The great aspect of solar cells is that they are producing non-polluting renewable energy by absorbing photons i.e. sunlight, and by extension converting the energy  of the photon to electrical energy for applications or conservation. 

One of the main challenges in the photovoltaic industry is the 33% efficiency limit for any traditional solar cell [1], which is where quantum dot solar cells (QDSCs) come into the spotlight as a solution to break the efficiency limit. 

The efficiency limit is due to excess energy lost to heat (thermalization loss), which could have been utilized [2]. The QDSC solves this issue with “quantum confinement” which will be elaborated further in the following.

What are quantum dot solar cells?

Quantum dot solar cells utilize crystalline nanoparticles known as quantum dots (QDs) as the absorbing photovoltaic material, and are investigated to replace common bulk materials such as silicon. QDs are typically in the size of 1-10 nanometers and are made from semiconductor materials or metals, e.g. cadmium selenide (CdSe) and lead sulfide (PbS). 

One of the biggest attractive attributes of QDSCs, is that they can cover the whole spectrum of wavelengths from sunlight. This is possible since the band gap i.e. the absorption spectrum is related to the size of QDs due to the quantum confinement effect. Therefore, the band gap can be tuned to match the solar radiation spectrum by adjusting the size of QDs, thereby providing great flexibility of light absorption and better efficiency in solar power generation [3]. 

Quantum dots made from lead sulfide are of most interest due to its great tunability [4]. In the case of bulk materials, the band gap is fixed in terms of what material is used, which makes thermalization loss inevitable.

Due to the very small size of QDs, it is also possible to print them into thin flexible sheets and make transparent solar cells more of a reality. These could possibly be incorporated into screens or windows as a form of electricity generation. The technology has also improved substantially in terms of efficiency, from 2.7% in 2010 to a reported 16.6% in 2020 [5].

Technical specifications of QDSCs

The QDSC is typically sandwich structured, with a photoanode, a counter electrode and an electrolyte. The photoanode consists of a transparent conducting electrode (TCE, typically indium tin oxide glass) and a metal oxide semiconductor (usually TiO2) coated with a layer of quantum dots [3]. 

The photoanode is where the sunlight is absorbed and as a result generates both positively charged electron vacancies (holes) and negatively charged electrons in the solar cell. The positive charges are transferred between the photoanode and the counter electrode with the use of an electrolyte. 

The purpose of the counter electrode is to transfer electrons from the external circuit (the electrical device) to the electrolyte, which results in catalyzing the redox reactions of the electrolyte [3]. A schematic of the QDSC is shown on Figure 1. 

Figure 1: A schematic of the quantum dot solar cell, which shows all the main components of a QDSC. The electrons transfer from the photoanode through the external circuit to the counter electrode. 

Outlook on Market

QDSCs are currently yet to be commercially viable in mass scale due to substantial challenges, such as reducing QD material cost and developing high-throughput deposition methods for mass production [6]. Although, several small companies have begun to produce QDSCs with the most notable ones being QD Solar, and Solterra Renewable Technologies, a subsidiary of the QD mass-manufacturer Quantum Materials Corp. 

Solterra gets a headstart in the immature market currently and has accordingly the proprietary technology to mass-produce QDSCs at hundreds of meters per minute using roll-to-roll printing technology. [7]

End note

QDSCs are currently seen as the next generation of photovoltaic devices, which has the most potential to break the thermodynamic efficiency limit of 33% with its excellent optoelectronic properties and the use of quantum confinement. QDSCs also have a maximum potential conversion efficiency of 66%, which is double the possible efficiency of traditional single junction solar cells [8], demonstrating that quantum dots could be the future of solar power.

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Sources

[1] Rühle, Sven “Tabulated values of the Shockley–Queisser limit for single junction solar cells”, published in ScienceDirect (2016).

[2]  Semalti, Pooja et. al.  “Advancements in Quantum Dot Solar Cells: Synthesis and Applications”, published in Sigma-Aldrich (2020). 

[3] Technical University of Denmark (DTU): Department of Chemistry. “Chemistry at the Nanoscale: Chapter 4”. (2020)

[4] Lutfullin, Marat et. al. (King Abdullah University of Science and Technology) . “Quantum Dots for Electronics and Energy Applications”, published in Sigma-Aldrich (2020)

[5] Hutchins, Mark. PV Magazine. “A quantum dot solar cell with 16.6% efficiency”. (2020)

[6] Jean, Joel. “Getting high with quantum dot solar cells”. Published in Nature (2020).

[7] Solterra. “Solterra: Business Objective”. (2020)

[8] NREL. “Quantum Dots Promise to Significantly Boost Solar Cell Efficiencies” (2013).

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Content

• What are hydrogen fuel cells?
• Nanoscale catalysts – Pt-dispersed on carbon nanotubes
• Nanoscale catalysts – non-precious metal (NPMCs) and metal-free catalysts (MFCs)
• Outlook on hydrogen fuel cells

What are hydrogen fuel cells? [i, ii, iii,iv]

HFCs utilize hydrogen as an energy carrier to produce electricity that can be used in various applications. The HFC is an electrochemical device that converts the chemical energy in hydrogen directly to electrical energy that can be used to power a motor without combustion. This makes the HFC a very attractive replacement to combustion engines, as the HFC only emits water vapor as a waste product. What makes HFCs very advantageous in automotive applications compared to purely electrical storage is the potential high ranges. This has been applied to drones, planes, trucks, and cars 

One of the most researched types of the HFC is the polymer electrolyte membrane fuel cell (PEMFC) due to its low operating temperature, very high efficiency, and low weight [i]. However, because of its low operating temperatures, the PEMFC needs a catalyst to generate useful currents due to sluggish kinetics [ii]. The most commonly used catalysts are made from platinum due to its high catalytic activity, but is expensive and of limited supply [iii], which makes it difficult for HFCs to be commercialized. Specifically, state-of-the-art platinum-based catalysts make around 25% of the overall fuel cell stack cost. [iv]

The solution to lowering costs is the use of nanotechnology. This is because most of the reactions in HFCs take place at the catalyst surface and by decreasing the size of it, you get a larger surface area per volume and therefore less material is used. 

Nanoscale catalysts – Pt-dispersed on carbon nanotubes [iii, v]

One of the most promising nanotechnologies to decrease the amount of Pt is the use of multi-walled carbon nanotube-graphene complexes (MWCNTs) atomically dispersed with Pt nanoparticles, which can overall increase the performance of catalysts. [iii] CNTs are cylinder-rolled graphene sheets i.e. carbon sheets with diameters in the range of a nanometer, in which MWCNTs consist of multiple rolled layers of graphene. The MWCNTs are used as catalyst supports in this case and have excellent electron conductivity, and with its high dispersion capability and surface area, less Pt is used and the large surface area ensures a high active site density. [v]

This nanotechnology can greatly reduce the amount of Pt used in the catalyst to 0.06 mg Pt per square meters from 0.125 mg Pt per square meters, a Pt reduction of 52%! [iii] However, MWCNTs do not exclude Pt totally, but they reduce the cost greatly and this is great for the commercialization of HFCs. 

Nitrogen doped on Pt-supported MWCNTs have also been introduced as a means to further increase performance by 8% in terms of maximum power density [iii], as nitrogen can enhance the properties of graphene. Additionally, there is currently research on whether Pt can be totally disregarded in catalysts, as to further reduce production costs of HFCs. 

Nanoscale catalysts – non-precious metal (NPMCs) and metal-free catalysts (MFCs) [vi, vii, viii, ix]

Current and past research shows that it is possible to build non-Pt solutions, which in favor avoids the cost of platinum altogether. However, catalyst activity and stability earlier were shown to be far more inferior compared to Pt-based catalysts in terms of kinetics. 

The earliest solutions utilized molecular catalysts such as iron-nitrogen complexes, but massive progress has been done with the introduction of N-doped carbon nanomaterials [vii, viii]. One of the most promising technologies is the transition metal-nitrogen-carbon catalyst due to it showing sufficient potential in durability and catalytic activity, in which iron is mostly used. [vi, vii]

However, the poor stability in PEMFC exhibits one of the greater challenges for NPMCs, which makes NPMCs not an ideal choice for PEMFCs currently. MFCs currently show interest and great promise in PEMFCs, as an N-doped CNT catalyst shows a stable PEMFC performance, though at a low activity [viii]. 

Outlook on hydrogen fuel cells 

Hydrogen fuel cells are slowly beginning to be commercialized and with solutions to the challenges of catalyst cost, the implementation can be accelerated. There are also other challenges to fuel cells than cost that affect their commercialization and attraction, such as carbon-monoxide poisoning, high energy cost of storage, and the use of mostly fossil fuels to produce hydrogen due to electrolysis having a kinetically sluggish oxygen evolution reaction (OER) [ix] etc. 

However, most of those challenges can be solved with nanotechnology, e.g. with electrolysis, the implementation of nanocatalysts can make electrolysis a more attractive option or the use of nanoparticles in photoelectrochemical water splitting can make hydrogen production more environmentally friendly [x]. While it is tempting to look at other energy options due to challenges of the HFC technology at present, the future advantages of the technology make it worth the investment in nano-solutions to solve them.

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References

[i] Dicks, Andrew L. Rand, David A. J.. (2018). Fuel Cell Systems Explained (3rd Edition). John Wiley & Sons. Retrieved from https://app.knovel.com/hotlink/toc/id:kpFCSEE01P/fuel-cell-systems-explained/fuel-cell-systems-explained 

[ii] Chen, Yuanjun et. al (2018). Enhanced oxygen reduction with single-atomic-site iron catalysts for a zinc-air battery and hydrogen-air fuel cell. Retrieved from https://www.nature.com/articles/s41467-018-07850-2 

[iii] Lou, Chong et. al (2014). A Review of the Application and Performance of Carbon Nanotubes in Fuel Cells. Retrieved from https://www.hindawi.com/journals/jnm/2015/560392/ 

[iv] Technische Universität Darmstadt (2020). Investigation of the degradation process of non-precious metal catalysts (NPMC) for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEM-FC). https://www.mawi.tu-darmstadt.de/ekat/ekat/research_2/stabilization/index.en.jsp 

[v]  Gupta, Chanchal et. al (2016). Development of multiwalled carbon nanotubes platinum nanocomposite as efficient PEM fuel cell catalyst. Retrieved from https://link.springer.com/article/10.1007/s40243-015-0066-5

[vi] Shen, Yue et. al (2014). Pt Coated Vertically Aligned Carbon Nanotubes as Electrodes for Proton Exchange Membrane Fuel Cells. Retrieved from https://www.sciencedirect.com/science/article/pii/S1877705813018134 

[vii] Asset, Tristan et. al (2020). Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells. Retrieved from https://www.cell.com/joule/pdf/S2542-4351(19)30587-2.pdf 

[viii] Xei, Longfei et. al (2018). Zigzag carbon as efficient and stable oxygen reduction electrocatalyst for proton exchange membrane fuel cells. Retrieved from https://www.nature.com/articles/s41467-018-06279-x 

[ix] Cheng, Yi et. al (2015). Advances in electrocatalysts for oxygen evolution reaction of water electrolysis-from metal oxides to carbon nanotubes. Retrieved from https://www.sciencedirect.com/science/article/pii/S1002007115001264 

[x] Mao, Samuel S. (2012). Nanomaterials for renewable hydrogen production, storage and utilization. Retrieved from https://www.sciencedirect.com/science/article/pii/S1002007112001463

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