mV In Acid Research Articles

Free-standing FeNiCuCoBMo high-entropy alloy bifunctional electrocatalysts for efficient pH-universal water electrolysis

Utilizing the synergistic effects of multi-components has been proven to be a feasible approach to enhance water splitting electrocatalytic activity, since developing efficient and stable bifunctional electrocatalysts within a wide pH range remains a significant challenge. Herein, we report a FeNiCuCoBMo high-entropy alloy (HEA) prepared by dealloying, showing outstanding bifunctional electrocatalytic performance for HER and OER in a pH-universal electrolyte. After dealloying for 7 h, the HER and OER overpotentials of the HEA strips in 1.0 M KOH decreased by 22 mV and 25 mV, respectively.Impressively, the free-standing strips with the optimal compositions achieve low HER (82 mV in acid and 101 mV in alkali at 10 mA·cm−2) and OER (311 mV in acid and 245 mV in alkali at 10 mA·cm−2) overpotentials. In a full hydrolysis experiment, a current of 10 mA·cm−2 can run for 100 h with only 1.58 V, which is attributed to the ample exposure of highly active sites resulting from the synergistic effect of each element. Theoretical calculations further support these findings, showing that the electronegativity differences of the metallic elements in FeNiCuCoBMo result in abnormal activity at specific locations (Fe and Mo).Additionally, charge rearrangement and enhanced electronic conduction at the interface reduce the activation energy barrier, thereby improving the performance of the electrocatalysts.

Bimetallic MoFe phosphide catalysts for the hydrogen evolution reaction

In this work, highly efficient and stable (bi)metallic phosphides were prepared by a facile, flexible, and controllable sol-gel method followed by sintering. This novel approach allowed to avoid complicated and dangerous phosporization using, e.g. red phosphorus. The doping of the MoP catalysts with Fe was used to further boost their catalytic performance. Monometallic MoP and Mo3P, as well as bimetallic MoFeP were tested in both acidic and alkaline environments and showed remarkable results. The incorporation of Fe enabled the creation of a scalable and cost-effective catalyst for the hydrogen evolution reaction due to the synergy between Fe and Mo in the bimetallic phosphides. Using MoFeP for catalysing the hydrogen evolution reaction, overpotentials of only -132 mV (in an acidic medium) and -142 mV (in an alkaline medium) were needed to reach current density of -10 mA.cm−2, indicating the possibility of use this catalyst in a large pH range. Its high catalytic activity was maintained even at current density of -100 mA.cm−2 as documented by overpotentials of -202 mV and -246 mV in acid and alkaline environment, respectively. This, along with excellent stability and durability confirms its promising potential for incorporation into industrially relevant applications. The experimental data were confirmed by computational (DFT) results, showcasing that incorporation of Fe caused an increase in the number of active sites with Gibbs adsorption energy close to zero.

High-Density Dual-Structure Single-Atom Pt Electrocatalyst for Efficient Hydrogen Evolution and Multimodal Sensing.

Herein, we report a high-density dual-structure single-atom catalyst (SAC) by creating a large number of vacancies of O and Ti in two-dimensional (2D) Ti3C2 to immobilize Pt atoms (SA Pt-Ti3C2). The SA Pt-Ti3C2 showed excellent performance toward the pH-universal electrochemical hydrogen evolution reaction (HER) and multimodal sensing. For HER catalysis, compared to the commercial 20 wt % Pt/C, the Pt mass activities of SA Pt-Ti3C2 at the overpotentials of ∼30 and 110 mV in acid and alkaline media are 45 and 34 times higher, respectively. More importantly, during the alkaline HER process, an interesting synergetic effect between Pt-C and Pt-Ti sites that dominated the Volmer and Heyrovsky steps, respectively, was revealed. Moreover, the SA Pt-Ti3C2 catalyst exhibited high sensitivity (0.62-2.65 μA μM-1) and fast response properties for the multimodal identifications of ascorbic acid, dopamine, uric acid, and nitric oxide under the assistance of machine learning.

Efficient hydrogen generation of a cobalt porphyrin-bridged covalent triazine polymer

Hydrogen production obtained by electrocatalytic water splitting exhibits great promise in addressing both energy shortage and environmental contamination. Herein, we prepared a novel cobalt porphyrin (CoTAPP)-bridged covalent triazine polymer (CoTAPPCC) by covalently linking CoTAPP with cyanuric chloride (CC) for catalytic hydrogen evolution reaction (HER). Both experimental techniques and density functional theory (DFT) calculations were used to evaluate the correlation of HER activity with molecular structures. Benefiting from the strong electronic interactions between the CC unit and the CoTAPP moiety, a standard current density at 10 mA cm−2 is obtained for CoTAPPCC with a low overpotential of 150 mV in acid, which is comparable to or better than the best records reported previously. Additionally, a competitive HER activity in basic medium is obtained for CoTAPPCC. The strategy reported herein is valuable for designing and developing porphyrin-based efficient HER electrocatalysts.

Efficient electrocatalysts with strong core-shell interaction for water splitting: The modulation of selectivity and activity

Optimum of electrocatalysts plays a key role in the design and development of cost-effective and environmentally benign technologies for energy conversion and storage. In this work, we study two kinds of core-shell structured electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in acid and alkali electrolyte. The electrocatalyst of cobalt@cobalt phosphide nanoparticles is grown on carbon nanotubes (Co@CoP/CNT) and shows a current density of 10 mA cm−2 at an overpotential of 67 mV in acid and 125 mV in alkali, demonstrating the strong metal-metal phosphide interaction facilitating hydrogen reduction. However, the in situ formed cobalt@cobalt oxide (Co@Co3O4/CNT) electrocatalysts from the Co@CoP/CNT can retard both the HER and OER performances through the metal-metal oxide interaction. The underlying mechanisms of the activity and selectivity of the core-shell catalysts towards water splitting reactions is further unveiled by density functional theory (DFT) simulation. The different catalytic behaviors of the core-shell structured catalysts in HER and OER are attributed to the different interactions between the metallic cores, shells and the adsorbed intermediate species. This study not only promises a facile and general strategy to fabricate high performance electrocatalysts but also sheds new light on crucial design principles of catalyst materials for intermediate-selective electrocatalytic reactions.

Evolution of interfacial coupling interaction of Ni-Ru species for pH-universal water splitting

Designing highly-efficient and pH-universal electrocatalysts for overall water-splitting reactions is important for the sustainable production of hydrogen. Herein, NiRu nanoparticles encapsulated into N-doped carbon (NiRu@NC) are fabricated with the assistance of MOF for hydrogen evolution reaction (HER) and also evolved into core–shell structured nanomaterials with abundant interfaces for oxygen evolution reaction (OER, Ni/RuOx@C). Benefiting from optimized electronic structure, interfacial synergy effect and special core–shell structure (~150 nm), the newly developed electrocatalysts present remarkable electrocatalytic activities with small overpotentials to drive 10 mA cm−2 for the HER (19 mV in alkaline, 80 mV in neutral, 20 mV in acid) and OER (252 mV, 316 mV, 211 mV). Moreover, the obtained electrocatalysts exhibit outstanding catalytic performances for overall water-splitting in a two-electrode system with 1.53 V and 1.67 V to drive 10 mA cm−2 in basic and neutral electrolyte, respectively. Remarkably, a small potential of 1.47 V is required to deliver 10 mA cm−2 in acidic media and no significant decay of the activity is observed. This work paves the way for designing a novel class of electrocatalyst that can not only exhibit remarkable electrocatalytic water splitting performances in various conditions, but also possess excellent stabilities for future generation of clean energy.

A novel worm-like micelles@MOFs precursor for constructing hierarchically porous CoP/N-doped carbon networks towards efficient hydrogen evolution reaction

Constructing electrocatalysts with plentiful active sites, great mass transfer ability, and high electrical conductivity is critical to realize efficient hydrogen evolution reaction (HER). Hierarchically porous cobalt phosphide/N-doped nanotubular carbon networks (CoP/NCNs) that have all the features were fabricated in this work. For the fabrication, the polymeric worm-like micelles (PWs) with a large aspect ratio were coated by a uniform nanolayer of Zn-Co zeolitic imidazolate frameworks (Zn-Co-ZIFs) on their surface, resulting in the hybrid nanofibers PWs@Zn-Co-ZIFs (HPWs). Inheriting the randomly curved and entanglement properity of PWs, the rigid HPWs formed hybrid networks with the packing voids sized tens to 200 nm. Then, the hybrid networks were treated by pyrolysis-oxidation-phosphidation and ZnO-removal processes, leading to the hierarchically porous CoP/NCNs. In the CoP/NCNs, there are plentiful CoP nanoparticles embedded on the surface of conductive carbon network and fully exposed. When used for HER electrocatalyst, the CoP/NCNs only need small overpotentials (98 and 118 mV in acid and alkaline electrolyte) at 10 mA cm−2. This novel strategy is instructive for tailoring hierarchically porous transition metal phosphide/carbon nanocomposites as promising electrocatalysts.

Modulation of the Electronic Structure of IrSe2 by Filling the Bi Atom as a Bifunctional Electrocatalyst for pH Universal Water Splitting

Although IrSe2 is a superior oxygen evolution catalyst, its electronic structure lacks active sites for hydrogen production and hinders its use as a bifunctional catalyst. The intercalation of Bi atoms into the IrSe2 framework regulates its electronic structure with the improvement of performance. Herein, novel bismuth inserted iridium‐based selenide (BiIr4Se8) is synthesized via solid‐state reaction. Interestingly, BiIr4Se8 exhibits impressive activities with dual functionality in pH‐universal electrolytes. It achieves small overpotentials at 10 mA cm−2 toward hydrogen evolution (85 mV in acid and 252 mV in alkali) and oxygen evolution (379 mV in acid and 364 mV in alkali), which overmatches the benchmarking noble metal‐based electrocatalysts. When served as the bifunctional electrocatalyst to drive a water electrolyzer, BiIr4Se8 affords 10 mA cm−2 at a voltage of 1.56 and 1.52 VRHE in 0.5 m H2SO4 and 1 m KOH, far exceeding the Pt/C||RuO2 and other published materials. Theoretical calculations show that a narrow bandgap is obtained due to the induced intermediate band derived from the combination of the p orbitals of Bi atoms and structure reconstruction, which promotes the electrocatalytical performance. This research introduces a new strategy to design channel structures and sheds light on their applications for water splitting.

Confinement Effect of Mesopores: In Situ Synthesis of Cationic Tungsten-Vacancies for a Highly Ordered Mesoporous Tungsten Phosphide Electrocatalyst.

Engineering defects in crystalline electrocatalysts is an effective approach to tailor the electronic structure and number of active sites, which are essential for the intrinsic activity of the hydrogen evolution reaction (HER). Unlike previously reported methods, we demonstrate a confinement effect using a mesoporous template for in situ fabrication of cationic W vacancies in as-prepared ordered mesoporous tungsten phosphide (WP) nanostructures by adjusting the nonstoichiometric ratio of the precursor elements. With a plenty of W vacancies and ordered mesoporosity, the as-prepared catalyst WP-Mesop exhibits better catalytic performance than the catalysts without mesopores and/or vacancies. The WP-Mesop shows an ultralow overpotential of 175 mV in acid and 229 mV in alkaline at 100 mA cm-2 and stability of 48 h without structural collapse in both acid and alkaline media. Meanwhile, density functional theory calculations further reveal that the activation barrier for HER can be lowered by introducing cationic W vacancies. This strategy can be extended to generate cationic defects in other transition metal phosphides to improve their HER activities.

Scalable Synthesis of Bimetallic Phosphide Decorated in Carbon Nanotube Network as Multifunctional Electrocatalyst for Water Splitting

It is challengeable to obtain a scalable method to synthesize nonprecious electrocatalysts with high efficiency and stability for overall water splitting, to replace the costly and scarce noble metal based electrocatalysts (e.g., Pt- and Ru-based materials). Herein, bimetallic (Fe, Co)P nanoparticles decorated in carbon nanotube network (FCP-CN) are synthesized through a facile and scalable spray drying and subsequent phosphorization process. The FCP-CN hybrid delivers excellent performance in hydrogen evolution reaction both in acidic and alkaline media, oxygen evolution reaction, and overall water splitting: it possesses an Pt-like hydrogen evolution reaction activity with an ultralow onset overpotential of 18 mV in acid; remarkably, it shows an ultrasmall Tafel slope of 38 mV dec–1 in oxygen evolution reaction; being employed as both cathode and anode, this catalyst demonstrates promising performance of overall water splitting with high long-term stability. The performance is superior among recently reported transition-metal-based catalysts for overall water splitting. This work provides a scalable and low-cost synthesis strategy to synthesize nonprecious and multifunctional transition-metal-based catalysts with unique nanoarchitecture and outstanding catalytic performance for water splitting.

General Method for Synthesizing Transition-Metal Phosphide/N-Doped Carbon Nanomaterials for Hydrogen Evolution.

Applications of effective and steady metal catalysts for the hydrogen evolution reaction (HER) via electrolysis of water have a huge potential to relax energy crisis and reduce carbon dioxide emission. Herein, we design a simple, facile, and general approach for the synthesis of a series of transition-metal phosphide nanoparticles embedded in N-doped carbon (NC) nanomaterials using metal salts, abundantly available hexamethylene diamine tetra(methyl phosphonic acid), and urea as precursors. The resultant transition-metal phosphide nanoparticles can serve as high-efficiency and steady HER catalysts. Particularly, when the current density is 10 mA cm-2, the overpotentials of the obtained RhP2@NC are 30, 85, and 70 mV in acid (0.5 M H2SO4), neutral (1 M PBS), and alkaline (1 M KOH) solutions, respectively. Besides, the RhP2@NC exhibits good stability after 10 h in aforementioned solutions. More importantly, it is suited to fabricate other transition-metal phosphide nanoparticles/NC heterostructures by this synthetic strategy. The obtained CoP@NC, FeP@NC, Ni2P@NC, and Cu3P@NC also show relatively high efficiency for HER. Hence, the versatile synthesis strategy opens a new route for the research and fabrication of transition-metal phosphide-based catalysts for HER.

Urchin-like CoP with Controlled Manganese Doping toward Efficient Hydrogen Evolution Reaction in Both Acid and Alkaline Solution

Splitting water to produce hydrogen through an efficient and low-cost way requires the development of catalysts based on earth-abundant elements. Using d-band theory to modify the band structure, we verified Mn atoms to be potent dopants to greatly reinforce the activity of urchin-like CoP, and this catalyst could reach a current density of 10 mA/cm2 with overpotentials of only 65 and 100 mV in acid and alkaline, which were much superior to that of pristine CoP and close to that of Pt/C catalyst. Also, the outperformance of its durability was tested for 20 h to maintain a current density of 10 mA/cm2. The increments of overpotentials were 2.4 and 1.1 mV in acid and alkaline, respectively. After introducing partial Mn atoms, the density functional theory calculation revealed that the Gibbs free energies of hydrogen adsorption (ΔGads,H) of Mn-doped CoP (−0.07 eV) was much smaller than pristine CoP (−0.157 eV). Furthermore, density of state analysis indicated that the strong interaction between Co atoms and ...

Cobalt encapsulated N-doped defect-rich carbon nanotube as pH universal hydrogen evolution electrocatalyst

Exploring efficient and economical Pt-free electrocatalysts is of great significance for the electrocatalytic hydrogen evolution reaction (HER). However, the rational design on an industrial scale is a formidable challenge. Herein, we reported a facile calcination at controlled temperatures to fabricate rationally assembled cobalt nanoparticles embedded in defect-rich N-doped carbon nanotubes (Co-NCNTs), which was derived from low-cost dicyanadiamide thermally polymerized with cobalt precursor forming metal-organic frameworks, then further calculation leading to final products. The as-obtained samples were endowed with high content of N as electrocatalytic active site, defect-rich structure and excellent synergistic effect between cobalt nanoparticles and carbon nanotubes toward electrocatalytic HER. As expected, Co-NCNTs were highly active and long-term stable with onset potentials of c.a. 15 mV in acidic electrolytes (0.5 M H2SO4), 70 mV in alkaline (1 M KOH) and 300 mV in neutral media (pH 7). Specially, to achieve the current density of 10 mA cm−2, the overpotential of 103 mV in acid, 204 mV in alkaline and 337 mV in neutral media was obtained. The enhanced HER performance was discussed in detail by adjusting the molar ratio of precursor and metal species. Moreover, the present synthetic route is easy to scale up and expand to other non-noble metal and alloy.

In-situ cobalt and nitrogen doped mesoporous graphitic carbon electrocatalyst via directly pyrolyzing hyperbranched cobalt phthalocyanine for hydrogen evolution reaction

Hydrogen energy as a promising renewable and sustainable energy has attracted growing attentions in recent years, in searching of low-cost electrocatalysts with high efficiency as alternatives of non-noble metals for hydrogen evolution reaction (HER). Herein we report an in-situ cobalt and nitrogen co-doped mesoporous graphitic carbon catalysts by directly pyrolyzing hyperbranched cobalt phthalocyanine (H-CoPc), which can efficiently catalyze HER in acidic, alkaline or neutral electrolytes. With only 0.62 at% of cobalt dopants, the outstanding Co@NG-800 catalyst exhibits excellent durability and HER activities with the overpotential as low as 286 mV in acid, 290 mV in alkaline and 480 mV in neutral, achieved at 10 mA cm−2 current density. These activities indicate that the Co@NG-800 catalyst can couple with the best available oxygen-evolving catalysts in the overall water-splitting. Meanwhile, the mesoporous structure derived from H-CoPc provides more active sites than un-hyperbranched precursor. This work may provide a novel organic metal complexes and simple route in designing highly active and durable transition metal and nitride co-doped carbonaceous electrocatalysts for HER.

Cobalt nanoparticles embedded in nitrogen-doped carbon for the hydrogen evolution reaction.

There is great interest in renewable and sustainable energy research to develop low-cost, highly efficient, and stable electrocatalysts as alternatives to replace Pt-based catalysts for the hydrogen evolution reaction (HER). Though nanoparticles encapsulated in carbon shells have been widely used to improve the electrode performances in energy storage devices (e.g., lithium ion batteries), they have attracted less attention in energy-related electrocatalysis. Here we report the synthesis of nitrogen-enriched core-shell structured cobalt-carbon nanoparticles dispersed on graphene sheets and we investigate their HER performances in both acidic and basic media. These catalysts exhibit excellent durability and HER activities with onset overpotentials as low as ∼70 mV in both acidic (0.5 M H2SO4) and alkaline (0.1 M NaOH) electrolytes, and the overpotentials needed to deliver 10 mA cm(-2) are determined to be 265 mV in acid and 337 mV in base, further demonstrating their potential to replace Pt-based catalysts. Control experiments reveal that the active sites for HER might come from the synergistic effects between the cobalt nanoparticles and nitrogen-doped carbon.

Electrochemistry of various substituted aminophenyl iron porphyrins

Iron tetrakis(o-aminophenyl)porphyrin (Fe(o-NH2)TPP), iron tetrakis(p-aminophenyl)porphyrin (Fe(p-NH2)TPP) and iron tetrakis(p-N,N'-dimethylaminophenyl)porphyrin (Fe(p-NMe2)TPP) were electropolymerized onto GC electrodes and were tested for catalysis of dioxygen reduction in aqueous solutions. They showed catalytic activity in the same potential ranges of the two reduction waves detected in deaerated solutions: +0.05 and −0.25 V for poly[Fe(o-NH2)TPP], −0.04 and −0.23 V for poly[p-NH2TPP] and −0.08 and −0.25 V for poly[Fe(p-NMe2)TPP] at pH 3. The electrocatalysis was more effective in acidic solutions than in basic solutions: the overvoltage of O2 reduction was reduced by about 450 mV in acid and by only 50 mV in base. Also the H2O2 yields reflected the different activities of the catalyst in the electrolyte solutions: 2.5% at pH 3 and 30% at pH 12 for O2 reduction at a high surface coverage of a poly[Fe(o-NH2)TPP] film. While the H2O2 yields for O2 reduction at the other two iron porphyrin films at pH 3 were similar (2.8 and 3.1% for poly[Fe(p-NH2)TPP] and poly[Fe(p)-NMe2)TPP], respectively), they were considerably higher at pH 12 (41 and 58% for poly[Fe(p-NH2)TPP] and poly[Fe(p-NMe2)TPP], respectively). Unlike the poly[Fe(P-NH2)TPP] and poly[p-NMe2)TPP] films, poly[Fe(o-NH2)TPP] was shown to reduce H2O2 catalytically at high pH, thus lowering the yield of H2O2 formed as an intermediate during O2 reduction in basic solutions.

Redox and electrocatalytic properties towards dioxygen reduction of ruthenium tetra(ortho-aminophenyl)porphyrin complexes with various axial ligands

The redox properties of ruthenium(II) tetra( ortho-aminophenyl)porphyrins (Ru( o-NH 2)TPP) with various axial ligands (L = CO, pyridine, acetonitrile and tetrahydrofuran) have been studied after dissolving them in CH 3CN (for ruthenium complexes with L= CO or pyridine) or HCL(g)-saturated CH 3CN solutions (L = acetonitrile or tetrahydrofuran). The formal potentials for oxidation of the Ru(II)( o-NH 2)TPP(CO) compound were +0.88 and +1.20 V. For the Ru(II)( o-NH 2)TPP(L) 2 com- plexes, two oxidation waves were observed in the 0 to +0.5 V range: E°' = +0.15, −0.38 V for the (pyridine) 2 complex; +0.12, +0.28 for the (CH 3CN) 2 complex; and +0.10, +0.50 V for the (tetrahy- drofuran) 2 complex. The carbonyl and pyridine porphyrin complexes, when adsorbed onto GC electrodes and tested for catalysis of dioxygen reduction in aqueous solutions, did not show any catalytic activity. In contrast, the adsorbed acetonitrile and tetrahydrofuran compounds catalysed the O 2 reduction. The electrocatalysis was more effective in strong acid (0.5 M H 2SO 4) than in strong base (1 M NaOH): the overvoltage of O 2 reduction was reduced by 450 mV in acid and by only 50 mV in base. Also the H 2O 2 yields reflected the different activities of the catalyst in the electrolyte solutions: 90% in base and insignificant ( < 0.5%) amounts of H 2O 2 in acid. Different reactivities were also found for the adsorbed Ru( o-NH 2)TPP towards H 2O 2 in acid and base: while H 2O 2 is catalytically reduced in 0.5 M H 2SO 4, it is catalytically disproportionated in 1 M NaOH. Although the half-wave potential for O 2 reduction in 0.5 M H 2SO 4 was less positive in the presence of adsorbed ruthenium porphyrin than that of the adsorbed cobalt analogue complex (Co(II)( o-NH 2)TPP) ( E 1 2 =+0.05 and +0.18 V vs. Ag/AgCl, respectively), the use of a dual coating, Ru( o-NH 2)TPP+Co( o-NH 2)TPP, achieved a relatively high reduction potential in 0.5 M H 2SO 4 (+0.10 V vs. Ag/AgCl) as well as almost full reduction of dioxygen to water.

Different views regarding the kinetics and mechanisms of oxygen reduction at Pt and Pd electrodes

Two diametrically different pH dependences have been reported for oxygen reduction at prereduced Pt electrodes. In the current density (cd) region with the Tafel slope of −2.3 RT/F, the pH dependence expressed as d E/dpH is, according to one group of workers, −90 mV, ie −3 × 2.3 RT/(2 F) and − 30 mV, ie −2.3 RT/(2 F) in acid and alkaline solutions, respectively. The same dependences, according to another group of workers are −60 mV, ie −2.3 RT/F in both acid and alkaline solutions. In the cd region with the Tafel slope of −2 × 2.3 RT/F, d E/dpH according to the first group of workers is − 120 mV and 0 mV in acid and alkaline solutions, respectively. However, according to the second group, this dependence is −60 mV in both acid and alkaline solutions. The reasons for the variation in pH dependences is discussed. A previous analysis for the pH dependence of − 2.3 RT/F at all pHs in both cd regions is in error. An account of the pH dependences of −3 × 2.3 RT/(2 F) and −2.3 RT/(2 F) in terms of Temkin conditions of adsorption is satisfactory and allows for the change of Tafel slopes and pH dependences with increasing cd.