Long-term Electrolysis Research Articles

Bimetallic NiCo boride nanoparticles confined in a MXene network enable efficient ambient ammonia electrosynthesis

Ambient electrocatalytic nitrogen fixation is an emerging technology for green ammonia synthesis, but the absence of optimized, stable and performant catalysts can render its practical application challenging. Herein, bimetallic NiCo boride nanoparticles confined in MXene are shown to accomplish high-performance nitrogen reduction electrolysis. Taking advantage of the synergistic effect in specific compositions with unique electronic d and p orbits and typical architecture of rich nanosized particles embedded in the interconnected conductive network, the synthesized MXene@NiCoB composite demonstrates extensive improvements in nitrogen molecule chemisorption, active area exposure and charge transport. As a result, optimal NH3 yield rate of 38.7 μg h−1 mgcat.−1 and Faradaic efficiency of 6.92% are acquired in 0.1 M Na2SO4 electrolyte. Moreover, the great catalytic performance can be almost entirely maintained in the cases of repeatedly-cycled and long-term electrolysis. Theoretical investigations reveal that the nitrogen reduction reaction on MXene@NiCoB catalyst proceeds according to the distal pathway, with a distinctly-reduced energy barrier relative to the Co2B counterpart. This work may inspire a new route towards the rational catalyst design for the nitrogen reduction reaction.

Beyond the catalyst: A robust and omnidirectional hydrophobic triple-phase architecture for ameliorating air-breathing H2O2 electrosynthesis and wastewater remediation

On-site H2O2 electro-generation via 2-electron O2 reduction reaction (2e−-ORR) presents tremendous potential in various H2O2-based industries. Primary efforts are dedicated in synthesizing ideal catalysts, whereas this is only part of the determinants because 2e−-ORR involves an interface of gas–liquid-solid triple phases. Beyond the catalysts, comprehending and establishing an appropriate reaction microenvironment is also crucial due to its significant roles in mediating the transport of 2e−-ORR species and boosting the kinetics of 2e−-ORR. Herein, a robust and omnidirectional hydrophobic triple-phase architecture is created to ameliorate the H2O2 electro-generation by facilely and reasonably regulating the proportion between catalysts and PTFE microparticles without energy-hungry high-temperature calcination. The positive roles of the hydrophobic triple-phase architecture are primarily ascribed to the prominently enhanced O2 mass transfer towards the reaction center, full utilization of electroactive sites, still satisfactory ion mobility, electron conductivity and liquid paths, as well as extensive triple-phase reaction zones with ultrahigh 2e−-ORR activity. Under natural air diffusion without energy-hungry aeration, the cathode with proper wettability reaches a preeminent H2O2 productivity at a near-industrial current density (44.6 mg h−1 cm−2 with satisfying current efficiency of 70.2% at 100 mA cm−2) and is dependable in the long-term electrolysis stability, which almost surpasses all the previous carbonaceous air-breathing cathodes. Considering various advantages (superior performance, economy energy, low cost, facile fabrication, etc.) of the air-breathing cathode, it also exhibits prominent behavior in wastewater remediation. This study highlights the significance of interface engineering and provides valuable inspiration for similar gas-involving electrocatalytic reactions.

Atomically Thin, Ionic-Covalent Organic Nanosheets for Stable, High-Performance Carbon Dioxide Electroreduction.

The incorporation of charged functional groups is effective to modulate the activity of molecular complexes for the CO2 reduction reaction (CO2 RR), yet long-term heterogeneous electrolysis is often hampered by catalyst leaching. Herein, an electrocatalyst of atomically thin, cobalt-porphyrin-based, ionic-covalent organic nanosheets (CoTAP-iCONs) is synthesized via a post-synthetic modification strategy for high-performance CO2 -to-CO conversion. The cationic quaternary ammonium groups not only enable the formation of monolayer nanosheets due to steric hindrance and electrostatic repulsion, but also facilitate the formation of a *COOH intermediate, as suggested by theoretical calculations. Consequently, CoTAP-iCONs exhibit higher CO2 RR activity than other cobalt-porphyrin-based structures: an 870% and 480% improvement of CO current densities compared to the monomer and neutral nanosheets, respectively. Additionally, the iCONs structure can accommodate the cationic moieties. In a flow cell, CoTAP-iCONs attain a very small onset overpotential of 40mV and a stable total current density of 212mAcm-2 with CO Faradaic efficiency of >95% at -0.6V for 11h. Further coupling the flow electrolyzer with commercial solar cells yields a solar-to-CO conversion efficiency of 13.89%. This work indicates that atom-thin, ionic nanosheets represent a promising structure for achieving both tailored activity and high stability.

Assembly of an improved hybrid cascade system for complete ethylene glycol oxidation: Enhanced catalytic performance for an enzymatic biofuel cell

We report an Enzymatic Fuel Cell (EFC) combining an enzyme that can cleave carbon-carbon bonds (oxalate oxidase (OxOx)) with an organic catalyst (Pyrene-TEMPO (TEMPO = 2,2,6,6-tetramethyl piperidinyl-N-oxyl)) immobilized on the surface of modified carboxylated multi-walled carbon nanotubes (MWCNT-COOH). This combination gave a hybrid bi-catalyst electrode for complete ethylene glycol (EG) oxidation. The hybrid electrode provided ninefold enhanced catalytic activity (0.17 ± 6 × 10−3 mA cm−2) in the presence of EG as compared to the electrode in the absence of EG (0.018 ± 3 × 10−5 mA cm−2), indicating that the enzyme combined with the organic catalyst improved energy generation through deep EG electrooxidation. Electrochemical impedance spectroscopy reveals that the addition of the enzyme in the electrode containing MWCNT–COOH–Pyrene-TEMPO increased the charge transfer resistance (Rct) and the capacitance of the double layer. Long-term electrolysis for 15 h showed that the hybrid electrode presented outstanding current density and stability. The EG oxidation products were identified and quantified by high-performance liquid chromatography (HPLC-UV/RID). The results confirmed complete EG oxidation in the presence of CO2 in the solution, allowing 10 electrons to be collected from the fuel. Overall, this study illustrates the development of a simple and improved hybrid bi-catalyst electrode for promising applications in small electronic devices.

High-Efficiency Ammonia Electrosynthesis on Anatase TiO2-x Nanobelt Arrays with Oxygen Vacancies by Selective Reduction of Nitrite.

Electrocatalytic reduction of nitrite to NH3 provides a new route for the treatment of nitrite in wastewater, as well as an attractive alternative to NH3 synthesis. Here, we report that an oxygen vacancy-rich TiO2-x nanoarray with different crystal structures self-supported on the Ti plate can be prepared by hydrothermal synthesis and by subsequently annealing it in an Ar/H2 atmosphere. Anatase TiO2-x (A-TiO2-x) can be a superb catalyst for the efficient conversion of NO2- to NH3; a high NH3 yield of 12,230.1 ± 406.9 μg h-1 cm-2 along with a Faradaic efficiency of 91.1 ± 5.5% can be achieved in a 0.1 M NaOH solution containing 0.1 M NaNO2 at -0.8 V, which also exhibits preferable durability with almost no decay of catalytic performances after cycling tests and long-term electrolysis. Furthermore, a Zn-NO2- battery with such A-TiO2-x as a cathode delivers a power density of 2.38 mW cm-2 as well as a NH3 yield of 885 μg h-1 cm-2.

Steam Electrolysis vs. Co-Electrolysis: Mechanistic Studies of Long-Term Solid Oxide Electrolysis Cells

High-temperature electrolysis using solid oxide electrolysis cells (SOECs) is an innovative technology to temporarily store unused electrical energy from renewable energy sources. However, they show continuous performance loss during long-term operation, which is the main issue preventing their widespread use. In this work, we have performed the long-term stability tests up to 1000 h under steam and co-electrolysis conditions using commercial NiO-YSZ/YSZ/GDC/LSC single cells in order to understand the degradation process. The electrolysis tests were carried out at different temperatures and fuel gas compositions. Intermittent AC- and DC- measurements were performed to characterize the single cells and to determine the responsible electrode processes for the degradation during long-term operation. An increased degradation rate is observed at 800 °C compared to 750 °C under steam electrolysis conditions. Moreover, a lower degradation rate is noticed under co-electrolysis operation in comparison to steam electrolysis operation. Finally, the post-test analyses using SEM-EDX and XRD were carried out in order to understand the degradation mechanism. The delamination of LSC is observed under steam electrolysis conditions at 800 °C, however, such delamination is not observed during co-electrolysis operation. In addition, Ni-depletion and agglomeration are observed on the fuel electrode side for all the cells.

Rigorous Evaluation of Liquid Products in High-Rate CO2/CO Electrolysis

In this work, the water crossover in CO2 electrolysis was systematically analyzed with hydrated ions transferring through ion-selective membranes as charge carriers in three-compartment devices. We demonstrate that the water crossover leads to a variation in the electrolyte volume, which plays an important role in the evaluation of the catalytic selectivity of liquid products. Without considering the water crossover, the catalytic selectivity of liquid products could be overestimated with the use of anion exchange membranes and underestimated with cation exchange membranes, especially when ions with large hydration numbers transport through membranes for long-term electrolysis. In addition, a protocol for reliably quantifying liquid products is proposed for high-rate CO2/CO electrolysis in three-compartment flow electrolyzers.

Stabilization of Cu+ via Strong Electronic Interaction for Selective and Stable CO2 Electroreduction.

Copper oxide-based materials effectively electrocatalyze carbon dioxide reduction (CO2 RR). To comprehend their role and achieve high CO2 RR activity, Cu+ in copper oxides must be stabilized. As an electrocatalyst, Cu2 O nanoparticles were decorated with hexagonal boron nitride (h-BN) nanosheets to stabilize Cu+ . The C2 H4 /CO ratio increased 1.62-fold in the CO2 RR with Cu2 O-BN compared to that with Cu2 O. Experimental and theoretical studies confirmed strong electronic interactions between the two components in Cu2 O-BN, which strengthens the Cu-O bonds. Electrophilic h-BN receives partial electron density from Cu2 O, protecting the Cu-O bonds from electron attack during the CO2 RR and stabilizing the Cu+ species during long-term electrolysis. The well-retained Cu+ species enhanced the C2 product selectivity and improved the stability of Cu2 O-BN. This work offers new insight into the metal-valence-state-dependent selectivity of catalysts, enabling the design of advanced catalysts.

Engineering a concordant microenvironment with air-liquid-solid interface to promote electrochemical H2O2 generation and wastewater purification

Electroreduction of O2 to H2O2 provides an appealing route for both renewable energy storage and wastewater purification. Although grand endeavors are devoted into synthesizing high-performance electrocatalysts, the understanding and engineering of local microenvironment around reactive sites are also critical. Herein, a concordant gas/liquid microenvironment with plentiful air–liquid-solid interfaces is constructed and evaluated, which not only mediates the harmonious transport of O2, H+ and electron (especially reducing the O2 diffusion layer thickness and raising O2 local concentration to accelerate O2 transport), but also induces massive highly active triphase boundary sites for high-rate H2O2 electro-generation. Consequently, even under natural air diffusion, the cathode with the polytetrafluoroethylene proportion of 20% achieves a preeminent H2O2 productivity (40.7 mg h−1 cm−2 at 100 mA cm−2, corresponding to the maximum H2O2 concentration of 813.8 mg L-1) with superior current efficiency (64.2%). Long-term electrolysis stability and reusability indicate the practicability of the air-breathing cathode without the energy-intensive external aeration. Efficient purification of dye wastewater by the modified cathode meets the expectation, showing its huge potential in environmental protection. This work deeply elucidates the effect mechanism of a concordant gas/liquid microenvironment with abundant air–liquid-solid interfaces, which has considerable guiding significance for similar gas-involving electrocatalysis.

Enhanced N2-to-NH3 conversion efficiency on Cu3P nanoribbon electrocatalyst

Ambient electroreduction of nitrogen (N2) is considered as a green and feasible approach for ammonia (NH3) synthesis, which urgently demands for efficient electrocatalyst. Morphology has close relationship with catalytic activity of heterogeneous catalysts. Nanoribbon is attractive nanostructure, which possesses the flexibility of one-dimensional nanomaterials, the large surface area of two-dimensional nanomaterials, and lateral size confinement effects. In this work, Cu3P nanoribbon is proposed as a highly efficient electrocatalyst for N2-to-NH3 conversion under benign conditions. When measured in N2-saturated 0.1 M HCl, such Cu3P nanoribbon achieves high performance with an excellent Faradaic efficiency as high as 37.8% and a large yield of 18.9 µg·h−1µmgcat.−1 at −0.2 V. It also demonstrates outstanding stability in long-term electrolysis test at least for 45 h.

One-pot electrodeposition synthesis of NiFe-phosphate/phosphide hybrid nanosheet arrays for efficient water splitting

Developing facile and cost-effective synthetic strategies for efficient overall water splitting (OWS) catalysts is critical to renewable energy industry. Herein, the one-pot electrodeposition synthesis of hybrid NiFe-phosphate/phosphide (NiFe-Pi/P) nanosheet arrays on Ni foam (NF) as bifunctional catalytic electrode is freshly proposed. Through electrodeposition process, nano morphology and bimetallic synergism of NiFe-Pi\\P can be well controlled, resulting in hybrid micro crystallization, nanosheet array structure, superhydrophilicity and superareophobicity characteristics, all of which guarantee high activity, rich active sites and quick mass transfer process on electrode surface. The fabricated electrode exhibits excellent activity on hydrogen evolution reaction (HER), the optimum one displays overpotential of 97 mV and 185 mV to acquire current responses to 20 mA cm−2 and 100 mA cm−2 respectively, which are close to those of Pt/C catalyst in alkaline medium. Besides, the electrode shows good activity for oxygen evolution reaction (OER) to deliver 20 mA cm−2 at 255 mV as well. More importantly, in long-term multi-step current electrolysis over 27 h, the symmetrically assembled cell adopting NiFe-Pi/P@NF electrode needs only 1.57 V to get 10 mA cm−2, and negligible voltage elevation is revealed with current density cyclic variations. This work provides practical insights into synthesis of hybrid nanosheet array OWS catalysts in efficient approach and on a large scale.

Bifunctional Mn-doped CoSe2 nanonetworks electrode for hybrid alkali/acid electrolytic H2 generation and glycerol upgrading

Electrolytic water splitting, as a promising route to hydrogen (H2) production, is still confronted with the sluggish anodic oxygen evolution reaction (OER) and its less value-added O2 production. Herein, we report a bifunctional electrode fabricated by in situ growth of Mn-doped CoSe2 nanonetworks on carbon fiber cloth (Mn-CoSe2/CFC), which shows attractive electrocatalytic properties toward glycerol oxidation reaction (GOR) in alkali and hydrogen evolution reaction (HER) in acid. A flow alkali/acid hybrid electrolytic cell (fA/A-hEC) was then developed by coupling anodic GOR with cathodic HER with the Mn-CoSe2/CFC bifunctional electrode. Such fA/A-hEC enables a rather low voltage of 0.54 V to achieve 10 mA cm−2, and maintain long-term electrolysis stability over 300-h operation at 100 mA cm−2 with Faraday efficiencies of over 99% for H2 and 90% for formate production. The designed bifunctional electrode in such innovative fA/A-hEC device provides insightful guidance for coupling energy-efficient hydrogen production with biomass upgradation.

High Entropy Alloy Electrocatalytic Electrode toward Alkaline Glycerol Valorization Coupling with Acidic Hydrogen Production.

Electrochemical glycerol oxidation reaction (GOR) is an attractive alternative anodic reaction to oxygen evolution reaction for a variety of electrolytic synthesis, thanks to the possibility of mass production of glycerol from biomass and the relative low thermodynamic potential of GOR. The development of high-activity cheap electrocatalysts toward GOR yet faces a daunting challenge. Herein, we experimentally prepare a new range of high entropy alloy (HEA) self-supported electrodes with uniform HEA nanoparticles grown on carbon cloth. The systematic electrochemical studies verify that the HEA-CoNiCuMnMo electrode exhibits attractive performance for GOR electrocatalysis with low overpotential and high selectivity toward formate products. The surface atomic configurations of HEA-CoNiCuMnMo are studied by a self-developed machine learning-based Monte Carlo simulation, which points out the catalytic active center to be Mo sites coordinated by Mn, Mo, and Ni. We further develop a hybrid alkali/acid flow electrolytic cell by pairing alkaline GOR with acidic hydrogen evolution reaction using the HEA-CoNiCuMnMo and the commercial RhIr/Ti as the anode and the cathode, respectively, which only requires an applied voltage of 0.55 V to reach an electrolytic current density of 10 mA cm-2 and maintains long-term electrolysis stability over 12 days continuous running at 50 mA cm-2 with Faraday efficiencies of over 99% for H2 in the cathode and 92% for formate production in the anode.

Performance and Degradation of Electrolyte-Supported Single Cell Composed of Mo-Au-Ni/GDC Fuel Electrode and LSCF Oxygen Electrode during High Temperature Steam Electrolysis

Ni-gadolinia-doped ceria (GDC) based electrode materials have drawn significant attention as an alternative fuel electrode for solid oxide cells (SOCs) owing to mixed ionic conductivity of GDC and high electronic and catalytic activity of Ni. Moreover, the catalytic activity and electrochemical performance of the Ni-GDC electrode can be further improved by dispersing small quantities of other metal additives, such as gold or molybdenum. Therefore, herein, we considered gold and molybdenum modified Ni-GDC electrodes and focused on the upscaling; hence, we prepared 5 × 5 cm2 electrolyte-supported single cells. Their electrochemical performance was investigated at different temperatures and fuel gas compositions. The long-term steam electrolysis test, up to 1700 h, was performed at 900 °C with −0.3 A·cm−2 current load. Lastly, post-test analyses of measured cells were carried out to investigate their degradation mechanisms. Sr-segregation and cobalt oxide formation towards the oxygen electrode side, and Ni-particle coarsening and depletion away from the electrolyte towards the fuel electrode side, were observed, and can be considered as a main reason for the degradation. Thus, modification of Ni/GDC with Au and Mo seems to significantly improve the electro-catalytic activity of the electrode; however, it does not significantly mitigate the Ni-migration phenomenon after prolonged operation.

Electrode Engineering for Electrochemical CO2 Reduction

Excessive carbon dioxide emissions cause severe global warming issues. One efficient way to deal with the problem is to convert CO2 into valuable fuels and chemicals. The electrocatalytic carbon dioxide reduction reaction (CO2RR) has an excellent potential to reduce carbon emissions. Over the past few decades, research in this field has focused on creating high-performance catalysts. However, the overall efficiency of the CO2RR process is limited at the device level, such as slow CO2 mass transportation causing insufficient current density output, which is an urgent issue to be overcome in the design of advanced CO2RR electrolyzers. Recently, gas diffusion electrodes have been intensively investigated to raise the CO2RR performance into commercially relevant current densities by boosting the CO2 mass transport rate. This review focuses on recent research progress on electrode engineering for increasing the CO2RR performance, along with electrode-stabilizing strategies for long-term CO2 electrolysis. Future directions are proposed to accelerate the development of advanced electrodes for industrial CO2 electrolyzers.

High reversible cycling performance of carbon dioxide electrolysis by flat-tube solid oxide cell

Solid oxide cells (SOCs) are promising for CO2 electrolysis. In this work, performance and stability of flat-tube SOCs during periodic cycles of CO2 electrolysis and hydrogen fuel discharge were studied. The cell underwent 26 reversible cycles of operation for 624 h. When the cell was electrolyzed with a current density of −200 mA/cm2 at 1.07 V, the CO yield was 205.79 sccm and the CO2 conversion rate reached 45.7%. When the cell was operated in electrolysis mode, the degradation rate was 0.74 mV/h. Excluding the air heat consumption, the energy conversion efficiency was 81.42%; when the cell was operated at 0.6 slm hydrogen and +200 mA/cm2, the degradation rate was 0.385 mV/h. Compared with long-term constant current electrolysis, the alternate operation of charge and discharge used in this study is more typical of the actual application conditions.

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Trinuclear Nickel Catalyst for Water Oxidation: Intramolecular Proton-Coupled Electron Transfer Triggered Trimetallic Cooperative O–O Bond Formation

Metal Supported Electrolysis Cells

Solid oxide electrolyser (SOE) technology can become a key player in energy systems, with increasing shares of electricity from fluctuating sources such as wind and solar, contributing to power grid balance and energy storage as well as providing green fuels for transportation. Most mature SOE configurations are electrolyte supported or fuel electrode supported. Metal supported SOE cell configurations are an interesting concept for decreasing costs and increasing robustness. The present study compares fuel electrode supported and metal supported cells in terms of performance and durability under SOE conditions. Special emphasis was on medium temperature operating conditions of 650 °C. Metal supported cells, fabricated using ceramic processing methods, showed a better performance compared to state-of-the-art (SoA) cells with Ni/YSZ fuel electrode supported configuration, fabricated by tape casting and screen printing, under steam electrolysis conditions at 700 and 650 °C. The area specific cell resistance (ASR) was lower by ca. 20% for the metal supported cell in 50% H2O in H2 vs. air at 650 °C. Furthermore, the metal supported cells showed a stable performance—even a slight activation—during long-term steam electrolysis tests over 500 h at 650 °C and −0.25 and −0.5 A/cm2, while the SoA reference cell degraded with 13%/1000 h under the same conditions.

CO induced phase-segregation to construct robust and efficient IrRux@Ir core-shell electrocatalyst towards acidic oxygen evolution

The development of highly active and durable electrocatalysts towards oxygen evolution reaction (OER) in acidic solution is essential for the commercialization of proton exchange membrane water electrolysis (PEMWE). Herein, we design a rational CO-induced phase-segregation strategy to efficient construction of core-shell structure electrocatalyst featured with IrRux core and Ir-rich shell (IrRux@Ir). X-ray photoelectron spectroscopy reveals that the intensive electronic interaction between IrRux core and Ir-rich shell induces the charge transfer from Ir to IrRux, thereby increasing the oxidation state of surface Ir sites and leading to an enhanced OER activity. Simultaneously, the Ir-rich shell efficiently protects the inner Ru atoms against dissolution during the long-term electrolysis, in turn significantly improving the structural stability. The resultant IrRux@Ir catalyst exhibits a low overpotential of ca. 288 mV at a current density of 10 mAcm−2 and desirable durability. Impressively, the membrane electrode assembly integrated with IrRux@Ir as anode maintains a steady value of approximately 1.83 ± 0.02 V during 400 h of electrolysis at a current density of 1 A cm−2, demonstrating that the IrRux@Ir is one of the most robust OER electrocatalyst to date. This study provides a new perspective to design high performance and robust OER electrocatalysts in PEMWE.

Heterostructured CoP·CoMoP nanocages as advanced electrocatalysts for efficient hydrogen evolution over a wide pH range

A sustainable and environmental-friendly method to produce hydrogen with high purity is the electrochemical water splitting, but its commercialization is challenged due to lack of cost-effective electrocatalysts for hydrogen evolution reaction (HER) over a wide pH range. Herein, a series of CoP·xCoMoP heterostructured nanocages (NCs) were prepared via a dissolution-regrowth and subsequent phosphorization process using metal-organic frameworks (MOFs) as template. The three-dimensional (3D) architecture of CoP·xCoMoP is constituted by the heterostructured nanosheets composed with CoP and CoMoP phase. These noble-metal-free earth-abundant transition metal phosphide (TMP) catalysts show a pH-universal HER activity with high efficiency. Under the optimal atom ratio of Co and Mo (6:5), CoP·5CoMoP NC catalysts can deliver a current density of 10 mA cm−2 at the overpotential of 72 mV with a Tafel slope of 60.3 mV dec−1 in 1.0 M KOH solution. The same current output requires overpotential of 44 mV in 0.5 M H2SO4 solution and 151 mV in1.0 M phosphate buffered solution (PBS), respectively. The superior HER activity of CoP·5CoMoP NC catalysts can be comparable to or even better than most of noble metal-free HER electrocatalysts reported recently. In addition, CoP·5CoMoP NC catalysts also show a fairly high HER stability over a wide pH range, and their HER activity can be well kept without significant loss for long-term electrolysis. The 3D CoP·5CoMoP heterostructured catalysts hold promise as efficient and low-cost catalysts for water splitting devices over a wide pH range.