The Chemistry of Renewable Energy: Solar Cells, Batteries, and Fuel Cells

According to the Cambridge dictionary, energy can be defined as “power to do work that produces light, heat, or motion, or the fuel or electricity used for power.” The First Law of Thermodynamic states that there is no way to destroy energy, only ways to convert it.

However, there is a noticeable distinction between energy and electricity. While energy can exist in different forms (chemical, thermal, mechanical, among others), electricity is the flow of current that can be converted into other forms of energy. Even during the Covid-19 pandemic in 2022, there was still significant energy and electricity consumption, as illustrated in Figure 1a, which shows the American Energy Pie chart, and Figure 1b, which details the various forms of electricity acquisition (ATKINS; De Paula, 2014).

 Figure 1 - a) Energy Sources in 2022 in the US); b) Electricity Sources in 2022 in the US

The graphs clearly illustrate our reliance on sources that require combustion and thermal energy to generate movement and electricity. Coal, natural gas, and petroleum are energy sources that release energy through exothermic reactions. This process emits carbon dioxide - especially during complete combustion with oxygen - which negatively impacts the environment by trapping solar radiation and contributing to global warming. In contrast, renewable energy sources, which are green and inexhaustible, still represent only a small fraction of our unsustainable energy supply. This article will further explore renewable energy sources, including solar cells, batteries, and fuel cells (SMIL, 2010).

Solar cells typically convert light into electricity through the photovoltaic effect. This process occurs in semiconductors, with P-type solar panels becoming the industry standard. These panels consist of an N-type and a P-type crystalline silicon (c-Si) wafer, which together form a p-n junction, as illustrated in Figure 2. The operation is as follows: when a photon strikes the cell, it excites an electron, which is then moved through the electric field, generating an electric current. Given that the sun is an inexhaustible resource (with billions of years of energy ahead), it is regarded as a clean and renewable energy source (SZE, 2007).

Figure 2 - Photovoltaic system 

Batteries store chemical energy and convert it into electrical energy by transferring electrons through oxidation-reduction (redox) reactions. In lithium-ion batteries, the anode (negative electrode) and cathode (positive electrode) are separated by an electrolyte. During discharge, electrons flow from the anode to the cathode through an external circuit, generating current. Simultaneously, lithium ions move through the electrolyte to maintain charge balance. Other battery types include alkaline, nickel-metal hydride (NiMH), and sulfuric acid-lead batteries. The following reaction illustrates how lead-acid batteries function (Winter; Brodd, 2004). Batteries are commonly used in electric cars, helping to reduce fossil fuel consumption and lower CO2 emissions from direct combustion.

(-) PbO2   +  HSO4-1  +  3H3O+1  +  2e-  →  PbSO4   +   5H2O

(+) Pb  +  HSO4-1  +  H2O  →  PbSO4  +  H3O+1   +   2e-

Global - Pb  +  PbSO4  +  2 HSO41-  +  2H3O+1   →  2 PbSO4  +  4 H2O

Reaction 01 - Acid and Lead battery

Fuel cells convert the chemical energy of a fuel and an oxidizing agent into electricity. As long as fuel is supplied, the process "splits" hydrogen (H₂) into protons (H⁺) and electrons (e⁻), as illustrated in Reaction 02. Figure 03 shows that the byproduct of this reaction is water (H₂O), making fuel cells a renewable energy source that is nearly twice as efficient as internal combustion engines. This efficiency leads to reduced fuel consumption in fuel cell vehicles. However, challenges such as chemical instability and sensitivity to moisture can impact this process. Additionally, research is underway to replace silicon with alternative materials to ensure that electric current flows effectively in new configurations (DURKIN, 2024).

(-) 2H2  →  4e- + 4H+

(+)  O2  +  4e- + 4H+  →  2H2O 

Global - 2H2 + O2 → 2H2O

Reaction 02 - Fuel Cells

Figure 03 - Fuel cells system

Therefore, with the overall growth of the energy and electricity market, innovative and promising options emerge. In this sense, technology and chemistry align to promote a better future, reducing environmental impact through disruptive ideas. Finally, with the growth and investment in renewables, they will become cheaper and accessible.

References

• Atkins, P. W., & de Paula, J. (2014). Atkins' Physical Chemistry. Oxford University Press.

• Smil, V. (2010). Energy Transitions: History, Requirements, Prospects. Praeger.

  Solar Cells

• Sze, S. M., & Ng, K. K. (2007). Physics of Semiconductor Devices. Wiley. Batteries

• Winter, M., & Brodd, R. J. (2004). What are Batteries, Fuel Cells, and Supercapacitors?. Royal Society of Chemistry.

• DURKIN, Kirill et al. Hydrogen-powered vehicles: comparing the powertrain efficiency and sustainability of fuel cell versus internal combustion engine cars. Energies, v. 17, n. 5, p. 1085, 2024.

• Cambridge Dictionary  -[online] Available at: https://dictionary.cambridge.org/dictionary/english/energy  

• Figure 1a -[online] (RubyHome, 2023) : https://share.google/images/hCGTTL0I0CN3du4Rs 

• Figure 1b -[online] (Third Way, 2023); Available at : https://share.google/images/xQoKq4cp6YJIO290p 

• Figure 2 -  [online]; Available at: https://www.eia.gov/energyexplained/solar/photovoltaics-and-electricity.php 

• Figure 3 - [online]; Available at:   https://sl.bing.net/dLMbnppJWNw