Perspective: Insights Into the Green Production of Biofuels

Introduction

In recent years there has been a strong motivation to turn away from fossil fuels for energy production. While publications have outlined many reasons for switching from fossil fuels, the concerns fall into two categories. First, there is a limited supply of fossil fuels. A popular model for resource depletion is based upon the Hubbert peak theory, which proposes that the use of a finite resource follows a bell-shaped curve over time. Many studies have applied this to fossil fuel consumption [1]. Aside from limited supply, another concern of burning fossil fuels is the production of greenhouse gases, such as CO₂. Climate scientists have shown that atmospheric CO₂ levels have exceeded capacity, resulting in detrimental environmental consequences [2]. This evidence shows a significant increase in CO₂ over recent years, highlighting the need for alternative methods of producing energy. These reasons are powerful motivators for the green production of chemical biofuels as a sustainable energy source.

As demand for energy and fuel continues to increase beyond the current means of supply, there is a critical need for the sustainable production of chemical fuels. The biofuel industry has emerged as a frontier for renewable energy production working toward a greener future. However, this industry faces significant challenges, such as competition for land use, resource sustainability, profitability, and infrastructure development [3]. For example, ethanol refinement is incredibly resource intensive, relying on energy from fossil fuels for thermal processing, and vast amounts of water for subsequent cooling [4]. Considering the relatively low energy density of ethanol, the energetic tradeoff for biofuel refinement from corn is impractical. Additionally, there is evidence that when factoring in land conversion, ethanol has a larger carbon footprint than gasoline, contradicting its environmental benefits in the bioeconomy [5]. Addressing these challenges requires a shift in the energy landscape to a circular bioeconomy that integrates chemistry and biotechnology to produce clean biofuels in a sustainable way.

Electrochemical conversion of CO₂ coupled with carbon capture technologies is an attractive long-term solution with dual benefits. Not only is this an economically competitive method for producing energy-dense biofuels, but it also removes atmospheric CO₂ in the process. This perspective will provide a focused assessment of opportunities for electrochemical CO₂ reduction (ECR) in the biofuel industry. A full overview will be delivered, including the fundamentals and challenges of CO₂ conversion, current standings and future directions in the industry, and a discussion of integrating electrochemistry and biotechnology in the green production of biofuel.

Fundamentals and Challenges of CO₂ Conversion

ECR presents several key advantages over conventional CO₂ reduction methods [6]. First, conventional methods of CO₂ reduction employ thermochemical, photochemical, or biological processes which are energy intensive. High input demand typically requires fossil fuel combustion as a reliable, cost-effective source of energy, thereby limiting the technology’s contribution to a carbon-neutral or carbon-negative economy. ECR promotes a carbon-negative economy by utilizing electrical energy as the input and employing catalysts to further reduce overpotentials.

In addition, the scalability of conventional methods is severely limited [7, 8]. Photochemical and biological processes are not scientifically sound outside of ideal laboratory conditions. For example, recent studies investigating algae-based carbon capture, utilization, and storage have come into light. However, this method is limited by low fixation rates relative to industrial demands and low biomass density, requiring excessive land use. Additionally, while some algae are more adaptive, temperature, nutrient, and light sensitivity must also be taken into consideration upon relying on biological experiments [9]. Thermochemical reduction is more commonly accepted as a scalable method of CO₂ reduction, but challenges arise in terms of selectivity and economic practicality, highlighting the need for technological advancement in catalyst design [10].

While conventional methods, such as thermochemical processing, do not allow for high selectivity among products, ECR allows for direct control over the system via choice of materials and experimental conditions [6]. Because CO₂ reduction involves many electron transfers and overcomes different levels of thermodynamic and kinetic barriers, various reduction pathways can be observed. CO₂ can be reduced to C₁ products (i.e. carbon monoxide, CO), C₂ products (i.e. ethanol, C₂H₅OH, and ethylene), or even C₃₊ products such as longer chain hydrocarbons. While longer chain hydrocarbons are more energy dense, most research is currently focused on C₂ products to integrate into the biofuel economy. As specific reduction pathways are favored by particular catalysts and voltages, varying the catalyst composition and applied potential can yield desired products such as ethylene and ethanol. Hence, ECR allows researchers to optimize catalyst activity not only in terms of efficiency and stability, but also in product selectivity, setting it apart from other methods.

Electrochemical cell types also have different advantages. Similar to catalyst composition, electrochemical design contributes to product selectivity by type of membrane and electrolyte [11, 12, 13]. These selections allow for control over local pH, reactant concentration, and diffusion rate, and these factors are governed by Faraday’s Law and the Nernst equation. Faradaic efficiency is critical in ECR because it represents the selectivity of the process by measuring the ratio of electrons used to yield the desired product versus the total number of electrons passed through the system. Therefore, by choosing specific materials and electrochemical cells, ECR allows for selective CO₂ reduction, generating high-commodity, energy-dense products. Further, electrochemical cell design allows for manipulation of experimental conditions, which can stabilize desired reaction intermediates and favor certain kinetics. Catalyst and electrochemical cell design could allow industry to develop C₂ products efficiently and cost-effectively. These products can then be used as chemical feedstock or biofuels, creating a circular carbon economy, making ECR an exciting emerging player in the biofuel industry.

Applications and Future Directions of ECR in the Biofuel Industry

The high impact of ECR lies in its energy storage capabilities. The potential to convert CO₂ into chemicals that store energy for a later use has the potential to change the trajectory of the clean energy field. While many avenues are being explored regarding the application of ECR, the two that are particularly exciting are CO₂ conversion to ethanol and CO₂ conversion to syngas.

Ethanol is a promising product because of its versatility. In addition to being used as a solvent it can also be used as a fuel or intermediate feedstock for producing more energy-dense biofuels and chemicals, making it an ideal starting point for ECR. The conversion of CO₂ to ethanol is a highly complex process that involves a 12-electron transfer. The overall reaction is shown below [14]:

CO₂ (g) + 12H⁺ + 12e^-   ->  C₂H₅OH (l)

In addition to optimizing reaction parameters such as applied potential, electrolyte composition, and electrochemical cell design, the development of a catalyst that is highly selective for ethanol is key. Research has found that copper catalysts tend to enhance ethanol selectivity by stabilizing key intermediates and providing additional active sites for C-C coupling. Recent advancements in the field have involved local environment modulation, the development of oxide-derived copper catalysts and nanostructured copper alloy catalysts, and the employment of tandem catalysts.

Local environment modulation refers to the stabilization of the metal ion and intermediate species by a dopant (usually nitrogen). Adding nitrogen to the system acts as a molecular “glue” for the catalyst by introducing covalent bonds between both the copper ion and the carbon backbone. The increased stability of the catalyst as a whole allows for more active sites to be introduced to the system and foster C-C coupling. In contrast, oxide-derived copper catalysts control the reaction pathway by introducing oxygen vacancies into the system. These vacancies serve as localized sites where electrons are highly concentrated. The high electron density at these sites can therefore be redistributed to force surrounding copper atoms in a lower valence state, further contributing to the high-density active centers (oxygen vacancies) that can now adsorb key intermediates more efficiently and stabilize them long enough for C-C coupling [15]. Nanostructured copper alloy catalysts follow the same principle of stabilizing intermediates, but using methodology related to physical structure rather than chemical environment [16]. Tandem catalysis is an interesting, relatively new concept that involves using a second metal center as a “spillover” site to “catch” intermediate species and serve as a site for C-C coupling [17]. 

While the energy density of liquid products, like ethanol, is higher, the conversion of CO₂ to syngas should not be discounted. Syngas is used as an intermediate feedstock to generate chemicals and fuels, but its most important role is in ammonia and fertilizer production [18]. As a mixture of H₂ and CO, syngas is the center of a circular economy, where CO₂ is converted to CO via ECR then reacted with steam to produce H₂ and CO₂. This water-gas shift reaction is shown below:

CO₂ (g) + H₂O (g)  ->  CO (g) + H₂ (g)

The hydrogen gas is then extracted to be used in the Haber-Bosch process. By employing ECR, the energy input for the Haber-Bosch process could be significantly decreased by the supply of hydrogen from the interconversion between CO₂ and CO, powered by renewable sources. At the same time, CO₂ emissions from the Haber-Bosch process are mitigated through ECR making this a powerful solution towards a circular bioeconomy.

Conclusions

This perspective presented a brief overview of the shift in the energy landscape from fossil fuels to a circular carbon economy utilizing biofuels. The need to reduce CO₂ emissions is clear from the data, and ECR powered by renewable energy is a reliable, economically competitive option. Coupled with carbon capture technologies, ECR has the potential to not only mitigate CO₂ emissions, but also act as the driving force of the biofuel industry, resulting in a sustainable bioeconomy.

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