Reaction In Acid Medium Research Articles

Platinum-complexed phosphorous-doped carbon nitride for electrocatalytic hydrogen evolution

Platinum is atomically dispersed within P-doped C3N4 forming Pt–N/P/Cl coordination interactions, and exhibits markedly enhanced electrocatalytic activity towards the hydrogen evolution reaction in acidic media, as compared to the P-free counterpart.

Towards stable and highly active IrO2 catalysts supported on doped tin oxides for the oxygen evolution reaction in acidic media

Iridium oxide is the preferred catalyst for water oxidation but it is required to maximize its utilization to deploy Proton Exchange Membrane Water Electrolyzers (PEMWEs) into the large-scale applications panorama. A promising pathway for dispersing this precious catalyst is on an electric conductive and stable support. However, there is a lack of understanding how the support-catalyst interactions affect the stability/activity of the electrocatalyst under anodic conditions. This work discloses a modified, easy-scalable, polyol synthesis protocol to produce a highly active and stable iridium-based catalyst, supported on metal-doped tin oxides. The loading of Ir was reduced 30 wt.% compared to the reference IrO2, and dispersed on Sb-SnO2 (IrOx/ATO), In-SnO2 (IrOx/ITO) and SnO2 supports. All synthesized electrocatalysts not only surpassed the OER-mass activity of a commercial catalyst (IrO2) – reference – but also reached higher electrochemical active surface areas and enhanced stability under the OER conditions. The highest performance was achieved with Ir NPs supported on ITO (176 A/gIr vs. 15.5 A/gIr for the reference catalyst @ 1.51 V vs. RHE) and both IrOx/ITO and IrOx/SnO2 catalysts demonstrated remarkable stability after cycling the electrode and performing long-term chronopotentiometry. ITO is, therefore, an auspicious support to serve Ir-based catalysts as it favors a good bargain between activity and stability, while drastically reducing the amount of noble metal.

Highly Durable Core(Nb4n5)–Shell(Nbox)-Structured Non-Precious Metal Catalyst for Oxygen Reduction Reaction in Acidic Media

Highly Durable Core(Nb4n5)–Shell(Nbox)-Structured Non-Precious Metal Catalyst for Oxygen Reduction Reaction in Acidic Media

IrO2/LiLa2IrO6 as a robust electrocatalyst for the oxygen evolution reaction in acidic media

The IrO2/LiLa2IrO6 electrocatalyst achieves a win–win situation for both activity and stability towards the OER in acidic media, attributed to the in situ formed IrOx and strong interaction between IrOx and bulk LiLa2IrO6.

NiS2 Nanoparticles Grown on Reduced Graphene Oxide Co-Doped with Sulfur and Nitrogen for Enhanced Hydrogen Evolution Reaction in Acid Media

NiS2 nanoparticle electrocatalysts grown on sulfur and nitrogen co-doped reduced graphene oxide (NiS2/S,N-rGO) were explored. Since the hybrid catalyst of NiS2/S,N-rGO had a large surface area and high conductivity, it demonstrated outstanding electrocatalytic properties. This NiS2/S,N-rGO hybrid electrocatalyst required only 95 mV overvoltage to reach the current density of 10 mA cm−2 in hydrogen evolution reaction (HER) in acidic solution. In addition, the 5000 cycles test experiment showed the excellent HER electrochemical stability of the NiS2/S,N-rGO catalyst. This work also provided new strategies for increasing the catalytic activity of non-precious metal electrocatalysts.

Ruco Alloy Nanoparticles as Efficient and Durable Electrocatalysts for Oxygen Reduction Reaction in Acidic Medium

Ruco Alloy Nanoparticles as Efficient and Durable Electrocatalysts for Oxygen Reduction Reaction in Acidic Medium

Microbial synthesis of efficient palladium electrocatalyst with high loadings for oxygen reduction reaction in acidic medium

Whereas limited amount of precious metal adsorbed by bacteria conflicting the needs of high loadings for better catalytic performances, cell disruption technology was adopted to smash Shewanella cells in this work, releasing abundant oxygen functional groups inside the cells for better adsorption of palladium ion. Then palladium catalysts were synthesized in two ways: 1) Pd catalyst supported on carbonized-broken-bacterial (Pd/FHNC) was obtained after direct carbonization and reduction; 2) Electrospinning technology was used to spin the broken Shewanella into fibers, and Pd nanoparticles supported on nitrogen-doped carbon nanofiber (Pd/NCNF) was prepared following carbonization and hydrogen reduction. The as-prepared catalysts exhibit excellent oxygen reduction reaction (ORR) electrocatalytic performance in the acid medium. The mass specific activities at 0.7 V of Pd/FHNC and Pd/NCNF were 0.213 A mg−1 and 0.121 A mg−1 which were 5.92 and 3.36 times than those of commercial Pd/C(0.036 A mg−1) respectively, and they also displayed higher stability than Pd/C. Furthermore, the Pd loadings of Pd/FHNC and Pd/NCNF were 21.52% and 17.13% respectively. An explanation for the improved performance is the co-doping of nitrogen and phosphorus, also the tight integration of Pd and broken-bacterial. Herein, we propose a novel and effective method for synthesis of ORR electrocatalysts.

RETRACTED: Atomic layer deposition of NiS2 nanoparticles on surface-modified MoS2 nanosheets for improved electrocatalytic hydrogen evolution reaction in acid media

Developing active and stable electrocatalysts for hydrogen evolution reaction (HER) is vital for the sustainable production of hydrogen (H2). In this report, NiS2 nanoparticles were deposited on the two-dimension (2D) MoS2 nanosheets by atomic layer deposition (ALD). The as-prepared MoS2/NiS2 composites were used as a working electrode for HER and displayed improved catalytic efficiency over the pristine MoS2 electrode. The electrochemical measurements demonstrated that the overpotential and the Tafel slope of the optimized MoS2/NiS2 composite were −114.6 mV and 45.4 mV/dec, which were very close to the benchmark Pt/C electrode. The improved HER activity of the MoS2/NiS2 composites could be due to the synergic effects between MoS2 and NiS2. This work could provide helpful guidance on how to design and construct efficient MoS2-based electrocatalysts for HER.

Facile Method to Synthesize a High-Activity S-Doped Fe/SNC Single-Atom Catalyst by Metal–Organic Frameworks for Oxygen Reduction Reaction in Acidic Medium

Fe/NC single-atom catalysts containing atomically dispersed FeN4 moieties have exhibited encouraging activity comparable to that of Pt in aqueous acids and outstanding performance in practical prot...

(Invited) Electrocatalytic Reduction of Highly Inert Redox Probes: Arsenates, Nitrates, Chlorates, As Well As Carbon Dioxide and Nitrogen in Acid Medium

Arsenates(V), nitrites(III), nitrates(V), chlorates(V), as well as carbon dioxide (CO2) and nitrogen (N2) are electrochemically highly inert systems during reductions. There is also a problem of the competitive hydrogen evolution reaction in acid media, particularly when noble metal electrocatalysts are considered. But arsenates, nitrates, chlorates and CO2 (but not N2) can adsorb and undergo activation on noble metal (platinum group metal and alloyed) catalytic nanoparticles. Among important issues is the presence of monoatomic hydrogen highly active centers and/or the feasibility of adsorptive activating interactions with metallic or metal oxide sites.On electrochemical grounds, noble metal nanoparticles (Pt, Rh, Ir, Pd, as well as bimetallic PtRu or PtSn), when deposited on an inert electrode substrates, exhibit electrocatalytic properties during reduction of arsenates, nitrates and chlorates in acid medium (e.g. 0.5 mol dm-3 H2SO4). For example, upon application of the appropriate potential, both As(III) and As(V) are preconcentrated and activated on surfaces of Pt-type nanoparticles. Consequently, the presence of respective adsorbates can monitored under voltammetric conditions. Population of protons does affect the systems’ electrochemical characteristics. The nature of the voltammetric reduction peaks of the adsorbates of arsenates(V), nitrates(III,V), chlorates(V), and even CO2 is strongly dependent on the preconcentration or activation potential. While the reduction and deposition of arsenite(III) and nitrate(III) is induced by adsorbed hydrogen on Pt formed at potentials lower than 0.25 V (vs. RHE), the adsorption, preconcentration and activation of arsenate(V) and chlorate(V) can be readily achieved at Pt oxides generated at Pt nanoparticles at potentials higher than 0.85 V (vs. RHE). The dynamics of the electroreduction of chlorates can be significantly enhanced through utilization of bimetallic PtRu or PtSn catalysts, in which specific interaction between Pt catalytic sites and the alloying metal (e.g. Ru or Sn) affect the electronic structure, chemisorptive properties and activity of platinum. Carbon dioxide is strongly adsorbed on surfaces of Pt-based catalysts and, subsequently, reduced to CO adsorbate; obviously it can be re-oxidized in the reverse positive-going voltammetric potential scan. While nitrogen does not interact specifically with noble metal catalytic centers, and its reduction is practically hindered by hydrogen evolution, immobilization of noble metal (e.g. Pd) nanostructures within polynuclear oxometallate networks leads to better selectivity and permits some electroreduction of nitrogen parallel to hydrogen evolution. A unique feature of the adsorbed As(III) and As(V) oxo species is the ability to agglomerate and form stable polynuclear electroactive conducting-polymer-like films on noble metal nanoparticles thus modifying their catalytic reactivity. Electrochemical experiments are supported by the data from scanning and transmission electron microscopies and Raman spectroscopy.

Role of surface steps in activation of surface oxygen sites on Ir nanocrystals for oxygen evolution reaction in acidic media

Ir and its oxide are the only available oxygen evolution reaction (OER) electrocatalysts with reasonably high activity and stability for commercial proton-exchange membrane electrolyzers. However, the establishment of structure–performance relationships for the design of better Ir-based electrocatalysts is hindered by their uncontrolled surface reconstruction during OER in acidic media. Herein, we monitor the structural evolution of two model Ir nanocrystals (one with a flat surface enclosed by (100) facets and the other with a concave surface containing numerous high-index planes) under acidic OER conditions. Operando X-ray absorption spectroscopy measurements reveal that the promotion of surface IrOx formation during the OER by the concave Ir surface with high-index planes results in a gradual OER activity increase, while a decrease in activity and limited oxide formation are observed for the flat Ir surface. After the activation process, the Ir concave surface exhibits ~ 10 times higher activity than the flat surface. Density functional theory computations reveal that Ir high-index surfaces are thermodynamically preferred for the adsorption of oxygen atoms and the formation of surface oxides under OER conditions. Thus, our work establishes a structure–performance relationship for Ir nanocrystals under operating conditions, providing new principles for the design of nanoscale OER electrocatalysts.

Covalently Bonded Ir(IV) on Conducted Blue TiO2 for Efficient Electrocatalytic Oxygen Evolution Reaction in Acid Media

The stability of anode electrode has been a primary obstacle for the oxygen evolution reaction (OER) in acid media. We design Ir-oxygen of hydroxyl-rich blue TiO2 through covalent bonds (Ir–O2–2Ti) and investigate the outcome of favored exposure of different amounts of covalent Ir–oxygen linked to the conductive blue TiO2 in the acidic OER. The Ir-oxygen-blue TiO2 nanoclusters show a strong synergy in terms of improved conductivity and tiny amount usage of Ir by using blue TiO2 supporter, and enhanced stability using covalent Ir-oxygen-linking (i.e., Ir oxide) in acid media, leading to high acidic OER performance with a current density of 10 mA cm−2 at an overpotential of 342 mV, which is much higher than that of IrO2 at 438 mV in 0.1 M HClO4 electrolyte. Notably, the Ir–O2–2Ti has a great mass activity of 1.38 A/mgIr at an overpotential 350 mV, which is almost 27 times higher than the mass activity of IrO2 at the same overpotential. Therefore, our work provides some insight into non-costly, highly enhanced, and stable electrocatalysts for the OER in acid media.

Designing of low Pt electrocatalyst through immobilization on metal@C support for efficient hydrogen evolution reaction in acidic media

• Introducing NiFe metal nanoparticles encapsulated in thin graphene layer as a catalyst support. • With 0.02 at Pt % this support promotes hydrogen evolution similarly to a commercial 20 wt-% Pt/C. • The support metal core and carbon shell have a crucial role for anchoring and activating Pt atoms. • The 0.02 at Pt % electrocatalyst with the metal core – carbon shell support shows high durability. Nanoparticles comprising of transition metals encapsulated in an ultrathin graphene layer (NiFe@UTG) are utilized to anchor very low amount of finely dispersed pseudo-atomic Pt to function as a durable and active electrocatalyst (Pt/NiFe@UTG) for the hydrogen evolution reaction (HER) in acidic media. Our experiments show the vital role of the carbon shell thickness for efficient utilization of Pt. Furthermore, density functional theory calculations suggest that the metal-core has a crucial role in achieving promising electrocatalytic properties. The thin carbon shell allows the desired access of Pt atoms to the vicinity of the NiFe core while protecting the metallic core from oxidation in the harsh acidic media. In acidic media, the performance of this Pt/NiFe@UTG catalyst with 0.02 at% Pt is the same as that of commercial Pt/C (10 and 200 mV overpotential to reach 10 and 200 mA cm −2 , respectively) with promising durability (5000 HER cycles). Our electrochemical characterization (cyclic voltammetry) shows no Pt specific peaks, indicating the existence of a very low Pt loading on the surface of the catalyst. Hence, this conductive core-shell catalyst support enables efficient utilization of Pt for electrocatalysis.

Enhancement of Activity and Development of Low Pt Content Electrocatalysts for Oxygen Reduction Reaction in Acid Media.

Platinum is a main catalyst for the electroreduction of oxygen, a reaction of primary importance to the technology of low-temperature fuel cells. Due to the high cost of platinum, there is a need to significantly lower its loadings at interfaces. However, then O2-reduction often proceeds at a less positive potential, and produces higher amounts of undesirable H2O2-intermediate. Hybrid supports, which utilize metal oxides (e.g., CeO2, WO3, Ta2O5, Nb2O5, and ZrO2), stabilize Pt and carbon nanostructures and diminish their corrosion while exhibiting high activity toward the four-electron (most efficient) reduction in oxygen. Porosity of carbon supports facilitates dispersion and stability of Pt nanoparticles. Alternatively, the Pt-based bi- and multi-metallic catalysts, including PtM alloys or M-core/Pt-shell nanostructures, where M stands for certain transition metals (e.g., Au, Co, Cu, Ni, and Fe), can be considered. The catalytic efficiency depends on geometric (decrease in Pt–Pt bond distances) and electronic (increase in d-electron vacancy in Pt) factors, in addition to possible metal–support interactions and interfacial structural changes affecting adsorption and activation of O2-molecules. Despite the stabilization of carbons, doping with heteroatoms, such as sulfur, nitrogen, phosphorus, and boron results in the formation of catalytically active centers. Thus, the useful catalysts are likely to be multi-component and multi-functional.

Acid leaching of LiCoO2 enhanced by reducing agent. Model formulation and validation

In this work, a model has been formulated to describe the complex process of LiCoO2 leaching through the participation of competing reactions in acid media including the effect of H2O2 as reducing agent. The model presented here describes the extraction of Li and Co in the presence and absence of H2O2, and it takes into account the different phenomena affecting the controlling mechanisms. In this context, the model predicts the swift from kinetic control to diffusion control. The model has been implemented and solved to simulate the leaching process. To validate the model and to estimate the model parameters, a set of 12 (in triplicate) extraction experiments were carried out varying the concentration of hydrochloric acid (within the range of 0.5–2.5 M) and hydrogen peroxide (range 0–0.6%v/v). The simulation results match fairly well with the experimental data for a wide range of conditions. Furthermore, the model can be used to predict results with different solid-liquid ratios as well as different acid and oxygen peroxide concentrations. This model could be used to design or optimize a LiCoO2 extraction process facilitating the corresponding economical balance of the treatment.

Nanocomposite of platinum and prussian blue: A highly active and stable electrocatalyst towards oxygen reduction reaction in acidic media

Deployment of proton exchange membrane fuel cells (PEMFCs) are of vital importance for the mitigation of fossil energy shortage and environment pollution. A significant amount of Pt on cathode is highly needed to accelerate the process of sluggish oxygen reduction reaction (ORR) and maintains a stable long-term performance. During ORR, the yield of undesirable hydrogen peroxide through a two-electron pathway not only decreases outer power but also weakens the long-term stability. Herein, Pt-prussian blue (PB) composite, featuring around structure, is firstly proposed and facilely fabricated via electrostatic self-assembly strategy. Pt1PB0.25/C shows 50% higher activity than commercial Pt/C, rationally ascribed to modulated electronic structure and higher selectivity towards four-electron pathway. Moreover, in contrast with the striking 38% loss in mass activity for Pt/C after accelerated degradation tests (ADTs), Pt1PB0.25/C shows only a 15% decrease possibly due to the elimination of hydrogen oxide and the anchor interaction between Pt and PB. The employment of PB with a synergy role paves an efficient way for the fabrication of brand-new Pt-based catalysts with high activity and stability.

Synthesis of 3D Sn doped Sb2O3 catalysts with different morphologies and their effects on the electrocatalytic hydrogen evolution reaction in acidic medium

Recently, the design and synthesis of highly effective, abundant and low-cost catalysts to produce molecular hydrogen through the hydrogen evolution reaction (HER) have been studied in a wide range of pHs to replace Pt. In this work, 3D flower-like and rod-like pure Sb2O3 and Sn: Sb2O3 particles by using co-precipitation methods were efficiently synthesized for electro-catalytic energy conversion applications. The morphologies and structure of the synthesized catalysts were investigated extensively. Electrochemical studies were carried out to investigate the catalytic performance of 3D flower-like and rod-like Sn: Sb2O3 and pure Sb2O3 particles for hydrogen evolution reactions (HER) in acidic environments using 0.5 M H2SO4 electrolyte. Among the prepared particles, 3D rod-like Sn: Sb2O3 show excellent electro-catalytic hydrogen evolution reactions (HER) at 0.5 M H2SO4. Moreover, we have obtained the high stability of the electrodes during chronoamperometric studies (current v/s time) approximately 2000 s at constant potential.

Self-supported amorphous iridium oxide catalysts for highly efficient and durable oxygen evolution reaction in acidic media

Developing highly active and stable electrocatalysts for acidic oxygen evolution reaction is still a great challenge. Herein, we developed a novel self-supported amorphous IrOx catalyst (IrOx/Ti) encapsulated with a small number of crystalline nanoparticles for efficient acidic OER. The resulting IrOx-50-AC/Ti electrode exhibits a superior OER performance with an overpotential of 250 mV at 10 mA cm−2 and Tafel slope of 50.4 mV dec−1 in 0.1 M HClO4. High contents of IrIII species and surface hydroxide groups, hierarchically porous structure and high defective density contribute to boost the OER activity. Importantly, the amorphous IrOx-50-AC/Ti also demonstrates a super long-term durability for 190 h due to the strong interaction between IrOx film and Ti. The unique amorphous-crystalline structure also enhances the stability and the activity by promoting the synergy between amorphous phase and crystalline phase. This work provides some valuable insights on developing efficient Ir-based catalysts for acidic water splitting.

A facile synthesis for nitrogen‐doped carbon catalyst with high activity of oxygen reduction reaction in acidic media

In this paper, we report a facile method to synthesize a nitrogen-doped carbon catalyst for oxygen reduction reaction (ORR). The catalyst was prepared via subsequent carbonization of the composite aerogel under NH3 and N2, which was synthesized by the hydrothermal treatment of resorcinol formaldehyde (RF) and graphene oxide (GO). The research focused on the effect of the heat treatment temperature on the carbon defects, doped nitrogen and ORR activity of the catalyst. The results show that more carbon defects can be formed under a relatively low temperature inside the catalyst which also possesses a high ratio of graphitic nitrogen. Interestingly, the catalyst shows a moderate ORR activity in alkaline (E1/2 = 0.78 V) but a good ORR activity in acids (E1/2 = 0.72 V), which should be attributed to its high content of carbon defects or graphitic nitrogen. Moreover, a direct methanol fuel cell with the nitrogen-doped carbon as cathode catalyst was fabricated and tested, which gives peak power densities of 18.27 and 16.14 mW cm−2 at 4 and 14 M methanol solution, respectively. All the results reveal that the carbonization of RF-GO composite aerogel is a promising facile method to prepare ORR catalysts with high activity.

The origin of the high electrochemical activity of pseudo-amorphous iridium oxides

Combining high activity and stability, iridium oxide remains the gold standard material for the oxygen evolution reaction in acidic medium for green hydrogen production. The reasons for the higher electroactivity of amorphous iridium oxides compared to their crystalline counterpart is still the matter of an intense debate in the literature and, a comprehensive understanding is needed to optimize its use and allow for the development of water electrolysis. By producing iridium-based mixed oxides using aerosol, we are able to decouple the electronic processes from the structural transformation, i.e. Ir oxidation from IrO2 crystallization, occurring upon calcination. Full characterization using in situ and ex situ X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and transmission electron microscopy allows to unambiguously attribute their high electrochemical activity to structural features and rules out the iridium oxidation state as a critical parameter. This study indicates that short-range ordering, corresponding to sub-2nm crystal size for our samples, drives the activity independently of the initial oxidation state and composition of the calcined iridium oxides.