One of the most promising future sources of alternative energy involves water-splitting photoelectrochemical cells (PECs) – a technology that could potentially convert sunlight and water directly to a clean, environmentally friendly, and cheap hydrogen fuel. Practical PEC- mediated hydrogen production requires robust and highly efficient semiconductors, which should possess good light-harvesting properties, a suitable energy band position, stability in harsh conditions, and a low price. Despite significant progress in this field, new semiconductors that entail such stringent requirements are still sought after.

Over the past few years, graphitic carbon nitride (CN) has attracted widespread attention due to its outstanding electronic properties, which have been exploited in various applications, including photo- and electro-catalysis, heterogeneous catalysis, CO2 reduction, water splitting, light-emitting diodes, and PV cells. CN comprises only carbon and nitrogen, and it can be synthesized by several routes. Its unique and tunable optical, chemical, and catalytic properties, alongside its low price and remarkably high stability to oxidation (up to 500 °C), make it a very attractive material for photoelectrochemical applications. However, only a few reports regarded the utilization of CN in PECs due to the difficulty in acquiring a homogenous CN layer on a conductive substrate and to our lack of basic understanding of the intrinsic layer properties of CN.

In this talk, I will introduce new approaches to grow CN layers with altered properties on conductive substrates for photoelectrochemical application^1-4. The growth mechanism and their chemical, photophysical, electronic, and charge transfer properties will be discussed.

Electrochemical energy storage technologies have been brought into the spotlight as they provide elegant and efficient approaches to store, transport, and deliver energy harvested from sustainable energy resources.^{1, 2} Typically, supercapacitors and batteries differ in electrochemical mechanisms, hence featuring almost opposite energy and power characteristics. However, the demand for power and energy supply is equally imperative in actual use and is keen to expand in the future. Thus it is highly desirable to design new electrode materials or rationally re-construct the recognized electrode materials for energy storage devices to mitigate the power-energy tradeoff.

2D layered materials are a class of materials with strong atom bonding in the basal plane and weak van der Waals (vdW) interaction between layers. These materials are equipped with versatile physical, chemical, electronic properties, as well as broad structural diversity. Importantly, the weak vdW interaction between the stacked layers enables layered materials with diverse possibilities for rational structure engineering, such as exfoliation into 2D nanoflakes, interlayer expansion with guest molecules, and hybrid structure construction. These structure engineering strategies are highly desired for layered materials to tailor their intrinsic properties (e.g., electronic structure, conductivity, and redox capability) and electrochemical behaviours (e.g., ion desolvation energy, solid-state ion diffusion kinetics, charge-storage mechanism) for diverse energy storage devices.³

Here, we will present our recent efforts in exploring 2D layered organic/inorganic materials for high-power energy storage applications.^{3, 4} We will show 2D redox-active carbon-rich frameworks as promising electrode alternatives for high-power energy storage devices by demonstrating 2D polyarylimide covalent organic framework (COF) as the first COF anode for Zn-ion aqueous batteries⁵ and dual-redox-site 2D conjugated metal-organic framework as a high-capacitance and wide-potential-window pseudocapacitive electrode. Moreover, we have demonstrated several interlayer engineering strategies for inorganic 2D layered materials to regulate the ion transport behaviors and boost the power-energy performance of the assembled energy storage devices.^{6, 7}

Two-dimensional materials are currently in the forefront of material research. The graphene and other 2D materials play key role in electrocatalysis as well as energy storage and related field. In this presentation will be discussed the influence of impurities present in graphene and other 2D nanomaterials nanomaterials on electrocatalysis. The synthesis procedures typically introduce various impurities of metallic and non-metallic ions. Their presence can dominate electrocatalytic properties like for hydrogen evolution reaction and oxygen reduction reaction. The other factor influencing electrocatalytic activity like structure, particle size and edge vs basal planes of two-dimensional materials will be discussed. The experiments using single crystal electrodes shows dominant effect of edges for the electrocatalytic properties.

Metallic transition metal dichalcogenides (MTMDCs) have manifested many intriguing properties in their bulk states, such as magnetism, charge density wave, and superconductivity. To propel the related applications, our group have also realized the direct syntheses of high-quality VX₂ and TaX₂ related MTMDCs materials on both conducting Au foils and insulating substrates.^[1-2]. Particularly, we have obtained thickness-tunable 2H-TaS₂ flakes and centimeter-size ultrathin films on an electrode material of Au foils. Extra high hydrogen evolution reaction (HER) efficiency was demonstrated on the CVD-grown 2H-TaS₂/Au foils.^[3] The first synthesis of vertically oriented 1T-TaS₂ nanosheets were also reported on nanoporous gold (NPG) substrates. For scalable production, a universal synthetic route was also developed for achieving high-quality 2D MTMDC (e.g., TaS₂, V₅S₈, and NbS₂) nanosheets using microcrystalline NaCl crystals as templates via a facile CVD route. ^[4] This synthetic route is perfectly compatible with a facile water dissolution−filtration process for obtaining high-purity MTMDC nanosheets powders towards high-performance energy-related applications.

It is well-known that, the as-prepared 2D MTMDCs are mostly environmentally unstable, and finding more stable 2D TMDCs has become a very critical issue. Herein, thickness-tunable and large-domain (∼1.5 mm) 1T-NiTe₂ were also achieved on the mica substrate. ^[5] Significantly, this 2D material presents ultrahigh conductivity (∼1.15 × 10⁶ S m−1) and high catalytic activity in pH-universal HER, and more interestingly robust environmental stability. We further uncover that, the CVD-derived 1T-VTe₂ nanosheets can serve as a high-performance electrode material thanks to its ultrahigh conductivity, and act as electro-catalysts with excellent electrocatalytic activity in HER. All these work should provide brand new insights into the direct syntheses and property investigations of nano-thick metallic 2D TMDCs crystals.

References

[1] Yanfeng Zhang*, et al., Chem. Soc. Rev. 44(2015), pp. 2587; Adv. Mater. 28(2016), pp. 10664; Coordin. Chem. Rev. 376(2018), pp. 1.; Nature Commun. 9(2018), pp. 979; ACS Nano 13 (2019), pp. 3649; Nano Lett. 17(2017), pp. 4908; Phys. Rev. B. 96(2017), pp075402.

[2] Yanfeng Zhang*, et al., Adv. Mater. 2017, pp. 1702359; ACS Nano 13 (2019), pp.885.
[3] Yanfeng Zhang*, et al., Nature Commun. 8(2017), pp. 958; Adv. Mater. 2018, pp. 1705916.

[4] Yanfeng Zhang*, et al., J. Am. Chem. Soc. 141(2019), pp 18694.

[5] Yanfeng Zhang*, et al., ACS Nano 15 (2021), pp.1858; ACS Nano 14 (2020), pp.9011.

Covalent functionalization of graphene leads to graphene derivatives with significantly modulated electronic, magnetic and surface properties with respect to pristine graphene. The graphene derivatives can be applied in various application including catalysis. A wide range of various approaches have been developed for covalent graphene functionalization so far. Despite the progress in direct covalent functionalization of graphene, this approach suffers from a low reactivity of graphene. Recently, we developed alternative route toward graphene derivatives based on chemistry of fluorographene (FG). FG is a stoichiometric graphene derivative (having ~C₁F₁ composition), which can be prepared by chemical delamination of graphite fluoride in a large scale. FG undergoes various chemical reactions at rather mild conditions [1], which lead to graphene derivatives. FG is susceptible for reductive defluorination, nucleophilic attack, Grignard [2], Bingel-Hirsch [3], photo Diels-Alder [4] and Sonogashira [5] reactions. The reactions result in homogeneously and densely surface functionalized graphene derivatives. Such materials can be utilized in a broad spectrum of applications. Hydroxyfluorographenes bear room-temperature antiferromagnetic or ferromagnetic ordering based on their composition [6, 7]. Cyanographene, i.e., graphene functionalized by nitrile groups, and graphene acid bearing carboxyl groups are well biocompatible materials suitable for further functionalization [8]. Conjugating graphene acid with redox active centers, e.g., ferrocene, leads to redox active heterogenous catalyst for arene CH insertion [9]. Pd nanoparticles with controllable size can be grown on graphene acid. The prepared nanohybrids were highly active catalysts in the Suzuki–Miyaura cross coupling reaction [10]. Anchoring Cu ions to cyanographene resulted in a mixed valence single-atom catalyst (SAC) very active in oxidative amine coupling reactions [11]. Graphene acid was covalently conjugated with dehydrogenase enzymes to a nano-bio catalyst exhibiting good performance in electrocatalytic reduction of CO₂ [12], also due to conductivity of graphene acid. It is worth noting that graphene acid shows metal free catalysis for alcohol oxidation, posing a new limit in carbocatalysis [13]. These examples demonstrate versatility of graphene derivatives in catalytic application.

ERC Consolidator grant (H2020, ID: 683024) 2D-Chem is gratefully acknowledged.

References

[1]          Zbořil R. et al., Small, 6 (2010) 2885; Dubecký M. et al., J. Phys. Chem. Lett., 6 (2015) 1430; Medveď M. et al., Nanoscale (2018), 4696; Chronopoulos D. et al., Appl. Mat. Today 9 (2017) 60.

[2]          Chronopoulos D. et al., Chem. Mater 29 (2017) 926.

[3]          Bakandritsos A. et al., Adv. Funct. Mater 28 (2018) 1801111.

[4]          Bares H. et al., Carbon 145 (2019) 251.

[5]          Chronopoulos D. et al., ChemComm 55 (2019) 1088.

[6]          Tuček J. et al., Nat. Commun. 8 (2017) 14525.

[7]          Tuček J. et al., ACS Nano 12 (2018), 12847.

[8]          Bakandritsos A. et al., ACS Nano 11 (2017) 2982.

[9]          Mosconi D. et al., Carbon 143 (2019) 318.

[10]        Blanco M. et al., Green Chem. 21 (2019) 5238.

[11]        Bakandritsos A. et al., Adv. Mater. 31 (2019) 1900323.

[12]        Seelajaroen H. et al., ACS Appl. Mater. Interfaces, 12 (2020) 250.

[13]        Blanco M. et al., Chem. Sci. 10 (2019) 9438.

      The development of industrial-scale, reliable, inexpensive production processes of graphene and related two-dimensional materials (GRMs)[1,2] is a key requirement for their widespread use in several application areas,[1-6] providing a balance between ease of fabrication and final product quality. In particular, in the energy sector, the production of GRMs in liquid phase [2,6] represents a simple and cost-effective pathway towards the development of GRMs-based energy devices, presenting huge integration flexibility compared to other production methods.

     In this presentation, I will first briefly introduce the key properties of GRMs. Then, I will present the strategy of BeDimensional in the production of GRMs by wet-jet milling [7] and the Industrial scale up. Afterward, I will provide a brief overview on some key applications of the as-produced GRMs, for anticorrosion coatings and energy conversion and storage devices. [3,8-15]

REFERENCES

[1] F. Bonaccorso, et. al., Adv. Mater. 28, 6136-6166 (2016).

[2] F. Bonaccorso, et al., Materials Today, 15, 564-589, (2012).

[3] F. Bonaccorso, et. al., Nature Photonics 4, 611-622, (2010).

[4] E. Pomerantseva, F. Bonaccorso, et al., Science 366 (6468) eaan8285 (2019).

[5] G. Iannaccone F. Bonaccorso, et al., Nature Nanotech 13, 183, (2018).

[6] A. C. Ferrari, F. Bonaccorso, et al., Nanoscale, 7, 4598-4810 (2015).

[7] A. E. Del Rio Castillo et. al., Mater. Horiz. 5, 890 (2018).

[8] F. Bonaccorso, et. al., Science, 347, 1246501 (2015).

[9] M. Garakani, et al. Energy Storage Materials 34, 1-11 (2020).

[10] S. Bellani, et al. Nano Lett.  18, 7155-7164 (2018).

[11] A. E. Del Rio Castillo, et al., Chem. Mater. 30, 506-516 (2018).

[12] S. Bellani, et al. Nanoscale Horizons 4, 1077 (2019).

[13] S. Bellani, et al. Adv. Funct. Mater. 29, 1807659 (2019).

[14] L. Najafi et al., Advanced Energy Materials 8 (16), 1703212 (2018).

[15] E. Lamanna et al., Joule 4, 865-881 (2020).

Although nanostructures contain tenths of thousands of atoms, the atoms at the interfaces make most of the impact on the functionality, especially regarding the catalytic activity. It is therefore an important mission to control the formation of interfaces at the atomic scale, but that is easier said than done. First, it is hard to characterize and understand what was obtained in the synthesis. Then, relating the specific structures to the macroscopic properties requires overcoming issues of adequate sampling and statistics, reproducibility to the atomic scale features and abundance of similar structures within a synthetic batch. Last, there are confusing cases where we synthesize a certain structure to best of our knowledge and spontaneous rearrangements produce other active particles. Here, I will present a few such cases.

Since the last decade, electrolysis of water to produce green and renewable hydrogen fuel was one of the main interests in clean energy field. While water molecules are decomposed to hydrogen and oxygen, the latter serves as a limiting factor because of its sluggish kinetics and various catalysts that can mend this impediment are known. However, catalytic materials under electrochemical operation are subject to harsh chemical environments since they are located in solution, and as a result mechanical may appear in the material. The big challenge is to understand the correlation between the mechanical characteristics of materials and their catalytic performance. In this research we use theoretical methods in the field of computational materials science in order to explore the catalytic performance of NiOOH, one of the best catalysts existed for oxygen evolution reaction (OER), at different interlayer arrangements. NiOOH is a material with inner-layers and outstanding catalytic performance. The ability to lower the overpotential of NiOOH even further could give exceptional results in increasing the efficiency of the OER.

Transition-metal dichalcogenides (TMDs) have garnered much current interest as inexpensive, earth-abundant alternatives to platinum-group metals for electrocatalysis. However, TMD-based catalysts have yet to demonstrate the levels of activity and selectivity required for practical applications. Here, we employ density functional theory calculations to understand the role of transition-metal dopants in enhancing the activity and selectivity of TMDs towards specific catalytic reactions. Specifically, we present examples of dopant engineering of MoSe₂ for the hydrogen evolution reaction (HER) and MoS₂ for the nitrogen reduction reaction (NRR), and show how dopants can not only favorably modify the electronic structures of these TMDs for the chosen reactions but also act as promoters of highly active defect sites. We also examine the effects of co-adsorbates on the thermodynamics of NRR and show how these may assist the reaction pathways.

The ability of transition metal atoms to reversibly change the oxidation state enables redox energy storage. Catalytic properties of some of the transition metals lead to use of their oxides in catalysis and electrocatalysis. A few oxides that are conductive find applications in transparent conductive layers in solar cells, thus contributing to energy harvesting. However, the majority of oxides have a low conductivity, limiting their applications in electrocatalysis, electrochemical energy storage, harvesting, and conversion. Two-dimensional (2D) carbides and nitrides of transition metals known as MXenes, with a thickness of a nanometer or less, have their surfaces terminated by oxygen or OH, forming compounds like Mo₂CO₂, Ti₃C₂(O,OH)_x, or Nb₄C₃O₂ with the surfaces resembling that of oxides or hydroxides. They are hydrophilic, form stable colloidal solutions in water, and work well with aqueous, ionic liquid or polar organic electrolytes. They chemically behave very much like the corresponding oxides/hydroxides; however, the fundamental difference is that electrons of the transition metal give them metallic conductivity, thus compensating for the key limitation of oxides and eliminating the need for conductive carbon additives. Moreover, the conductivity of titanium carbide MXene with O, OH, and/or F terminated surface is outstanding and can exceed 20,000 S/cm. A combination of high electronic conductivity with hydrophilicity and 2D structure facilitates electronic and ionic transport, allowing extremely fast charge/discharge with the electron transfer. Naturally, this opens new opportunities in energy storage, electrocatalysis, and other fields where a combination or surface redox with electrical conductivity is required. The family of 2D transition metal carbides and nitrides (MXenes) has been expanding rapidly since the discovery of Ti₃C₂T_x (T stands for surface terminations) in 2011 [1]. Approximately 30 different stoichiometric MXenes have been synthesized, and the structures and properties of numerous other MXenes have been predicted using density functional theory (DFT) calculations [2]. Furthermore, the availability of solid solutions on M and X sites, control of surface terminations, and the discovery of in-plane and out-of-plane ordered double-M MXenes (e.g., Mo₂TiC₂T_x) offer a potential for synthesis of dozens if not hundreds of new materials. The versatile chemistry of the MXene family renders their properties (conductivity, work function, surface charge, etc.) tunable for a large variety of catalytic and energy-related applications. Particularly, they are very promising candidates for energy storage [2], but applications in electrocatalysis, transparent conducting layers, biosensors, capacitive water desalination, and other fields are equally exciting [3].