Uncovering Key Factors in Graphene Aerogel-Based Electrocatalysts for Sustainable Hydrogen Production: An Unsupervised Machine Learning Approach

The application of principal component analysis (PCA) as an unsupervised learning method has been used in uncovering correlations among diverse features of aerogel-based electrocatalysts. This analytical approach facilitates a comprehensive exploration of catalytic activity, revealing intricate relationships with various physical and electrochemical properties. The first two principal components (PCs), collectively capturing nearly 70% of the total variance, attested the reliability and efficacy of PCA in unveiling meaningful patterns. This study challenges the conventional understanding that a material's reactivity is solely dictated by the quantity of catalyst loaded. Instead, it unveils a complex perspective, highlighting that reactivity is intricately influenced by the material's overall design and structure. The PCA bi-plot uncovers correlations between pH and Tafel slope, suggesting an interdependence between these variables and providing valuable insights into the complex interactions among physical and electrochemical properties. Tafel slope stands to be positively correlated with PC1 and PC2, showing an evident positive correlation with the pH. These findings showed that the pH can have a positive correlation with the Tafel slope, however, it does not necessarily reflect a direct positive correlation with the overpotential. The impact of pH on current density (j)and Tafel slope underscores the importance of adjusting pH to lower overpotential effectively, enhancing catalytic activity. Surface area (from 30 to 533 m2 g-1) emerges as a key physical property, inclusively inverse correlation with overpotential, indicating its direct role in lowering overpotential and increasing catalytic activity. The introduction of PC3, in conjunction with PC1, enriches the analysis by revealing consistent trends despite a slightly lower variance (60%). This reinforces the robustness of PCA in delineating distinct characteristics of graphene aerogels, affirming their potential implications in diverse electrocatalytic applications. In summary, PCA proves to be a valuable tool for unraveling complex relationships within aerogel-based electrocatalysts, extending insights beyond catalytic sites to emphasize the broader spectrum of material properties. This approach enhances comprehension of dataset intricacies and holds promise for guiding the development of more effective and versatile electrocatalytic materials.

One-pot self-assembled bimetallic sulfide particle cluster-supported three-dimensional graphene aerogel as an efficient electrocatalyst for the oxygen evolution reaction

The preparation of an electrocatalyst for the oxygen evolution reaction (OER) with high catalytic activity, good long-term durability and rapid reaction kinetics through interface engineering is of great significance. Herein, we have developed a bimetallic sulfide particle cluster-supported three-dimensional graphene aerogel (FeNiS@GA), which serves as an efficient electrocatalyst for OER, by a one-step hydrothermal method. Profiting from the synergy of the FeNiS particle cluster with high capacitance and GA with its three-dimensional porous nanostructure, FeNiS@GA shows a high specific surface area, large pore volume, low contact resistance, and decreases the electron and ion transport routes. FeNiS@GA exhibits outstanding OER activity (when the current density is 50 mA cm-2, the overpotential is 341 mV), low Tafel slope (63.87 mV dec-1) and remarkable stability in alkaline solutions, outperforming FeNiS, NiS@GA, FeS@GA and RuO2. Due to its simple synthesis process and excellent electrocatalytic performance, FeNiS@GA shows great potential to replace noble metal-based catalysts in practical applications.

Investigation of mechanical properties and structural integrity of graphene aerogels via molecular dynamics simulations

Graphene aerogel (GA), a 3D carbon-based nanostructure built on 2D graphene sheets, is well known for being the lightest solid material ever synthesized. It also possesses many other exceptional properties, such as high specific surface area and large liquid absorption capacity, thanks to its ultra-high porosity. Computationally, the mechanical properties of GA have been studied by molecular dynamics (MD) simulations, which uncover nanoscale mechanisms beyond experimental observations. However, studies on how GA structures and properties evolve in response to simulation parameter changes, which provide valuable insights to experimentalists, have been lacking. In addition, the differences between the calculated properties via simulations and experimental measurements have rarely been discussed. To address the shortcomings mentioned above, in this study, we systematically study various mechanical properties and the structural integrity of GA as a function of a wide range of simulation parameters. Results show that during the in silico GA preparation, smaller and less spherical inclusions (mimicking the effect of water clusters in experiments) are conducive to strength and stiffness but may lead to brittleness. Additionally, it is revealed that a structurally valid GA in the MD simulation requires the number of bonds per atom to be at least 1.40, otherwise the GA building blocks are not fully interconnected. Finally, our calculation results are compared with experiments to showcase both the power and the limitations of the simulation technique. This work may shed light on the improvement of computational approaches for GA as well as other novel nanomaterials.

Graphene aerogels via hydrothermal gelation of graphene oxide colloids: Fine-tuning of its porous and chemical properties and catalytic applications

Recently, 3D graphene aerogel has garnered a high interest aiming at benefiting of the excellent properties of graphene in devices for energy storage or environmental remediation. Hydrothermal gelation of GO dispersion is a straightforward method that offers many opportunities for tuning its properties and for processing it to devices. By adjusting hydrothermal gelation and drying conditions, it is possible to tune the density (from ~3 mg cm^-3 to ~2 g cm^-3), pore volume, pores size (micro to macropores), pore distribution, surface chemical polarity (hydrophobic or hydrophilic), and electrical conductivity (from ~0.5 S m^-1 to S cm^-1). Besides other well explored applications in energy storage or environmental remediation, graphene aerogels have excellent prospects as support for catalysis since they combine the advantages of graphene sheets (high surface area, high electrical conductivity, surface chemistry tunability, high adsorption capacity…) while circumventing their drawbacks such as difficult separation from reaction media or tendency to stacking. Compared to other 3D porous carbon materials used as catalyst support, graphene aerogels have unique porous structure. The pore walls are the thinnest to be expected for a carbon material (the thickness of monolayer graphene is 0.335 nm), hence leading to the highest exposed surface area per weight and even per volume for compacted aerogels. This has the potential to maximize the catalytic site density per reactor mass and volume while minimizing the pressure drop for continuous reactions in flow. Herein, different strategies to control the porous texture, chemical and physical properties are revised along with their processability and scalability for the implementation into different morphologies and devices. Finally, the application of graphene aerogels in the catalysis field are overviewed, giving a perspective about future directions needing further research.

A ternary hybrid of Zn-doped MoS2-RGO for highly effective electrocatalytic hydrogen evolution

Modification of MoS₂-based catalysts is effective in solving the overdependence of hydrogen evolution reactions (HERs) on noble metal catalysts. In this work, a Zn-doped molybdenum disulfide-reduced graphene oxide (Zn-MoS₂-RGO) hybrid was synthesized in one step employing a hydrothermal method. By substituting the position of Mo, uniform doping with Zn improved the catalytic activity of MoS₂ for HER. The interlayer spacing of MoS₂ increased from 0.65 to 0.75 nm, demonstrating RGO effectively interpolate into MoS₂ nanosheets. This prevented aggregation and exposed more edge active sites of MoS₂. According to density functional theory (DFT) calculations, the layered structure of the MoS₂ nanosheets doped with Zn and intercalated with RGO promoted charge transfer and resulted in outstanding hydrogen evolution activity. Compared with MoS₂ (6.86 eV), the Zn-MoS₂-RGO hybrid (5.47 eV) with a considerably lower energy level value exhibited excellent electrocatalytic performance. Under optimal conditions, at a potential of -0.3 V vs. RHE, the current density reached -169 mA cm^-2 in a 0.5 M H₂SO₄ solution, 4.78 μmol of H₂ was produced in 6 h, and the Faraday efficiency reached 92%. The results obtained herein indicated that Zn-MoS₂-RGO was a promising candidate for application in electrocatalytic HER.

Graphene Aerogels for In Situ Synthesis of Conductive Poly(para-phenylenediamine) Polymers, and Their Sensor Application

In this study, macroporous graphene aerogels (GAs) were synthesized by chemical reduction of graphene oxide sheets and were used as a support material for in situ synthesis of conductive poly(para-phenylenediamine) (p(p-PDA)). The in situ synthesis of p(p-PDA) in GA was carried out by using a simple oxidation polymerization technique. Moreover, the prepared conductive p(p-PDA) polymers in the networks of GAs were doped with various types of acids such as hydrochloric acid (HCl), nitric acid (HNO3), sulfuric acid (H2SO4), phosphoric acid (H3PO4), respectively. The prepared GA and different acid-doped forms as GA/p(p-PDA) composites were characterized by FT-IR, TGA, and conductivity measurements. The observed FT-IR peaks at 1574 cm-1, and 1491 cm-1, for stretching deformations of quinone and benzene, respectively, confirmed the in situ synthesis of P(p-PDA) polymers within GAs. The conductivity of GAs with 2.17 × 10-4 ± 3.15 × 10-5 S·cm-1 has experienced an approximately 250-fold increase to 5.16 × 10-2 ± 2.72 × 10-3 S·cm-1 after in situ synthesis of p(p-PDA) polymers and with HCl doping. Conductivity values for different types of acid-doped GA/p(p-PDA) composites were compared with the bare p(p-PDA) and their undoped forms. Moreover, the changes in the conductivity of GA and GA/p(p-PDA) composites upon CO2 gas exposure were compared and their sensory potential in terms of response and sensitivity, along with reusability in CO2 detection, were evaluated.

3D Carbon Foam Supported Edge-Rich N-Doped MoS2 Nanoflakes for Enhanced Electrocatalytic Hydrogen Evolution

Molybdenum disulfide (MoS₂ ) is one of the most promising alternatives to the Pt-based electrocatalysts for the hydrogen evolution reaction (HER). However, its performance is currently limited by insufficient active edge sites and poor electron transport. Hence, enormous efforts have been devoted to constructing more active edge sites and improving conductivity to obtain enhanced electrocatalytic performance. Herein, the 3D carbon foam (denoted as CF) supported edge-rich N-doped MoS₂ nanoflakes were successfully fabricated by using the commercially available polyurethane foam (PU) as the 3D substrate and PMo₁₂ O₄₀ ^3- clusters (denoted as PMo₁₂ ) as the Mo source through redox polymerization, followed by sulfurization. Owing to the uniform distribution of nanoscale Mo sources and 3D carbon foam substrate, the as-prepared MoS₂ -CF composite possessed well-exposed active edge sites and enhanced electrical conductivity. Systematic investigation demonstrated that the MoS₂ -CF composite showed high HER performance with a low overpotential of 92 mV in 1.0 m KOH and 155 mV in 0.5 m H₂ SO₄ at a current density of 10 mA cm^-2 . This work offers a new pathway for the rational design of MoS₂ -based HER electrocatalysts.

In Operando Stacking of Reduced Graphene Oxide for Active Hydrogen Evolution

Despite the remarkable electronic and mechanical properties of graphene, improving the catalytic activity of the atomically flat, inert, and stable carbon network remains a challenging issue in both fundamental and application studies. In particular, the adsorption of most molecules and ions, including hydrogen (H₂ or H⁺), on graphene is not favorable, underlining the challenge for an efficient electrochemical catalytic reaction on graphene. Various defects, edges, and functionalization have been suggested to resolve the catalytic issue in graphene, but cost-effectiveness and active catalysis with graphene have not been achieved yet. Here, we introduce dynamic stacking of reduced graphene oxide (rGO) with spontaneously generated hydrogen bubbles to form an efficient electrochemical catalyst with a graphene derivative; the in operando stacking of rGO, without using a high-temperature-based heteroatom doping process or plasma treatment, creates a large catalytic surface area with optimized edges and acidic groups in the rGO. Thus, the uniquely formed stable carbon network achieves active hydrogen evolution with a Tafel slope of 39 mV·dec^-1 and a double layer capacitance of 12.41 mF·cm^-2, which breaks the conventional limit of graphene-based catalysis, suggesting a promising strategy for metal-free catalyst engineering and hydrogen production.

Graphene Nanoarchitectonics: Recent Advances in Graphene-Based Electrocatalysts for Hydrogen Evolution Reaction

Under the double pressures of both the energy crisis and environmental pollution, the exploitation and utilization of hydrogen, a clean and renewable power resource, has become an important trend in the development of sustainable energy-production and energy-consumption systems. In this regard, the electrocatalytic hydrogen evolution reaction (HER) provides an efficient and clean pathway for the mass production of hydrogen fuel and has motivated the design and construction of highly active HER electrocatalysts of an acceptable cost. In particular, graphene-based electrocatalysts commonly exhibit an enhanced HER performance owing to their distinctive structural merits, including a large surface area, high electrical conductivity, and good chemical stability. Considering the rapidly growing research enthusiasm for this topic over the last several years, herein, a panoramic review of recent advances in graphene-based electrocatalysts is presented, covering various advanced synthetic strategies, microstructural characterizations, and the applications of such materials in HER electrocatalysis. Lastly, future perspectives on the challenges and opportunities awaiting this emerging field are proposed and discussed.

Promoting Electrocatalysis upon Aerogels

Electrocatalysis plays a prominent role in renewable energy conversion and storage, enabling a number of sustainable processes for future technologies. There are generally three strategies to improve the efficiency (or activity) of the electrocatalysts: i) increasing the intrinsic activity of the catalyst itself, ii) improving the exposure of active sites, and iii) accelerating mass transfer during catalysis (both reactants and products). These strategies are not mutually exclusive and can ideally be addressed simultaneously, leading to the largest improvements in activity. Aerogels, as featured by large surface area, high porosity, and self-supportability, provide a platform that matches all the aforementioned criteria for the design of efficient electrocatalysts. The field of aerogel synthesis has seen much progress in recent years, mainly thanks to the rapid development of nanotechnology. Employing precursors with different properties enables the resulting aerogel with targeted catalytic properties and improved performances. Here, the design strategies of aerogel catalysts are demonstrated, and their performance for several electrochemical reactions is reviewed. The common principles that govern electrocatalysis are further discussed for each category of reactions, thus serving as a guide to the development of future aerogel electrocatalysts.

Electrodeposition of amorphous molybdenum sulfide thin film for electrochemical hydrogen evolution reaction

Amorphous molybdenum sulfide (MoS_x) is a highly active noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER). The MoS_x was prepared by electrochemical deposition at room temperature. Low-cost precursors of Mo and S were adopted to synthesize thiomolybdates solution as the electrolyte. It replaces the expensive (NH)₂MoS₄ and avoid the poison gas (H₂S) to generate or employ. The (MoO₂S₂)²⁻, (MoOS₃)²⁻ and (MoS₄)²⁻ ions were determined by UV–VIS spectroscopy. The electrodeposition of MoS_x was confirmed with XRD, XPS and SEM. The electrocatalyst activity was measured by polarization curve. The electrolyte contained (MoO₂S₂)²⁻ ion and (MoOS₃)²⁻ ion electrodeposit the MoS_x thin film displays a relatively high activity for HER with low overpotential of 211 mV at a current density of 10 mA cm⁻², a relatively high current density of 21.03 mA cm⁻² at η = 250 mV, a small Tafel slope of 55 mV dec⁻¹. The added sodium dodecyl sulfate (SDS) can efficient improve the stability of the MoS_x film catalyst.

CZTS Decorated on Graphene Oxide as an Efficient Electrocatalyst for High-Performance Hydrogen Evolution Reaction

Cu2ZnSnS4 (CZTS) was synthesized by the sonochemical method using 2-methoxyethanol as the solvent and subsequently decorated onto graphene oxide (GO synthesized by the modified Hummers' method) using two different approaches such as in situ growth and ex situ synthesis followed by deposition. Preliminary characterizations indicated that the synthesized CZTS belongs to the kesterite structure with a sphere-like morphology. The in situ-synthesized CZTS/GO (I-CZTS/GO) composite is used as an efficient electrocatalyst for hydrogen evolution reaction (HER) which revealed superior electrocatalytic activity with a reduced overpotential (39.3 mV at 2 mA cm-2), Tafel slope (70 mV dec-1), a larger exchange current density of 908 mA cm-2, and charge transfer resistance (5 Ω), significantly different from pure CZTS. Besides, the I-CZTS/GO composite exhibits highest HER performance with high current stability of which shows no noticeable degradation after i-t amperometry. The catalytic activity demonstrates that the I-CZTS/GO composite could be a promising electrocatalyst in hydrogen production from their cooperative interactions.

Graphene/graphene nanoribbon aerogels decorated with S-doped MoSe2 nanosheets as an efficient electrocatalyst for hydrogen evolution

Graphene/graphene nanoribbon (GGNR) aerogels facilitate fast electron and ion transfer while S-doping improves the activity of catalytic sites for efficient hydrogen evolution.

PbTe quantum dots as electron transfer intermediates for the enhanced hydrogen evolution reaction of amorphous MoSx/TiO2 nanotube arrays

Amorphous molybdenum sulfides (a-MoSx) have been demonstrated as economic and efficient hydrogen evolution catalysts for water splitting. Further improvements of their hydrogen evolution reaction (HER) activities could be achieved by coupling them with appropriate electron transfer intermediates via interfacial engineering. In this study, a novel ternary composite electrode comprising PbTe quantum dots (QDs), a-MoSx and TiO2 nanotube arrays (TNAs) was successfully fabricated by a facile combination of successive ionic layer adsorption and reaction (SILAR) and electrodeposition routes. Investigation of the microstructures and electrocatalytic properties of the a-MoSx/PbTe QD/TNA hybrid material show that PbTe QDs can work as electron temporary storage and electron transfer intermediates between the electrocatalyst a-MoSx and electrode-based material TiO2 that significantly lower the impedance of electrode process, enhance the energy band bending at the interface between the electrolyte and electrode surface, and increase the electrochemically active surface area. The electron interphase crossing from a-MoSx to electrolyte and electron transport inside the electrode are greatly strengthened. The ternary PbTe@MoSx/TNA electrode demonstrates lowered onset potential and Tafel slope and superior electrocatalytic activity and cyclic stability towards HER.

Recent advances in three-dimensional graphene based materials for catalysis applications

This review presents recent theoretical and experimental progress in the construction, properties, and catalytic applications of 3D graphene-based materials.

Rationally Incorporated MoS2/SnS2 Nanoparticles on Graphene Sheets for Lithium-Ion and Sodium-Ion Batteries

Herein, we have designed and first synthesized a unique ternary hybrid structure by simultaneously growing SnS₂ and MoS₂ particles on graphene sheets (denoted as MoS₂/SnS₂-GS) via one-pot hydrothermal route. The charge incompatibility between MoO₄^2- and graphene oxide with negative charged functional groups on surface can be compromised with the aid of Sn⁴⁺ cations, which renders the final formation of SnS₂ and MoS₂ on GS surface. This is the first report of the cohybridization of MoS₂ and SnS₂ with GS matrix from anionic and cationic precursors in the absence of premedication of graphene surface. When MoS₂/SnS₂-GS acts as anodes for lithium-ion batteries, the hybrids exhibit much better cycling stability than MoS₂-GS and SnS₂-GS counterparts. The compact adhesion of MoS₂/SnS₂ nanoparticles helps offset the undesired result of destruction of electrode materials resulting from volume expansion during repeated cycles. Furthermore, by combination with their synergetic effect on interface and the presence of discrepant asynchronous electrochemical reactions for SnS₂ and MoS₂, MoS₂/SnS₂-GS hybrids are endowed with improvement of electrochemical capabilities. Besides, they also showed outstanding Na-storage ability.

Solvent-Assisted Oxygen Incorporation of Vertically Aligned MoS2 Ultrathin Nanosheets Decorated on Reduced Graphene Oxide for Improved Electrocatalytic Hydrogen Evolution

Three-dimensional oxygen-incorporated MoS2 ultrathin nanosheets decorated on reduced graphene oxide (O-MoS2/rGO) had been successfully fabricated through a facile solvent-assisted hydrothermal method. The origin of the incorporated oxygen and its incorporation mechanism into MoS2 were carefully investigated. We found that the solvent N,N-dimethylformamide (DMF) was the key as the reducing agent and the oxygen donor, expanding interlayer spaces and improving intrinsic conductivity of MoS2 sheets (modulating its electronic structure and vertical edge sites). These O dopants, vertically aligned edges and decoration with rGO gave effectively improved double-layer capacitance and catalytic performance for hydrogen evolution reaction (HER) of MoS2. The prepared O-MoS2/rGO catalysts showed an exceptional small Tafel slope of 40 mV/decade, a high current density of 20 mA/cm(2) at the overpotential of 200 mV and remarkable stability even after 2000th continuous HER test in the acid media.

Amorphous Molybdenum Sulfide on Graphene-Carbon Nanotube Hybrids as Highly Active Hydrogen Evolution Reaction Catalysts

In this study, we report on the deposition of amorphous molybdenum sulfide (MoSx, with x ≈ 3) on a high specific surface area conductive support of Graphene-Carbon Nanotube hybrids (GCNT) as the Hydrogen Evolution Reaction (HER) catalysts. We found that the high surface area GCNT electrode could support the deposition of MoSx at much higher loadings compared with simple porous carbon paper or flat graphite paper. The morphological study showed that MoSx was successfully deposited on and was in good contact with the GCNT support. Other physical characterization techniques suggested the amorphous nature of the deposited MoSx. With a typical catalyst loading of 3 mg cm(-2), an overpotential of 141 mV was required to obtain a current density of 10 mA cm(-2). A Tafel slope of 41 mV decade(-1) was demonstrated. Both measures placed the MoSx-deposited GCNT electrode among the best performing molybdenum sulfide-based HER catalysts reported to date. The electrode showed a good stability with only a 25 mV increase in overpotential required for a current density of 10 mA cm(-2), after undergoing 500 potential sweeps with vigorous bubbling present. The current density obtained at -0.5 V vs SHE (Standard Hydrogen Electrode potential) decreased less than 10% after the stability test. The deposition of MoSx on high specific surface area conductive electrodes demonstrated to be an efficient method to maximize the catalytic performance toward HER.

Integrated Three-Dimensional Carbon Paper/Carbon Tubes/Cobalt-Sulfide Sheets as an Efficient Electrode for Overall Water Splitting

The development of an efficient catalytic electrode toward both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is of great significance for overall water splitting associated with the conversion and storage of clean and renewable energy. In this study, carbon paper/carbon tubes/cobalt-sulfide is introduced as an integrated three-dimensional (3D) array electrode for cost-effective and energy-efficient HER and OER in alkaline medium. Impressively, this electrode displays superior performance compared to non-noble metal catalysts reported previously, benefiting from the unique 3D array architecture with increased exposure and accessibility of active sites, improved vectorial electron transport capability, and enhanced release of gaseous products. Such an integrated and versatile electrode makes the overall water splitting proceed in a more direct and smooth manner, reducing the production cost of practical technological devices.

Vertically aligned oxygen-doped molybdenum disulfide nanosheets grown on carbon cloth realizing robust hydrogen evolution reaction

A synergistically optimized oxygen-incorporated MoS₂/carbon cloth hybrid catalyst was successfully fabricated, realizing enhanced hydrogen-evolving activity and superior stability.

In situ growth of MoS2 nanosheets on reduced graphene oxide (RGO) surfaces: interfacial enhancement of absorbing performance against electromagnetic pollution

Electromagnetic pollution is rising all over the world.