Partially oxidized cobalt species in nitrogen-doped carbon nanotubes: Enhanced catalytic performance to water-splitting

It's still an ongoing research challenge to explore non-precious metal-based catalysts for substituting precious metal catalysts during full water electrocatalysis. Herein, we reported the partially oxidized cobalt species in nitrogen-doped carbon nanotubes hierarchical structures to produce dual-functionality towards oxygen/hydrogen evolution reactions. The in situ transformation of carbon nanotubes and well-exposed metal-oxide contributes to mass diffusion and greater electrolyte-accessible surface area. The as-synthesized catalyst displays low overpotentials of 287 mV and 171 mV for oxygen and hydrogen evolution reactions at 10 mA cm^?2 of current density with remarkable performance during long-term stability. Furthermore, when employed as cathode and anode, a respectable performance of 1.68 V demonstrated our catalyst as an efficient bifunctional material for conducting water-splitting operation.     

A review on the recent developments in zirconium and carbon-based catalysts for photoelectrochemical water-splitting

In the recent years, considerable interest in the development of clean and renewable alternative energy resources has been observed to overcome the problems of dwindling fossil reserves, environmental pollution and increasing energy demand for a sustainable future. In this respect, hydrogen is considered a sustainable, clean, and energy-rich fuel. Photoelectrochemical (PEC) water-splitting is deemed to be a very promising technology hydrogen production. A number of research endeavors have been dedicated to develop efficient catalysts for this process. An optimum photoelectrocatalyst drives down the energy needed for the disassociation of water by lowering the overpotential of the process and make it competent for commercial applications. Recently, a lot of Zirconium (Zr) and Carbon (C) based compounds have been analyzed for PEC water-splitting. This review article intends to offer insight and timely reference for the progress on Zr and C based catalyst for practical PEC water-splitting in a comprehensive and concise manner. With emphasis on the photoelectrochemical performance, relative design strategies and different approaches to improve or optimize the photoelectrocatalyst materials with Zr and C are discussed. Research approach and recommendations for future PEC water-splitting are also proposed.     

Understanding water-splitting thermochemical cycles based on nickel and cobalt ferrites for hydrogen production

Two step water-splitting cycles by using metal ferrites are considered as a clean and sustainable hydrogen production method, when concentrated solar energy is used to drive the thermochemical reactions. This process involves the reduction at very high temperature of the ferrite, followed by the water reoxidation to the original phase at moderate temperature, with the release of hydrogen. In order to decrease the temperature required to decompose the oxide, mixed ferrites of the type MFe₂O₄ with spinel crystal structure have been examined. In this sense, ferrites with the partial substitution of Co and Ni for Fe appear as successful materials in terms of hydrogen production and cyclability. In this work, commercial Ni and synthetic Co ferrites have been subjected to two water splitting cycles. The solid products obtained after thermal reduction and water decomposition reactions have been chemically and structurally characterized by WDXRF, XRD, XPS and SEM techniques, in order to get a deeper understanding of the mechanisms controlling the water splitting process. This knowledge contributes to improve the process involved in thermochemical cycles and to understand the lower efficiencies (H₂/O₂) for Co ferrite thermochemical cycles in comparison with those corresponding to Ni ferrite.     

Effects of intrinsic defects on the photocatalytic water-splitting activities of PtSe2

PtSe₂ monolayer is previously predicted to be a two-dimensional water-splitting photocatalyst. However, the weak van der Waals (vdW) interaction between H₂O and the basal surface of PtSe₂ significantly undermines its photocatalytic water-splitting activities. In this work, we explore the possibility of various intrinsic defects of PtSe₂ in remedying this deficiency on the basis of first-principles calculations. It is interesting to find that the introduction of Pt@Se, Se@Pt, and Se interstitial defect not only fully retain the water redox abilities of pure PtSe₂ and realize spatial separation of photogenerated electrons and holes, but also can extend optical absorption range and absorption coefficients. Moreover, introduction of the three kinds of defects increase the initial weak vdW interactions between H₂O and the PtSe₂ surface to different extent. In particular, Pt@Se anti-site defect transform the initial weak vdW to strong chemical interaction between H₂O and PtSe₂ surface, and function as active reaction site. These insights demonstrate that introduction of intrinsic defects, especially the Pt@Se anti-site defect, are effective means for improving the photocatalytic water-splitting activities of PtSe₂ monolayer.     

Thermochemical two-step water-splitting reactor with internally circulating fluidized bed for thermal reduction of ferrite particles

A thermochemical two-step water-splitting cycle using a redox system of iron-based oxides or ferrites was examined on hydrogen productivity and reactivity of ferrite in order to convert solar energy into hydrogen in sunbelt regions. In the present paper, a new concept is proposed for a windowed thermochemical water-splitting reactor, using an internally circulating fluidized bed of NiFe₂O₄/m-ZrO₂ particles, and thermal reduction of the bed is demonstrated on a laboratory scale by using a solar-simulating Xe-beam irradiation. The concept is that concentrated solar radiation passes through the transparent window and directly heats the internally circulating fluidized bed. The fluidized bed reactor enabled the NiFe₂O₄/m-ZrO₂ sample to remain in powder form without sintering and agglomerating during direct Xe-beam irradiation over 30 min. Approximately 45% of the NiFe₂O₄ was converted to the reduced phase by the solar-simulated high-flux beam, and was then completely reoxidized with steam at 1000 °C to generate hydrogen.     

MoS2 confined on graphene by triethanolamine for enhancing electrocatalytic hydrogen evolution performance

The reduction of active sites due to reunion and slow electron transfer rates and low electronegativity greatly reduced the catalytic performance of many two-dimensional materials. In this paper, we synthesized composites for partially reducing graphene oxide and molybdenum disulfide (MoS₂@prGO) by one-step hydrothermal method. With the addition of triethanolamine, MoS₂ is highly dispersed on the prGO carrier and converted into the 1T phase MoS₂ (50.4%). Meanwhile, it helps to increase the electron transfer rate of the MoS₂@prGO composites. MoS₂@prGO composites presents a high electron cloud density due to the existence of N atoms and prGO, which promotes the occurrence of hydrogen ion conversion hydrogen reaction and decreases the electrocatalytic hydrogen evolution overpotential. MoS₂@prGO composites exhibits an overpotential of 263 mV at 10 mA/cm² and a small Tafel slope of 60 mV/dec. This work is devoted to offer a new prospect and direction for the improvement of electrochemical HER performance.     

Cobalt and cobalt oxide supported on nitrogen-doped porous carbon as electrode materials for hydrogen evolution reaction

A facile method is proposed to prepare cobalt supported on nitrogen-doped porous carbon material with high graphitization degree by using chitosan as carbon source, urea as soft template and poloxamer as dispersant. The prepared cobalt-carbon material (Co@NPC) shows that uniformly distributed cobalt nanoparticles are encapsulated in nitrogen-doped porous carbon bundle with large specific surface area. When Co@NPC is applied as electrode in hydrogen evolution reaction, it exhibits superior electrocatalytic performance with low overpotential (η₁₀ = 259 mV), small Tafel slope (99 mV dec⁻¹) and high stability with 83% of its original current density remained after 6 h electrochemical test in 1 M potassium hydroxide electrolyte.     

Nanocomposite of graphene oxide with nitrogen-doped TiO2 exhibiting enhanced photocatalytic efficiency for hydrogen evolution

The application of hydrogen energy potentially addresses energy and environmental problems. In order to improve the photocatalytic efficiency, nanocomposite of N-doped TiO₂ with graphene oxide (NTG) is prepared and characterized with Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectra, X-ray photoelectron spectroscopy (XPS), atomic force microscope (AFM), photoluminescent spectra. The application of NTG to hydrogen evolution exhibits high photocatalytic efficiency of 716.0 or 112.0 μmol h⁻¹ g⁻¹ under high-pressure Hg or Xenon lamp, which is about 9.2 or 13.6 times higher than P25 photocatalyst. This is mainly attributed to the N-doping of TiO₂ and the incorporation of graphene oxide resulting in narrow band gap, together with the synergistic effect of fast electron-transporting of photogenerated electrons and the efficient electron-collecting of graphene oxide retarding charge recombination. These results provide a significant theoretical foundation for the potential application of N-doping photocatalysts to hydrogen evolution.     

Electronically tuned defective Prussian-blue on graphene for electrochemical water splitting

Exploiting high-performance and stable bifunctional electrocatalysts is highly desirable for water splitting applications to obtain large-scale renewable and clean fuels. Herein, a defective Prussian-blue (δ-PB or Fe^III_1.23[Fe^II_1.12(CN)₆]_0.87·δ_0.13) on graphene composite electrocatalyst was fabricated through a facile hydrolytic precipitation process, followed by a single-step carbonization treatment at 600 °C (denoted as δ-PB/G-600). The resultant optimized δ-PB/G-600 exhibits remarkable electrocatalytic water splitting in alkaline media, producing lower overpotentials of 189 and 105 mV at a current density of 10 mA cm^?2 for the oxygen evolution reaction and hydrogen evolution reaction, respectively. Most importantly, the δ-PB/G-600 shows remarkable durability for both reactions. The remarkable electrocatalysis was attributed to the abundant active sites and high electrical conductivity with a defective nature, which not only facilitate the electrolyte flux but also maintain the structural stability of δ-PB/G-600. Additionally, the high surface area confirms the facile mass transport and prompts the gaseous release of the composite.     

In-situ synthesis of PdAg/g-C3N4 composite photocatalyst for highly efficient photocatalytic H2 generation under visible light irradiation

Novel PdAg bimetallic alloy nanoparticle modified graphitic carbon nitride (g-C₃N₄) nanosheet was designed and prepared by an in situ chemical reduction procedure. By optimizing the loading content of the PdAg alloy NPs, the PdAg/g-C₃N₄ composite photocatalyst showed a champion photocatalytic hydrogen generation rate of 3.43 mmol h⁻¹ g⁻¹, and the apparent quantum yield (AQY) was determined to be 8.43% at 420 nm. Moreover, the photoluminescence and photoelectrochemical experimental results suggest that a higher separation efficiency of photo-induced charge carriers (e^- and h⁺) was obtained after loading PdAg alloy NPs on g-C₃N₄. The experimental outcomes indicate that there is a synergistic effect formed between PdAg and g-C₃N₄, which could significantly promote the charge transfer photo-induced charge carriers in the hybrid sample. A reasonable catalytic mechanism for the enhanced photocatalytic performance of the composite photocatalyst was proposed and verified by TRPL measurement, which could be taken as a guidance for the development of novel high performance catalytic system.     

Nitrogen/oxygen co-doped graphene bonded nickel particles composite for dehydrogenation of formic acid at near room temperature

Low-cost non-noble metal catalyst for dehydrogenation of formic acid to hydrogen at near room temperature is considered as a key to promoting commercial technology for clean energy. We have constructed reduced graphene oxide (RGO) self-assembly bonded nickel particles for synthesis of graphene nanosheets embedded with nickel nanoparticles architecture Ni@xRGO. Nitrogen and oxygen co-doped graphene facilitates the adsorption of hydrogen protons from formic acid. Electron transfer ability of Ni@xRGO with active sites is enhanced via Ni–C bond in the interface between the RGO nanosheets and nickel particles, which promoted the C–H bond breaking for dehydrogenation of formic acid. The Ni@0.20RGO has excellent catalytic performance for hydrogen production from formic acid at near room temperature (the yield of hydrogen, 240.0 mL g^?1 h^?1 at 50 °C), comparable to the most active non-noble metal catalysts.     

Self-standing and efficient bifunctional electrocatalyst for overall water splitting under alkaline media enabled by Mo1-xCoxS2 nanosheets anchored on carbon fiber paper

The layered MoS₂ nanostructures have been widely used in the electrochemical hydrogen evolution reaction (HER), but rarely applied in overall water splitting application for their ignorable oxygen evolution reaction (OER) activity. To address this issue, a novel self-standing and bifunctional electrocatalyst, consisting of Co-doped MoS₂ nanosheets anchored on carbon fiber paper, has been prepared via hydrothermal method. Taking advantage of conductive substrate of carbon fiber paper, sufficient-exposed active edges of MoS₂ sheets, and metallic character caused by Co-doping, our electrode exhibits high-efficient bifunctional activities for the overall water splitting in alkaline electrolyte (1 M KOH), which can produce a current density of 20 mA cm⁻² at an overpotential of 197 mV for HER and 235 mV for OER.     

Catalytic performance of a high-cell-density Ni honeycomb catalyst for methane steam reforming

The catalysis of methane steam reforming (MSR) by pure Ni honeycombs with high cell density of 2300 cells per square inch (cpsi) was investigated to develop efficient and inexpensive catalysts for hydrogen production. The Ni honeycomb catalyst was assembled using 30-μm-thick Ni foils, and showed much higher activity than that of a Ni honeycomb catalyst with cell density of 700 cpsi at a steam-to-carbon ratio of 1.36 and a gas hourly space velocity of 6400 h^?1 in a temperature range of 873–1173 K. Notably, the activity increased approximately proportional to the increasing geometric specific surface area of the honeycombs. The turnover rate of the Ni honeycomb catalyst was higher than that of supported Ni catalysts. The changes in chemical state of the Ni catalyst during hydrogen reduction and MSR reaction were analyzed by in situ X-ray absorption fine structure spectroscopy, which revealed that deactivation was mainly due to oxidation of the surface Ni atoms. These results demonstrated that the high-cell-density Ni honeycomb catalyst exhibits good performance for MSR reaction, and easy regeneration of the deactivated Ni honeycomb catalyst is possible only via hydrogen reduction.     

Tungsten promoted nickel phosphide nanosheets supported on carbon cloth: An efficient and stable bifunctional electrocatalyst for overall water splitting

It is of great significance to develop a highly active, durable and inexpensive bifunctional electrocatalyst for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report a tungsten-doped nickel phosphide nanosheets based on carbon cloth (W–Ni₂P NS/CC) as an efficient bifunctional catalyst through simple hydrothermal and phosphorization for overall water splitting in 1 M KOH. The W–Ni₂P NS/CC exhibits excellent electrochemical performance with low overpotentials for HER (η₁₀ = 71 mV, η₅₀ = 160 mV) and OER (η₂₀ = 307 mV, η₅₀ = 382 mV) in 1 M KOH, as well as superior long-term stability. Moreover, W–Ni₂P NS/CC as a bifunctional catalyst reveals remarkable activity with a low voltage of 1.55 V to reach a current density of 20 mA cm⁻². This work provides a viable bifunctional catalyst for the overall water splitting.     

In situ induced growth strategy of Co2P nanocrystals encapsulated into stable 3D carbon network as a bifunctional electrocatalyst for Zn-air battery

Rational design of inexpensive and robust carbon-based bifunctional catalysts is of considerable interest for practical application of rechargeable Zn-air battery (ZAB) technology. Herein, a facile in-situ induced growth strategy is developed to construct Co₂P nanocrystals encapsulated into a stable 3D carbon nanotube-modified graphene network (Co₂P@NPCNG). Specifically, cobalt tetranitrophthalocyanine (CoPc(NO₂)₄) is employed not only as the coupling agent to form and complex Co₂P nanocrystals with graphene, but also as the inducer to catalyze the graphitization of melamine to grow the uniform Co₂P nanocrystal-encapsulated CNTs on graphene in situ. Encouragingly, the as-synthesized Co₂P@NPCNG exhibits favorable bifunctional oxygen electrocatalytic activity, fast reaction kinetics and excellent stability. Impressively, both liquid ZAB and all-solid-state ZABs used Co₂P@NPCNG as air-cathode catalysts display considerable open-circuit voltage, charge-discharge property and long lifetime. Significantly, density functional theory (DFT) calculations demonstrate that the superior properties of Co₂P@NPCNG originate to the synergetic contributions between the stable configuration of 3D conductive carbon network and high metallic density of Co₂P. This work may provide feasible and facile avenues to strategically construct high-efficient 3D carbon-based bifunctional electrocatalysts for portable and even wearable devices.     

Noble metal-free cobalt oxide (CoOx) nanoparticles loaded on titanium dioxide/cadmium sulfide composite for enhanced photocatalytic hydrogen production from water

In this present paper, cobalt oxide (CoO_x) is studied as an effective cocatalyst in a photocatalytic hydrogen production system. CoO_x-loaded titanium dioxide/cadmium sulfide (TiO₂/CdS) semiconductor composites were prepared by a simple solvothermal method and characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), and X-ray photoelectron spectroscopy (XPS). Photocatalytic hydrogen production was studied using the as-synthesized photocatalysts in aqueous solution containing sodium sulfide (Na₂S)/sodium sulfite (Na₂SO₃) as hole scavengers under visible light irradiation (λ > 400 nm). The optimal cobalt content in CoO_x-loaded TiO₂/CdS composite is determined to be 2.1 wt% and the corresponding rate of hydrogen evolution is 660 μmol g⁻¹ h⁻¹, which is about 7 times higher than TiO₂/CdS and CdS photocatalysts under the same condition. Visible light-driven photocurrents of the semiconductor composites were further measured on a photoelectrochemical electrode, revealing that the photocorrosion of CdS can be prevented due to the presence of TiO₂–CoO_x.     

Exploring the new electrocatalyst based on pyrochlore manganese phosphate/graphene composite for hydrogen evolution and oxygen evolution reaction: An experimental and computational study

An advanced energy conversion technology of water splitting involves two half-reactions of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this work, we synthesized pyrochlore manganese phosphate (Mn₂P₂O₇) nanoplatelets by a simple coprecipitation method and made a composite with two-dimensional graphene nanosheets (GNS) through the ultrasonication process. Various characterization techniques like PXRD, FT-IR, Raman, XPS, SEM, and HR-TEM confirmed the formation of Mn₂P₂O₇-GNS composite material. The catalyst reveals outstanding performance towards HER activity with 0.333 V (vs. RHE) in an acidic medium and OER activity with 1.47 V (vs. RHE) in an alkaline medium at a current density of 10 mA/cm². The combined experimental study with DFT calculations reveals that the superior electrochemical action may originate from the synergistic effects associated with the high electronic conductivity of GNS and the interfacial belongings formed within Mn₂P₂O₇ and GNS. Also, the GNS plays an essential role in the composite to prevent the aggregation of nanoplatelet particles, thereby increasing active sites and reducing the overpotential. This work proposed recent advances of Mn₂P₂O₇-GNS based materials modified on glassy carbon electrode (GCE) as a stable and durable bifunctional electrocatalyst for the perspectives of the future development of a clean energy landscape.     

Synergetic catalysis of CuO and graphene additives on TiO2 for photocatalytic water splitting

A highly efficient and visible-light-responsive CuO/TiO₂-GR photocatalyst had been synthesized by a two-step process. The as-prepared CuO/TiO₂-GR composites were characterized by X-ray diffraction, N₂-physisorption, transmission electron microscope, X-ray photoelectron spectroscopy, Raman spectra, UV–vis diffuse reflectance spectra and Photoluminescence spectra. The results indicated that a chemical bond formed between GR and TiO₂ in CuO/TiO₂-GR composites. CuO/TiO₂-GR composites had a higher photocatalytic activity for hydrogen production due to a synergistic effect between CuO and GR. The synergistic effect could efficiently suppress charge recombination, improve interfacial charge transfer, enhance visible-light adsorption and provide plentiful phtotocatalytic reaction active sites. The maximum hydrogen evolution rate of CuO/TiO₂-GR-0.5 was 2905.60 μmol/(h·g), which was 20.20 times larger than pure P25.     

Enhanced catalytic efficiency of bimetallic Pt-Pd on PAMPs-modified graphene oxide for formic acid oxidation

A catalyst composed of 2-acrylamido-2-methyl-1-propane sulfonic acid (PAMPs) modified graphene oxide (GO) as the supporting material (PAMPs/GO) and electrodeposited monometallic and bimetallic catalysts (Pt and/or Pd) as the active catalytic component was fabricated to enhance the formic acid oxidation. The morphology of the prepared catalysts was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while the chemical compositions were identified by energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS). Functional groups of the supporting materials and catalysts were identified by Fourier transform infrared (FT-IR) spectroscopy. The electrochemical measurements of the synthesized catalysts were evaluated by cyclic voltammetry (CV) and chronoamperometry (CA), respectively. The kinetics of formic acid oxidation on the synthesized catalysts were determined by Tafel extrapolation of linear sweep voltammograms. The CO tolerance of the electrocatalyst was examined by CO stripping measurements. The results showed that the catalysts with PAMPs exhibited much higher electrocatalytic activity and longer-term stability for formic oxidation than the catalyst without PAMPS. In addition, the 3Pt3Pd/10%PAMPs/GO catalyst showed the greatest catalytic activity, stability, and fastest charge transfer kinetics when compared to other bimetallic catalysts and monometallic catalysts. In conclusion, modifying the GO surface with PAMPs can improve the efficiency of the electrocatalyst activity of Pt/Pd catalysts. The 3Pt3Pd/10%PAMPs/GO catalyst is a promising electrocatalyst for the enhancement of formic acid oxidation.     

La-doped cobalt supported on mesoporous alumina catalysts for improved methane dry reforming and coke mitigation

The promotional La₂O₃ effect on the physicochemical features of mesoporous alumina (MA) supported cobalt catalyst and its catalytic performance for methane dry reforming (MDR) was examined at varied temperature and stoichiometry feedstock. The Co₃O₄ nanoparticles were evidently scattered on fibrous mesoporous alumina with small crystal size of 8–10 nm. The promotion behavior of La₂O₃ facilitated H₂-reduction by providing higher electron density and enhanced oxygen vacancy in 10%Co/MA. The addition of La₂O₃ could reduce the apparent activation energy of CH₄ consumption; hence, increasing CH₄ conversion up to 93.7% at 1073 K. The enhancement of catalytic activity with La₂O₃ addition was also due to smaller crystallite size, alleviated H₂-reduction and the basic character of La₂O₃. Lanthanum dioxycarbonate transitional phase formed in situ during MDR was accountable for mitigating deposited carbon via redox cycle for 17–30% relying on reaction temperature. Additionally, the oxygen vacancy degree increased to 73.3% with La₂O₃ promotion. The variation of H₂/CO ratios within 0.63–0.99 was preferred for downstream generation of long-chain olefinic hydrocarbons.