FIELD

[0001] Innovations are disclosed in the field of oxidative electrochemical catalysis.

BACKGROUND

[0002] For decades, natural gas lagged coal and oil as an energy source, but today its consumption is growing rapidly as countries seek to lower greenhouse gas emissions by displacing coal for heating and power generation. The recent discoveries of vast shale gas reserves in the United States and widespread use of hydraulic fracturing, has seen natural gas prices decrease and today it supplies -22% of the global energy need. ¹ In its 2020 report, the International Energy Agency (IEA) projected global demand for natural gas to increase by 29% by 2040 and it would be the largest energy source among fossil fuels. ² In addition, the main component of natural gas i.e. , methane (CH ₄ ) is a well-established and widely available feedstock to produce several important commodity chemicals such as methanol, hydrogen, ammonia, and formaldehyde. Typical valorization of CH ₄ takes place with a combination of steam methane reforming and Fischer-Tropsch synthesis. These are highly endothermic processes requiring high temperatures (700°C-1100°C), pressures (10-40 bars) and suffer from a tradeoff between conversion and selectivity. Moreover, they are highly capital-intensive processes, requiring many unit operations and centralized infrastructure, thus hindering their implementation on a small scale. Therefore, it seems of great importance to developing a low-cost sustainable method for the direct partial oxidization of CH ₄ to useful chemicals and fuels under ambient conditions.

[0003] In this context, electrochemical partial oxidation of CH ₄ to oxygenates such as methanol (CH ₃ OH) and formic acid (HCOOH) is particularly attractive. The electrochemical conversion of CH ₄ under ambient conditions also offers a route to store renewable electricity addressing a major challenge of intermittency. The ability to control the potential to alter the selectivity of the reaction is another appealing factor to develop an electrochemical route for partial oxidation of CH ₄ to oxygenates. Since electrochemical devices are highly modular, and because the productivity scales directly with electrode size and current, an electrochemical route can provide an efficient and cost-effective solution that can be deployed in both large-scale industries and small-scale remote applications (such as those needed in remote oil fields).

[0004] There have been attempts to develop catalysts for electrochemical CH4 oxidation using metal/metal-oxides in various reaction conditions, however with limited success. The reported current densities or reaction rates are low (pA cm ^-2 to 1 mA cm ^-2 ), without any analysis of the Faradaic efficiency (FE) and reaction mechanism. A major difficulty arises due to the stable non-polar tetrahedral molecular geometry of CH ₄ and high C-H bond energy (AH _C -H = 439.3 kJ mol ^-1 ). Once this high activation energy for C-H bond dissociation is attained, it is difficult to control the partial oxidation to oxygenates, which are intermediate products, and avoid the terminal and more thermodynamically favourable pathway of CO2 production. ³ The competitive oxygen evolution reaction (OER) poses additional challenges to attain high selectivity towards CH ₄ oxidation products. Conventional alkaline water electrolysers operate at room temperature, with the hydroxide ion (HO-) generally functioning as the oxidant. Nevertheless, HO- has a negligible activity for protons abstraction from CH ₄ at mild conditions. ⁴ Attempts for electrochemical CH ₄ oxidation have also been made by utilizing high-temperature oxygen-ion conducting solid oxide electrolysis cells (SOECs). However, due to the use of high temperature, the reported selectivity towards oxygenates is negligible.

SUMMARY

[0005] Catalytic systems are disclosed for electrochemical CH ₄ oxidation to formate, including systems that function under ambient conditions. Operando spectroelectrochemistry studies reveal that the Fe ^lv =O species are active sites for electrochemical CH ₄ oxidation. Although electrochemical oxidation of Fe ^IH to Fe ^lv can be achieved at high potentials (> 1.4 V versus reversible hydrogen electrode, VRHE), high overpotentials lead to overoxidation of CH ₄ to CO2. Therefore, we demonstrate herein the use of reactive oxygen species (e.g., generated via partial electrooxidation of H2O2 on Ni) to mediate Fe ^IH oxidation to Fe ^lv at lower overpotential, suppressing unwanted overoxidation. Furthermore, we reveal the key role of Cu as a co-catalyst in preventing the complete oxidation of CH ₄ to CO2 by increasing the activation energy of the intermediate step. A CuFeNi catalyst is accordingly provided that exhibits electrochemical CH4 oxidation to formate at high current density (32 mA cm- ² ), Faradaic efficiency (42 %) and liquid oxygenate selectivity (100 %) using a low applied potential (0.9 V _RH E). A self-sustaining electrolyser system is provided, comprising an oxygen reduction reaction (ORR) to H2O2 on the cathode and CH4 oxidation on the anode, operating at high current density and low voltage to produce formate.

[0006] Methods for using electrolytic cells are accordingly provided for electrochemically oxidizing methane to formate. The methods involve providing a methane supply to an alkaline aqueous anolyte medium including hydroperoxyl anions, where the anolyte medium is in contact with an oxidation catalyst anode, where the oxidation catalyst anode includes CuFe oxide catalytic centres supported on a nickel substrate; and, supplying an anodic current to the oxidation catalyst anode in the anolyte medium, to electrolytically oxidize methane to formate in an anodic oxidation reaction.

[0007] Implementations of the present methods and electrolytic cells may include one or more of the following features. The nickel substrate may be a nickel foam substrate. The O2 reactant may be electrolytically reduced to form a H2O2 product. The cathodic ORR may be carried out by feeding the O2 reactant through a carbon-based gas-diffusion layer (GDL). The GDL may include NO and/or SO2 functional groups. The method may further include directing the H2O2 product of the ORR to the anolyte medium to provide the hydroperoxyl anions. The H2O2 product may be generated by the cathodic ORR at a peroxide product generation rate, and hydroperoxyl anions may be consumed in the anodic oxidation reaction at a peroxide consumption rate, and the peroxide product generation rate may be greater than or equal to the peroxide consumption rate. The cathodic ORR may be carried out in a catholyte chamber and the anodic oxidation reaction may be carried out in an anolyte chamber, where the catholyte chamber is in electrochemical communication with the anolyte chamber through an anion exchange membrane (AEM). The A peroxide concentration gradient may be maintained across the AEM between the catholyte chamber and the anolyte chamber, where the peroxide concentration gradient mediates the flow of hydroperoxyl anions from the catholyte chamber to the anolyte chamber. The AEM may include hydrophilic cation group side chains on a hydrophobic polymer backbone. The anodic oxidation reaction may be carried out at faradaic efficiencies of at least 40%. The anodic oxidation reaction may be carried out at a formate liquid product selectivity of at least 90%, optionally at least 99%. The anodic oxidation reaction may be carried out at current densities of at least 30 mA cm ^-2 . Electrolytic oxidization of methane to formate in the anodic oxidation reaction may be carried out under ambient conditions, and/or at a temperature of 5°C to 45°C or 15°C to 30°C or 20°C to 25°C; and/or a pressure of 50-115 kPa, or 80-110 kPa or 95-105 kPa.

[0008] Accordingly, one general aspect of the present methods and electrolytic cells involves the use of an anodic CuFeNi oxidation catalyst to electrochemically oxidize methane to formate, for example where the nickel substrate is a nickel foam substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 Biological examples for CH4 hydroxylation. (A) Diiron diamond-core structure of the intermediate Q proposed in sMMO and a proposed CH4 hydroxylation reaction via a radical rebound mechanism. Glu, glutamate aminoacid residue. (B) Heme structure of the intermediate I proposed in cytochrome P450 and a proposed CH4 hydroxylation reaction. Cys, cysteine thiolate ligand.

[0010] Figure 2 Electrochemical CH4 oxidation reaction, eCH4OR. (A) Linear sweep voltammetry in H-type cell using CuFe/NiF electrode in 1 .0 M KOH with and without the addition of 50 mM H2O2. (B) ¹ H NMR spectra after CH4 electrolysis performed using NiF, Fe/NiF, Cu/NiF, and CuFe/NiF electrodes at a constant voltage of 1 .0 VRHE for 1 hour in 1 .0 M KOH with 50 mM H2O2 (inset: ¹³ C NMR spectrum of ¹³ CH4 electrolysis performed using the CuFe/NiF electrode at a constant voltage of 1 .0 VRHE for 1 hour in 1 .0 M KOH with 50 mM H2O2). (C) eCH4OR products faradaic efficiencies and corresponding current densities obtained at different applied potentials on the CuFe/NiF in 1 .0 M KOH with 50 mM H2O2. (D) eCH4OR products faradaic efficiencies and corresponding current densities obtained at different applied potentials on the CuFe/NiF in 1 M KOH (without the addition of H2O2). (E) Chronoamperometry of CuFe/NiF at 0.9 VRHE showing the consumption of H2O2 after 260 min and the effect of adding the same amount of H2O2 in restoring the current. (F) Radar chart showing a comparison of eCH ₄ OR faradaic efficiency, selectivity towards the target product, liquid oxygenates production rate, applied potential, and current density against selected best reports from the literature.

[0011] Figure 3 Structural characterization of the catalyst. (A) XRD pattern of the CuFe/NiF in comparison with standard XRD patterns. (B) and (C) Fe l_3,2-edge and Cu l_3,2-edge of the CuFe/NiF electrode, respectively. (D) HRTEM image of the CuFe/NiF electrode. (E) and (F) Core-level spectra for Fe 2p and Cu 2ps/2 of the CuFe/NiF electrode, respectively. Black empty circles: experimental points, solid lines: fitted data.

[0012] Figure 4 Mechanistic understanding of the eCH ₄ OR. (A) Schematic of the proposed reaction mechanism of CH ₄ oxidation on the CuFe/NiF with the use of H2O2. (B) Operando spectroelectrochemical integral absorbance of CuFe/FTO at 1 .4 VRHE with and without the addition of 100 mM H2O2 in 1.0 M KOH (inset: integral absorbance at 2.0 VRHE without the addition of H2O2). The baseline was recorded under open circuit potential (OCP) conditions.

[0013] Figure 5 Cathodic oxygen reduction reaction. (A) Linear sweep voltammetry of unmodified Freudenberg GDL in 1 .0 M KOH as a cathode and switching gases between N2 and O2 in a flow cell configuration. (B) Raman spectrum of the Freudenberg GDL. (C) ATR-FTIR spectrum and SEM (inset) of the microporous side of Freudenberg GDL. (D) H2O2 production rates and corresponding current densities at different applied potentials.

[0014] Figure 6 Self-sustaining electrochemical system. (A) H2O2 generation rates via ORR using carbon-based GDL cathode and consumption rates via eCH ₄ OR using CuFe/NiF anode as a function of total charge passed during 50 minutes of each experiment. (B) Schematic of the coupled electrochemical process of cathodic ORR and anodic eCH ₄ OR (denoted as eCH ₄ OR-ORR). (C) Chronoamperometry of eCH ₄ OR-ORR and faradaic efficiencies demonstrating the self-sustaining process at 1 .3 VRHE with different initial concentration gradients of H2O2 in 1 .0 M KOH between catholyte and anolyte (anolyte was 1 .0 M KOH with an initial addition of 50 mM H2O2). Real experimental data are shown in pale colors. Inset: a schematic showing the rate of generation of H2O2 in the catholyte (Rgene), rate of H2O2 consumption in the anolyte (Rcons) and rate of H2O2 migration through the AEM (Rmem). Note that H2O2 predominantly presents as HOO“ in our case since we are using alkaline electrolytes. (D) TEA analysis showing plant-gate minimum selling price (MSP) of HCOOH produced from the coupled eCH ₄ OR-ORR process as a function of HCOOH faradaic efficiency and electricity price. The dashed line indicates the market price at $740 ton’ ¹ HCOOH.

[0015] Figure 7 is a schematic illustration of processes disclosed herein.

DETAILED DESCRIPTION

[0016] Selective partial oxidation of methane to liquid oxygenates has been a long-sought goal, due to the intrinsic chemical inertness of its C-H bonds. With the continuous reduction in renewable electricity prices, the electrochemical partial oxidation of methane is gaining momentum globally. Inspired by the catalytic sites in cytochrome P450 and soluble methane monooxygenase (sMMO) metalloenzymes, here we disclose a highly active multi-metal CuFeNi electrocatalyst for selective electrochemical methane oxidation reaction (eCH ₄ OR) to formate at room temperature. Mechanistic studies using operando spectroelectrochemistry measurements revealed the synergistic effect of nickel, iron, and copper to selectively oxidize CH ₄ . Specifically, the analysis revealed the presence of high valent Fe ^lv as the active site for CH ₄ oxidation, attained by the reactive oxygen species generated during the partial oxidation H2O2 at low overpotentials compared to water oxidation reaction (OER) on nickel. Furthermore, the critical role of copper in preventing the overoxidation of valuable oxygenates to CO2 is disclosed. We achieved Faradaic efficiencies of ~ 42 % and liquid product selectivity of 100 % at current densities of 32 mA cm ^-2 using a low applied potential of 0.9 V versus reversible hydrogen electrode. A self-sustaining electrochemical system is also disclosed, wherein the H2O2 required for the CH ₄ oxidation is generated in situ through the cathodic oxygen reduction reaction (ORR) in a flow cell configuration that operates at high current density and low voltage to produce formate. The system is schematically illustrated in Figure 7, and described in detail in the following Examples. Examples

Electrochemical CH4 oxidation reaction (eCl-LiOR).

[0017] Electrochemical oxidations were performed in a 3-electrode H-type cell, in an alkaline environment (1.0 M KOH) using a hydrothermally grown CuFe oxide on nickel foam, denoted as CuFe/NiF (synthesis procedure is discussed in the Supplementary Information). Fig. 2A shows the linear sweep voltammetry (LSV) curves under a continuous purge of Ar and CH4 in the anolyte, with and without the addition of H2O2. Regardless of the purge gas used, the onset potential of the oxygen evolution reaction (OER) in the absence of H2O2 is seen at high anodic potentials (> 1 .5 VRHE), triggered by the transition of p-Ni(OH)2 to p-NiOOH as seen by the peak centred at 1 .42 VRHE. With the addition of 50 mM H2O2, the anodic current starts to increase at an onset potential of - 0.8 VRHE, i .e. , at an overpotential of - 0.107 V, as the standard oxidation potential of H2O2 is 0.693 VRHE (H2O2(aq) 02(g) + 2H ⁺ (a _q ) + 2e ^_ ). The anodic current increases gradually and at higher voltages emerges as combined oxidation current from H2O2 oxidation and OER. The two-step increase in current density is due to the decrease of the alkalinity at the electrode surface because of the presence of protons released during the potential sweep. ⁶ This behaviour was verified when the reaction was performed under stirring conditions in which the current continuously increased. It is worth mentioning that since the pH of the electrolyte is above 9, H2O2 exists mainly as HOC- (hydroperoxyl anion), which will be referred to interchangeably herein.

[0018] The current observed at a low potential regime (-0.8-1 .5 VRHE) is driven mainly from the H2O2 oxidation on the nickel surface. This was confirmed due to the negligible oxidation current (< 1 mA cm ^-2 ) from CuFe catalysts which were grown on titanium foam (denoted as CuFe/TiF) following the same synthesis method as the CuFe/NiF electrode. Interestingly the current density with and without (i.e. , Ar purge) purging CH4 did not change (Fig. 2A), however, upon analysing the electrolyte with ¹ H NMR (Fig. 2B), after 60 min of reaction under chronoamperometric conditions, indicated formate production. Chronoamperometry tests using isotopical ly labelled ¹³ CH4 and the ¹³ C NMR analysis, as shown in the inset of Fig. 2B, further confirmed the CH4 to be the carbon source. Following that, we carried out control experiments at the same condition using NiF, Fe/NiF and Cu/NiF as catalysts which revealed that only the CuFe/NiF catalyst was able to oxidize CH ₄ to formate (Fig. 2B), indicating the synergistic effect of Ni, Fe and Cu in selective partial oxidation of CH ₄ to formate. It is important to highlight that while NiF and Cu/NiF did not produce any oxidation products (formate or CO2), the Fe/NiF catalyst produced CO2 indicating electrochemical oxidation of CH ₄ (see SI). However, unlike the CuFe/NiF catalyst, the LSV curves and chronoamperometry measurements using Fe/NiF, revealed higher current density when the purge gas of the anolyte is switched from Ar to CH ₄ . [0019] We then carried out chronoamperometry tests using the CuFe/NiF electrode with the addition of 50 mM H2O2, at different voltages, (Fig 2C). Irrespective of the voltage applied, formate was the only liquid product detected and O2 was the only gas product that comes from H2O2 oxidation as the applied voltages were below those needed for water oxidation. Since the reaction was done in alkaline conditions, it was assumed that any CO2 produced would be captured in the form of carbonate (CO3 ^2- ). The quantification of CO3 ^2- was performed using the total alkalinity method which revealed that the total FE for CH ₄ oxidation (HCO“ + CO2) was very similar (~ 50 %) irrespective of the applied voltages. We believe this behaviour is due to the limited availability of CH ₄ in the electrolyte because of its low solubility (23 mg L ^-1 water), thereby limiting the total FE for CH ₄ oxidation. With increasing voltages there was a tradeoff between current density and faradaic efficiency for HCOO-, decreasing from 42% at 0.9 VRHE to 13% at 1 .2 VRHE. On the flipside, faradic efficiency for COs ^2- increased from 8.3% at 0.9 VRHE to 41 % at 1 .2 VRHE-

[0020] Further control experiment under open circuit potential (OOP) conditions, i.e., without an electrochemical bias, did not reveal any products via the ¹ H NMR analysis, indicating that the oxidation products detected (HCOO- + CO2) were directly/indirectly results of an electrochemical reaction. The CH ₄ oxidation was also conducted without the addition of H2O2 in the OER potential window. Fig. 2D shows that at high anodic bias (> 1 .8 V), CO2 was the only CH ₄ oxidation product detected (FE of 21 .6 % at 1 .8 VRHE). While this confirmed the possibility to oxidize CH ₄ without the use of H2O2, it also revealed the undesirable overoxidation of CH ₄ to CO2 due to the high voltages. Upon stepping to a higher applied potential (2.6 VRHE), the CO2 FE dropped to 5.1 % due to the limited availability of CH ₄ in the electrolyte. [0021] The harsh oxidizing conditions warranted stability testing of our CuFe/NiF catalyst. We confirmed the stability of our catalyst by chronoamperometry measurements at 0.9 VRHE (Fig. 2E) whereby the drop in current density was due to H2O2 consumption and the addition of the same amount of H2O2 led the current to return to its initial value, while the HCOO“ faradaic efficiency remained steady at -42%. We summarized the electrochemical performance metrics of the CuFe/NiF catalyst in a radar chart (Fig. 2F), wherein we compared our results with previously reported literature. We demonstrated a current density of -32 mA cm ^-2 , at an applied potential of 0.9 V _RH E, total eCF OR faradaic efficiency of 50.7% and a liquid oxygenate selectivity of 100%. There are only a handful of reports, wherein faradaic efficiency for eCF OR was reported at very low current densities ( A cm ^-2 ) and/or high applied potentials (>1.4 VRHE).

Catalyst Characteristics

[0022] X-ray diffraction (XRD) analysis was conducted to determine the crystallinity of the CuFe/NiF electrode. The XRD pattern (Fig. 2A) shows that three large diffraction peaks at 44°, 52° and 76° are due to the (111), (200) and (220) facets of the nickel scaffold, while the ones at 30°, 35° and 37° match those for the tetragonal CuFe2O4 (JCPDS: 34-0425). However, other diffraction peaks are also present which can be assigned to CuO and Fe2Os. A previous report by Inamdar et al. confirmed that the hydrothermally grown CuFe on nickel foam at 105 °C forms a bimetallic composite of crystalline Cu matrix incorporated with Fe. ⁷ Our XRD data also suggests that our catalyst is a composite system of amorphous CuFe oxides and tetragonal CuFe2O4.

[0023] X-ray absorption spectroscopy (XAS) was conducted to gain more understanding of the electronic and oxidation states of each element in the electrode. The XAS spectrum of the Fe Ls-edge presents two main peaks at 708 and 711 eV while the Fe l_2-edge shows peaks at 720 and 722 eV (Fig. 3B). ⁹ The peak at - 710 eV represents confirms the presence of both octahedral and tetrahedral sites of Fe ^IH as compared with the spectrum of Fe2Os reference. The suppression of the peak at 708 eV is attributed to the contributions from Cu ¹ cations. The Cu l_3,2-edge spectrum of the CuFe/NiF coincide with the absorption spectrum of the CuO, which shows that copper presents mainly as Cu" on the electrode (Fig. 3C). However, the peak at 935 eV also signifies the existence of Cu ¹ species.

[0024] Further, the chemical state of the CuFe/NiF electrode was studied by X-ray photoelectron spectroscopy (XPS). The high-resolution spectrum of Fe 2p in Fig. 3E reveals two peaks at 721 .4 and 710.75 eV which correspond to Fe 2pi/2 and Fe 2p3/2 and spin-orbit states, respectively. ¹⁰ ’ ¹¹ This observation confirms the presence of the Fe ¹¹¹ state in the CuFe/NiF electrode. The Cu 2p spectrum in Fig. 3F shows two satellite shake-up peaks at 954.4 and 951 .0 eV and two peaks at 932.6 and 931.2 eV, confirming that Cu has a combination of 1+ and 2+ oxidation states on the surface of CuFe/NiF electrode. ¹² ’ ¹³ The core-level O 1s further confirms the presence of the metal oxides on the catalyst surface.

[0025] Field emission scanning electron microscopy (FE-SEM) images of the CuFe/NiF electrode illustrated the following characteristics. The synthesized CuFe consists of randomly interconnected compact nanoflakes covering the NiF substrate. Energy dispersive X-ray (EDX) spectroscopy proved the existence of Fe, Cu and O in the CuFe/NiF electrode with an atomic ratio for Cu/Fe at 1 .65. This observation confirms that the CuFe composite is Cu-rich even though an equimolar amount of Cu and Fe precursors were used during the hydrothermal synthesis. A high-resolution transmission electron microscopy (HRTEM) image of the CuFe/NiF electrode is shown in Fig.3D. The lattice fringes with a distance of 2.4 A is associated with the (311) facet of CuFe2O4 while the fringes with lattice distances of 2.1 A and 1.8 A correspond to the (111) and (200) facets of Fe and Cu, respectively. ⁷ ’ ¹⁴

Mechanistic study ofeCI- OR

[0026] The control experiments as discussed earlier in Fig. 2A indicates that the eCH4OR only occurred in presence of iron, while the synergistic use of H2O2 and copper helped in its selective partial oxidation to formate. To further gain insights into the mechanism, the eCH4OR were conducted in an operando spectroelectrochemical setup which allowed to monitor changes in the absorption spectra as a function of applied voltage. The electrode was prepared by hydrothermally growing a thin layer of CuFe on fluorine-doped tin oxide (denoted as CuFe/FTO). At first, the experiment was performed under water oxidation reaction, without the addition of H2O2. The baseline was recorded under open circuit potential (OCP) conditions. The inset of Fig. 4B shows a broad integrated absorption (AAbs) band centred at 600 nm when the potential applied on the CuFe/FTO was held at 2.0 VRHE. This peak can be assigned to the high-valent Fe ^lv =O and is similar to those previously reported in the literature for the a-Fe2Os on FTO. ^{6 15-17}

[0027] Upon adding H2O2, the same peak can be observed at 1 .4 VRHE (~ 600 mV lower overpotentials) with much higher intensity (Fig. 4B). Without the use of H2O2, this peak is only observable at much higher voltages (> 1 .9 VRHE). This observation shows that the formation of the high-valent Fe ^lv =O species is obtained at lower overpotentials compared to the OER.

[0028] The mechanisms of C-H bond dissociation of CH ₄ can be classified into two categories: dehydrogenation and deprotonation. The dehydrogenation mechanism is generally observed for strong oxidizing catalysts such as high-valent metal oxo species as in the Fe ^lv =O. ²⁰ The mechanism occurs via the surface nucleophilic oxygens, i.e., electron saturated species (O ² “), which act as H ⁺ acceptors and abstract a hydrogen atom (»H) from CH ₄ . In contrast, the deprotonation mechanism usually occurs on metal complexes with low oxidation states metal centres and accessible H ⁺ acceptors. ²¹ In addition to the dehydrogenation and deprotonation mechanisms, CH ₄ oxidation could go through the Fenton pathway in which the reaction is initiated by free radicals that are accompanied by a Fenton reagent, such as Fe" which generates OH radicals. A DFT study by Szecsenyi et al. have shown the existence of a combination of dehydrogenation, deprotonation, and Fenton pathways. ²¹ The complexity of their reaction mechanism is due to the presence of multiple oxidation states of Fe. It has been found that Fe" and Fe ¹¹¹ favour the deprotonation and Fenton pathways, while the Fe ^lv =O would promote the dehydrogenation pathway. ²⁰ Adopting electrochemical means assure steady generation of Fe ^lv sites that can prevent unwanted competing reactions at the Fe" ¹ sites to take place through the Fenton pathway. ²⁰

[0029] These high valent Fe ^lv species detected during in situ spectroelectrochemical measurements of Fig. 4B , have been reported to be active sites for OER in alkaline conditions, using operando XAS and Mdssbauer spectroscopies. ^{22 23} An operando infrared spectroscopy study performed by Zandi and Hamann showed that the rate-limiting step for the water oxidation is the oxidation of Fe ^IH -OH to Fe ^lv =O. ²⁴

Fe" ¹ + H ₂ O Fe ^IN -OH + H ⁺ (1)

Fe ^IH -OH Fe ^lv =O + H ⁺ + e (2)

[0030] Therefore, even without the use of H2O2, one can generate the high-valent metal oxo Fe ^lv =O species, and in return dissociate the C-H bond. However, due to the use of high voltages, there will be a competitive OER, in addition to the high possibility of the produced oxygenates being overoxidized to CO2. This supports what was observed during the electrochemical tests discussed in Fig. 2D. Based on our combined experimental and mechanistic study, we can propose the following mechanism for eCH ₄ OR using our CuFe/NiF electrode: The generation of high valent Fe ^lv =O species is dependent on the availability of the reactive oxygen species whereby H2O2 would predominantly be oxidized on nickel to generate the reactive oxygen species of hydroperoxyl radicals (»OOH). The Fe ^lv =O is then generated by radical addition as mentioned in Equation (4):

Fe ^IH + H ₂ O Fe ^IH -OH + H ⁺ (3)

Fe ^IH -OH + »OOH Fe ^lv =O + O ₂ + H ⁺ + 2e (4)

[0031] Hence, Fe ^lv =O would easily be formed since H2O2 oxidation happens at low applied potential compared to H ₂ O oxidation. The copper centres will modulate the reaction environment and prevent the over-oxidation of the produced oxygenates to CO2 by reducing the excess of radicals.

Cathodic oxygen reduction reaction (ORR)

[0032] Intending to supply the required H2O2 for CH ₄ oxidation at the anode, here we focus our efforts to electrochemically generate H2O2 from cathodic ORR. The current industrial production of H2O2 is mainly obtained via the anthraquinone process. However, this process requires precious-metal catalysts and high-pressure H ₂ in the complementary hydrogenation step. Furthermore, it involves multi-step reactions and separations, which lead to generating organic wastes and enormous energy consumption. [0033] The electrochemical generation of H2O2 is obtained via the 2-electron oxygen reduction reaction (ORR). Recently, metal-free carbon materials have been received increasing attention for H2O2 generation in alkaline media. ^25-28 In this study, we have performed ORR experiments in an alkaline medium (1.0 M KOH) using an unmodified carbon-based gas-diffusion layer (Freudenberg GDL). We adopted a flow cell configuration instead of the H-type cell to diminish both the mass transfer limitation and the effect of the low solubility of O2 (40 mg L ^-1 water) by continuously feeding the reactant through the gas-diffusion layer. The study was initiated by conducting linear sweep voltammetry (LSV) in the potential range between 0.8 and - 0.5 V _R HE while flowing N2 and then O2 (Fig. 5A). The current steadily increased with the sweep of the applied potential when O2 was flowed, confirming the occurrence of the ORR.

[0034] Our results show that Freudenberg GDL has higher catalytic activity for oxygen reduction to H2O2 than previously reported Printex 6L and Vulcan XC-72R carbon in an alkaline medium. ²⁹ ’ ³⁰ Pristine carbons are inactive, so it is important to activate inert surfaces of carbon either by heteroatoms doping or by creating defective carbon sites. The activity of Freudenberg GDL is due to its partial graphitization. The edges of the graphitic carbons have been shown to exhibit superior electrolytic performance than basal carbons due to the alteration of their local electronic structure. The fibres used in Freudenberg GDL are also designed to preserve a certain degree of un-graphitized carbon to retain the hydrophobicity that is required for the gas diffusion layer. The Raman spectrum of the GDE has dominant bands at ca. 1360 cm ^-1 and 1580 cm ^-1 corresponding to the disordered sp ² carbon (defect, D band) and the resonant phonon vibrations of the crystalline graphitic carbon (graphitic, G band), respectively (Fig. 5B). ³¹ ’ ³² This confirms the presence of graphitic species along with sp ² hybridization of partially oxidized carbonaceous species in GDE as also shown by the ratio of D/G intensities (I D/IG) that is ~ 1.1.

[0035] The presence of NO and SO2 functional groups was found to enhance the electrolytic activity of the carbon-based electrocatalyst for ORR. Sulfonic acid functions were found to provide effective Bronsted acid sites which enhance the ionic conductivity. ³³ Also, the nitrogen-doped carbons exhibit a more positive onset potential as reported for the nitrogen doping of carbon nanotubes (CNTs) and graphene materials. ³⁴³⁵ Fourier transform infrared attenuated total reflection (ATR- FTIR) spectroscopy was performed to confirm the existence of the NO and SO2 functional groups. As shown in Fig. 5C, the band at 1080 cm ^-1 is related to the bending vibrations of S=O2 while the band observed at 1029 cm ^-1 corresponds to the N=O stretching vibrations. Similar observations were reported for the Vulcan XC 72R and Printex L6 Carbons. ²⁹³⁶ The ORR happens on the triple-phase boundary sites with the help of the micropores present on the surface of the Freudenberg GDL that are responsible for supplying oxygen during the reaction as shown in the inset of Fig. 5C. The higher conductivity of KOH compared to NaOH when used as a supporting electrolyte may also contribute to the higher quantities of H2O2 and higher current densities. ³⁷

[0036] Chronoamperometry studies were performed at constant potentials of - 0.07 and 0.60 VRHE, and the produced H2O2 was spectroscopically quantified by the cerium (IV) ion titration method. At 0.131 VRHE, the current density obtained was stable at -110 mA cm ^-2 for 100 min at a Faradaic efficiency towards H2O2 of ~ 90 %. Moving to a more negative bias at -0.07 VRHE has increased the current density to - 152 mA cm ^-2 with an H2O2 Faradaic efficiency of 84 %. As illustrated in Fig. 5D, the production rate of H2O2 also increased from 1 .83 to 2.57 mmol h ^-1 cm ^-2 upon increasing the applied potential from 0.60 to -0.07 VRHE.

Full cell self-sustaining electrolyser for eCl- OR

[0037] A coupled system is provided, comprising an oxygen reduction reaction (ORR) to provide H2O2 on the cathode and an eCH4OR on the anode (eCH4OR- ORR). In this way, in situ generated H2O2 from the cathodic ORR on unmodified GDE may be used as an oxidant for the CH4 oxidation reaction at the anode. We estimated the H2O2 production and consumption rates individually based on a fixed value of charge passed through the system (Fig. 6A). As shown in the figure, the coelectrolysis can be operated sustainably since the H2O2 generation rate is higher than the consumption rate at different chosen charge transfers. It is important to note that a continuous supply of H2O2 from the cathodic ORR for the anodic CH4 oxidation reaction can be achieved by maintaining the concentration gradient between the cell compartments. This will force HOO- to steadily migrate through the anion exchange membrane (AEM) from the catholyte to the anolyte chamber. AEMs consist of hydrophilic cation groups that are directly incorporated or anchored as side chains into the hydrophobic polymer backbone. ³⁸ The anions and water can migrate through the dense AEM by the water-filled hydrated ionic domains in the polymer matrix. Vehicular transport of the anions occurs due to both diffusion and electro-osmosis mechanisms. ³⁹ Diffusion of the anions is driven by the concentration gradient while electro-osmosis of those charged species occurs in response to the electrical potential gradient.

[0038] Using a flow cell with an AEM (Figs. 5B and C), we operated a self- sustaining coupled experiment using 1.0 M KOH as anolyte and catholyte and unmodified carbon GDL and CuFe/NiF as cathode and anode, respectively. The anolyte was 1 .0 M KOH with the addition of 50 mM H2O2 to initiate the overall reaction, which will generate H2O2 at the GDE cathode simultaneously. We note that for a self-sustaining process, the rate of H2O2 (Rgene) generation in the catholyte has to be more than or equal to the rate of H2O2 consumption (R _CO ns) in the anolyte chamber (Rgene>R _con s). However, if the rate of H2O2 migration through the AEM (Rmem) is less than R _CO ns, H2O2 will continuously be consumed, leading to a non- sustainable process (inset of Fig. 5C). To solve this issue, R _me m is increased by introducing an initial concentration of H2O2 in the catholyte chamber, since Rmem is directly proportional to the concentration gradient across the membrane (AC _H2 o ₂ )- Different concentrations of H2O2 in 1 .0 M KOH were introduced to the catholyte chamber to study the effect of H2O2 concentration gradient across the AEM on the sustainability of the process. We noticed that the current stabilized at higher values upon increasing AC _H2 o ₂ - When we maintained 450 mM concentration gradient of H2O2 across the membrane, the current was stabilized at 20 mA cm ^-2 for 10 hours. Increasing the initial concentration gradient of H2O2 across the membrane to 950 mM resulted in levitating the current from 82 mA cm ^-2 to 110 mA cm ^-2 during the first 175 minutes and was stable for more than 500 minutes of the reaction.

[0039] A detailed techno-economic analysis (TEA) on a prospective embodiment of the coupled eCH ₄ OR-ORR process yields a minimum selling price (MSP) of the target product, as illustrated in Fig. 6d, which shows the MSP of HCOOH as a function of Faradaic efficiencies at different electricity costs, showing profitable regions as a function of energy efficiency and electricity cost.

Conclusion

[0040] A route for selective partial oxidation of methane at ambient conditions is disclosed herein, which avoids unwanted overoxidation to CO2. Operando potential- controlled spectroelectrochemistry showed that Fe ^lv can be obtained with the help of reactive oxygen species generated via the partial electrooxidation of H2O2 at lower overpotentials. Cu is disclosed to have a crucial role in protecting the produced liquid oxygenates from overoxidation to CO2. A trimetallic catalyst of CuFeNi is provided that is demonstrated to be capable of Faradaic efficiencies of -42% and liquid product selectivity of 100% at current densities of 32 mA cm ^-2 . We also present a coelectrolysis cell to generate H2O2 required for the CH4 oxidation through the cathodic oxygen reduction reaction (ORR), which showed current densities of 110 mA cm ^-2 at 1 .3 VRHE for more than 8 hours of reaction.

[0041] Although various embodiments of the invention are disclosed herein, many adaptations and modifications may be made within the scope of the invention in accordance with the common general knowledge of those skilled in this art. Such modifications include the substitution of known equivalents for any aspect of the invention in order to achieve the same result in substantially the same way. Numeric ranges are inclusive of the numbers defining the range. The word "comprising" is used herein as an open-ended term, substantially equivalent to the phrase "including, but not limited to", and the word "comprises" has a corresponding meaning. As used herein, the singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a thing" includes more than one such thing.

[0042] Citation of references herein is not an admission that such references are prior art to the present invention. Any priority document(s) and all publications, including but not limited to patents and patent applications, cited in this specification are incorporated herein by reference. All documents cited or referenced in herein cited documents, together with any manufacturer’s instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention. More specifically, all referenced documents are incorporated by reference to the same extent as if each individual publication were specifically and individually indicated to be incorporated by reference herein and as though fully set forth herein. The invention includes all embodiments and variations substantially as hereinbefore described and with reference to the examples and drawings.

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