the alkaline HER activities of catalysts based on 2D transition metal dichalcogenides (TMDs) (MoS2 and WS2). A series of ultrathin 2D-hybrids are synthesized via facile controllable growth of 3d metal (Ni, Co, Fe, Mn) hydroxides on the monolayer 2D-TMD nanosheets. The resultant Ni(OH)2 and Co(OH)2 hybridized ultrathin MoS2 and WS2 nanosheet catalysts exhibit significantly enhanced alkaline HER activity and stability compared to their bare counterparts. The combined theoretical and experimental studies confirm that the formation of the heterostructured boundaries by suitable hybridization of the TMD and 3d metal hydroxides is responsible for the improved alkaline HER activities because of the enhanced water dissociation step and lowers the corresponding kinetic energy barrier by the hybridized 3d metal hydroxides. (2) Nitrogen-coordinated iron atoms on carbon matrix (Fe-N-C) materials are the most active Pt-group-metal-free ORR catalysts but still suffering their low stability and relatively lower activity compared to platinum-based materials. In the second work, Fe and Ni dual sites atomically dispersed in hierarchically ordered macroporous carbon support (Fe-Ni/N-HOMC) was designed and successfully prepared. Isolated atomic Fe- N4 and Ni-N4 active sites were confirmed via various characterizations. The ORR activity and stability of Fe-Ni/N-HOMC in both acid and alkaline electrolyte were much higher than commercial Pt/C and the mono-Fe doping counterpart, which was among the state-of-the-art ORR electrocatalysts. In addition, this 3D ordered interconnected macroporous structure with abundant mesopores and micropores could greatly increase the accessible ORR active site and also enhance the mass transport during the ORR process. When employed as cathodes for PEMFC, we found the excellent ORR activity of Fe-Ni/N-HOMC was completely translated to the cathode in the fuel cell. (3) High-performance bifunctional electrocatalysts with ORR and OER activity is the key to developing efficient rechargeable Zn-air batteries. In the third work, a high-performance bifunctional electrocatalysts for both OER and ORR were synthesized via further hybridizing as-prepared Fe-Ni/N-HOMC with NiFe layer double hydroxides (LDHs). Layered double hydroxides (LDHs) have been reported to be promising OER electrocatalysts with ultrahigh OER performances. The as-synthesized new composites exhibited almost the same ORR activity as Fe-Ni/N-HOMC, revealing that hybridization of NiFe-LDHs would not deteriorate the initial ORR activity. Moreover, the remarkable enhancement of OER activity was observed after the hybridization, which was attributed to the strong coupling of uniformly dispersed small NiFe-LDH nanoparticles with the carbon substrate. The prototype Zn-air battery was assembled using these new composites, which displayed the ultralow voltage gap and long-term stability. (4) Compared with Fe-N-C or Co-N-C based ORR electrocatalysts, the Cu-nitrogen-carbon composites were attracted little attention. However, the natural multicopper oxidases (MCOs) enzymes, such as laccase, can serve as efficient ORR catalyst with almost no overpotential. Inspired by their tris-copper centers in MCO, one novel Cu-nitrogen-carbon composite (Cu SAs/N-CS) with atomic Cu coordination sites were synthesized via the pyrolysis of the Cu-involved metal-organic-framework. The copper contents in Cu SAs/N-CS reaches as high as 3.17 wt.%, and the average distances of adjacent copper sites was around only 3.1 Å. Due to the synergetic effect of abundant single atomic copper active sites with closer distance and ultrathin carbon nanosheet structure, Cu SAs/N-CS exhibited superior ORR activity exceeding commercial Pt/C catalyst, methanol tolerance, and long-term stability in both alkaline and neutral electrolyte. In summary, four kinds of new composites were successfully designed and prepared as high-performance electrocatalysts for HER, ORR and OER. Multi-dimensional heterostructures, atomic metal coordination sites and 3D hierarchically porous structure were designed and observed, which contributed greatly to improve activities of these composites. This thesis suggests several new viewpoints in the design of electrocatalysts based on earth-abundant materials: (i) offering new strategies for the preparation of novel 2D and 3D heterostructures as electrocatalysts; (ii) expanding methods for the synthesis of atomic metal coordination sites and evaluating their activities for ORR; (iii) evaluating the practical performances of achieved electrocatalysts in proton exchange membrane fuel cell and Zn-air battery; (iv) attempting to explain reaction mechanisms of some electrocatalysts by DFT calculation.

Three-dimensional hierarchically porous iridium oxide-nitrogen doped carbon hybrid: An efficient bifunctional catalyst for oxygen evolution and hydrogen evolution reaction in acid

Highly disordered cobalt oxide nanostructure induced by sulfur incorporation for efficient overall water splitting

The growing demand to shift energy production to sustainable sources poses some serious challenges. One of the biggest are the intermittent energy (wind and sun). To mitigate this issue, there is a critical need for large energy storage technologies, to store excess energy during the peak hours and carry it over for the rest of the day, weeks, and seasons. The best way to do so is through the Hydrogen Economy. At the heart of this scheme is green hydrogen production using water electrolysis.There are two distinct chemical reactions that take place in water electrolyzers: the cathodic hydrogen evolution reaction (HER), and the anodic oxygen evolution reaction (OER). Both reaction require catalysts to execute at high rates, and while the HER is considered to be relatively facile and takes place at low overpotentials, the OER requires relatively high overpotentials and high loadings of precious metal catalysts. It is considered the bottleneck reaction. The OER is a four electrons oxidation reaction per generated O2 molecule and proceeds in four distinct reaction steps. This leads to a very sluggish reaction kinetics and high overpotentials to reach viable current densities.In recent years, more and more non-precious metal OER catalyst have been developed. Most notably is the family of mixed nickel-iron oxyhydroxides (NiFeOOH), which are relatively cheap, selective and efficient catalysts in alkaline media, and their performance has been increased by optimizing the Ni:Fe ratio. One challenge that still remains is to increase the NiFeOOH surface area, and by that the electrochemically active site density (EASD).In this regard, one class of materials that has been attracting the attention of materials’ scientists in recent years are aerogels. Aerogels can be made from many different materials, such as silicates, carbons, metal organic materials, bio-inspired molecules, metals, and metal oxides. They consist of distinct units which form a porous 3D covalent framework (COF). Because of their diversity, aerogels have many different applications, e.g. as insulators, sensors, or catalysts.In this talk we will report the synthesis of NixFeyMzOOH aerogels, with a modified easy synthetic method via an epoxide route. These aerogels show much higher utilization of the material and overall increase in mass activity when catalyzing the OER when compared to other NiFeOOH derived materials. They were tested for their OER electrocatalytic activity and to the best of our knowledge these are the first aerogel materials that propagate OER themselves, rather than being used merely as support material for OER catalysts. The catalytic activity depends largely on the Ni:Fe ratio and not the surface area, which can lead to mass transport limitations when too high, showing an optimum for the ratio and the surface area.

Nanostructured Ni films were synthesized from two distinct baths and were assessed as electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in 1 M KOH. Herein, Ni was electrodeposited from two separate solvents, the aqueous acetate buffer and ethaline solvent as a kind of deep eutectic solvents (DESs), and both the deposited films were investigated as electrocatalysts for HER and OER. The electrodeposition parameters such as pH and deposition potential were studied. The electrodeposition process was performed using chronoamperometry technique and Ni deposits were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Fabricated Ni@PGE deposit from ethaline only requires an overpotential of − 154 mV and 350 mV to achieve a current density of 10 mA cm−2 for HER and OER, respectively. While, Ni@PGE from acetate requires an overpotential of − 164 mV and 400 mV to produce the current density of 10 mA cm−2 for HER and OER.Graphical abstract

Environmentally safe and efficient production of renewable fuels are of great demand to meet the current global energy crisis and global warming. As a fuel, hydrogen has been considered as the most suitable source of clean and sustainable energy, which indeed promote the importance of electrochemical water splitting [1-3]. Efficient and stable electrocatalyst for both hydrogen and oxygen evolution reactions are inevitable for the commercialization of water electrolyzer. Comparatively, the oxygen evolution reaction (OER) is more sluggish than the hydrogen evolution reaction (HER), which in turn affect the overall efficiency of the water electrolysis [4]. The non-noble metal oxides-based catalysts in alkaline solution have been shown promising OER activity [5] however, the high alkaline condition and high oxidation potential affects the stability of the metal oxides.Bimetallic oxide of NiFe system has been found as an efficient catalyst for OER [6]. Various methods were demonstrated for the synthesis of NiFe catalyst and studied their OER activity. In the present work, we made NiFe catalyst through electrochemical deposition method. We deposited NiFe over Ni foam and modified Ni foam through the constant potential method and voltammetric method. The OER activity and stability of these catalysts were measured in alkaline solution. Among the various catalysts synthesized, the NiFe deposited over modified Ni foam through constant potential method has shown better OER activity, attained 10 mA/cm2 at 1.45 V (vs RHE) but after electrolysis the potential increased to 1.47 V, indicating the poor catalytic stability. Whereas the NiFe catalyst prepared through the voltammetric method has attained 10 mA/cm2 at 1.46 V and remained unchanged after electrolysis, indicating better catalytic activity and stability (Figure 1). Figure 1. Linear sweep voltammogram measured before and after electrolysis in 1 M KOH of electrodeposited NiFe catalyst prepared through (a) constant potential method and (b) voltammetric method. Acknowledgement This presentation is based on results obtained from a project commissioned by the New Energy and Industrial Technology Development Organization (NEDO) References. 1 N. T. Suen, S. F. Hung, Q. Quan, N. Zhang, Y. J. Xu and H. M. Chen, Chem. Soc. Rev. 46, 337–365 (2017).2 M. I. Jamesh and X. Sun, J. Power Sources. 400, 31–68 (2018).3 S. Jung, C. C. L. McCrory, I. M. Ferrer, J. C. Peters and T. F. Jaramillo, J. Mater. Chem. A. 4, 3068–3076 (2016).4 M. Tahir, L. Pan, F. Idrees, X. Zhang, L. Wang, J. J. Zou and Z. L. Wang, Nano Energy. 37, 136–157 (2017).5 F. Song, L. Bai, A. Moysiadou, S. Lee, C. Hu, L. Liardet and X. Hu, J. Am. Chem. Soc. 140, 7748–7759 (2018).6 M. Gong and H. Dai, Nano Res. 8, 23–39 (2014). Figure 1

Metal-organic framework derived Fe-Co-CN/reduced graphene oxide for efficient HER and OER

Amorphous materials are used in multitude of catalytic applications, including electrocatalytic water-splitting. Identification and investigation of active sites in amorphous catalysts are rarely reported, mainly owing to the complexity of the systems. Herein, we report an amorphous bifunctional Co–W–B electrocatalyst for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The optimized Co–W–B catalyst showed promising overpotential values of 97 mV (HER) and 292 mV (OER), respectively, to achieve 10 mA cm–2 in 1 M KOH, with good stability. The promoting effect of W in Co–B was investigated experimentally, while computational tools were used to identify all the possible catalytic sites in an amorphous Co–W–B model and classify the most preferred sites for HER and OER. The presence of multi-catalytic sites with specific selectivity toward HER and OER was observed, which explained the bifunctional activity of Co–W–B. This study will foster better understanding of the origin of catalytic activity in similar amorphous systems.

Exploring the electrocatalytic performance of silicene and single atom doped silicene for HER, OER and ORR activity using density functional theory

With the ever-increasing global energy demand, the development of renewable energy technologies is crucial to fulfilling the energy requirements in a sustainable manner. Hydrogen has emerged as a promising energy carrier that can provide emission-free energy for end-use applications. Among the several pathways of hydrogen production, green hydrogen production by electrochemical water splitting using renewable energy is an emission-free, efficient energy conversion technology. Hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) occur at the cathode and anode, respectively. Water splits into hydrogen and oxygen at the thermodynamic potential of 1.23 V; however, overpotentials are observed experimentally due to the kinetic barriers, which increases the required potential for water splitting. Catalysts lower the required overpotentials, making the process more energy efficient. A significant issue with large-scale hydrogen production by water electrolysis is that the state-of-the-art catalysts for HER and OER are based on precious metals and thus are limited by their high cost and net production capacity. Thus, there is a lot of interest in developing noble-metal-free electrocatalysts based on earth-abundant materials that can efficiently lower the associated overpotentials for the two halves of the water-splitting reaction.A large number of late transition metal-based ternary complex oxides have been investigated for their catalytic performance towards water splitting, where fine-tuning of the properties using compositional engineering has been demonstrated. Binary sulfides, selenides, and phosphides have also been explored as candidates for water splitting. Recently, early transition metal-based ternary complex materials have emerged as a novel class of electronic materials with applications in infrared optics and photovoltaics. However, similar chalcogenide compositions based on late-transition metals remain underexplored.We have synthesized a series of LaMS3 (M = Mn, Fe, Co, Ni) materials by high-temperature sulfurization of corresponding ternary oxides, using carbon disulfide (CS2) as the sulfurizing agent. The LaMS3 compounds are isostructural and crystallize in the hexagonal P 63 space group. When tested for catalytic performance towards water splitting, they demonstrate good activity towards both HER and OER, with LaNiS3 (LNS) being the most active material, and a periodic activity trend is observed, where LNS > LCS >> LFS > LMS. For OER in alkaline electrolyte, LNS demonstrates an overpotential of 373 mV at 10mA/cm2 in 1.0 M KOH, and a low Tafel slope value of 48 mV/dec. The long-term stability was tested using chronopotentiometry measurements, and LNS shows excellent stability, with negligible change in overpotential over several hours. For HER testing in both acidic and alkaline electrolyte, LNS again seems to be the most active material, and a periodic activity trend in the HER activity of LaMS3 materials, similar to the OER activity trend, is observed. LNS has an overpotential of 340 mV in 0.5 M H2SO4 and 362 mV in 1.0 M KOH, indicating that LNS is catalytically active over a wide range of pH levels. LNS also shows excellent long-term stability towards HER in both acidic and alkaline electrolyte. We also compared the activity of LaMS3 materials with their ternary oxide counterparts from which they were synthesized, and the LaMS3 compounds outperform the LaMO3 compounds, demonstrating lower overpotentials and Tafel slope values for both HER and OER, with the HER overpotentials for LaMO3 materials being > 250 mV higher compared to LaMS3, for both acidic and alkaline electrolytes. The catalytic activity of the LaMS3 series towards both HER and OER, along with their excellent long-term stability, makes these materials promising candidates for bifunctional electrocatalysts for water splitting while offering a wide compositional range that allows for further optimization of their performance.

Nitrogen-doped porous carbon encapsulated nickel iron alloy nanoparticles, one-step conversion synthesis for application as bifunctional catalyst for water electrolysis

N-doped defect-rich porous carbon nanosheets framework from renewable biomass as efficient metal-free bifunctional electrocatalysts for HER and OER application

Polymorphic ternary metal chalcogenide solid solution nanopowder as electrocatalyst for hydrogen and oxygen evolution reaction

Multidimensional regulation of Ni3S2@Co(OH)2 catalyst with high performance for wind energy electrolytic water

Keplerate polyoxomolybdate nanoball mediated controllable preparation of metal-doped molybdenum disulfide for electrocatalytic hydrogen evolution in acidic and alkaline media