A Perspective on Solar-Driven Electrochemical Routes for Sustainable Methanol Production

Abstract

The transition towards sustainable and renewable energy sources is imperative in mitigating the environmental impacts of escalating global energy consumption. Methanol, with its versatile applications and potential as a clean energy carrier, a precursor chemical, and a valuable commodity, emerges as a promising solution within the realm of renewable energy technologies. This work explores the integration of electrochemistry with solar power to drive efficient methanol production processes, focusing on electrochemical reduction (ECR) of CO₂ and methane oxidation reaction (MOR) as pathways for methanol synthesis. Through detailed analysis and calculations, we evaluate the thermodynamic limits and realistic solar-to-fuel (STF) efficiencies of ECR and MOR. Our investigation encompasses the characterization of multijunction light absorbers, determination of thermoneutral potentials, and assessment of STF efficiencies under varying conditions. We identify the challenges and opportunities inherent in both ECR and MOR pathways, shedding light on catalyst stability, reaction kinetics, and system optimization, thereby providing insights into the prospects and challenges of solar-driven methanol synthesis, offering a pathway towards a cleaner and more sustainable energy future.

1. Introduction

The escalating global population, coupled with the surging energy demands projected by the International Energy Agency, necessitates a paradigm shift towards sustainable and renewable energy sources. Current trends, as indicated by the energy progress report, reveal that a substantial 85.33% [1] of the world’s energy consumption is derived from non-renewable sources, contributing to alarming levels of carbon dioxide emissions. While renewable resources, including wind and solar power, have witnessed a commendable surge, constituting 12.84% of global energy consumption in 2021 [2], challenges persist. The intermittent nature of solar and wind energy, along with the logistical hurdles in large-scale electricity storage and transmission, underscores the imperative for alternative solutions [3]. Hydrogen, touted as a clean fuel, faces practical limitations due to storage complexities and high costs [4]. In this context, the methanol economy emerges as a promising avenue [5,6]. Methanol, with its single carbon structure, liquid state at room temperature, and diverse applications, offers a compelling solution to store surplus energy efficiently. Unlike hydrogen, methanol can seamlessly integrate into existing infrastructure, making it a viable and practical choice for a sustainable energy future. Moreover, methanol’s versatility extends beyond its role as a fuel [3]. Its high-octane number makes methanol an ideal additive to gasoline, offering a cleaner and more environmentally friendly option for the transportation sector. Additionally, methanol serves as a critical precursor and commodity chemical for numerous industries, including plastics, textiles, pharmaceuticals, and agriculture. Its versatility as a feedstock allows for the production of a wide array of essential products [7], thereby contributing to various sectors of the economy. In addition to its role as a critical precursor and commodity chemical for numerous industries, methanol’s versatility extends to the realm of energy storage and utilization. Methanol serves as a vital component in direct methanol fuel cells (DMFCs), offering a clean and efficient means of electricity generation [8]. DMFCs harness the electrochemical oxidation of methanol to produce electricity, providing a promising avenue for decentralized power generation and mobile applications. By embracing methanol as a cornerstone of the renewable energy transition, we unlock its potential as a sustainable precursor for a multitude of industries, paving the way for a greener and more resilient future.

The utilization of electrochemistry, harnessing the power of affordable electrons to drive chemical reactions, emerges as a highly attractive and innovative approach within the methanol economy. This method holds the promise of transforming renewable energy, particularly solar energy, into a potent driver for sustainable chemical processes. By coupling electrochemistry with solar energy, we unlock a synergistic relationship that not only addresses the intermittent nature of sunlight but also enhances the overall efficiency of energy conversion. Solar-driven electrochemical processes offer a pathway to utilize abundant and cost-effective electrons, providing a clean and renewable energy source for the production of methanol. An electrochemical system is primarily integrated with solar energy in two different ways: (1) coupling of a photovoltaic (PV) cell with an independent electrochemical cell (PV–EC) [9,10]. (2) Incorporating a photoelectrocatalyst (PEC) in an electrochemical system [11,12]. While a PEC offers a compact design and uses fewer materials compared to a PV–EC system, PECs to date have been limited by their efficiency to out-compete PV–EC systems for higher throughput [13]. Considerable efforts have also been devoted to achieving the solar-driven electrochemical conversion of methane/CO₂ to methanol through photocatalysis [14,15,16,17] incorporating a PEC system. The efficient functioning of a photocatalyst involves absorption units for initial light absorption, generating electron–hole pairs, and active centers that facilitate methane/CO₂ activation, leading to methanol production. This coupled approach, using a single photocatalyst for both functions, is cost-effective and compact, but limits the selection of optimal absorber and catalyst in a PEC system. The decoupled approach, using independent components in PV–EC systems, offers flexibility but tends to be costlier, as it allows the selection of the best absorber and catalyst separately, enhancing efficiency at a higher cost. Either of these integrations, however, are still advantageous to implement compared to a standalone electrochemical system to reduce the carbon footprint of such processes. Solar integration not only contributes to the reduction of greenhouse gas emissions, but also aligns with the broader goals of creating a circular and sustainable energy ecosystem.

A multitude of methods, spanning thermochemical [18,19], electrochemical [20,21,22], photoelectrochemical [23,24], and photocatalytic [25,26] reactions, have been explored for the reduction of CO₂ into valuable fuels. Among these, electrochemical reduction (ECR) of CO₂ emerges as the most promising due to its distinct advantages [27]. The conversion of CO₂ to CH₃OH through electrochemical means is particularly noteworthy, given its environmental sustainability and its potential role in establishing a methanol economy. Notably, Jouny et al. conducted a techno-economic analysis of ECR technology, indicating the feasibility of large-scale production of carbon monoxide (CO) and formic acid (HCOOH) [28]. The identified ECR reaction pathways for CO₂ to CH₃OH conversion involve complexities related to catalyst stability and reaction kinetics [29,30,31]. However, extensive research into the different types of electrocatalysts and novel electrolysis techniques promises the commercial viability of the ECR technology. Transition metals and their compounds, notably metal complexes and alloys, are extensively studied as electrocatalysts in CO₂ electrochemical reduction due to their favorable electronic properties [29]. Additionally, reports on the electrochemical reduction of CO₂ over semiconductor electrodes and photocatalysts expand the scope, producing not only CH₃OH but also other valuable products [32,33]. Additionally, researchers are increasingly focusing on solar power-driven electrochemical reduction of CO₂ to methanol. Barton et al. demonstrated selective solar-driven reduction of CO₂ to methanol using a catalyzed p-GaP-based photoelectrochemical cell with a quantum efficiency of 41% under 365 nm illumination [34]. Other studies have explored solar-driven photoelectrochemical reduction of CO₂ to methanol using various photocathodes, showcasing high faradaic efficiencies and low overpotentials [35,36]. This ongoing research collectively underscores the potential of electrochemical approaches, both traditional and solar-driven, in paving the way for a sustainable and efficient pathway from CO₂ to methanol, offering insights into both fundamental reaction mechanisms and practical applications. This positive progression in research of the ECR technology has also been confirmed by recent successful demonstrations of commercial-scale and cost-effective methanol production by companies like Shunli [37] and Oxylus Energy [38].

Parallel to the progress in reducing CO₂ to methanol, methane oxidation reaction (MOR) to methanol has surfaced as a promising pathway for efficient methanol production. Electrochemical strategies, particularly those operational at low to mild temperatures, have garnered attention for their potential [39,40]. MOR to methanol represents a sustainable approach with multifaceted environmental benefits. Methane, a potent greenhouse gas with a global warming potential 23 times higher than CO₂, poses a significant environmental challenge. By converting methane to methanol, not only can we mitigate its detrimental impact on climate change, but we can also address the issue of methane flaring, where excess methane is simply burned, contributing to additional CO₂ emissions. Leveraging methane, a decentralized resource, for methanol production offers a compelling solution, tapping into existing methane sources while reducing dependence on fossil fuels. Furthermore, methanol’s characteristics as a safe and convenient transporter make it an attractive alternative to compressed methane, enhancing its potential as a sustainable energy carrier. Through electrochemical methane oxidation, we not only curb greenhouse gas emissions but also unlock the potential of methane as a renewable feedstock, ushering in a more sustainable era of chemical synthesis and energy utilization. In that framework, Mustain et al. utilized a mixed oxide catalyst of NiO/ZrO₂ at room temperature, showcasing its efficacy in converting CH₄ to various oxygenates [41]. Sun et al. reported a CH₃OH production rate of 25 μmol/gcat/h with a Faradaic efficiency of 89% using a NiO/Ni catalyst at room temperature [42]. Surendranath et al. achieved a production rate of 268 μmol/gcat/h and a selectivity of 69% using the Pt(II):Pt(IV) catalyst [43]. Transition metal oxides, including NiO/ZrO₂, Ni(OH)₂/ZrO₂, and Co₃O₄/ZrO₂, have been investigated for room temperature electrochemical oxidation of methane in carbonate-based systems [41,44,45]. The utilization of membrane electrode assemblies (MEAs) in methanol production has shown promise, allowing for advanced control over catalyst environments and selective reactant transport. Moreover, akin to CO₂ reduction, solar-driven methane oxidation to methanol has garnered attention. Moon et al. demonstrated a solar cell-powered electrochemical CH₄ conversion, achieving an energy-efficient and environmentally friendly process that produced 7165.0 μmol/gcat of CH₃OH at ambient pressure and 21,986.6 μmol/gcat at 10 bar pressure in a 12 h reaction [46]. These collective advancements underscore the multifaceted and evolving landscape in electrochemical methane oxidation, contributing to the diversification of sustainable methanol synthesis methodologies. Owing to the low rates of methanol production in both the ECR and MOR technologies, it is also imperative that robust experimental protocols are established for analyzing the catalyst performance. Standardized reaction parameters such as temperature, pressure, and reactant concentrations must be precisely maintained. Comprehensive characterization methods, including spectroscopic and microscopic analyses, should be employed to understand catalyst morphology and identify active sites. Control experiments, such as blank runs or use of inert materials should be conducted to help discern genuine catalytic effects from external factors. Additionally, ensuring reproducibility through repeated trials is vital for establishing the reliability of observed results. These protocols collectively contribute to the accurate evaluation of catalyst performance, promoting consistency and comparability across different studies in the pursuit of advancing sustainable methanol production processes. In general, the robust practices are even more important with additional complexity of coupling electrochemical processes such as the ones discussed in this work with either a photoelectrocatalyst (PEC) or a photo-voltaic (PV) system. Some of the crucial best practices are highlighted in the works by Bonchio et al. [47], and Seger et. al [48], addressing the complexity of different electrochemical systems.

Schematics of typical aqueous electrochemical systems used in the literature showing ECR and MOR are shown in Figure 1. Since these are aqueous electrochemical systems, the methanol producing reactions from both ECR and MOR are also competing with hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode. A significant research effort in designing electrocatalysts for such systems is catered towards suppressing these parasitic side reactions for better ECR and MOR performance [49]. As advancements continue in electrochemical systems, aimed at producing methanol from CO₂ or CH₄, it becomes essential to assess their integration with renewable energy sources like solar power. These reactions, driven by electrical energy, stand to benefit from solar energy, further mitigating their carbon footprint. While several efforts have explored utilizing multijunction light absorbers to convert CO₂ into fuels using sunlight [50,51,52,53], there is a scarcity of literature on solar-driven CH₄ conversion.

The limited exploration of the solar driven electrochemical processes can mainly be attributed to a number of limitations primarily including: (1) Inadequate performance of the light absorbers. (2) Instability of PEC systems for long duration ECR or MOR in aqueous media. (3) Losses in a PV–EC system associated with direct current to direct current (DC–DC) conversion from PV to EC systems. Hence, there are research efforts focused on circumventing these limitations. The literature predominantly highlights electrocatalyst and photoelectrocatalyst optimization for high efficiency and longevity [54,55], surface modifications, and the development of protective layers are underway [56,57]. Furthermore, electrolyte engineering and exploring advanced electrolysis techniques like tandem catalysis [58,59,60,61], and improving electrode designs for to enhance mass transport of the reactant [62] are also being explored. Acknowledging the current scope for progress and the limitations of the solar-driven electrochemical systems, this article aims to serve as a “catalyst” to initiate and expand the research and discussions around solar driven electrochemical methanol production. This article thus aims to offer insights into the thermodynamic and realistic STF efficiency of ECR and MOR as promising pathways for methanol production, fostering a sustainable and circular energy ecosystem. Despite ECR being at a higher Technology Readiness Level (TRL) compared to MOR, it is an opportune moment to evaluate the solar-to-fuel (STF) efficiencies of both processes. The subsequent sections are structured as follows: The Methods section delineates the mathematical expressions for determining Shockley–Queisser (SQ) limits of multijunction light absorbers, the utilization of thermoneutral potential to describe an ideal electrochemical cell, and the computation of STF efficiencies for ECR and MOR to methanol. The Results and Discussion section examines the current-voltage characteristics of multijunction light absorbers, ideal STF efficiencies for methanol production via ECR and MOR, and realistic STF efficiencies for both pathways. Finally, the Conclusions section offers insights and recommendations for implementing such integration in solar-driven electrochemical methanol production systems.

2. Methods

Current-voltage (JV) characteristics of a Multijunction Light Absorber: We consider ideal, intrinsic light absorbers with up to 5 junctions of optimal bandgaps to assess the performance limits of ideal solar-driven ECR and MOR to methanol. The JV characteristic of each junction in the light absorber is determined by a detailed balance of photons, encompassing thermal generation of carriers, electron-hole recombination, and total absorption of photons with energy exceeding the band gap of the junction. In our ideal condition calculations, we disregard extrinsic losses such as light reflection, contact shadowing, series resistance, inefficient collection of electrons and holes, nonradiative recombination, and temperature rise. The JV characteristics of an ideal multijunction light absorber are derived by applying bias across each junction for a terrestrial air mass (AM) 1.5 spectrum at 1 sun, expressed as follows:

$$V\left(J\right)=\frac{kT}{e}{\mathsf{\Sigma }}_{i=1}^n\left[\frac{J_{sc,i}-J}{J_{0,i}}+1\right]$$

(1)

where $V\left(J\right) \left(\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{t}\right)$ represents the total potential generated across the light absorber, $e (\mathrm{C})$ denotes the electronic charge, $k (\mathrm{J}{\mathrm{K}}^{-1})$ denotes the Boltzmann constant, $T (\mathrm{K})$ stands for the temperature, $n$ denotes the number of junctions, $J (\mathrm{m}\mathrm{A} \mathrm{c}{\mathrm{m}}^{-2})$ represents the photocurrent density, $J_{sc,i} (\mathrm{m}\mathrm{A} \mathrm{c}{\mathrm{m}}^{-2})$ denotes the short-circuit current density of the ith junction, and $J_{0,i}\left(\mathrm{m}\mathrm{A} \mathrm{c}{\mathrm{m}}^{-2}\right)$ is the saturation current density of the ith junction. Further details and comprehensive calculations are available in our prior work [63].

Thermoneutral potential: The operation of an ideal electrochemical cell can be conceptualized in two ideal states: equilibrium or adiabatic. The equilibrium potential is defined as:

$$V_{eq}=\frac{\Delta G^0}{nF}=\frac{1}{nF}(\Delta H^0-T\Delta S^0)$$

(2)

where $V_{eq} (\mathrm{V}\mathrm{o}\mathrm{l}\mathrm{t})$ represents the equilibrium potential, $\Delta G^0 (\mathrm{k}\mathrm{J}\mathrm{m}\mathrm{o}{\mathrm{l}}^{-1})$ denotes the standard Gibbs free energy, $\Delta H^0$ signifies the standard enthalpy, $\Delta S^0$ represents the standard entropy of formation of the electrochemical reaction under consideration, $\mathrm{n}$ is the number of electrons per mol of the product formed, and $F=\mathrm{96,485} (\mathrm{C} \mathrm{m}\mathrm{o}{\mathrm{l}}^{-1})$ is the Faraday’s constant. It is crucial to note that under equilibrium, an electrochemical cell does not generate a net current and thus, cannot yield a nonzero STF efficiency for any configuration of solar integration with an electrochemical cell. However, an electrochemical cell operating under adiabatic (or isentropic) conditions can produce a net positive current at potentials higher than $V_{eq}$. This operating potential is termed the thermoneutral potential and is defined as:

$$V_{th}=\frac{\Delta H^0}{nF}$$

(3)

where $V_{th}$ represents the thermoneutral potential.

STF efficiency: The STF efficiency is defined as the ratio of power generated as fuel from ECR or MOR to the incident solar power and is expressed as:

$${\eta }_{STF}\left(\%\right)=\frac{J_{op}{\eta }_F\times V_{th}\times A_{EC}}{P_s\times A_{LA}}\times 100$$

(4)

where ${\eta }_{STF}$ denotes the STF efficiency, ${\eta }_F$ represents the Faradaic efficiency of methanol, $J_{op} (\mathrm{m}\mathrm{A} \mathrm{c}{\mathrm{m}}^{-2})$ signifies the operating current density of the electrochemical cell at a given potential, $P_s=100 (\mathrm{m}\mathrm{W} \mathrm{c}{\mathrm{m}}^{-2})$ denotes the average power of solar insolation per unit area for irradiation at the ground level, $A_{EC} \left(\mathrm{c}{\mathrm{m}}^2\right)$ is the electrode area of the electrochemical cell, and $A_{LA} \left(\mathrm{c}{\mathrm{m}}^2\right)$ is the area of illumination of the light absorber. Unless mentioned otherwise, the calculations highlighted in this work assume $A_{EC}=A_{LA}$ for simplicity.

3. Results and Discussion

The assessment of the solar integration of any electrochemical system can be initiated with the consideration of the most ideal scenario, namely, determining the thermodynamic limits. In this context, the thermodynamic limits for STF efficiency in methanol production through either ECR or MOR can be assessed by conducting detailed balance calculations for SQ limits to observe the JV characteristics of multijunction light absorbers followed by determining the thermoneutral potentials of ECR and MOR. The SQ limits were determined using Equation (1), and Figure 2 illustrates the JV characteristics of ideal multijunction light absorbers. Notably, the current density decreases with an increase in cell voltage (or electrochemical load) due to a decrease in the fraction of solar radiation absorbed at higher voltages. Additionally, current density decreases with an increase in the number of junctions, a consequence of restrictions imposed by current matching. These JV characteristics emphasize that within the evaluated range of electrochemical load, double and triple junctions exhibit higher current densities across a broader range of cell voltages, making them favorable starting points for the solar integration of any electrochemical reaction.

Table 1. Thermoneutral potentials of ECR and MOR to methanol.

	Reaction	$\mathit{e}$	$\mathsf{\Delta }{\mathit{H}}^0 \left(\mathbf{k}\mathbf{J} \mathbf{m}\mathbf{o}{\mathbf{l}}^{\mathbf{-}\mathbf{1}}\right)$	${\mathit{V}}_{\mathit{t}\mathit{h}} (\mathbf{V})$
ECR	$C{O}_2+2H_{2}O\to C{H}_{3}OH+\frac{3}{2}{O}_2$	6	728.74	1.259
MOR	$C{H}_4+H_{2}O\to C{H}_{3}OH+H_2$	2	121.94	0.632

presents the determination of thermoneutral potentials for ECR and MOR using Equation (3). It is essential to note that the assessment of thermoneutral potentials considers these reactions under near-ideal conditions, assuming adiabatic reactions and higher potential than equilibrium due to no ohmic or Nernstian losses, which can be experimentally determined.

With both the JV characteristics and thermoneutral potentials calculated, the STF efficiency for methanol production through ECR and MOR can be calculated using Equation (4). The maximum current density obtained from an ideal light absorber for a given ideal electrochemical reaction will be at the thermoneutral potential of the reaction. Figure 3 depicts the STF efficiency of methanol production for multijunction light absorbers through ECR and MOR routes.

ECR with a higher thermoneutral potential demonstrates the highest efficiency at 39.54% and 29.01% for double and triple junction light absorbers, respectively. Meanwhile, MOR exhibits the highest efficiencies at 29.27% and 21.83% for single and double junction light absorbers, respectively. The variation in STF efficiencies between each route for the same light absorbers arises from differences in thermoneutral potentials. STF efficiencies are lower for higher-order junction light absorbers due to poor matching of current densities among the junctions, as depicted in Figure 2. It is also noteworthy that leaves of plants contain two photosystems that function akin to a double junction light absorber, optimizing sunlight utilization for photosynthesis [24].

After evaluating the thermodynamic limits of ECR and MOR for methanol production, it is crucial to translate these findings into more realistic conditions. Figure 4A presents a comparison of the JV characteristics of a state-of-the-art triple junction light absorber (InGaP/GaAs/Ge) with the ideal triple junction light absorber. The significant disparity between these JV curves stems from extrinsic losses such as light reflection, contact shadowing, series resistance, inefficient collection of electrons and holes, nonradiative recombination, and temperature rise. Consequently, the real triple junction light absorber exhibits a lower maximum current density and a reduced stable current density over a narrower range of electrochemical load. Moreover, to enhance its utility across a broader range of operating potentials, these light absorbers can be connected in series. Connecting two of these real light absorbers in series expands the operating voltage range but at the expense of decreased maximum current density as seen in Figure 4B. It is pertinent to note that the term “current density” in the context of light absorbers implies that the current is measured per unit area of the light absorber. The light absorber area can always be increased to match the higher currents required to operate more realistic electrochemical systems. Presently, our capabilities are constrained by the efficiency of today’s light absorbers, although electrochemical cells have progressed to a higher TRL for ECR (and to some extent, MOR). Ongoing research endeavors can continuously improve light absorber efficiency to reduce material costs in the future while still matching the currents of an electrochemical system by augmenting the area of the light absorbers utilized today.

After comprehending the JV characteristics of a realistic light absorber, the next step is to consider non-ideal, realistic electrochemical reactions to evaluate the STF efficiencies of ECR and MOR for methanol production.

Table 2. Literature highlights for the performance of ECR and MOR to methanol.

	Catalyst	Electrolyte	Applied Potential (V)	Methanol Current Density (mA/cm2)	Faradaic Efficiency (%)	References
CO2	CuGa2 (GDE)	CO2 gas with 1 M KOH	−0.3 vs. RHE	21.4	77.26	[65]
	RuO2/TiO2 nanotubes (NTs)	0.5 M NaHCO3	−0.8 vs. SCE	1.2	60.5	[66]
	PtZn nano-alloys	0.1 M NaHCO3	−0.90 vs. RHE	3.75	81.4	[67]
	n-GaAs-crystal-(111)As	0.2 M Na2SO4	−1.20 to −1.40 vs. SCE	0.16–0.2	100	[68]
	Pd83Cu17 bimetallic aerogel	25 mol% [Bmim]BF4 and 75 mol% water	−2.1 vs. Ag/Ag+	31.8	80	[69]
	Pre-oxidized Cu foil (1 h, 130 °C)	0.5 M KHCO3	−0.9 vs. SCE	0.069	33.36	[70]
	RuO2:TiO2 (35:65)	0.05 M H2SO4	−0.05 vs. SCE	0.061	76	[71]
	Cu1.63Se0.33	[Bmim]PF6 (30 wt %)/CH3CN/H2O	−2.1 V vs. Ag/Ag+	30	80	[72]
CH4	TiO2-RuO2	0.1 M Na2SO4	2.1 vs. SCE	13	30	[73]
	V2O5-SnO2	Sn0.9In0.1P2O7	0.9	4	61.4	[74]
	TiO2/RuO2/V2O5	0.1 M Na2SO4	2.0 V vs. SCE	-	56	[75]
	3.0 NiO/Ni	0.1 M NaOH	1.40 V vs. RHE	3.0	14	[42]
	Cu-Ti	1 M KCl	3.07 V vs. RHE	6.24	16	[40]
	Cu-Ti	0.1 M KH2PO4-K2HPO4	2.15 V vs. RHE	0.3	7	[39]

presents literature works on ECR and MOR to methanol. It is evident that significantly more progress has been made from ECR to methanol compared to MOR. Despite MOR having a much lower thermoneutral potential than ECR, it is kinetically challenging due to the non-polar, stable, and insoluble nature of CH₄ as a reactant. There may be more fundamental efforts needed to develop more efficient MOR electrochemical systems before it can go to higher TRL levels. However, another attractive intermediate route that has garnered attention lies between the two reactions considered here. A tandem approach of reducing CO₂ to CO which is already at high TRL levels followed by utilizing green H₂ to perform CO hydrogenation, yielding green methanol [64]. The utilization of green H₂ in the subsequent CO hydrogenation step ensures the overall environmental sustainability of the process. Green H₂ is produced through renewable energy sources, such as wind or solar, or even as a byproduct of ECR avoiding the carbon footprint associated with conventional hydrogen production methods. While the tandem electrochemical CO₂-to-CO and green methanol production pathway shows promise, several challenges need to be addressed. These include optimizing catalyst stability, improving energy efficiency, and scaling up the process for industrial applications. Furthermore, the economic viability of the overall system needs careful consideration. Deeper insight into this process may become a separate study on its own, therefore, we acknowledge the potential of tandem approach while focusing on ECR and MOR for the scope of this work.

The potentials reported in Table 2 correspond to the working electrode potentials, unlike the total cell potentials used in the calculations depicted in the preceding figures. Consequently, it is reasonable to assume that the total cell potentials would exceed the reported values in this table. Bearing this in mind, we varied the overpotential to ~2 volts above the thermoneutral potential to evaluate the STF efficiency under realistic conditions, as illustrated in Figure 5A. In the case of ideal light absorbers, as the overpotential increases, the most efficient light absorber transitions from double to triple junction. However, due to extrinsic losses, the realistic efficiency remains around 20% at the highest overpotentials.

Under realistic conditions, it is important to understand how a PV–EC system operates when the two individual components -PV light absorber and electrochemical cell- are integrated. This is highlighted in Figure 5B, where we see the JV characteristics of a realistic triple junction light absorber (from Figure 4A) of various light absorber areas along with 2 hypothetical electrochemical reactions. From this figure it can be seen that a PV–EC integrated system will operate where the light absorber and the electrochemical reaction JV curves intersect, which is also evident in some recent experimental work on integrated PV–EC systems [76,77]. As the area increases, the current at any given potential also increases linearly for the light absorber. The hypothetical electrochemical reaction 1, in this case, appears to be an efficient reaction where the EC system is able to drive higher currents at lower voltages. For such efficient reactions, the PV–EC system is limited by the light absorber JV characteristics. Hence, increasing the area of a light absorber can increase the operating current and consequently, increase the STF efficiency. On the other hand, the hypothetical electrochemical reaction 2 is an inefficient reaction. Here, the EC system appears to have sluggish JV curve and increasing the light absorber area will not increase the current and the STF efficiency significantly. In this case, increasing the area of the electrocatalyst can help reach higher operating currents leading to higher STF efficiency. It should be noted that scaling both PV and EC systems is a non-trivial task and may lead to emergent phenomena, such as uneven temperature distribution and localized current distribution [78], and may require various engineering controls to mitigate scaling effects. Alternatively, optimizing this EC reaction to make it more efficient like reaction 1, would also be beneficial. This analogy can be extrapolated to ECR and MOR as well, since ECR has higher TRL (resembling reaction 1), and MOR has a lower TRL (resembling reaction 2). The design principles discussed above can be directly applied to integrated PV–EC systems for ECR and MOR. The insights from Figure 5B provide valuable guidance for designing integrated PV–EC systems tailored for specific electrochemical reactions, facilitating informed decision-making and optimization strategies. By leveraging these insights, researchers can navigate the design space effectively, advancing the development and deployment of efficient solar-driven electrochemical systems for sustainable energy conversion and storage applications.

4. Conclusions

The exploration of solar-driven electrochemical processes for methanol production presents a promising avenue towards sustainable energy solutions. This study delves into the integration of electrochemistry with renewable energy, particularly solar power, to drive efficient chemical reactions for methanol synthesis. The analysis highlights the feasibility and potential of both electrochemical reduction (ECR) of CO₂ and methane oxidation reaction (MOR) as pathways for methanol production. ECR is currently particularly promising, exhibiting higher efficiencies and technological readiness levels compared to MOR. However, challenges persist in both processes, including catalyst stability and reaction kinetics. Our findings highlight the importance of considering realistic conditions in evaluating solar-to-fuel (STF) efficiencies. While ideal scenarios demonstrate significant potential, extrinsic losses and practical limitations temper the achievable efficiencies. The advances in light absorber technology and electrochemical systems offer avenues for continual improvement and optimization and demonstrate a promise to compensate for the higher cost of this decoupled electrochemical system. The integration of PV light absorbers with electrochemical cells in realistic conditions necessitates a nuanced understanding of their operational dynamics, as explained by the intersection of JV characteristics of these individual systems to determine the operating point of an integrated PV–EC system. This integration shows the significance of optimizing light absorber and electrochemical reaction geometric areas to enhance system performance and STF efficiency. Efficient electrochemical reactions highlight the importance of maximizing light absorber areas, while inefficient electrochemical reactions necessitate focus on electrocatalyst areas or urges the field to direct efforts toward developing more efficient EC systems. The principles delineated herein, furnish a framework for tailored PV–EC system designs, guiding optimization strategies and informed decision-making. By leveraging these insights, researchers can direct the development and deployment of solar-driven electrochemical systems towards sustainable energy conversion and storage applications. Looking ahead, future research efforts should focus on addressing key challenges in catalyst design, reaction kinetics, and system optimization to further enhance STF efficiencies. Additionally, efforts to integrate solar-driven electrochemical processes into larger energy systems and industrial applications are essential for practical implementation. Collaboration across disciplines, including material science, electrochemistry, and renewable energy engineering, will be crucial in driving innovation and fostering the transition towards a sustainable methanol economy. In conclusion, this study provides insights into the thermodynamic and practical considerations of solar-driven electrochemical methanol production. By elucidating the complexities and opportunities inherent in ECR and MOR pathways, we pave the way for informed decision-making and targeted research efforts aimed at realizing the full potential of solar-driven methanol synthesis in shaping a cleaner and more sustainable energy future.