A Review on Synthesis of Methane as a Pathway for Renewable Energy Storage With a Focus on Solid Oxide Electrolytic Cell-Based Processes

Environmental issues related to global warming are constantly pushing the fossil fuel-based energy sector toward an efficient and economically viable utilization of renewable energy. However, challenges related to renewable energy call for alternative routes of its conversion to fuels and chemicals by an emerging Power-to-X approach. Methane is one such high-valued fuel that can be produced through renewables-powered electrolytic routes. Such routes employ alkaline electrolyzers, proton exchange membrane electrolyzers, and solid oxide electrolyzers, commonly known as solid oxide electrolysis cells (SOECs). SOECs have the potential to utilize the waste heat generated from exothermic methanation reactions to reduce the expensive electrical energy input required for electrolysis. A further advantage of an SOEC lies in its capacity to co-electrolyze both steam and carbon dioxide as opposed to only water, and this inherent capability of an SOEC can be harnessed for in situ synthesis of methane within a single reactor. However, the concept of in situ methanation in SOECs is still at a nascent stage and requires significant advancements in SOEC materials, particularly in developing a cathode electrocatalyst that demonstrates activity toward both steam electrolysis and methanation reactions. Equally important is the appropriate reactor design along with optimization of cell operating conditions (temperature, pressure, and applied potential). This review elucidates those developments along with research and development opportunities in this space. Also presented here is an efficiency comparison of different routes of synthetic methane production using SOECs in various modes, that is, as a source of hydrogen, syngas, and hydrogen/carbon dioxide mixture, and for in situ methane synthesis.


INTRODUCTION
The global energy consumption has been increasing ever since the dawn of industrialization. Such an increase can be attributed to the expeditious economic growth across the globe and the rising population. World Energy Outlook 2019 [International Energy Agency (IEA) 2019 executive summary] and others (Criqui and Kouvaritakis, 2000;Newell et al., 2018) report that in 2018, worldwide energy consumption increased by 2.3%, which is almost twice the average rate of growth since 2010. The deleterious aftermath of such higher energy consumption is an overall CO 2 emission of 33.1 Gt, which is 1.7% higher than that in 2017. This is equivalent to 2.4 ppm rise in the total concentration of atmospheric CO 2 as stated by the IEA (Capuano, 2018) and others (Saracoglu et al., 2018). According to studies conducted by National Aeronautics and Space Administration (NASA) and recent UN reports, such a steady acceleration of CO 2 levels will increase the earth's temperature by 1.5°C by 2052, leading to the melting of the Arctic ice and subsequent perilous rise in sea level. Such effects coupled with the gradual depletion of fossil fuels has made a compelling case for the development and deployment of renewable energy (RE) sources such as solar, wind, tidal, and geothermal energy.
A substantial reduction in the manufacturing and capital costs associated with solar and wind energy plants has been observed in the last decade. Renewables 2018 (Saracoglu et al., 2018) predicts that by 2023, there would be 1,460 TW h of electricity generation from solar photovoltaic and 1,880 TW h of electricity generation from offshore and onshore wind turbines (Renewables, 2018). An analysis of the world energy scenario in terms of fuel type shows that consumption of renewables has expanded at an annual growth rate of about 12% over the past 10 years, driven by the move of electricity generation toward renewables (Tanaka, 2010).
Despite impressive numbers, inherent intermittency and uneven geographical distribution of RE sources remain a barrier for a solid commercial case of clean energy technologies, and therefore, reliance on fossil fuel continues. As such, the key enablers for renewable technology will be pathways for efficient and cost-effective storage as well as transportation of RE from the supply areas rich in RE to the demand areas like cities and industrial establishments. In this context, the conversion of RE into storable and transportable energy carriers is being considered a promising avenue to balance the energy supply-demand equation.
An emerging technology in this area is broadly defined as Power-to-X, where X can be a fuel such as hydrogen, ammonia, methanol, and dimethyl ether, or a commodity chemical like lubricants and aviation fuels (Foit et al., 2017;Rego de Vasconcelos and Lavoie, 2019;Dueñas et al., 2020). Hydrogen is a zero carbon-emission fuel with high gravimetric energy density (33.3 kW h/kg) that can be synthesized using various electrolytic, photocatalytic, or solar thermochemical processes (Chen and Shangguan, 2013;Acar and Dincer, 2019) and can be efficiently converted back to energy without carbon emission. However, the major hurdles in a hydrogen-based RE economy are difficulties in the storage and transportation of hydrogen owing to energyintensive processes of compression or liquefaction and boil-off losses during transportation. For instance, up to 12% of the energy content of hydrogen is lost while compressing the gas to the required pressures (Makridis, 2017;Bruce et al., 2018). Although hydrogen has a very high gravimetric heat content (lower heating value (LHV) of 33.3 kWh/kg), its volumetric heat content is very low (2.73 Wh/L). Further, several challenges related to the mechanical properties of storage materials need to be addressed (Christian et al., 2013;Prabhukhot Prachi et al., 2016).
A last link of the supply chain technology is the conversion of hydrogen back to energy that also needs substantial bottoms-up development. Most of the current infrastructure for utilization of carbon fuels such as gasoline, diesel, or natural gas cannot be used with hydrogen in high concentration due to issues related to hydrogen embrittlement, flame travel distance, and a high autoignition temperature (Verhelst and Wallner, 2009).
In response to these challenges, storing and transporting energy in the form of hydrogen carriers appears interesting, where methane is being considered one of the promising and practical energy cum hydrogen carriers due to its moderately high heat content (LHV of 13 kW h/kg) as compared to other prospective alternatives such as gasoline (LHV of 12 kW h/kg) and ammonia (LHV of 5.2 kW h/kg), lower compression energy (0.56 kW h/kg at the usual compression pressure of 350 bar) (Makridis, 2017), and facile synthesis using RE and waste CO 2 ( Figure 1A).
Methane is a primary constituent of natural gas (50-90%) and can be effectively used for any application where natural gas is being currently used. Apart from this, methane finds potential application in different chemicals production as well. Global Energy Statistical Yearbook states that from 2010 to 2018, global natural gas consumption increased by 11%, and the IEA has predicted that this value will rise to almost 55% with a total gas consumption of ∼53,330 TW h by 2040 (Capuano, 2018). Recently, there has been an interest in utilizing methane as a probable RE carrier, especially in the context of Australia's abundant RE potential and its vision as an exporter of the same to other countries.
In the genre of electrolytic routes, a comparatively early-stage technology that offers scalable, economic, and highly efficient solution to produce either pure hydrogen or syngas (a mixture of CO and H 2 ) for methanation is based upon solid oxide electrolysis. More interestingly, it can be used to directly synthesize methane from steam and CO 2 co-electrolysis with a right combination of electrocatalyst and process conditions. However, this route of methane synthesis is not yet well established, with a dearth of knowledge on the fundamental mechanism, reaction pathways, and development of materials tailored to improve the reaction kinetics, product selectivity, and process efficiency. Further, the scale and lifetime of solid oxide electrolyzers remain limited compared to those of more established alkaline and proton exchange membrane (PEM) electrolyzers.
For methane synthesis using solid oxide electrolytic cell (SOEC) coupled with a thermochemical methanator, the challenges are essentially those related to hydrogen and syngas production in SOECs. This review elucidates those issues followed by a brief discussion on the emerging area of one-step methane generation in SOEC via co-electrolysis of steam (instead of hydrogen) and CO 2 . An efficiency comparison of methane synthesis using SOEC in all possible configurations has also been presented at the end. For such comparison, previous studies Luo et al., 2018;Wang et al., 2018; have focused on SOECs only as a source of hydrogen and syngas (H 2 /CO), but here we have considered SOECs as a source of H 2 /CO 2 mixture and for in situ methanation as well.

MAJOR PATHWAYS OF METHANE SYNTHESIS
As opposed to other emerging hydrogen carriers, the infrastructure for the synthesis, storage, transport, and utilization of methane is fairly well-established. Several pathways of methane synthesis have emerged over the past few decades that can be broadly categorized as shown in Figure 1B and have been briefly described in this review with a detailed discussion on SOECs. Synthetic methane production routes can be classified as follows: (1) Purely thermochemical process in a catalytic reactor using biosyngas or industrial syngas, or other non-electrolytic sources of H 2 and CO 2 (discussed in Purely Thermochemical Route of Methane Synthesis) (2) Combined thermo-electro-chemical process where H 2 produced from electrolytic routes [using alkaline electrolytic cells (AELs) or PEM electrolytic cells (PEMECs) or solid oxide electrolytic cells (SOECs)] or syngas produced from steam/CO 2 co-electrolysis in SOECs is utilized for thermochemical methanation in a conventional catalytic reactor (discussed in Combined Thermo-Electro-Chemical Route of Methane Synthesis and Challenges and Advancement in the Electrolytic Production of Hydrogen and Syngas in Solid Oxide Electrolytic Cell Integrated With Thermochemical Reactor for Methane Synthesis) (3) Purely electrochemical process of in situ methane synthesis in high-temperature SOECs (discussed in State of the Art of Purely Electrochemical In Situ Methane Synthesis in Solid Oxide Electrolytic Cell) As can be well perceived from the above classifications, synthetic methane production requires a source of carbon along with an electrolytic or non-electrolytic source of hydrogen. Several industrial processes yield highly pure and concentrated CO 2 off-gas, which can be separated from impurities and sequestered (Farla et al., 1995). These processes include production of alcohols (ethanol and methanol), ammonia synthesis, synthetic fuel production, and production of several organic chemicals. Other sources of CO 2 in high concentration include iron and steel plants (Kim and Worrell, 2002) and cement industries (Taylor et al., 2006). Process heaters, fluidized catalytic cracking units, and steam methane reforming of hydrogen production units in refineries (Van Straelen et al., 2010) are also important sources of CO 2 .
A naïve yet promising process for producing CO 2 is direct air capture, which is an industrial process of trapping CO 2 from the ambience with subsequent conversion to pure CO 2 stream that can be either sequestered or recycled back for other industrial processes ( Figure 2). Processes available for this emerging technology include absorption of CO 2 by ion exchange membranes with humidity-swing regeneration (Lackner, 2003), solid amines on a mesoporous silica substrate (Gray et al., 2008), and alkaline aqueous solution (Keith, 2009).

PURELY THERMOCHEMICAL ROUTE OF METHANE SYNTHESIS
Most of the current industrial applications of methane are essentially in petroleum or natural gas markets, and as such, synthetic methanation is limited to removal of CO 2 and CO during steam methane reforming in processes such as ammonia production. Typically, methanation is carried out in a thermochemical catalytic reactor in the temperature range of 250-350°C and 25 bar pressure by either CO 2 hydrogenation (Mills and Steffgen, 1974;Wei and Jinlong, 2011;Junaedi et al., 2014;El Sibai et al., 2015) via the Sabatier process (Eq. 1) or CO hydrogenation (Anderson et al., 1984;Schulz, 1999;Van Der Laan and Beenackers, 1999;Méndez and Ancheyta, 2020) via the Fischer-Tropsch process (Eq. 2). Reaction kinetics and product selectivity are heavily affected by the type of catalyst and support, whereas overall reactor efficiency depends on its design and operating conditions, all of which have been widely studied till date and are not within the scope of our present discussion, and readers are referred to reviews and books (Mills and Steffgen, 1974;Rönsch et al., 2016;Mebrahtu et al., 2019).
Both CO 2 and CO methanation are highly exothermic reactions accompanied by a reduction in volume, thus favorable at high pressures and low temperatures (LTs).
As can be determined from Eqs 1 and 2, the stoichiometric methanation of CO 2 and CO is accompanied by the evolution of 2.03 and 2.55 kW h heat per cubic meter of methane produced at normal temperature and pressure of 298 K. The thermodynamics and kinetics of both CO and CO 2 methanation processes have been widely studied by many FIGURE 2 | Exhaust from vehicles and other sources of greenhouse gas can be captured from air, and the CO 2 thus separated can be recycled back to fuel synthesis sources. researchers (Gao et al., 2012;Sahebdelfar and Takht Ravanchi, 2015;Rönsch et al., 2016). As reported by Gao et al. (2012), a maximum yield of CH 4 (>90%) from CO methanation is obtained in the temperature range of 200-300°C. In case of the Sabatier reaction as well, methane yield maintains a steady value (>90%) for temperatures up to 300°C, irrespective of pressure. Above 450°C, the reverse water gas shift (RWGS) reaction (Eq. 3) becomes dominant due to its endothermic nature and results in an upsurge of the CO by-product with a commensurate decrease in methane yield. Thus, the overall process of CO 2 conversion is dictated by competing reactions 1 and 3. A perfect combination of LT, high pressure, appropriate H 2 :CO 2 ratio, and suitable catalyst plays an important role for maximum methane yield from the Sabatier reaction. Compared to CO 2 , CO hydrogenation is more exothermic and releases 3.58 MW h heat for each ton of methane produced, and reaction kinetics is also expected to be faster as the reactivity of CO is likely to be higher than that of CO 2 molecule. Thus, methane synthesis from renewably produced syngas might be a better option instead of CO 2 hydrogenation with only hydrogen produced renewably (Sabatier process), as further discussed in Energy Efficiency of Methane Synthesis via Different Electrochemical Routes.

COMBINED THERMO-ELECTRO-CHEMICAL ROUTE OF METHANE SYNTHESIS
Green hydrogen generation for synthetic methane production has been widely studied using different electrolysis cells or electrolyzers that have been developed over the past few decades. These electrolyzers can be classified based upon the type of electrolyte (aqueous or solid) and ionic species transported during electrolysis, and operating temperature regimes as shown in Figures 3A,B. The aqueous solutionbased electrolytes were the original choice for electrolytes, and the most technologically matured electrolyzer design, AEL is based upon an aqueous electrolyte which transports hydroxyl ions (OH − ). Further, the family of electrolyzers based upon solid membrane electrolytes emerged where electrolytes are nonporous solids, which can either be proton (H + ) or oxide ion (O 2− ) conductors. This group of electrolyzers can operate over a wide range of temperature and can be broadly classified into low temperature (LT; <100°C), intermediate temperature (100-500°C), and high temperature (HT; >500°C). In addition to these common types, electrolyzers based upon molten carbonate electrolytes have been evaluated to some extent where the electrolyte is a molten carbonate material which operates at temperatures above 600°C (Hu, 2016;Hu et al., 2016). PEMECs are most widely used for LT operation. Polymer composite electrolyte membranes, beta alumina-based electrolyte membranes, and proton-conducting solid oxide electrolytic cells (H-SOECs) are used for intermediate temperature range and emerging solid oxide electrolytic cells (SOECs) used for HT operation ( Figure 3B). The basic working principles of AELs, PEMECs, SOECs, and H-SOECs are depicted in Figure 3C. Each of these electrolyte systems is at a different stage of technological maturity and comes with its own merits and demerits as discussed in reviews (Carmo et al., 2013;Santos et al., 2013;Rashid et al., 2015). In the section below, we have briefly discussed AEL and PEM technology followed by a detailed discussion on SOEC technology.
The setup is relatively simple with two electrodes, usually a mild steel cathode and an Ni anode immersed in a liquid alkaline electrolyte (20-30% potassium hydroxide, KOH), and separated by a porous diaphragm permeable to both OH − ions and water molecules. The operating temperature of AELs varies from 70 to 90°C, and hydrogen can be produced at pressures from 1 to 30 bar (Ursua et al., 2012). However, few of the shortcomings of AELs include corrosiveness of the electrolyte that reduces cell lifetime, and a low current density (100-300 mA per cm 2 of the electrode area) due to high ohmic losses which increase plant footprint (Mazloomi and Sulaiman, 2012). Current density is an important figure of merit used for comparison of the electrolytic systems as it is directly proportional to the rate of hydrogen production. More importantly, the limited ability of AELs for dynamic response to the fluctuations in the electricity supply is a key issue when it is powered by RE sources like solar and wind, which are intermittent in nature. Any additional requirement for energy storage devices such as batteries and related electronics significantly increases the capital and process costs, leading to an increased levelized cost of hydrogen production. Some opportunities for further improvement of this electrolyzer are in the improvement of diaphragm materials and electrocatalysts.

Proton Exchange Membrane Electrolyzer
To overcome the limitations of AELs, the concept of replacing the electrolytic solution by a solid polymeric membrane having proton conductivity emerged, and the electrolyzer came to be known as PEMEC, more popularly PEM (Xu and Scott, 2010;Carmo et al., 2013;Paidar et al., 2016;Jayakumar, 2019;Yang et al., 2019). In PEM electrolyzers, oxygen is released at the anode, and the H + ions migrate to the cathode through the solid nonporous polymeric membrane, where they recombine with e − to release hydrogen gas. As the anode and cathode are separated by the polymeric electrolyte membrane, this guarantees minimum gas crossover with the yield of very high purity hydrogen. The most commonly used polymer membrane electrolyte material is Nafion sold by Dupont Ltd. On the anode side, a titanium (Ti)-based electrode mesh or foam with iridium (Ir)-based oxygen evolution electrocatalyst is used, and on the cathode side, platinum (Pt) metal nanoparticles supported on a high surface area carbon support are used. With improvements in design and materials, the PEM technology has made substantial progress, and the operating current densities can be as high as 1.5 A cm −2 and hydrogen can be produced at pressures above 30 bar (Ayers et al., 2010). The design of PEM is relatively compact and modular in nature, allowing for flexibility in scale and configuration of the system. Considerable progress has been made over the past decade in terms of technological maturity with systems up to 3 MW scale available commercially. Despite such progress, key technical challenges still remain unsolved, which include high costs associated with components fabrication, reliance on precious metal group catalysts, and performance degradation over the lifetime requiring incremental electrical energy input.
The levelized cost of hydrogen production per kilogram by PEM is in the range of $ 6.08-7.43 and for AEL, about $4.78-5.84 (Bruce et al., 2018). These costs are expected to decrease as the scale of the system increases. The cost of hydrogen from electrolysis is somewhat challenging considering the variables involved; however, it is generally agreed that cost of electricity is a major contributor with up to 65% of the cost of hydrogen (James et al., 2016;McDonagh et al., 2018). Clearly, the reduction in electrical energy input and lower cost of electricity can bring a shift change in the cost of electrolysis.
The minimum theoretical energy required for water electrolysis with PEM or AEM is about 40 kW h/kg of hydrogen. On top of that, losses associated with electrolyte resistance and electrode reactions (electrode polarization) and system-level losses add up. The total energy required at the system level can be as high as 5 kW h per Nm 3 of hydrogen, which equates to about a whopping 60 kW h/kg of hydrogen.
SOECs have a great potential to overcome the limitations of AELs and PEM, in particular energy requirements (Luo et al., 2018;. Further, it allows electrolysis of not only water but also CO 2 and co-electrolysis of steam/CO 2 mixture (Bandi et al., 1995;Laguna-Bercero, 2012;Hansen, 2015;Zheng et al., 2017). SOEC operation at HTs (above 600°C) requires a substantially lower electric energy input per kilogram of fuel produced due to favorable cell thermodynamics, which has been elaborated in a subsequent section. A detailed comparison of the characteristics of AELs, PEMECs, and SOECs is available in the literature (Bertuccioli et al., 2014;Bhandari et al., 2014;Peterson and Miller, 2016;Schmidt et al., 2017;Buttler and Spliethoff, 2018). Figure 4A shows the basic working principle of SOECs. The electrodes are separated by a dense ion-conducting ceramic Frontiers in Energy Research | www.frontiersin.org September 2020 | Volume 8 | Article 570112 6 membrane. The feed gases, that is, steam or CO 2 or both, are supplied to the cathode where it reacts with electrons provided by an external power supply to produce H 2 and CO, respectively, and oxygen ions, which are transported through an ionconducting electrolyte to the anode, where they combine and release the electrons (e − ) again (Bandi et al., 1995;Zheng et al., 2017;Pandiyan et al., 2019). The electrochemical reactions taking place in the SOECs are given as follows: Cathode i. Steam electrolysis:

BASIC WORKING PRINCIPLE OF SOLID OXIDE ELECTROLYTIC CELLS AND THEIR APPLICATION IN SYNTHETIC METHANE PRODUCTION
ii. CO 2 electrolysis: iii. Steam/CO 2 co-electrolysis: Anode Oxygen evolution: The voltage gradient across the ion-conducting membrane drives the reaction, and the minimum operating voltage (V 0 ) required for the process is equal to the change in the Gibbs free energy (ΔG): where ΔH is the enthalpy change given by the heat of the reaction and TΔS is the supply of external heat, T being the temperature and ΔS the entropy change.
As can be forecasted from Eq. 8, with an increase in T, ΔH remains almost unaffected, while ΔG (electrical energy input) decreases almost linearly with T ( Figure 4B) since both steam and CO 2 decomposition are highly endothermic reactions (Eqs 4 and 5). Hansen (2015) reported that the ratio of ΔG to ΔH is about 93% at 80°C and about 77% at 750°C. This reduction in ΔG is significant, as it constitutes over 64% of the total cost of electrolytic hydrogen production, as discussed earlier. More importantly, the heat (TΔS) can be sourced either as low-grade waste heat from industrial processes, or as high-grade heat from the solar thermal concentrators.
The operating voltage of the cell is governed by its ohmic resistance and resistances offered by the activation of electrodes, charge transfer, diffusion of the reactant and the product gases, and gas conversion. All these resistances together are often referred to as area-specific resistance of the cell and dictate the efficiency and, in turn, the cost of fuel production.

Solid Oxide Electrolytic Cell Materials, Designs, and Modes of Operation for Methane Synthesis
The solid oxide electrolyte in SOECs can be either an oxygen ion conductor or a proton conductor. A major development of SOECs revolves around oxygen ion-conducting electrolytes, but there has been renewed interest in proton-conducting electrolytes for SOEC application (Ding et al., 2019;Duan et al., 2019;Vøllestad et al., 2019). Despite the impressive performance, H-SOECs are still at infancy and limited to the laboratory scale relative to the oxygen ion conductor-based technology.
Most of the materials and methods of fabrication for SOECs are derived from those used for solid oxide fuel cells (SOFCs), which is already a commercialized technology. For state-of-theart SOECs, choice of the electrolyte is an oxygen ion-conducting ceramic known as 8 mol% yttria (Y 2 O 3 )-stabilized zirconia (ZrO 2 ) or YSZ (Smart and Weissbart, 1967;Bandi et al., 1995;Graves et al., 2011). As shown in Figure 4A, the electrolyte is sandwiched between the fuel electrode (cathode) and the oxygen electrode (anode). Cathodes are generally composed of porous composites (commonly known as cermets) of Ni metal with YSZ, which is made by mixing NiO with YSZ. The state-of-the-art anode (oxygen evolution electrode) materials for SOECs are lanthanum strontium manganite (La 0.8 Sr 0.2 MnO 3−δ , commonly known as LSM) and YSZ composite (Kim et al., 2001;  2004; Murakami et al., 2010;Kaur et al., 2018b), or lanthanum strontium cobalt ferrite (La 0.8 Sr 0.2 Co 0.2 Fe 0.8 O 3δ , commonly known as LSCF) (Laguna-Bercero et al., 2011;Zheng et al., 2014). Several new materials are being developed and trialed as SOEC electrodes such as mixed conductor perovskites (Ebbesen and Mogensen, 2009;Laguna-Bercero, 2012). In particular, there has been significant research on new cathode materials  with regard to methanation, as discussed in Challenges and Advancement in the Electrolytic Production of Hydrogen and Syngas in Solid Oxide Electrolytic Cell Integrated With Thermochemical Reactor for Methane Synthesis.
SOECs can be fabricated in two different configurations, namely, planar and tubular (Shi et al., 2015), as shown in Figures 5A,B. Planar cells are in the form of square or rectangular flat plates or circular discs ( Figure 5A) that can be stacked on top of one another and interconnected in series with metallic or ceramic plates referred to as interconnects, which are electronic conductors in both reducing and oxidizing atmospheres. The planar cells can either be electrode supported (Knibbe et al., 2010) where the anode is first prepared followed by casting a thinner layer of electrolyte atop ( Figure 5C), or electrolyte supported (Ursua et al., 2012;Ghaib and Ben-Fares, 2018) where the electrolyte is first fabricated followed by electrode coating ( Figure 5D). In electrodesupported cells, the electrolyte is few microns thick, which remarkably reduces the ohmic resistance. However, such thin electrolytes are vulnerable to mechanical stresses and prone to failure under continually variable loading conditions which are common with renewables. Another design under consideration is a metal-supported cell where a porous metallic plate acts as a support and very thin layers of electrode and electrolyte can be coated on it (Tucker, 2010;Leah et al., 2017).
The planar design ensures high volumetric hydrogen production and a facile fabrication process. However, major concerns include issues with the hermetic sealing required to separate fuel gas from the air and compressive stresses lowering the ability of stack to cope up with thermal cycling.
Alternatively, a tubular design ( Figure 5B) is available, where either an electrolyte or a cathode (fuel electrode) is formed in the shape of a tube, typically by processes such as isostatic bag pressing, extrusion, or freeze casting. Once the starting component is prepared, the remaining electrodes/electrolytes can be coated by a variety of processes, including dip coating, spray coating, or brush painting. The tubular cells are freestanding and can be stacked side by side in a row or as a bundle of several tubes. In the case of electrolyte-supported tubular cells, the requirements for sealing are less stringent, and the ability to cope with thermal cycling is shown to be better (Kaur et al., 2018a). The downsides of this tubular geometry are a lower volumetric current density leading to a larger footprint of the system and issues with designing and fabrication of current collectors for current collection from the tube interior.
SOECs can be operated in four different modes or configurations for methane synthesis as shown in Table 1. The first two modes belong to thermo-electro-chemical routes of methane synthesis, whereas modes 3 and 4 constitute purely electrochemical pathways ( Figure 1B). In modes 1 and 2, SOECs simply act as a source of hydrogen or syngas coupled with a separate conventional SR or methanation reactor (MR), respectively, whereas modes 3 and 4 encompass methanation carried out in a single reactor without the need for a separate downstream process. The differences between modes 3 and 4 are the operating temperature and role of cathode toward the entire process. In mode 3, the cathode itself acts as a methanation catalyst, or the catalyst is in direct physical contact with the cathode, referred to as bilayer electrodes. With bilayer cathodes, both steam/CO 2 co-electrolysis and methanation occur on the

Mode
Process description Mode 1 SOEC supplies hydrogen produced by steam electrolysis to a conventional Sabatier reactor. Heat from the Sabatier reactor can be used for the SOEC reactor (i.e., steam production and maintaining temperature). Mode 2 SOEC supplies syngas produced by steam/CO 2 electrolysis to a conventional methanation reactor. Heat from the methanation reactor can be used for the SOEC reactor (i.e., steam production, heating reactant gases, and maintaining temperature).

Mode 3
In situ methanation in a single temperature zone cell where both steam/CO 2 co-electrolysis and methanation occur on the cathode itself. One-step process without a downstream reactor.

Mode 4
In situ methanation in a dual temperature zone cell where syngas generation from steam/CO 2 co-electrolysis occurs on the cathode maintained at a high temperature followed by its methanation on a catalyst bed kept at a lower temperature, but both housed inside the same cell.
Frontiers in Energy Research | www.frontiersin.org September 2020 | Volume 8 | Article 570112 cathode itself, the key feature being that the SOEC can be maintained at one single temperature. Contrarily, in mode 4, the catalyst is placed as a separate bed in proximity of the electrode (at some distance from the electrode) in cooler regions of the cell but still within the same cell. So, in this case, the cell features two zones: a high-temperature electrolysis zone, where syngas is produced from coelectrolysis, followed by a catalyst-laden lower temperature zone, where the syngas along with unconverted CO 2 undergoes methanation. As is expected, the energy efficiency and cost of methane production will vary for each of these pathways (Luo et al., 2018), and a comparison on the same has been provided in detail in Energy Efficiency of Methane Synthesis via Different Electrochemical Routes.

Challenges and Advancement in the Electrolytic Production of Hydrogen and Syngas in Solid Oxide Electrolytic Cells Integrated With Thermochemical Reactor for Methane Synthesis
For modes 1 and 2, mentioned above, the materials and design challenges are very similar to the state-of-the-art SOECs, and research has been progressing at different paces consistent with "waves" in hydrogen research and development (R&D) in general. The advantageous use of SOECs with reduced electrical energy input has been experimentally demonstrated almost 40 years ago by Erdle et al. (1992), followed by Quandt and Streicher (1986) in the 1980s. The key technical issues for SOECs as hydrogen or syngas generators include limited lifetime due to degradation at the electrodes, sealing issues, and high capital costs. Capital cost is a function of several factors, such as production volume and scale of the units, and material supply chain. One might expect a drastic reduction in capital costs with improved cell designs such as microtubular cells Chen et al., 2019;Monzón and Laguna-Bercero, 2019), advent of new manufacturing processes such as 3D printing Wei et al., 2019), and advanced automation which can be implied if the technology gains market acceptance. The challenges associated with materials are more fundamental in nature, and the degradation mechanism in SOECs is not yet well understood. It is generally agreed that more R&D is warranted toward designing new electrode materials like double perovskites (Shin et al., 2015;Afroze et al., 2019;Tian et al., 2020) or modification of existing electrodes. When CO 2 is added to steam for generation of syngas (mode 2), the issues related to the stability of the electrode become even more challenging. SOEC cathodes have been under intense investigation over the recent few years as both energy-intensive reactions in SOECs, namely, steam and CO 2 splitting (Eqs 4 and 5), occur on this electrode. The state-of-the-art cathode material is a porous cermet comprising YSZ and metallic nickel, as mentioned earlier in Solid Oxide Electrolytic Cell Materials, Designs, and Modes of Operation for Methane Synthesis. Ni provides the electronic conduction pathways and catalytic activity, while the YSZ matrix prevents Ni particle grain growth and provides ionic conduction pathways leading to an increased triple-phase boundary area (Bandi et al., 1995;Sridhar and Vaniman, 1997;Holtappels et al., 1999;Graves et al., 2011). Although Ni-YSZ is the common choice of fuel electrode due to the high catalytic activity of Ni in H-H (Bourcet and Tantardini, 1994) bond breaking, its pitfalls include accelerated coarsening, agglomeration, and migration away from the electrode-electrolyte interface particularly under electrolysis condition; limited catalytic activity for CO 2 splitting; poor stability under variable loading conditions; and reduced mechanical strength in the presence of steam (Hauch et al., 2008;Knibbe et al., 2010b;Iwanschitz et al., 2010;Khan et al., 2016;Wang Y. et al., 2019). As Ni gets oxidized upon exposure to steam or CO 2 , an additional supply for partial recycling of generated H 2 is required to maintain Ni in the metallic state. Product gas circulation is not a critical challenge in small lab-scale experiments; however, at the system level, it could pose significant challenges as it adds to the complexity of the plant. If the operation of SOECs is desired without recirculation of the product gas, the operating window must ensure that oxygen partial pressure in the inlet chamber (cathode chamber) always remains below 10 −14 bar. This, in turn, requires SOEC operation with or above particular current densities. In the case of fluctuating electricity supply, such a requirement may lead to curtailing of hydrogen/syngas production if the availability of electricity cannot meet the minimal demand. Alternatively, a complex system control or a backup power can be used, however, at a substantial capital cost penalty. Usually, operation with recirculated gas like hydrogen requires a low steam-tohydrogen ratio in the feed gas with the hydrogen content above 10%, preferably up to 20% hydrogen with steam. Significant degradation in the performance has been reported by Eguchi et al. (1996) at a higher steam concentration. In a similar observation for CO 2 electrolysis, Green et al. (2008) have shown that for a decreasing CO 2 /CO ratio (creating a more reducing environment), polarization resistance offered by an Ni-YSZ cathode first decreases and then increases, which is contrary to the general expectation. This is due to the Nicatalyzed Boudouard reaction leading to enhanced coke deposition that dampens cathode activity. Thus, steam electrolysis demands the cathode to be stable in low H 2 partial pressures, but co-electrolysis or CO 2 electrolysis poses the additional challenge that cathode should be CO tolerant and coking resistant. Thus, a considerable portion of the current SOEC material development focuses on the development of an alternative cathode which will be stable under a variety of operating conditions. The development strategies involve either a modification of the Ni-YSZ or a complete substitution with new materials. The modification includes either tailoring of the microstructure or addition of a catalytic phase to the Ni-YSZ, which can improve the catalytic performance of Ni toward CO 2 splitting. Some researchers (Ishihara et al., 2010;Wang et al., 2013;Hong et al., 2015;Kim et al., 2016;Neofytidis et al., 2019;Yu et al., 2020) have tried to evaluate the alloying of Ni with a second metal as promoter, which alters the work function of Ni, thereby improving its catalytic activity toward steam or CO 2 splitting and minimizing issues related to carbon deposition. For example, As for the complete substitution of Ni-YSZ, a wide variety of materials such as perovskites, fluorites, and composites have been studied as prospective cathodes for both hydrogen and syngas generation Wang Y. et al., 2019;Jiang, 2019;Pandiyan et al., 2019). Since the key role of Ni in Ni-YSZ is to provide electronic conduction and catalytic activity toward electroreduction, a pertinent alternative is to seek perovskites that can potentially exhibit these two features. Such ceramics, mainly fluorites and perovskites, belong to the genre of mixed ionic electronic conductors. A plethora of investigations on perovskite electrodes have been reported for SOFCs (Zhu and Deevi, 2003a;Faes et al., 2012;Mahato et al., 2015); however, for SOECs, the number of publications and patents in the area have started increasing only over the past few years. Yang and Irvine (2008) used strontium and cobalt-stabilized lanthanum manganite (La 0.8 Sr 0.2 Cr 0.5 Mn 0.5 O 3−δ , commonly referred as LSCM) as the cathode with YSZ as the electrolyte and lanthanum strontium ferrite (La 0.7 Sr 0.3 FeO 3 , commonly LSF) as the oxygen electrode for steam electrolysis, and achieved startup from a very low hydrogen concentration (4% by volume) without having to overcome high ohmic and polarization resistances as witnessed in Ni-based cathodes prior to the reduction of NiO to Ni. Another example is doped titanium oxide or titanate cathodes. Titanate perovskites (Yang et al., 2014;Gan et al., 2016;Barnett et al., 2019;Dogu et al., 2019) have been investigated for several years as an alternative to Ni-YSZ. Titanates lack catalytic activity for electrolysis, and the addition of a second catalytic phase becomes necessary. Marina and Pederson (2008) reported a composite ceria-titanate cathode (La 0.35 Sr 0.65 TiO 3 -Ce 0.5 La0.5O 2 ) which demonstrated substantially lower polarization resistance (0.2-0.28 Ω cm 2 ) than Ni-YSZ (0.4 Ω cm 2 ). Ceria provides additional catalytic activity, and the titanate phase provides electronic conductivity.
Recently, Deka et al. (2019) used Ni-and Co-doped lanthanum strontium ferrite (LSF) cathode for co-electrolysis at 800°C. Co-electrolysis performed for 110 h on a La 0.7 Sr 0.3 Ni 0.1 Co 0.1 Fe 0.8 O 3 cathode showed appreciable stability of this composition to coking. Co-doped LSF showed the lowest Faradaic efficiency, whereas Ni-doped LSF gave ∼100% Faradaic efficiency (see Table 2 for the performance). Their in situ X-ray near-edge structure studies revealed that during co-electrolysis, Co ions may get oxidized, thereby decreasing the number of oxygen vacancies in the material and lowering its electrochemical activity. Moreover, unlike Ni-doped cathodes, Co-doped ones showed evidence of graphitic carbon formation that could have further reduced the electrochemical performance of the cell. Doped ceria-based fluorite oxides and related composite cathodes have been gaining popularity over the last few years. It is generally accepted that Ceria (CeO 2 ), and in particular aliovalent metal (gadolinium, samarium, or praseodymium)doped ceria such as gadolinia-doped ceria (GDC) or samariadoped ceria, is immune to coking due to the presence of highly mobile surface oxygen species that react with any deposited carbon and suppress coke build-up through gasification (Livermore et al., 2000;Goodenough and Huang, 2007;Cimenti and Hill, 2010;Lee et al., 2016;Elleuch and Halouani, 2020). The presence of such mobile oxygen species is the synergistic effect of oxygen vacancies imparted by the dopant and the in situ reduction of Ce 4+ to Ce 3+ at HTs and in a reducing atmosphere. Although not as widely studied for CO 2 electrolysis or co-electrolysis, ceria-based electrodes have already proven their merit in SOFCs. Yue and Irvine (2012) compared the cathode performance of LSCM/YSZ and LSCM/GDC composites for CO 2 electrolysis and concluded that GDC itself undergoes reduction under applied load, thus offering much lesser polarization resistance and enhanced cathode activity than YSZ. Doped ceria has a conductivity of up to 1 s/cm in a reducing atmosphere, and as such, either mixing with an electronically conducting phase is required or an additional current collection layer needs to be coated on the top of the ceria cathode. Recently, LSM and GDC composite was evaluated by Kaur et al. (2018b) as an SOEC cathode for CO 2 electrolysis, and it not only exhibited better electrochemical performance but was also found to be significantly more stable. The LSM and related structures provided required electronic conductivity. Kulkarni et al. (2017) observed that at 800°C, a Pd-doped La 0.8 Sr 0.2 Co 0.2 Fe 0.8 O 3−δ (LSCF) cathode with samaria-doped ceria interlayer and YSZ electrolyte rendered a high Faradaic efficiency of 97% with minimal overpotential losses ( Table 2) for CO 2 electrolysis. Ag-GDC composites were evaluated for electrolysis of CO 2 (Xie et al., 2015). Ag possesses high electrical conductivity and chemical stability in oxidizing atmospheres above 200°C, and GDC has high ionic conductivity and good catalytic properties toward CO 2 reduction in CO/CO 2 atmospheres, and their composite has shown promising results for CO 2 electrolysis. However, any adverse effect of the cell performance due to evaporation of Ag and possible electromigration needs to be evaluated in long-term tests.
Another important component that has drawn sufficient research interest is the SOEC electrolyte. As of date, YSZ is the most commonly used electrolyte (Graves et al., 2011b;Hansen, 2015b) due to its high ionic conductivity (∼0.016 s/cm) along with thermal and chemical stability at the usual operating temperatures of SOECs (800-1,000°C). However, it fails to exhibit sufficient ionic conductivity in the intermediate temperature range (500-800°C), which is a major drawback. There are some reports even on the failure of the YSZ electrolyte, which is otherwise thought to be stable under SOFC operating conditions Park et al., 2019).
Over the past few decades, substantial efforts have been made to develop an oxygen ion conductor electrolyte which works at Frontiers in Energy Research | www.frontiersin.org September 2020 | Volume 8 | Article 570112  (Mat et al., 2019;Pesaran et al., 2019;Pham et al., 2019;Zhigachev et al., 2019;Wang et al., 2020); however, the high cost of scandium (several times higher than that of yttria) is a major issue for practical applications. Another prominent electrolyte family is ceria-based materials Raza et al., 2020). The technical issues with these electrolytes for SOEC application are their propensity toward electronic conduction under high applied potential or reducing atmosphere, phase stability in the presence of steam and CO 2 , and lower mechanical strength than YSZ. Thus, as of now, YSZ continues to be the choice of electrolyte material for commercial SOECs and the most widely used electrolyte for laboratory-scale studies. Table 2 shows observations from some of the key cathodes and electrolytes that have been tested for coelectrolysis and CO 2 electrolysis.
The established air electrodes for SOFCs are also materials of choice for SOEC anodes, as previously mentioned in Solid Oxide Electrolytic Cell Materials, Designs, and Modes of Operation for Methane Synthesis. However, rapid degradation due to electrode delamination and changes in electrode-electrolyte interfacial phase assemblage has been reported with LSM-YSZ composites (Chen and Jiang, 2011;Graves et al., 2011;Rashkeev and Glazoff, 2012;Kim et al., 2013;Graves et al., 2015). It is now well-established that a high current density leads to the build-up of high internal oxygen partial pressure at the anode-electrolyte interface, resulting in the entrapment of nano-sized oxygen bubbles that finally causes microstructural damage and electrode delamination (Virkar, 2010;Tietz et al., 2013;Graves et al., 2015;Chatzichristodoulou et al., 2016;Khan et al., 2017). In response to the degradation issues, LSCF-based materials, which are mixed ionic electronic conductors, are being evaluated in SOECs (Guan et al., 2006;Hjalmarsson et al., 2013;Kim and Choi, 2014;Singh et al., 2015). According to one such research conducted by Singh et al. (2015), CO 2 electrolysis was carried out at 1,000°C using Ni-GDC cathode and YSZ electrolyte with two different anodes (LSCF and LSM-YSZ). Under open circuit conditions, LSCF showed much lower anodic polarization resistance (∼0.074 Ω cm 2 ) than LSM/YSZ (∼0.13 Ω cm 2 ). Also, the LSCF anode remained stable for a constant operation of 9 h at a current density of 1.2 A cm −2 . In another work by GE (Guan et al., 2006) using Ni-YSZ as fuel electrode, YSZ as electrolyte, and different perovskites as oxygen electrodes for steam electrolysis at 800°C and 1.3 V for 100 h, the cell degradation rate followed the order: LSCF < LSF < LSM. Although LSCF shows promising results at intermediate current densities, long-term stability testing at high current density is still under investigation. As oxygen evolution is relatively less energy intensive in SOECs, the research emphasis in on engineering the porosity and microstructure of the cathodes (Chatzichristodoulou et al., 2016;Khan et al., 2017) to avoid delamination from the electrolyte. Also, changes in cell operating conditions are being explored. For example, Graves et al. (2015) recently proposed that electrolysis-induced degradation can be reduced by reversibly switching between electrolysis and fuel-cell modes. They reported that for steam electrolysis at a current density of 1 A cm −2 , the cell voltage increased from 1.33 to 1.73 V for 420 h of constant operation, whereas in the reversible mode operation (1 h electrolysis followed by 5 h fuelcell mode), the cell voltage during the electrolysis part of the cycles remained stable at 1.33 V over the same span of 420 h.
In addition to electrodes and electrolyte, there are issues associated with cost reduction of interconnects and sealing stability in the presence of steam and gases like CO 2 (Zhu and Deevi, 2003b;Lessing, 2007). These are persistent issues with SOFC technology; however, they can be more challenging for SOECs as understandings on the performance and degradation behavior of these components in SOECs are limited. The typical sealing being explored in planar SOEC stacks is edge seal, glass seal, or compressive seal (Lessing, 2007). For the edge sealing, typically a metal frame of ferritic stainless steel is used. Most sealing metal frames have thermal expansion coefficients higher than zirconia (10.5 K −1 × 10 −6 K −1 ), which led to the development of chromium (Cr)-based alloys with lower thermal expansion coefficients. However, Cr gets oxidized to Cr 2 O 3 followed by vaporization and condensation on the electrode and electrolyte surfaces, increasing cell resistance. Thus, low Cr content ironbased steels are presently used for edge sealing and interconnect because of their low cost, easy fabrication methods, and reasonably low thermal expansion coefficient. The development work is also continuing with ceramic materials and coatings like doped lanthanum chromate (LaCrO 3 ) which has the potential to attenuate the Cr poisoning effect of metallic interconnects (Cable et al., 2011). Other major challenges include stack design and assemblage, and optimization of manufacturing processes at scale. These challenges are not specific to synthetic methane production but need focused R&D to meet the performance and life targets for SOECs at a realistic price point. These issues are discussed in detail in SOFC reviews (Balachandran et al., 1989;Lessing, 2007;Shaigan et al., 2010;Ebbesen et al., 2014;Mahato et al., 2015;Mah et al., 2017;Pandiyan et al., 2019;Wang Y. et al., 2019), which apply to SOECs as well.

State of the Art of Purely Electrochemical In Situ Methane Synthesis in Solid Oxide Electrolytic Cells
A relatively large body of literature work is available on SOEC cathodes for hydrogen or CO, or more recently on syngas production; however, very limited experimentation has been carried out on the in situ synthesis of hydrocarbons where the cathode needs to play an additional role of synthetic catalyst to promote reactions such as methanation. Lately, researchers have Frontiers in Energy Research | www.frontiersin.org September 2020 | Volume 8 | Article 570112 started investigating in situ methane synthesis in SOECs in a single reactor (Jensen et al., 2003;Bierschenk et al., 2011;Xie et al., 2011;Li et al., 2013;Chen et al., 2014;Chen et al., 2017;Lei et al., 2017;Jensen et al., 2019;Luo et al., 2020) with only CO 2 and steam as feedstock ( Figure 6) employing either mode 3 or 4 as described in this section. Xie et al. (2011) were one of the first to perform in situ methanation using a composite of lanthanum-doped strontium titanate (La 0.2 Sr 0.8 TiO 3+δ , commonly LST) and GDC as cathode on YSZ electrolyte and LSM/YSZ composite anode. They used an additional layer of iron catalyst placed in direct contact with the cathode (Figure 6A). At 650°C, about 2.8% methane was generated at atmospheric pressure. They suggested that methane yield can be further improved with adequate optimization of reactor design, proper manipulation of pressure, possibly by the usage of backpressure, and catalyst infiltration into cathode instead of placing it atop the cathode.
The effect of temperature, as encountered in in situ methanation, is still ambiguous and has been systematically studied by only a few researchers. According to some studies, in situ methane synthesis should be carried out in dualtemperature zone SOECs (mode 4 described in Basic Working Principle of Solid Oxide Electrolytic Cells and Their Application in Synthetic Methane Production), where the high-temperature region favors endothermic steam/CO 2 co-electrolysis, whereas a catalyst-laden cooler region within the same cell favors exothermic methanation reactions of the in situ-generated syngas ( Figure 6B). In one such design, Chen et al. (2014) conducted in situ methanation using an Ni-YSZ cathode, YSZ electrolyte, and LSM-YSZ anode, where the SOEC part was operated at 800°C and 1.3 V, and the temperature was gradually decreased to 250°C in the Fischer-Tropsch (F-T) regime. Methane yield remained almost constant from 200 to 400°C for the F-T section, followed by a sharp decrease with further increase in temperature up to 800°C, as HT does not favor exothermic CO methanation reaction (Eq. 2); rather, it promotes methane steam reforming (Eq. 9), which is a competing reaction occurring in the F-T section.
CH 4 + H 2 O CO + 3H 2 ΔH 1,023K 206 kJ mol −1 (9) In the SOEC section, as expected, increasing the operating temperature improved the ionic conductivity of the electrolyte, thereby sprucing up the methane output flux. In another study by Lei et al. (2017) using a microtubular SOEC consisting of Ni-YSZ cathode, YSZ electrolyte, and LSM-YSZ anode, the electrolysis zone was operated at 800°C and the temperature decreased to 200°C in the F-T regime. They obtained a methane yield of 23.1% with an inlet gas composition of 21.3% CO 2 , 58.7% H 2 , and 20.0% H 2 O under an electrolysis current density of 0.32 A cm −2 . Luo et al. (2020) recently proposed a mechanism for in situ methanation in dual-temperature zone SOECs. They said it takes place in three active regions: on the high-temperature zone cathode surface driven by the reaction between inflowing hydrogen and CO 2 , on the cathode-electrolyte interface driven by rapid hydrogenation of the CO produced from the electroreduction of CO 2 , and on the low-temperature zone cathode surface driven by purely heterogeneous catalyst-mediated CO 2 /CO methanation.
Regarding the role of temperature, another school of thought advocates the use of a single temperature (550-650°C) zone SOEC, where methanation occurs on or in the near vicinity of the cathode itself (mode 3 described in Basic Working Principle of Solid Oxide Electrolytic Cells and Their Application in Synthetic Methane Production). Li et al. (2013) used a button-cell configuration with Ni-YSZ cathode, ScSZ electrolyte, and LSM-ScSZ composite anode at 650°C and reported trace amount (0.29%) of methane formation. On similar lines, Bierschenk et al. (2011) showed that reducing SOEC operating temperature from 750 to 600°C increases methane yield from a merely traceable amount to 14.3%. They further predicted from thermodynamic calculations that increasing the pressure to 10 atm would further raise the yield to 26.7%. Jensen et al. (2003) showed that the equilibrium methane yield can be raised to over 50% at 650°C and an operating pressure >15 bar, and this conclusion also indicates that an intermediate temperature SOEC should be more suitable for methane synthesis. They proposed that co-electrolysis carried out at a pressure as high as 150 atm and 650°C would generate 85% CH 4 and 15% H 2 with minimal CO and CO 2 . Jensen et al. (2019) further carried out in situ methanation in an SOFCMAN 301 stack with 30 NiO-YSZ|YSZ|GDC|LSCF-GDC planar cells maintained at 700°C and 18.7 bar. They reported a methane yield of 18% at a current density of 0.17 A cm −2 . In another study, Luo et al. (2020) carried out in situ methanation in a tubular reactor with a Ni-YSZ|Ni-ScSZ bilayer cathode, ScSZ electrolyte, and LSM-ScSZ composite anode under varying conditions of pressure. They showed that under an applied current of 2 A, methane yield of 7% at 1 atm increased to 28.7% at 4 atm. However, with technology related to in situ methanation in SOECs being at a nascent stage, the effect of pressure on methane yield is debated. Contrary to calculations of Jensen and coworkers, Chen et al. (2017) observed that from 1 to FIGURE 6 | In situ methane synthesis in SOEC in a single temperature zone configuration with catalyst layer above the cathode (A) and dual temperature zone configuration with catalyst beside the cathode (B).
Frontiers in Energy Research | www.frontiersin.org September 2020 | Volume 8 | Article 570112 2.7 bar, the conversion ratio of methane increased and then remained unchanged with further increase in pressure. They explained this phenomenon in terms of the synergistic effect of pressure on methanation reaction rate and current density. On one hand, methanation, being a volume contraction reaction (Eqs 1 and 2), is favored at high pressure, and on the contrary, increase in pressure increases the required voltage for steam and CO 2 coelectrolysis, thereby slightly reducing the current density that pulls down the syngas production rate. In addition to temperature and pressure effects, investigations on reaction pathways are also equivocal. Li et al. (2013) proposed that under higher applied voltage, CO generated in situ dissociates and deposits on the cathode surface as carbon that undergoes hydrogenation to methane. Another interesting possibility is a combinative effect of electric field and catalytic activity on the reaction kinetics by a phenomenon termed as a non-Faradaic electrochemical modification of catalytic activity (NEMCA). In the context of SOECs, it is believed that catalytic activity is enhanced due to the promotion of the work function of catalytic surfaces generated by oxygen ion pumping to/from the electrolyte onto the catalyst surface (Yentekakis and Bebelis, 1992;Yentekakis et al., 1994;Varkaraki et al., 1995;Yentekakis et al., 1995;Frantzis et al., 2000;Cavalca, 2006;Anastasijevic, 2009;Fan, 2012;Theleritis et al., 2012;González-Cobos et al., 2017;López et al., 2019). Unlike structural promoters that improve the dispersion and the chemical stability of the active catalyst phase, electronic promoters directly enhance the catalytic activity of the catalyst itself ( Figure 7A).
For in situ methanation, Anastasijevic (2009) investigated CO 2 hydrogenation in light of the NEMCA effect using Ru||YSZ||Au between 200 and 300°C under an anodic polarization of −1.3-1.3 V. Under open-circuit conditions, the rate of CH 4 formation was 10 −4 mmol s −1 , which increased to ∼2.4 mmol s −1 × 10 −4 mmol s −1 under an applied potential of 1.3 V and decreased to ∼0.6 mmol s −1 × 10 −4 mmol s −1 under −1.3 V. However, the formation rate and selectivity of CO exhibited exactly opposite trends, as shown in Figure 7B. There was a monotonic decrease in the rate of CO formation from ∼0.7 mmol s −1 × 10 −4 mmol s −1 under −1.3 V to ∼0.5 mmol s −1 × 10 −4 mmol s −1 under 1.3 V. The authors concluded that supplying O 2− to the Ru catalyst surface increases its work function, which, in turn, strengthens the Ru-H bond and weakens the Ru-CO bond. This suppresses RWGS (Eq. 3) and promotes methanation, thereby increasing methane yield. Contrarily, stripping O 2− away from the Ru surface decreases its work function, promoting RWGS, and hence CO formation. In another study, Fujiwara et al. (2018) conducted steam/CO 2 co-electrolysis on button cells at 600°C using Ni-GDC cathodes, LSCF-GDC anodes, and Hionic ™ substrate (oxide ion conductor) electrolyte. The cathodes were doped with Pd and Ru (0.3 mmol/g Ni-GDC). They reported that both CO 2 conversion and methane selectivity increased with an increase in applied potential up to ∼4 V, which they explained in light of the NEMCA effect. According to them, polarization enriched the cathode surface with electrons, which could have enhanced its catalytic activity toward CO 2 conversion via the RWGS reaction (Eq. 3). Nevertheless, there is a dearth of studies that address the NEMCA effect on CO 2 methanation, especially for in situ methane synthesis in SOECs.
A summary of the key findings of in situ methanation studies carried out till date is provided in Table 3. The crux of the researches conducted so far on in situ methanation in SOECs is that all of temperature, pressure, operating voltage, inlet gas composition, and most essentially electrode and/or electrocatalyst play pivotal roles in determining the yield of methane. However, to interpret and optimize the operating parameters and determine the type of methanation electrocatalyst that would render the highest yield, a clear perception of the actual reaction mechanism is imperative. Thus, the application of SOECs to produce methane requires further R&D activities focused on a clear perception of the governing reactions, and reaction kinetics thereof, choice of electrode-electrolyte-catalyst combination, and finally, optimization of the operating conditions. Nonetheless, it is worth mentioning that in situ methanation is one of the most efficient ways for synthetic methane production. We have presented a comparison of the energy efficiency of in situ methanation in SOECs with four other routes of methane synthesis in the following section.

ENERGY EFFICIENCY OF METHANE SYNTHESIS VIA DIFFERENT ELECTROCHEMICAL ROUTES
In this section, we compare the energy requirements and efficiencies of five different routes of methane synthesis FIGURE 7 | A) Schematic of the basic working principle of non-Faradaic electrochemical modification of catalytic activity effect when ions (oxide ions or cations) are pumped to the catalyst working electrode. (B) Pictorial depiction of the trends followed by methane and CO production rates when CO 2 hydrogenation is carried out over a range of negative to positive polarization of the active catalyst surface. Here, both the y axes have the same scale. (Figure 8) which involve the use of PEM, AELs, and SOEC electrolyzers. For SOECs, we considered different modes of operation. In route 1, water electrolysis is carried out in AELs or PEM at near-standard operating conditions (80°C and 1 bar) to produce hydrogen, which along with CO 2 is fed to a Sabatier reactor (SR) for methane synthesis. In route 2, steam electrolysis is carried out in SOECs to produce hydrogen, which is fed to a SR for methane generation. In route 3, steam and CO 2 are coelectrolyzed in SOECs to generate syngas, which is supplied to a MR for methane synthesis. Route 4 involves in situ synthesis of methane in a two-temperature zone SOEC, where syngas generation from co-electrolysis of steam-CO 2 occurs in the high-temperature zone followed by its subsequent methanation in the low-temperature zone. In route 5, dry CO 2 electrolysis in SOECs produces CO that undergoes WGS reaction with steam in a thermochemical reactor to generate a H 2 /CO 2 mixture that then undergoes methanation in a subsequent SR.
So routes 2-5 involve SOECs, here assumed to be at 800°C and 1 bar, which can be essentially operated in multiple configurations for methane synthesis. It can be used as a source of hydrogen (route 1) or CO (route 5) or syngas (route 3) that undergoes subsequent methanation in a SR or a MR operated at 250°C and 25 bar pressure in the presence of state-ofthe-art Ni catalyst. Further, it is possible to use SOECs for onestep methanation (route 4) directly from steam and CO 2 .
The schematic of the five routes considered here are shown in Figure 8 and their corresponding energy efficiencies in Figure 9A. The process energy efficiencies (ƞ) are calculated by employing basic enthalpy balance: Process energy efficiency η Energy content of product Total energy input × 100 (10) It is to be noted now that SOECs provide the opportunity to recycle back the exothermic heat of methanation reactions (Eqs 1 and 2); however, the recirculation of heat requires an additional balance of plant which can increase capital costs. So, for each of routes 2, 3, and 5, two cases of η and energy demand are portrayed ( Figures 9A,B): one with heat recycled back from SR or MR to the SOEC being operated at 800°C and the other one with no such heat recycle, whereas routes 1 and 4 have only one value of η; for route 1, there is no scope for heat recycle with AELs/PEM, and for route 4, exothermic heat is generated in situ within the SOEC itself and is thus available at all times. Although methane itself is a well-established fuel, it can also be used as a source of hydrogen (at energy penalty) using an established steam methane reforming process in conventional thermochemical reactors or membrane separation catalytic reactors (Kim et al., 2018;Simakov and Román-Leshkov, 2018). When methane was produced via routes 1, 2, and 5 (Eq. 10), the energy efficiency of the process was estimated to be approximately 67, 73, and 80%, respectively, whereas the highest efficiency of ∼89% was calculated for in situ methanation (route 4), as shown in Figure 9A. The round-trip efficiency (RTE) of producing methane from hydrogen and back  to hydrogen through steam reforming ( Figure 9C) was also maximum for route 4 (∼62%), followed by route 3 (∼58%). Similar RTE (∼65%) was reported by Jensen et al. (2015) while operating solid oxide cells in reversible mode at nearatmospheric pressure. They modeled the system where methane was synthesized as well as utilized using the same device. However, at high pressure (15 bar), higher efficiencies of up to 70% were reported by them. In another similar study, Butera et al. (2019) reported that during the reversible operation of solid oxide cells at higher pressures, the RTE can be further increased to 80% through proper adjustment of the H/C ratio of the gases being purged into the cell during both electrolysis and fuel-cell modes. From preliminary energy balance calculations and considering minimal balance of plant requirements, it can be stated that in situ methanation in SOECs is the most efficient yet relatively cheaper option for methane synthesis as well as hydrogen recovery. For large-scale methane producing plants, the capital costs associated with the integration of separate reactors is substantial. In contrast, in situ methanation in SOECs eliminates the need for separate reactors and auxiliary components, which makes the system relatively compact and reduces plant footprint.
Calculations are based on the following major assumptions: • During actual cell operation, voltage and current density would dictate the input energy, and thus the energy efficiency of the process. However, what have been reported here are simplistic calculations purely based on thermodynamics, so no current density is involved in this case. • System losses incurred from the integration of various components such as catalytic recuperator, condenser, humidifier, and gas splitter have not been considered here. • Heat loss from reactor and pipelines is 5% of the total heat generated due to exothermic reactions occurring in the SR or MR or WGSR (heat exchanger efficiency 95%). • 90% of each of the gases produced through electrolysis is available for further reactions. • 100% of the products generated in the SR, MR, and WGSR are obtained at the reactor outlets. • In route 5, H 2 /CO 2 , produced in the WGSR at the ratio of 1: 1, flows to the gas separator from where H 2 /CO 2 at a ratio of 4:1 is sent to the SR and the remaining CO 2 is recycled back to the SOEC. • Electrolyzers are thermally insulated and prevent heat loss to the surrounding. • For RTE, no losses have been considered for methane compression and transportation, and efficiency of the steam methane reformer for converting methane to hydrogen is 70%. • RTE calculations are based on the best-case scenario, that is, with heat recycled back from the SR or MR to the SOEC. • LHV has been used for both methane (55 KJ/g) and hydrogen (120 KJ/g).

CONCLUDING REMARKS AND TECHNOLOGY OUTLOOK
An ever-increasing global energy demand with subsequent development in solar and wind energy systems has made the compelling case for investigations on renewably powered synthetic reactors for the production of hydrogen and hydrogen carriers as a means of energy storage and transport. Methane, the major component of natural gas, is one of the favorite contenders if the carbon source is waste CO 2 produced from industrial or biological processes, or collected from the atmosphere using direct air capture technology, and hydrogen is sourced using renewable sources of energy. Methane itself is a well-established fuel but can potentially be a part of a circular hydrogen economy acting as a source of hydrogen that can be transported over long distances for various applications. The steam methane reforming for hydrogen production is a wellestablished process.
Methane can be synthesized in a SR from CO 2 and hydrogen. CO 2 can be acquired from carbon capture and sequestration stations and hydrogen from electrolysis of water in AELs or PEM. AEM and PEM are becoming mature technologies for hydrogen production, but both suffer from certain shortcomings, the major one being the high electrical energy demand. In such a pretext, SOECs are being considered eligible candidates for methane synthesis. Although some preliminary research corroborates the huge potential of in situ methane synthesis in SOECs, rigorous investigations on the electrochemical and chemical reaction mechanisms and kinetics, and cell material behavior under the process conditions are required to advance the technology. A balance between SOEC kinetics and a thermodynamically favorable operating window for methane synthesis needs to be achieved, which is a nontrivial challenge.
The areas requiring detailed inspection and development include electrocatalytically active materials selection, cell design, the establishment of overall mechanism and reaction pathways of methanation processes, and optimization of process conditions.
One of the prime constraints of SOEC is the high-temperature operation that tends to reduce the cell materials lifetime and is counterproductive for methanation reaction. This calls for innovation of novel materials as electrolytes that will possess appreciable ionic conductivity at intermediate temperatures (350-600°C), good chemical stability to endure the redox environment of the SOEC and finally excellent adhesion to the electrodes. Development of electrode materials (cathode) with the desired combination of electrocatalytic properties, mechanical strength, phase stability and electrical conductivity at SOEC operating temperature and environment is another prerequisite.
The choice for cathode materials for one-step methane synthesis (on the same electrode) becomes even more challenging as in addition to the requirements for electrodes to perform co-electrolysis, the activity of methanation also needs to be considered. Materials need to be chosen such that they have low CO adsorption energies so that the CO produced in situ can undergo further hydrogenation to yield methane. Oxygen evolution being relatively less energy intensive in SOECs, the research emphasis is on engineering the porosity and microstructure of the anodes to avoid delamination from the electrolyte, rather than materials itself. A second limitation of methane synthesis using SOECs is that the rate-limiting step for electrolysis of CO 2 or coelectrolysis of CO 2 /H 2 O mixture in SOECs has not yet been unanimously established or determined. For syngas production through co-electrolysis, the role of CO 2 electrolysis (Eq. 5) vs. RWGS reaction (Eq. 3) as the major pathway of CO generation is still debated. As of date, two notions prevail regarding the role of the RWGS reaction in the co-electrolysis of steam and CO 2 . Some researchers believe that with steam electrolysis being much faster than CO 2 electrolysis, only H 2 O splits into hydrogen which then undergoes RWGS reaction with CO 2 to produce CO, as RWGS is thermodynamically more favorable. However, others believe that CO 2 is electrolyzed to CO. Establishment of the reaction mechanism for in situ methanation poses additional challenges as it follows a complex pathway consisting of primarily four different steps: 1) dissociative adsorption of CO 2 on the electrode followed by either its hydrogenation to CH 4 (methanation) or CO (RWGS), or even surface reduction to CO; 2) electroreduction of CO 2 to CO at the triple-phase boundary; 3) hydrogenation of the CO produced in situ through steps 1 and 2 to CH 4 ; and 4) electroreduction of the steam generated during in situ methanation to prevent dissociation of methane via steam reforming. The kinetically slowest and hence rate-limiting step will be dictated by the electro-kinetics of the whole process subject to temperature, and such understanding is required to enable material design and optimization of processing conditions to enhance reaction kinetics and methane yield to practical levels.
Other parameters requiring optimization are SOEC operating conditions such as temperature, pressure, current density, voltage, and inlet gas composition. The effect of temperature is still debated; most of the investigations conducted with dualtemperature zone SOECs show that the operation of the SOEC section at higher temperatures improves syngas generation, which would consequently enhance methane yield in the subsequent low-temperature F-T section. However, some other investigations clearly indicate that a single temperature zone SOEC operated at an intermediate temperature ensures improved methane yield. Equally ambiguous is the effect of pressure as discussed in State of the Art of Purely Electrochemical In Situ Methane Synthesis in Solid Oxide Electrolytic Cells. The contribution of the NEMCA effect toward the enhanced catalytic activity of electrocatalysts during in situ methane synthesis is also yet to be established.
A sustainable operation meeting the required SOEC kinetics in the temperature range suitable for methanation (300-500°C) warrants the development of new materials and cell designs functioning effectively in the temperature range. The electrochemical performance targeted for the electrodes and the electrolyte will vary with the design, and this will in turn affect the costs associated with the cell fabrication process. The electrolytesupported cells can be limited in current densities (proportional to hydrogen or syngas production); however, the production costs are usually lower than that of anode-supported cells. On the other hand, the electrode-supported cells can be operated at very high current densities, even exceeding 1 A cm −2 ; however, cost and lifetime are the key challenges. In the opinion of the authors, the capital costs of US$1,000 per kilogram (hydrogen) at 1 MW scale can make a compelling commercial case for SOECs, considering competing technologies and hydrogen cost targets.
In a nutshell, exhaustive studies and improvisation, therefore, can make in situ methanation in SOECs an indispensable part of Power-to-Methane technology that is bound to play a key role in the future energy sector.

AUTHOR CONTRIBUTIONS
SahB, major write up and review; AK, conception, outline, major write up, and review; SG, critical review, content suggestions, and proofreading; SanB, critical review, content suggestions, and proofreading.

FUNDING
This work received funding from the Australian Renewable Energy Agency (ARENA) as part of ARENA's Research and Development Program-Renewable Hydrogen for Export, and also from CSIRO Hydrogen Energy Systems Future Science Platform, CSIRO Research Office, and a PhD scholarship from Monash University. The funder was not involved in the study design; collection, analysis, and interpretation of data; the writing of the manuscript; or the decision to submit it for publication.