Electrochemical Reduction Of Water Research Articles

Simultaneous electrochemical reduction of water and oxidation of carbon in a solid oxide cell for separately producing hydrogen and carbon monoxide

Hydrogen can be produced from reduction of water at the cathode of a solid oxide electrolysis cell (SOEC) and electricity and CO gas can be co-generated from the oxidation of carbon at the anode of a direct carbon solid oxide fuel cell (DC-SOFC). In the present work, a solid oxide cell (SOC) combining the cathode reaction of the SOEC and the anode reaction of the DC-SOFC is proposed, prepared, and investigated. It is demonstrated that hydrogen and CO can be separately produced from the cathode and anode through simultaneous reduction of water and oxidation of carbon in such a H2O-C SOC. Resistances from electrode polarizations are analyzed through measuring current-voltage characteristics, overpotentials, and AC impedances of the cell. The advantage of the H2O-C SOC over two separately operated SOEC and DC-SOFC is verified by the resistance analysis. An electrolyte-supported H2O-C SOC, with a cermet of silver and gadolinium-doped ceria (Ag-GDC) as the symmetrical electrode material and a steam concentration of 50 % at the cathode, gives an operating current density of 0.67 A cm-2 at the thermal neutral voltage (0.7 V). It stably operates at 0.2 A cm-2 under an applied voltage of only 0.16 V, producing pure hydrogen from the cathode and CO with a concentration of 88 % from the anode. The performance is significantly improved by using an anode-supported H2O-C SOC which gives an operating current density of 2.27 A cm-2 at the thermal neutral voltage.

Improving NiFe Electrocatalysts through Fluorination-Driven Rearrangements for Neutral Water Electrolysis.

Neutral electrolysis to produce hydrogen is prime challenging owing to the sluggish kinetics of water dissociation for the electrochemical reduction of water to molecular hydrogen. An ion-enriched electrode/electrolyte interface for electrocatalytic reactions can efficiently obtain a stable electrolysis system. Herein, we found that interfacial accumulated fluoride ions and the anchored Pt single atoms/nanoparticles in catalysts can improve hydrogen evolution reaction (HER) activity of NiFe-based hydroxide catalysts, prolonging the operating stability at high current density in neutral conditions. NiFe hydroxide electrode obtains an outstanding performance of 1000mA cm-2 at low overpotential of 218mV with 1000 h operation at 100mA cm-2. Electrochemical experiments and theoretical calculations have demonstrated that the interfacial fluoride contributes to promote the adsorption of Pt to proton for sustaining a large current density at low potential, while the Pt single atoms/nanoparticles provide H adsorption sites. The synergy effect of F and Pt species promotes the formation of Pt─H and F─H bonds, which accelerate the adsorption and dissociation process of H2O and promote the HER reaction with a long-term durability in neutral conditions.

Space confined growth of RuO2/Ru carbon nanorods for highly efficient electrocatalytic hydrogen evolution

Exploring efficient and robust electrocatalysts with low cost for hydrogen evolution reaction (HER) is critical for practical application of electrochemical water splitting. Ru-based platinum group electrocatalysts shows great potential in HER, however, Ru suffer from sluggish kinetics of water dissociation for electrochemical reduction of water to hydrogen in alkaline environments. Here, oxygen atoms are introduced into Ru by simple secondary heat treatment to form Ru–O–C bonds to weak the strength of Ru–H bond. The overpotential of the catalyst is only 19 mV at 10 mA cm−2 and the stability is greatly improved. Theoretical calculations reveal that the Ru–O–C structure formed in RuO2 facilitates the dissociation of H–OH, which is the rate-determining step in alkaline electrolytes, leading to significant improvement in HER activity.

Synthesis, Characterization, and Photoelectric and Electrochemical Behavior of (CH3NH3)2Zn1-xCoxBr4 Perovskites.

We report the synthesis, characterization, and photoelectric and electrochemical properties of (CH3NH3)2Zn1-xCoxBr4 (x = 0.0, 0.3, 0.5, 0.7, and 1.0) samples. X-ray powder and single-crystal diffraction confirm the formation of solid solution across the entire range. Additionally, as the cobalt concentration increases, the crystallinity of the samples decreases, as indicated by the powder diffraction patterns. All samples remain stable up to 560 K, beyond which they decompose into CH3NH3Br and the respective bromide. The semiconductor behavior of the compounds is confirmed through optical absorption measurements, and band gap values are determined by using the Tauc method from diffuse reflectance spectra. Raman spectroscopy reveals a slight redshift in all vibration modes with increasing cobalt content. Finally, photovoltaic measurements on solar cells constructed with (MA)2CoBr4 perovskite exhibit modest performance, and electrochemical measurements indicate that the compound with the composition (MA)2Zn0.3Co0.7Br4 exhibits the highest current for electrochemical water reduction during oxygen evolution.

A first-principles study on two-dimensional tetragonal samarium nitride as a novel photocatalyst for hydrogen production

The prospect of using two-dimensional tetragonal samarium nitride (t-SmN) in photocatalytic applications is being reported. First principles calculations were performed in order to study structural, electronic, thermal and photocatalytic properties of the material. The phonon band structure and cohesive energy computed using density functional perturbation theory revealed dynamical stability of the monolayer. Ab-initio molecular dynamics (AIMD) simulations were carried out to check the thermal stability of the monolayer which pointed that the material is stable above 1000 K and chemically inert at room temperature. The electronic structure calculations revealed SmN as ferromagnetic semiconductor with a direct bandgap of 1.41eV that predicts applications of the material in spintronics. The band alignment pointed towards suitability of band offsets for electrochemical reduction of water splitting at neutral pH. The optical properties indicated decent light-harvesting ability of the material from visible and ultraviolet regions of solar spectrum. The micro-mechanisms of water decomposition and photocatalytic hydrogen formation on the SmN monolayer are unraveled to confirm water splitting and hydrogen generation.

Development of polyoxometalate-based Ag-H2biim inorganic-organic hybrid compounds functionalized for the acid electrocatalytic hydrogen evolution reaction.

The electrocatalytic hydrogen evolution reaction (HER) is an ideal method for hydrogen production. Transition metal complex electrocatalysts exhibit poor HER activity due to excessive or weak adsorption of H during the electrochemical reduction of water to molecular hydrogen in acidic environments. Developing specific functional complex materials as desired catalysts is challenging. Here, an electrochemical surface restructuring strategy of polyoxometalate (POM)-modified Ag materials toward the HER with a dramatically decreased overpotential under acidic aqueous conditions is established. We prepared two POM [SiW12O40]4- (SiW12)/[P2W18O62]6- (P2W18)-based Ag-2,2'-biimidazole (H2biim) inorganic-organic hybrid compounds (1 and 2) via the hydrothermal method and these two compounds undergo an electrochemical restructuring process in 0.5 M H2SO4 during the HER, in which Ag nanoparticles are in situ formed with the basic structures of SiW12 and P2W18 being maintained. The activated catalysts (1-AC-RDE and 2-AC-RDE) exhibit good electrocatalytic activity for the HER with good long-term stability, and the required overpotentials at a current density of 10 mA cm-2 are 112 mV (1-AC-RDE) and 91 mV (2-AC-RDE) with Tafel slopes of 77 mV dec-1 and 65 mV dec-1, respectively. The excellent electron-proton storage and transferability of SiW12 and P2W18 may provide a solution for the insufficient capture of H by Ag, leading to an effective self-optimizing behavior and superior acidic HER activity.

Activation of metal-free porous basal plane of biphenylene through defects engineering for hydrogen evolution reaction

The biggest challenge in the commercial application of electrochemical reduction of water through the hydrogen evolution reaction (HER) is hampered due to the scarcity of inexpensive and efficient catalysts. Herein, we propose a metal-free biphenylene nanosheet, a recently proposed two-dimensional (2D) carbon allotrope, as an excellent HER electrocatalyst. The dynamical and thermal stability of biphenylene nanosheet is validated through phonon dispersion and abinitio molecular dynamics (AIMD) calculations, respectively. At a low H coverage (1/54), the biphenylene nanosheet shows excellent catalytic activity with the Gibbs free energy (ΔGH∗) of 0.082 eV. The Bdoping and C-vacancy in biphenylene further improve ΔGH∗ to −0.016 eV and 0.005 eV, respectively. The interactions between the H atom and the nanosheet are explained through the relative position of the p-band center. Our study opens new possibilities to use non-metallic porous materials as highly efficient electrocatalysts for HER.

Reversible hydrogen spillover in Ru-WO3-x enhances hydrogen evolution activity in neutral pH water splitting

Noble metal electrocatalysts (e.g., Pt, Ru, etc.) suffer from sluggish kinetics of water dissociation for the electrochemical reduction of water to molecular hydrogen in alkaline and neutral pH environments. Herein, we found that an integration of Ru nanoparticles (NPs) on oxygen-deficient WO3-x manifested a 24.0-fold increase in hydrogen evolution reaction (HER) activity compared with commercial Ru/C electrocatalyst in neutral electrolyte. Oxygen-deficient WO3-x is shown to possess large capacity for storing protons, which could be transferred to the Ru NPs under cathodic potential. This significantly increases the hydrogen coverage on the surface of Ru NPs in HER and thus changes the rate-determining step of HER on Ru from water dissociation to hydrogen recombination.

Barrel-Shaped-Polyoxometalates Exhibiting Electrocatalytic Water Reduction at Neutral pH: A Synergy Effect.

Two copper-based barrel-shaped polyoxometalates (POMs), namely, [{H3O}4{Na6(H2O)22}][{CuI (H2O)3}2{CuII (H2O)}3{B-α-BiIIIWVI9O33}2]·7H2O (NaCu-POM) and Li4[{NH4}2{H3O}3{Li(H2O)5}][{CuII(SH)}{(CuIICuI1.5)(B-α-BiIIIWVI9O33)}2]·9H2O (LiCu-POM) have been synthesized and structurally characterized. The single-crystal X-ray diffraction analyses of NaCu-POM and LiCu-POM reveal the presence of penta- and hexa-nuclear copper wheels per formula units, respectively; these copper wheels are sandwiched between two lacunary Keggin anions {B-α-BiIIIWVI9O33}9- (BiW9) to form the barrel-shaped title POM compounds. In both the compounds NaCu-POM and LiCu-POM, the mixed-valent copper centers are present in their respective penta- and hexa-nuclear copper wheels, established by X-ray photoelectron spectroscopy (XPS) as well as by bond valence sum (BVS) calculations. Compound LiCu-POM additionally shows the presence of a sulfhydryl ligand (SH-), coordinated to one of the copper centers of its {Cu6}-wheel, that is expected to be generated from the in situ reduction of sulfate anion present in the concerned reaction mixture (lithium-ion in ammonia solution may be the reducing agent). Interestingly, the title compounds, NaCu-POM and LiCu-POM exhibit an efficient electrocatalytic hydrogen evolution reaction (HER) by reducing water at neutral pH. Detailed electrochemical studies including controlled experiments indicate that the active sites for this electrocatalysis are the W(VI) centers of the title compounds, not the copper centers. However, a relevant tri-lacunary Keggin cluster anion {PVWVI9O33}7- (devoid of copper ion) does not show comparable HER as shown by the title compounds. The intra-cluster cooperative interactions of the mixed-valent copper centers (CuII/CuI) with the tungsten centers (W6+) make the overall system electrocatalytically active toward water reduction to molecular hydrogen at neutral pH. High Faradaic efficiencies (89 and 92%) and turnover frequencies (1.598 s-1 and 1.117 s-1) make the title compounds NaCu-POM and LiCu-POM efficient catalysts toward electrochemical water reduction to molecular hydrogen.

Electrochemical Pretreatment of Solid–Electrolyte Interphase Formation for Enhanced Li 4 Ti 5 O 12 Anode Performance in a Molten Li–Ca Binary Salt Hydrate Electrolyte

“Significant improvement in the cycle performance of a Li4Ti5O12 electrode was achieved using a Li–Ca binary salt hydrate (LCH) electrolyte in combination with an optimized electrochemical pretreatment process. The LCH electrolyte provided less-water-soluble Ca-based SEI components, and careful pretreatment process enabled the formation of a thicker SEI layer on the Li4Ti5O12 electrode, leading to 95.5 % capacity retention over 50 cycles…“ Learn more about the story behind the research featured on the front cover in this issue's Cover Profile. Read the corresponding Research Article at 10.1002/celc.202200061.

Front Cover: Electrochemical Pretreatment of Solid‐Electrolyte Interphase Formation for Enhanced Li 4 Ti 5 O 12 Anode Performance in a Molten Li−Ca Binary Salt Hydrate Electrolyte (ChemElectroChem 11/2022)

The Front Cover illustrates that less-water-soluble Ca-based solid electrolyte interphase formed in a Li–Ca binary salt hydrate electrolyte effectively mitigates electrochemical reduction of water, leading to improvement in the cycle performance of a Li4Ti5O12 electrode. More information can be found in the Research Article by S. Kondou et al.

High throughput screening driven discovery of Mn5Co10Fe30Ni55Ox as electrocatalyst for water oxidation and electrospinning synthesis

The electrochemical reduction of water or CO2 holds great potential for storage of intermittently produced renewable energy and requires highly active electrocatalyst to speed up water oxidation at anode. Metal oxides containing multiple components are promising electrocatalyst for the oxygen evolution reaction (OER); but their development demands exhaustive experiments, resulting in long term cycles and high cost in research. In this paper, a high-throughput experimental scheme is employed for searching effective OER oxide electrocatalysts. Combining inkjet printing technology and scanning electrochemical microscopy, a material library of Mn-Co-Fe-Ni oxide is rapidly prepared and its OER activity is screened. The composition of Mn5Co10Fe30Ni55Ox has been found to exhibit the highest catalytic activity. Nanowire catalyst of the corresponding composition has been prepared by electrospinning method, which achieves a low overpotential of 280 mV at current density of 10 mA·cm−2 and a Tafel slope of 60.4 mV·dec−1.

Dibenzyldithiocarbamate-Functionalized Small Gold Nanoparticles as Selective Catalysts for the Electrochemical Reduction of CO2 to CO

Dibenzyldithiocarbamate-functionalized gold nanoparticles (DBDTC-Au NPs) were synthesized in the laboratory, and the effects of the DBDTC ligand on the Au electrocatalytic activity for CO2 reduction reaction to CO were investigated in CO2-saturated KHCO3 electrolyte. Our synthesis route produced sub-2.0 nm Au particles, with the number of gold atoms in the range of 67–120, and with strongly bonded and stable DBDTC ligands on the surface. Online differential electrochemical mass spectrometry (DEMS) experiments on the DBDTC-Au nanoparticles showed higher faradic current and higher CO/H2 ratio of their ionic signals compared to those for citrate-capped Au NPs. Quantitative analyses via gas chromatography (GC) showed that our DBDTC-Au catalyst converts CO2 into CO with a faradic efficiency of approximately 100 ± 1% at −0.8 V and 93 ± 0.5% at −1.0 V vs RHE. The higher activity of the synthesized DBDTC-Au electrocatalyst was attributed to the following important functionalities of the DBDTC ligand: (i) the strong bonding to the surface stabilizes the Au NPs, inhibiting the formation of large agglomerates, and (ii) the hydrophobic character of the dibenzyl moieties allows the permeation of CO2, but repels water from the Au surface, minimizing the electrochemical reduction of water and, therefore, enhancing the faradic efficiency for CO formation.

Electrocatalytic hydrogen evolution by molecular Cu(II) catalysts

Two mononuclear copper complexes [Cu(QCl-tpy)(OAc)(Cl)] (1′), and [Cu(4Ql-tpy)(OAc)(OH2)]Cl (2′) (QCl-tpy corresponds to 2-choloro-3-(2,6-di(pyridin-2-yl)pyridine-4-yl) quinoline and 4Ql-tpy represents 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)quinoline) have been synthesized and characterized using various analytical techniques like HRMS, EPR, single-crystal XRD, CV etc. Both the complexes were employed for electrochemical proton reduction in 95:5 DMF/H2O (v/v) in the presence of AcOH as a proton source. Among the two complexes, 2′ exhibits higher catalytic activity for proton reduction and achieves a TOF value of 1682 s−1. The complexes also performed electrochemical water reduction in 0.1 M phosphate buffer at environmentally benign pH 7.0, with lower overpotential than that of a blank solution by 400 mV and 350 mV for complex (1′) and (2′) respectively. The acid-base equilibria study in Britton- Robinson buffer predicts that both the complexes show comparable pKa value around 4.4, which is for [CuII(L)(OAc)(H2O)]+ ⇆ [CuII(L)(OAc)(OH)] + H+, where L is QCl-tpy or 4Ql-tpy. The Nyquist plot shows that the Heyrovsky step is the rate-determining step in both the catalysts. As a result, both the complexes follow the same mechanistic cycle, wherein the deprotonated species [CuII(L)(OAc)(OH)] on PCET generates [Cu0(L)(OAc)(OH2)]-. The [Cu0(L)(OAc)(OH2)]- converts into the CuII-H, in the presence of H+ and further CuII-H evolves H2 in the presence of H2O to complete the cycle.

Scalable Synthesis of Hollow MoS2 Nanoparticles Modified on Porous Ni for Improved Hydrogen Evolution Reaction

Developing an inexpensive non-noble metal catalyst is essential in the electrochemical water reduction. However, to choose the support for hydrogen-producing catalyst is still a problem that needs to be solved. Herein, we report a novel route combining powder metallurgy and hydrothermal synthesis to fabricate Ni/MoS2 catalysts for boosting the hydrogen evolution reaction performance. Powder metallurgy produces the suborbicular pores on the surface and inside Ni once the sintering temperature reaches 1000 °C. The hollow MoS2 nanoparticles are successfully modified on the surface of porous Ni by hydrothermal synthesis. The hollow structure of MoS2 nanoparticles provides more active sites for electrochemical reaction and the porous Ni matrix with the porosity of 16.5% exhibits higher electronic conductivity, which endows the Ni/MoS2−1000 catalyst with an excellent hydrogen evolution reaction activity. The Ni/MoS2−1000 shows a low overpotential of 229 mV at the current densities of 10 mA·cm−2 with a small Tafel slope of 76 mV·dec−1. This study provides guidelines on the large-scale synthesis of nanostructured electrocatalysts with porous Ni as the support.

Ruthenium nanodendrites on reduced graphene oxide: an efficient water and 4-nitrophenol reduction catalyst

A green and efficient protocol is reported for the elegant design of reduced graphene oxide (rGO)-supported Ru nanodendrites for promotion of electrochemical water reduction in a wide pH range as well as for environmental remediation.

Electrochemically assisted polyamide deposition at three-phase junction

A three-phase junction formed between a solid electrode placed between solutions of the organic phase and the aqueous phase is employed for localized and electrochemically assisted nylon-6,6 deposition. The fluorine-doped tin oxide (FTO) conductive support (serving as the working electrode) is immersed into the cell filled with the aqueous solution of 1,6-hexanediamine (1,6-DAH) and the organic solution of adipoyl chloride (AC) dissolved in 1,2-dichloroethane. The interfacial polycondensation reaction between these two reagents occurs spontaneously at the liquid–liquid interface at elevated pH and is inhibited when diamine is fully protonated. With this in mind, we have designed a system where the pH close to the three-phase junction was increased using electrochemical water reduction triggering very localized polyamide deposition.

Nickel phosphide catalysts for hydrogen generation through water reduction, ammonia-borane and borohydride hydrolysis

Using hydrogen as fuel requires efficient production and retrieval from energy carriers. Here, we describe the synthesis of various pure phases of nickel phosphide and describe the growth mechanisms. A comparative study illustrates the phases’ catalytic activity towards hydrogen production through electrochemical water reduction as well as hydrogen retrieval by hydrolysis of hydrogen storage materials (ammonia-borane and NaBH4). Charge separation between the Niδ+ and Pδ− sites in the various Ni–P phases plays a key role in achieving the desired efficacy of the catalytic reaction. Ni2P exhibited a significant enhancement towards the hydrogen evolution reaction, with an overpotential of 126 mV at J= 10 mA cm−2 in acid and 180 mV in alkaline. Ni12P5 was the most efficient catalyst, with a turnover frequency (TOF) = 23.0 min−1 for hydrogen evolution from ammonia-borane, and TOF= 17.3 min−1 from NaBH4, which is in accordance with noble metal nanoparticles.

Hydrogen Production from Water Electrolysis Driven by High Membrane Voltage of Reverse Electrodialysis

The voltage produced from the salinity gradient in reverse electrodialysis (RED) increases proportionally with the number of cell pairs of alternating cation and anion exchange membranes. Large-scale RED systems consisting of hundreds of cell pairs exhibit high voltage of more than 10 V, which is sufficient to utilize water electrolysis as the electrode reaction even though there is no specific strategy for minimizing the overpotential of water electrolysis. Moreover, hydrogen gas can be simultaneously obtained as surplus energy from the electrochemical reduction of water at the cathode if the RED system is equipped with proper venting and collecting facilities. Therefore, RED-driven water electrolysis system can be a promising solution not only for sustainable electric power but also for eco-friendly hydrogen production with high purity without CO2 emission. The RED system in this study includes a high membrane voltage from more than 50 cells, neutral-pH water as the electrolyte, and an artificial NaCl solution as the feed water, which are more universal, economical, and eco-friendly conditions than previous studies on RED with hydrogen production. We measure the amount of hydrogen produced at maximum power of the RED system using a batch-type electrode chamber with a gas bag and evaluate the interrelation between the electric power and hydrogen energy with varied cell pairs. A hydrogen production rate of 1.1 × 10<sup>−4</sup> mol cm<sup>−2</sup> h<sup>−1</sup> is obtained, which is larger than previously reported values for RED system with simultaneous hydrogen production.

1000-Fold Preconcentration of Per- and Polyfluorinated Alkyl Substances within 10 Minutes via Electrochemical Aerosol Formation.

We present a simple and efficient method for preconcentrating per- and polyfluorinated alkyl substances (PFAS) in water. Our method was inspired by the sea-spray aerosol enrichment in nature. Gas bubbles in the ocean serve to scavenge surface active material, carrying it to the air-ocean interface, where the bubbles burst and form a sea-spray aerosol. These aerosol particles are enriched in surface-active organic compounds such as free fatty acids and anionic surfactants. In our method, we in situ generate H2 microbubbles by electrochemical water reduction using a porous Ni foam electrode. These H2 bubbles pick up PFAS as they rise through the water column that contains low concentration PFAS. When these bubbles reach the water surface, they burst and produce aerosol droplets that are enriched in PFAS. Using this method, we demonstrated ∼1000-fold preconcentration for ten common PFAS in the concentration range from 1 pM to 1 nM (or ∼0.5 ng/L to 500 ng/L) in 10 min. We also developed a diffusion-limited adsorption model that is in quantitative agreement with the experimental data. In addition, we demonstrated using this method to preconcentrate PFAS in tap water, indicating its potential use for quantitative analysis of PFAS in real-world water samples.