Direct Electrochemical Reduction Research Articles

Product Distribution of Steady-State and Pulsed Electrochemical Regeneration of 1,4-NADH and Integration with Enzymatic Reaction.

The direct electrochemical reduction of nicotinamide adenine dinucleotide (NAD+) results in various products, complicating the regeneration of the crucial 1,4-NADH cofactor for enzymatic reactions. Previous research primarily focused on steady-state polarization to examine potential impacts on product selectivity. However, this study explores the influence of dynamic conditions on the selectivity of NAD+ reduction products by comparing two dynamic profiles with steady-state conditions. Our findings reveal that the main products, including 1,4-NADH, several dimers, and ADP-ribose, remained consistent across all conditions. A minor by-product, 1,6-NADH, was also identified. The product distribution varied depending on the experimental conditions (steady state vs. dynamic) and the concentration of NAD+, with higher concentrations and overpotentials promoting dimerization. The optimal yield of 1,4-NADH was achieved under steady-state conditions with low overpotential and NAD+ concentrations. While dynamic conditions enhanced the 1,4-NADH yield at shorter reaction times, they also resulted in a significant amount of unidentified products. Furthermore, this study assessed the potential of using pulsed electrochemical regeneration of 1,4-NADH with enoate reductase (XenB) for cyclohexenone reduction.

Direct electrochemical reduction of graphene oxide thin film for aptamer-based selective and highly sensitive detection of matrix metalloproteinase 2

Simple and low-cost biosensing solutions are suitable for point-of-care applications aiming to overcome the gap between scientific concepts and technological production. To compete with sensitivity and selectivity of golden standards, such as liquid chromatography, the functionalization of biosensors is continuously optimized to enhance the signal and improve their performance, often leading to complex chemical assay development. In this research, the efforts are made on optimizing the methodology for electrochemical reduction of graphene oxide to produce thin film-modified gold electrodes. Under the employed specific conditions, 20 cycles of cyclic voltammetry (CV) are shown to be optimal for superior electrical activation of graphene oxide into electrochemically reduced graphene oxide (ERGO). This platform is further used to develop a matrix metalloproteinase 2 (MMP-2) biosensor, where specific anti-MMP2 aptamers are utilized as a biorecognition element. MMP-2 is a protein which is typically overexpressed in tumor tissues, with important roles in tumor invasion, metastasis as well as in tumor angiogenesis. Based on impedimetric measurements, we were able to detect as low as 3.32 pg mL−1 of MMP-2 in PBS with a dynamic range of 10 pg mL−1 – 10 ng mL−1. Further experiments with real blood samples revealed a promising potential of the developed sensor for direct measurement of MMP-2 in complex media. High specificity of detection is demonstrated – even to the closely related enzyme MMP-9. Finally, the potential of reuse was demonstrated by signal restoration after experimental detection of MMP-2.

Investigation on the application of platinum anode towards direct electrochemical reduction of solid metal oxides in CaCl2–CaO melt

Investigation on the application of platinum anode towards direct electrochemical reduction of solid metal oxides in CaCl2–CaO melt

A novel electrochemical strategy to detect hydrogen peroxide by utilizing peroxidase-mimicking activity of cerium oxide/graphene oxide nanocomposites

We herein describe a novel electrochemical strategy to detect hydrogen peroxide (H2O2) by utilizing the peroxidase-mimicking activity of cerium oxide nanoparticles (CeO2 NP) and reduced graphene oxide (rGO). Particularly, CeO2 NP/rGO nanocomposites were deposited on the commercial electrode by a very convenient and direct electrochemical reduction of graphene oxide. Due to the peroxidase-mimicking activity of CeO2 NP and the outstanding electrochemical properties of reduced graphene oxide, the reduction current of H2O2 was greatly enhanced. Based on this strategy, we reliably determined H2O2 down to 1.67 μM with excellent specificity and further validated its practical capabilities by robustly detecting H2O2 present in heterogeneous human serum samples. We believe that this work could serve as a new facile platform for H2O2 detection.

Fast and Sensitive Detection of Ammonia from Electrochemical Nitrogen Reduction Reactions by 1 H NMR with Radiation Damping.

A facile NMR method is reported for analysis of ammonia from the electrochemical reduction of nitrogen, which compares a calibrated colorimetric method, a calibrated 1 H NMR method and two 1 H NMR direct measurements using external reference materials. Unlike spectrophotometric methods, 1 H NMR requires less bench time and does not require separation of ammonia from the electrolyte. A novel approach to the problem of radiation damping in NMR measurements considered the specific role of hardware tuning. Radiation damping is suppressed improving signal-to-noise ratio and detection limit (1.5µgL-1 ). The method is demonstrated to be effective for the analysis of ammonia from direct electrochemical nitrogen reduction in KOH, and from lithium-mediated nitrogen reduction in a non-aqueous solution. An uncertainty budget is prepared for the measurement of ammonia.

Electrochemical Ammonia Production from Wastewater in a Flow Cell Reactor

Although its high carbon footprint, the Haber-Bosh (HB) process mainly dominates the NH3 production market. Indeed, H2 is mainly produced by fossil fuels steam reforming processes and the catalytic reaction between N2 and H2 requires high energy input, being carried out at high temperatures and pressures. Moreover, it should be considered that HB plants are not evenly distributed in the world, thus NH3 and its derivates transportation to the final user is a large portion of the overall environmental impact.1 Moreover, as the population is growing, the NH3 demand is increasing fast to sustain the massive fertilizer utilization.For these reasons, in the last years, the research has focused on the possibility of producing ammonia from direct nitrogen electrochemical reduction (NRR) in aqueous electrolytes under ambient conditions exploiting renewable energy. This process is limited by low selectivity at high current densities and low yield, due to the high dissociation energy of the N2 triple bond and the unavoidable hydrogen evolution reaction (HER).2 On the other hand, NO3 - can be easily converted into NH3 thanks to the lower activation energy, which makes the reaction thermodynamically favoured compared to N2 reduction.3 Due to the massive agriculture, NO3 - is one of the most abundant contaminants of underground waters and high levels in the human body can cause many diseases. Thus, the use of NO3 - to produce NH3 under ambient conditions can be carried out with lower energy consumption, but can also address the water pollution issue.4 The aim of our work is to assess the catalytic activity of commercial MoS2 and a synthesised Bi-based catalyst in a gas-diffusion electrode flow cell of 10 cm2 geometrical area. Such a setup has the advantage of guaranteeing a better mass transport of the active species and of being scaled up. The electrodes are mainly made by airbrush deposition of a catalyst ink on a carbon paper support, which allows obtaining a high electrochemical active surface as a result of its high porosity. Catalyst loading and NO3 - concentration was varied to find the optimal condition in terms of Faraday efficiency (FE) and yield. MoS2 catalyst showed good catalytic activity towards NH3, with a FE between 62%-77% and yield between 2.9-13 mmol g-1 h-1. On the other hand, synthesised Bi material showed 21% FE and 0.41 mmol h-1 g-1, probably due to Bi final oxidation state, which is not so active towards NH3 production. NH3 quantification has been carried out through UV-vis colourimetric method, using both Nessler’s and Berthelot’s reagents. Even if the concentration of NH3 is high enough to exclude that it can derive from contamination, more sensitive quantification is needed for these experiments, together with the quantification of side products, such as NO2 -, H2 and N2.

Direct Electrochemical Regeneration of NADH and Its Biomimetics

Enzyme-catalyzed redox transformations are an attractive approach for conversion of unactivated alkanes into value-added products under mild reaction conditions. This is especially true since advancements in the area of enzyme engineering has led to a dramatic expansion in the use of modified oxidoreductases for selective functionalization of commodity petrochemicals in non-aqueous media. Despite their appeal, many redox enzymes rely on stoichiometric quantities of expensive coenzymes, such as nicotinamide adenine dinucleotide (NADH), that are not easily regenerated. The use of direct electrochemistry to catalytically regenerate NADH has long been an aspiration of chemists as a convenient and straightforward method that would enable use of the expensive cofactor in enzyme-catalyzed synthesis on an industrial scale. Unfortunately, direct electrochemical reduction of the oxidized form of nicotinamide adenine dinucleotide (NAD+) results in unstable radical intermediates that rapidly form biologically inactive byproducts. Recent progress has been made towards the development of inexpensive artificial NADH biomimetics as replacements for the natural coenzyme, but the strategies to regenerate these biomimetic coenzymes is equally limiting. A primary focus of our research is to design synthetic analogues of NAD+ and NADH that are both enzymatically active and electrochemically regenerable. We will present our recent progress in understanding the structure-reactivity relationships that lead to dimerization over the preferred formation of 1,4-dihydronicotinamide in NAD+ and its biomimetics, and we will describe strategies to prevent dimerization of electrochemically reduced NAD+.

Sulfur poisoning on the La0.75Sr0.25Cr0.5Mn0.5O3-δ-Gd0.1Ce0.9O2-δ fuel electrode for direct electrochemical CO2 reduction in solid oxide electrolysis cells

As a promising energy conversion technology, solid oxide electrolysis cells (SOECs) can efficiently convert CO2 to CO by utilizing renewable energy sources and industrial waste heat. Sulfur poisoning of fuel electrode is a significant drawback to the electrochemical activity and long-term electrolysis performance of SOECs using sulfur-containing fuels. Traditional Ni-based cermet electrodes are susceptible to sulfur poisoning due to the low activation barrier and large enthalpy of the adsorbed sulfur species on the Ni surface. La0.75Sr0.25Cr0.5Mn0.5O3-δ-based materials which exhibit excellent redox stability have been considered as the promising candidates for substitution of Ni-based cermet electrodes. Herein, the sulfur poisoning behavior on the La0.75Sr0.25Cr0.5Mn0.5O3-δ-Gd0.1Ce0.9O2-δ (LSCM-GDC) fuel electrode under CO2–SO2 condition and the mechanism of sulfur poisoning are investigated. The results show that the LSCM-GDC fuel electrode possesses stable electrocatalytic activity as SO2 content up to 10 ppm. With increasing of SO2 content, metallic sulfide, amorphous sulfur and sulfate impurities can be detected on the electrode surface, which decrease the reactive sites and hinder the charge transfer and oxygen ions transport pathway, leading to degradation of the electrolysis performance. The plausible sulfur poisoning mechanism is divided into two categories, namely, thermodynamic and electrochemical processes. For thermodynamic process, SO2 molecules adsorb into oxygen vacancies and couple to coordinated oxygen lattices to form sulfates; for electrochemical process, the formation of amorphous sulfur and metallic sulfide impurities is caused by the binding of SO2 molecules to surface polarons at three phase boundaries under cathodic polarization. This work provides new insights into sulfur poisoning effect on the perovskite oxide electrodes.

Electrochemical surface plasmon resonance approach to probe redox interactions between microbial extracellular polymeric substances and p-nitrophenol

Microbial extracellular polymeric substances with redox functional groups play a crucial role in the bio-conversion of pollutants, which can affect their reactivity toward diverse pollutants. However, the redox interactions between microbial EPS and pollutants have not addressed in depth due to the absence of essential analytical methodologies. In this study, we have developed an electrochemical-surface plasmon resonance (EC-SPR) system to investigate the interactions between EPS and p-nitrophenol (PNP) by simultaneously monitoring the electrochemical reaction and the binding kinetics. Moreover, in vitro PNP degradation experiments were performed in the presence of EPS across varying redox states to provide further verification of PNP reduction by EPS. The results indicated that direct electrochemical treatment successfully converted raw EPS (EPSraw) into reductive EPS (EPSred) and oxidized EPS (EPSox), respectively. The EC-SPR system served as a powerful tool for probing redox interactions between EPS at distinct redox states and PNP. The binding affinity of EPS to PNP was related to the redox states of EPS, following the order of EPSred > EPSraw > EPSox. EPS exhibited the capability to reduce PNP to p-aminophenol by donating electrons, and the reductive process highly depended on the redox states of EPS, primarily determined by their electron donating capacity. Importantly, direct electrochemical reduction treatment of EPS leads to a substantial improvement in the PNP removal efficiency from 33.8% (EPSraw) to 56.9% (EPSred). This work contributes to a comprehensive understanding of the critical role of EPS redox property in the conversion of refractory pollutants in aquatic environments.

Unlocking long-chain hydrocarbons (C2–7) via direct electrochemical CO2 and CO reduction on balanced Au/Ni electrodes

Electrochemical (EC) CO2 reduction method has been widely used as a green energy and environmental solution strategy. The use of Au/Ni electrodes was introduced to showcase a new concept of the direct EC Fischer-Tropsch (dEC F-T) synthesis pathway. This pathway involves the combination of electrodes that produce H2 and CO (syngas) during electrochemical CO2 reduction. The introduction of Au on the Ni electrode surface led to an increase in CO production and a gradual decrease in H2 production. When the interface was balanced, a pronounced F-T synthesis pathway was observed, resulting in the production of a series of hydrocarbons (CnH2n and CnH2n+2, n = 2–7). The dEC F-T synthesis was evaluated under different conditions, including electrolytes, concentrations, metal supports (Co and Fe), various overlayer metals (Ag and Cu), light irradiation, and isotope effects. The process was elucidated through surface C−C coupling polymerization reactions based on Anderson-Schulz-Flory weight distribution analysis. Additionally, the F-T synthesis was demonstrated through EC CO reduction via direct CO and H adsorption. The dEC F-T path provides a novel strategy for energy and environment by producing high-value long-chain hydrocarbons.

Enriching Metal-Oxygen Species and Phosphate Modulating of Active Sites for Robust Electrocatalytical CO2 Reduction.

Direct electrochemical reduction of CO2 (CO2 RR) into value-added chemicals is a promising solution to reduce carbon emissions. The activity of CO2 RR is influenced deeply by the reaction microenvironment and electronic properties of the catalysts. Herein, the surface PO4 3- anions are tuned to modulate the local microenvironment and the electronic properties of indium-based catalyst with abundant metal-oxygen species enabling efficient electrochemical conversion of CO2 to HCOO- . Indium nanoparticles coupled with PO4 3- anions (PO4 3- -In NPs) achieved a high selectivity of HCOO- up to 91.4% at a low potential of -0.98 V versus reversible hydrogen electrode (vs. RHE), and a high HCOO- partial current density of 279.3 mA cm-2 at -1.1 V vs. RHE in the electrochemical flow cell. In-situ and ex-situ characterizations confirmed the PO4 3- anions kept stable on the surface of indium during CO2 RR, accelerating the generation of OCHO* intermediate. From density functional theory (DFT) calculations, PO4 3- anions enriched the metal-oxygen species on the substrate to optimize the electronic structure of the catalysts and induced a local microenvironment with massive K+ ions on the interface, thus reducing the activation energy barrier of CO2 RR. This article is protected by copyright. All rights reserved.

(Keynote) Electrochemical CO2 Reduction on NiNC Single Metal Atom Catalysts Under Alkaline to Acidic pH Conditions

The science and technology of the direct electrochemical CO2 reduction reaction on both model electrodes in liquid-electrolyte H-cells and Gas Diffusion Electrodes (GDEs) in flow electrolyzers offer as many formidable challenges as intriguing opportunities. Controlling the selectivity (faradaic efficiency), while maximizing the energy efficiency by lowering the kinetic overpotentials remains key to turn this complex reaction into a practical process embedded in a process chain. In this talk, I will highlight some of our recent advances in the design and characterization of NiNC single site electrocatalysts for the electrochemical reduction of CO2 to CO and CO2/CO mixed feeds into value-added fuels and chemicals in H-cell and Gas Diffusion Electrode (GDE) single cell electrolyzers. Further focus will be placed on the diagnosis of carbonate transport processes during electrolyzer cell operation using the experimentally accessible carbon crossover coefficient, CCC. Performance and its limitations of cell operation in alkaline and acid conditions will be contrasted and discussed.

Simultaneous Operability of Thermocatalytic and Electrocatalytic CO2 Reduction Reactions

The direct electrochemical reduction reaction of carbon dioxide (eCO2RR) captured from the ambient air into base chemicals and fuels holds great potential for an elegant implementation of a net-zero emission carbon cycle. This assumes that the energy used to drive the capturing and conversion process is largely free of carbon dioxide emissions, i.e., from renewables such as wind and photovoltaics. Moreover, high Faradaic efficiency for the desired products at sufficiently high current density in eCO2RR has to be demonstrated to compete with more conventional multi-step Power-to-X conversion schemes both in terms of overall energy efficiency and cost.[1]eCO2RR is known for its wide variety of products.[2] However, hydrogen evolution reaction (HER), which is more thermodynamically reactive, inevitably seizes protons and electrons in the cathode. In addition, the reaction pathway of C2+ products is more complex than that of C1 products because it is strongly dependent on the catalyst surface and electrocatalytic operating conditions. For example, the formation of C-C bonds is interfered with the formation of C-H and C-O bonds on the catalyst surface, resulting in the direct generation of multi-carbon products technically challenging.[3] Due to these kinetic barriers, the C2+ products show much lower the energy efficiency, Faraday efficiency, and partial reduction current density of than those of the C1 products. The mechanistic aspects of the reaction pathways for the various C2+ products have not been fully deciphered, which ultimately limits the commercial application of eCO2RR conversion to C2+ products.[4]Noting that industrial thermocatalytic processes produce and supply long-chain hydrocarbons in a long-term stable manner, we propose the production of C2+ products by combining simultaneous thermocatalysis and electrocatalysis. In the ongoing work, hydrogen from the water electrolysis is further converted to syngas via a reverse water-gas shift reaction (rWGS), followed by an established conversion route to fuels and chemicals such as Fischer-Tropsch or methanol synthesis and methanol to olefins, gasoline or jet fuel.[5] Here, a reactor and test bench capable of operating at 20 bar and 200 degrees Celsius have been fabricated to enable simultaneous thermocatalytic and electrocatalytic production of C2+ products. Furthermore, different catalysts can be employed to explore the catalytic processes on the catalyst surface.

Electrochemical Reduction of Amine-Captured CO2 in Aqueous Solutions

Technologies that can capture CO2 and enable conversion into value-adding chemicals and fuels or stable minerals for sequestration are vital for transitioning towards net zero or even negative greenhouse gas emissions. Conventional approaches for electrochemically converting CO2 have utilized a decoupled approach of first capturing and concentrating CO2, and then using the concentrated CO2 as a feedstock for conventional electrochemical processes. Direct electrochemical reduction of amine-captured CO2 1,2 can potentially offer advantages by removing the need to thermally regenerate the amine capture solution, which can be energy intensive and typically uses thermal energy from nonrenewable sources. In this talk, we share our recent work on the electrochemical reduction of amine-captured CO2 to value-adding products like CO and stable minerals like carbonates. We discuss the influence of the capture environment on the resulting capture solution chemistry, and how to alter the capture solution speciation through electrolyte design. We further consider the detailed CO2 reduction mechanisms in these amine-containing solutions and provide design strategies for increasing the Faradaic efficiency of CO2 reduction vs. the competitive hydrogen evolution reaction (HER), as well as decreasing the overpotential of CO2 reduction.

(Invited) A Review of Electrochemical Methods for Extraction of Oxygen From Regolith

The surface of many planetary bodies, including the Moon and Mars, is composed primarily of metal oxides. A number of competing technologies have been proposed to extract oxygen from the dust on the surface. These methods include reduction of the iron- and silicon-containing species by hydrogen or by electrons. The direct electrochemical reduction methods run at different temperatures, from approximately room temperature to over the melting point of iron. Some of these methods produce liquid metal, and others solid metal powders. The latter can optionally extend the electrolysis to recover oxygen from aluminum and titanium species, although at an energetic cost. This talk will review recent progress and current capabilities for these processes.

(Invited) Green Ammonia-Mediated CO2 Capture and Direct Electrochemical Reduction to Formate

Direct electrochemical conversion of CO2 capture solutions (instead of gaseous CO2) into valuable chemicals can circumvent the energy-intensive CO2 regeneration and pressurization steps. While commonly used CO2 capture agents include alkali and amine solutions, ammonia has been rarely investigated. In another aspect, mismanagement of reactive nitrogen (Nr) in waste has emerged as a major problem in water pollution to our ecosystems, causing severe eutrophication and health concerns. Sustainably recovering Nr [such as nitrate (NO3 −)-N] and converting it into green ammonia (NH3) could mitigate the environmental impacts of Nr and reduce the NH3 demand from the carbon-intensive Haber-Bosch process, as well as a possible CO2 capture agent due to its alkaline nature.In this talk, we will present our rencet research on integration of electrodialysis and electrocatalysis for ammonia synthesis from dilute waste Nr sources, and green ammonia-mediated CO2 capture (to ammonium bicarbonate, NH4HCO3) and subsequent reduction to ammonium formate (NH4HCO2) as a new approach to CO2 capture and utilization (CCU). We have demonstrated a record-high NO3 −-to-NH3 performance in a scalable, versatile, and cost-effective membrane-free alkaline electrolyzer (MFAEL): an unprecedented NH3 partial current density of 4.22 ± 0.25 A cm−2 with a faradaic efficiency of 84.5 ± 4.9%. We also discovered that an ammonium bicarbonate (NH4HCO3)-fed electrolyzer with an anion exchange membrane (AEM) outperforms the state-of-the-art KHCO3 electrolyzer with a bipolar membrane (BPM) owing to its favorable thermal decomposition property, which allows for a 3-fold increase in the in situ CO2 concentration, a maximum 23% increase in formate faradaic efficiency, and a 35% reduction in cell voltage by substituting BPM with the AEM. Our integrated process by combining NH4HCO3 electrolysis with CO2 capturing by on-site generated green ammonia from the electro-reduction of nitrate in MFAEL has shown a remarkable 99.8% utilization of CO2 capturing agent. Such a multi-purpose process may offer a sustainable route for the simultaneous removal of Nr wastes and streamlined CO2 capturing and upgrading to valuable chemicals.

N‐(aminobenzyliden)phthalimides as a New Redox Mediator Family for the Selective Indirect Reduction of Benzyl or Allyl Halides

AbstractThe chemical stability of the electrogenerated radical anion of N‐(aminobenzyl) phthalimide 1 in aprotic organic media at room temperature is reported in this work. The redox couples 1/1⋅− and its imine precursor 2/2⋅− can carry out reductive redox catalysis of benzyl or allyl halides indirectly. This reaction gives easy access to reactive alkyl radicals, which in the presence of diethyl fumarate yields 2‐benzylsuccinate diethyl ester in moderate yields; this reaction is unattainable by direct electrochemical reduction. Spin density calculations at the DFT level of theory showed that the spin density of 1⋅− and 2⋅− is located on the phtalimide ring, and a dimeric association was evidenced (Eint=23.47 kcal/mol and Eint=80.42 kcal/mol respectively), via π−π stacking interactions. The redox mediators presented here showed good functional group selectivity, redox potential tunability and easy preparation from readily available commercial materials with excellent yields.

Can We Decarbonise Methanol Production by Direct Electrochemical CO2 Reduction?

AbstractMethanol is one of the most important chemical feedstocks, with an increase in global demand of more than 20 million tonnes since 2017. The current production of methanol is primarily from fossil fuel derived synthetic gas (syngas), producing up to 2.97 tonnes of CO2 per tonne of methanol. The direct electrochemical CO2 reduction reaction (eCO2RR), i. e. systems in which CO2 is reduced to methanol at the cathode coupled to the oxygen evolution at the anode, is one of the most attractive approaches for decarbonising methanol production, using renewable energy sources as the power source. In this review, we critically discuss the progress and challenges associated with eCO2RR to methanol, with the aim of addressing the question of whether this approach can provide a feasible route to the decarbonisation of this key chemical commodity.

Direct electrochemical formation of carbonaceous material from CO2 in LiCl-KCl melt

The electrochemical reduction of CO2 on a Ni electrode in LiCl-KCl melt at 723 K was studied. The Ni electrode which was partially immersed in the melt was used for electrochemical measurement in order to investigate the relationship between electrical wettability and electrodeposition behavior of carbon from CO2 at the three-phase interface of the Ni electrode/LiCl-KCl melt/CO2 gas. It was found that solid carbon was obtained in the wetted area of the Ni electrode by potentiostatic electrolysis. Under high polarization, a super meniscus was formed at the three-phase interface where the direct electrochemical reduction of CO2 to solid carbon progressed. In addition, SEM and TEM observations with EDS analyses showed that electrodeposited carbon was multi-walled carbon nanotubes with a diameter of 30–50 nm. The dependence of applied potential and electrolytic time on morphology and crystallinity of electrodeposited carbon was also investigated. Based on the experimental results, we have proposed the model of the formation mechanism of the multi-walled carbon nanotube on the Ni electrode. The reported data indicate that carbonaceous materials from CO2 can be obtained by electrochemical technique in molten salt and suggest that the technique contributes to developing the carbon recycling system against global warming.

Synergies of the Direct Electrochemical Reduction and Pd-Catalyzed Hydrogenation in a Sequential Membrane for an Efficient N-Nitrosodimethylamine Removal

Synergies of the Direct Electrochemical Reduction and Pd-Catalyzed Hydrogenation in a Sequential Membrane for an Efficient <i>N</i>-Nitrosodimethylamine Removal