Carbon derived from biomass: a precursor-based approach towards enhanced carbon surface chemistry

(1)

(2)

Dedication

No human endeavor is ever truly solitary, without exception, we all form part in a long chain of fate and coincidence, of nature and nurture, that identifies us biologically and spiritually as humans. Plenty of philosophers have argued and speculated on the ultimate alienation that man experiences, or more accurately, suffers as consequence of the implications of mere existence. I believe such notions are patently false. The tenets of molecular biology and chemistry teach of us an unbroken chain of

inheritance that links all of us together as part of a larger, brighter, phenomenon. The truth is not that man is isolated, instead, we’re so unbearably close to each other that we’re desperate to pretend it isn’t so. The task of acknowledging ourselves within the other is so spiritually daunting than resigning oneself to an intellectual exile is easier by comparison. When a person undertakes a scientific inquiry such as this one, they are joined by the efforts of everyone else who generated the knowledge

necessary as a starting point, this deeply humbling task echoes from the microcosm of a lab (specially one as tiny as ours!) to the larger macrocosm of humanity. As discrete as the individual might be, from any reasonable vantage point, it is evident that our power comes from the aggregate force of our efforts. Research is, if done consciously, a reminder of how close we are to and how much we need each other. In that sense I would like to dedicate this humble piece of work to all of us who struggle to make sense out of the cacophony of the cosmos in order to bring insights that result in a better tomorrow for humanity.

(3)

Acknowledgements

The first acknowledgement is to my advisor and dear friend Victor Pérez PhD, without whom I wouldn’t have been able to enter the program. His constant guidance permeates every bit of work showcased in this text.

The second one is to Tecnológico de Monterrey and specifically to Dr. Serio Omar Martínez, for taking a leap and accepting my candidacy and my tuition scholarship. The economical support from Tec, especially under these pandemic conditions, has been heartwarming.

I acknowledge CONACyT’s economic support through my scholarship and extend my hopes that they realize that the work produced at private institutions remains worthy of funding.

An important acknowledgement goes out to Javier Bojorquez and the Department of Sciences for their contributions to the work on blood-derived carbon.

I also wish to acknowledge the support of Dr. Fernando Rodriguez Macías, Dra. Grissel Trujillo and Dr. Mario Moises Álvarez for their constant collaboration and support to my research endeavors. The work realized in experimental collaboration with MSc. Luis Castañeda Maldonado and MSc. Analuisa Rubacalba Medina (both of whom finished their M.Sc. with Dr. Rodriguez-Macías) forms the backbone of this thesis.

Finally, an important acknowledgement, to my lab mates, who provided all sorts of miscellaneous guidance and training that helped me throughout my research; Braulio Cardenas, Alejandro Lujambio, Binny Jind, Dr. Sunshine Holmberg, Dr. Martin Jiménez, Dr. Roberto Gallo, Dr. Euth Cardenas and Cinthia Ramírez.

(4)

6

Carbon derived from biomass: a precursor-based approach towards enhanced carbon surface chemistry

by

Julio Fredin Ortiz

Abstract

The present thesis describes a systematic effort to synthesize, through pyrolysis, carbonaceous materials derived from readily available biomass, specifically, lyophilized fruit, dog hair and porcine blood. The characterization procedures and the search for suitable engineering applications, from separation to electrochemical sensing and energy storage are also discussed in this text. All samples with the exception of the canine hair were analyzed through scanning electron microscopy. Blood samples underwent elemental analysis, Fourier-transform infrared spectroscopy and powder x-ray diffractography and the dog hair samples underwent preparation as electrodes and evaluation through cyclic voltammetry and Electrochemical Impedance Spectroscopy. While the circumstances of the 2020 COVID-19 pandemic limit the scope of the results, preliminary work points towards two major insights. First, the applicability of the biomass derived carbon towards filtration systems due to its richness of oxygen groups. Second, the necessity of a secondary activation or annealing process for their potential energy related applications.

(5)

7

List of Figures

1. Lignocellulose Materials ... 14

2. Quality vs Cost ... 16

3. Ragone Plot ... 18

4. Electrochemical Capacitor ... 19

5. Nyquist Plot ... 22

6. EDLC Capacitance Model ... 24

7. Pore Size Capacitance ... 25

8. MnO2 Cyclic Voltammetry ... 26

9. Hybrid Lic Capacitor ... 27

10. Composite Capacitor ... 28

11. Hummer’s Method ... 29

12. MXene production ... 32

13. Generator Power Smoothing ... 37

14. Ostwald Ripening of electrodes ... 38

15. Pyrolyzed Blood Carbon ... 41

16. SEM of Blood Carbon ... 42

17. Elemental Analysis of Blood Carbon ... 43

18. IR Spectra of Blood Carbon ... 44

19. XRD of Blood Carbon ... 45

20. Freeze Dried Pineapple 100 micron ... 46

21. Freeze Dried Pineapple 20 micron ... 46

22. Pineapple Carbon 100 μm ... 47

23. Pineapple Carbon 10 μm ... 47

24. Pineapple Carbon 5% FeCl3 100 μm ... 48

28. Dog hair Carbon x Polyurethane Nyquist plot ... 51

29. Dog hair Carbon x Polyurethane CV plot ... 52

30. Biomass Carbon CV plot ... 52

31. Electrode Assembly ... 52

(6)

8

List of Tables

1. Methods of activation ... 34 2. Biomass EDLC literature results ... 35 3. Blood carbon elemental analysis ... 43

(7)

9

The present work’s literary perspective varies, as needed, from first and third person in an effort to be as clear and detailed as possible in the actual contribution of critical third parties, and the general inference and rationale behind the undertaken throughout the present body of work. I am well aware of just how contentious the issue of third person versus first person writing is on the scientific literature. However, given the nature of this thesis as a guide for others to continue my work, I find it important to judiciously and sparingly use the first person to be able to differentiate scientific fact from my own intuition towards experimentation. For that reason, the use of the first person will be limited but present within this thesis, most specially when discussing the less technical aspects of the literature and the actual experimentation of this thesis.

Chapter 1 Introduction

To think of chemistry is to think of carbon. Even though carbon-based compounds have their own self denominated branch of chemistry, organic chemistry, carbon effectively plays an important role in each and every other branch of chemistry. From its pivotal role in biochemistry, it’s undeniable virtues as an electrochemical electrode, it’s unparalleled allotrope count (and the great variety of uses that implies) in materials chemistry, as a ligand in inorganic chemistry and its role as a model surface system in physical chemistry, the amount of complexity that can be accomplished with the simple permutations of carbon atoms is awe-inspiring. The present work seeks to leverage the broad spectrum of carbon chemistry at the intersection between biochemistry, organic chemistry and materials chemistry. While the engineering and economic value of carbonaceous materials will be discussed in detail further ahead in this text, what is remarkable from a purely scientific perspective is how complex macromolecules generated through highly efficient biological processes with a variety of structural and transport purposes can be modified through chemical and physical means to serve an entirely different, useful, purposes. The blood running through the heart of a living being can be transformed into a carbon electrode that operates at the heart of a fuel cell. That is the unparalleled flexibility inherent to the unique chemistry of carbon and the core of the research undertaken in this thesis.

1.1 Motivation

The Sensors and Devices group at Tec de Monterrey has developed a great deal of expertise with the fabrication of carbonaceous materials synthesized through the use of electrospun polymers³. These carbons are often used as electrodes¹, which makes nitrogen-doped carbons made in the group through the electrospinning of polyacrylonitrile fibers to form non-woven fabrics of particular interest given their improved electrochemical activity. The broader insight from this body of research is that the

(9)

precursor’s unique composition has an effect on the resulting carbon’s chemistry and morphology, which means that, by selecting the right precursor, it is possible to get highly tailored chemistries and morphologies without the need of complicated annealing processes and without the need to utilize external chemical dopants.

1.2 Problem Statement and Context

To talk of modern society in terms of “wasteful” or “environmentally-unconscious” is to echo a general sentiment of inconformity with the lack of importance given to the environment, however the scientific perspective demands a proper effort to give dimension to the massive amount of waste currently produced by modern human society.

An estimated 13 billion tons (13x10¹² kg) of food are wasted every year², that is the same mass as 144,444 aircraft carriers (at 90,000 tons each³), the largest moving machine ever produced by man; there are only 22 carriers in service across all the navies of the world⁴. From another, more human, perspective, the estimated weight of the entire human wet biomass of the world turns in at an estimated 287 million tons (excluding children)⁵. That means that humanity wastes over 45 times its weight in food every year. And that is just speaking about food waste, another 2 billion tons of trash are thrown away by humanity every year⁶. Calling modern human society “wasteful” is a serious understatement of the problem.

The absurd amount of waste produced by our society can be best seen as an unwanted side-effect to the demographic explosions that resulted from the advent of industrialized mass farming and manufacturing⁷. These world-changing technologies, in spite of their many blessings, created a deeply inefficient system where waste abounds due to a variety of consumer demand and supply chain related phenomena^8–10. Energy and mass have a very different flow in a natural ecosystem than in our industrialized society¹¹. In nature, autotrophs generate biomass from readily available building blocks and energy (often solar) which kickstarts a complex food web that includes both heterotrophs (which consume autotrophs or other heterotrophs for their mass and energy needs) to the critically important detritivores (which feed on the remains of other living beings). The interactions of these macrosystems are self-mediated and regulated to an astounding level of stability and regularity¹², in contrast modern manufacturing is barely starting to catch up with the complex systems found in nature¹³. From the original, linear, processes that could be described as “from factory to trash heap”, back in the beginning of the industrial revolution, to modern, highly efficient supply chains that allows for both returns and end of life recycling processes, mankind is still struggling to replicate the succinct elegance of nature’s own interconnected systems.

A mindset that seeks to emulate the efficiency of nature has gained a lot of traction as the nations of the world have noticed the economic potential burrowed within the heaps of what we mistakenly denominate as waste. Economist Gunter Pauli codified this trend into the tenets of a blue economy where private and public entities seek to reform and transform what was once denominated as trash into subproducts that can be further

(10)

refined and processed to give them new life and economic worth as specialty materials or fuel¹⁴. It is at this level where the research done on carbon electrodes derived from polymer fibers can take on a new role through the transformation of subproducts into highly-valued functional or energy materials^15,16. Within this new paradigm, discarded, moldy and perhaps even rotten food, could be freeze-dried, pyrolyzed and turned into filters or electrodes for a variety of interesting chemical reactions and processes^15,17. Similarly, the trimmed hair at the barber shop or the pet salon can be collected and processed into hollow tube molecular sensors¹⁸. The blood that is merely poured down the drain at the slaughterhouse can instead be processed into platinum group metal free electrodes for oxygen reduction reactions and oxygen evolution reactions in fuel cells.

Through the use of chemistry, physics and material engineering, all these possibilities and more are available to help transition our society into a mindful inhabitant of the Earth.

1.3 Research Question

With the context discussed in section 1.2 in mind, and the expertise the Sensors and Devices group has accrued working with carbon electrodes and materials the right questions to ask are, how do we choose which readily available waste materials we use and how do we process them into value added products that can be used by the private sector?

1.4 Overview of the Proposed Solution

To evaluate which materials to use as carbon precursors, two fundamental parameters were taken into account, readily available worldwide and available at negligible costs.

Essentially, the scaling costs of the research presented in this thesis are in processing of the raw material, not so much for its acquisition. With that in mind, Dr. Rodriguez-Macias’

group at Tecnológico de Monterrey settled on a variety of fruits (in this thesis there was a focus on mangoes, pineapples and bananas but a variety of berries have also been tested) that are easy to export and import and are likely to have a percentage of them go to waste due to lead times in the supply chain and variation on consumer demand¹⁹. Similarly, Dr. Rodriguez-Macias’ research group obtained dog hair and I procured pig blood specifically because both items contains a variety of chemically and electrochemically interesting heteroatoms, such as sulphur, nitrogen, oxygen and iron.

With these three precursors selected, I designed two pyrolysis processes with the purpose of enhancing the surface area, improving the precursor to char ratio, and promoting a given microscopic morphology for an application. After the production of the biomass-derived carbon, the resulting chemical and structural properties were analyzed through Fourier transform infrared spectroscopy (FT-IR), powder X-ray diffractography (XRD) and scanning electron microscopy (SEM), different applications from advanced filtering to energy storage were to be tested (with most still pending due to the 2020 COVID-19 pandemic).

(11)

Chapter 2 Literature Review

As discussed in the previous chapter, waste management and reuse has become a staple of modern supply chains and developed economies. As companies compete to extract every ounce value from their raw materials, an interesting opportunity for material scientists has arisen because of this research and market trend⁸. Transforming waste into value added products is a strategy that has been implemented with great success across a variety of industries, from catalyzing the transformation of reaction byproducts into valuable precursor molecules²⁰, the rise of commercial bioplastics²¹ and plastic substitutes²² and the pyrolysis of organic matter into both fuel gas²³ and carbon derivate products²⁴, there abounds a literature of successful conversion of waste into valuable products through the use of chemical and physical means^9,15,25.

What is important to emphasize is that biomass is the only renewable source of carbon that can be transformed into gas, liquid and solid product¹⁷. Biomass is estimated to already provide 14% of the world’s energy consumption but large quantities of it have no specific use and end up burned in open air or dumped was waste²⁶. These bad agricultural practices result on lung-harming dusts, and acid rain produced from the sulphur and nitrogen oxides that are produced from ill-planned disposal of biomass²⁷. A better alternative is to develop processes that enable a transformation from waste to value added products.

Biomass derived carbons are of particular interest due to the broad application of hard carbons (carbons that cannot be graphitized at temperatures lower than 3000°C) across electro-sensing²⁸, electrocatalysis²⁹ and molecular sieving³⁰. Activated carbon has an active industrial role in air³¹ and water filters³² and the supercapacitor industry³³ while more graphitic carbons are often employed as anodes in lithium batteries³⁴ and hydrogen fuel cells³⁵. Various groups have reported carbons with high active surface area together with the corresponding excellent capacitance³⁶(250 F g^-1at 10mV). There have also been attempts with moderate success to test animal derived hard carbon for different half cells for battery applications³⁷. Finally, there is also plenty of animal waste being utilized for general waste adsorption. Feathers³⁸, bones³⁹, and wool⁴⁰ are just some of the examples of bio-sourced materials reported for the creation of functional carbon materials.

From a practical perspective, carbon materials obtained from the pyrolysis of biomass, are often found in either a turbostratic structure or an amorphous structure⁴¹, these resulting materials can be broadly split into two areas of application after chemical of physical activation, filtration or energy storage. While both applications are of critical importance and hold economic value, the focus of this literature survey and review, being more on line with previous work done at the Sensors and Devices, will lean towards electrical storage applications. However, for the sake of completeness a small overview of the advances in traditional filtering and molecular sieving applications of biomass-

(12)

derived activated carbons will be provided. Readers are pointed towards the thesis of MSc. Luis Castañeda-Maldonado for a more detailed account on separation applications.

2.1 Biomass-derived carbon for filtering and molecular sieving

The chemical and petrochemical industries rely heavily on separation of mixtures as an important unit of their manufacturing processes, the necessity for high purity products ensures a stable demand of gas separation processes for the foreseeable future^42,43. Adsorption is a physical technique with a track record for its effectiveness at separation and purification, however it’s generally understood to be a costly process^39,44. Utilizing low cost, biomass-derived carbons as an alternative provides an avenue to make the whole process more cost effective. Lignocellulosic biomass is an attractive precursor for activated carbons for adsorption processes¹⁷.

Lignocellulosic materials are comprised of three main natural polymers: cellulose (C6H10O5)x, hemicelluloses (C5H8O4)m and lignin [C9H10O3·(OCH3)0.9–1.7]n as shown in Figure 1.

Fig.1 Structure of Lignocellulosic materials in wood⁴⁵

Lignin is the name of a class of aromatic polymers most often found in the cell wall of a large variety of biomass, especially wood species. Its function is to bind cellulose fiber in plants. Worldwide production of lignin amounts to 40 to 50 million tons per year⁴⁶. This is of special interest given that pyrolyzed lignin’s carbon structures are is similar to that of bituminous coal⁴⁷, which makes suitable for manufacturing of activated carbons.

(13)

From a broader perspective, pyrolysis is applied to the precursor to generate char, oil and gaseous products from biomass. During the process moisture and volatile compounds are removed from the structure, leading to high porosity, large surface area and ample pore distribution and volumes⁴⁷. Hence, the char from pyrolysis can itself be treated further to prepare activated carbon and carbon molecular sieves (CMS). A CMS is best understood as a carbonaceous material with a very narrow micropore size distribution which allows it to discriminate molecules on the basis of their size⁴⁸. Biomass-derived CMSs have been utilized for the separation of nitrogen and oxygen from air, propane and propylene⁴⁹, benzene and cyclohexane from their mixtures⁵⁰.

There is a wide variety of thermal processes that can be utilized for the production of the char and its activation. Pyrolysis, to minimize losses to combustion processes, is always done under inert gas flow (often nitrogen)⁴⁷. Lignin decomposition starts between 160–

170°C (while decomposition of hemicellulose and cellulose occurs at a much broader range between 200 and 400°C) and heating processes are often executed at slow heating rates (100°C h^-1), to limit mass loss of char¹⁷. Typical terminal temperatures range from 900 to 1000°C38,40,51,52. Generally speaking, higher lignin content results in higher char yield⁵³. For this reason, materials like wood and nut shells are favored for this type of processes⁴⁷. After the production of char which already has a high surface area (somewhere between 400 and 700 m²/g depending on precursor and conditions), a secondary thermal process is done to further enhance surface area and pore volume.

H3PO4, HCl, H2SO4, ZnCl2 and both water and CO2 gas comprise the chemical agents commonly used in the activation of char⁵⁴. In contrast to the standard two step method there are also reports on the literature of one pot approaches where the activation agent and the precursor are mixed together prior to a single step pyrolysis. While this approach often yields carbons with less surface area than the two-step process^55–57, often the amount of time saved throughout the process can justify the use of a one-step approach⁵⁸. Depending on the combination of parameters surface areas above 2000 m²/g are possible¹⁷, but it is important to emphasize that there is not one single characteristic of the carbon that ensures its superiority for a given adsorption process. Different carbons, with different manufacturing processes often perform better or worse for a given separation task. Custom optimization of both the production of the char and the activation must be undertaken for any given adsorption process.

For the creation of CMS, there are two reported approaches in the literature, chemical vapor deposition (CVD) adjustment of pore size and controlled pyrolysis. The CVD procedure seeks to enhance an activated carbon and turn it into a CMS through the deposition of additional layers of carbon through a controlled injection of benzene, acetylene or methane gas under temperatures around 1000°C and anaerobic atmosphere conditions⁵⁹, this approach allows for tailoring of micropore sizes with very homogenous distributions however, the scalability of this procedure is challenged given the costs associated with industrial use of CVD. This technique only makes sense when utilized for the separation high value-added molecules. Otherwise, controlled pyrolysis with CO2 provides an alternative where the process of regular char

(14)

activation is finely tuned to reduce the pore size distribution of the resulting activated carbon.

Blood has been reported as a precursor for carbonaceous materials twice in the literature.

The first work ever published was for a generic electrocatalyst (when decorated with cobalt oxide)⁶⁰ and second work described hemoglobin derived quantum dot structures for the detection of hydrogen peroxide⁶¹.

2.2 Biomass-derived carbon for energy storage applications

While there is a wide variety of electronic applications for biomass-derived carbon, such as the one previously explored by Sensors and Devices group through the use of human hair as hollow tube molecular sensors¹⁸, as mentioned previously, the main focus of this text will be on energy storage devices, more specifically electric double layer capacitors (EDLC). There is interesting and important literature discussing the use of carbon nanotubes⁶², glassy carbon⁶³, highly oriented pyrolytic graphite (HOPG)⁶⁴ and other forms of carbon for molecular sensors⁶⁵, however I believe biomass-derived carbon is mismatched for sensor applications as a sensor due to its general lack of unmodified selectivity and the bulk nature of its precursor. Figure 2 summarizes how much quantities of a given material are necessary for a given application. Sensors often require just the tiniest quantities of active material to work, which means that biomass-derived carbons have to compete with highly engineered micro and nanomaterials that can only be produced reliably in small batches. In contrast, for energy applications where critical mass is key, biomass-derived carbons do not need to compete with materials that require expensive manufacturing techniques given that producing them at industrial scale (millions of tons per year) would be prohibitively expensive (or outright impossible with current technology). I wouldn’t say that it is not possible to find a commercial detection niche where biomass-derived carbons could build a foothold, but that would that be a challenge well beyond the scope of this text. The concept of playing towards one’s own strengths is key, biomass-derived carbon can be produced at massive scales because of the abundance of agricultural “waste”.

Fig. 2 The overview of applications in terms of their quantity vs cost⁶⁶

(15)

For the energy storage device to be built with biomass-derived carbon EDLCs were chosen over other energy storage means such as batteries or fuel cells due to high surface area discussed in the prior subsection. Another important consideration is that, even though graphite anodes are the norm in many types of batteries, including the industry-standard Li-ion battery, biomass-derived carbon rarely graphitizes at 1000°C unless external stress is exerted during the pyrolysis process⁶⁷. That is because the fibrous arrays of carbon atoms in the natural biopolymers discussed previously in section 1.1 do not allow for an easy transition towards graphitic (plane-aligned) carbon⁴⁷. The disordered, turboestratic, multiplane structure observed in biomass-derived carbon is the reason for the remarkable surface area biomass-derived carbons display even before activation. As discussed previously, general pyrolysis processes stop before 1000°C which allows the biopolymer fibers to retain the unaligned inner structure the results from the exothermic reactions and that occur during the pyrolysis procedure. For agricultural waste carbons to become graphitic, substantially higher temperatures are required, often between the range of 2000 and 3000°C, because only then are carbon atoms free to rearrange into graphite, their most thermodynamically stable bulk structure⁶⁸. Using biomass-derived graphitic carbon would prove a losing commercial proposition given that other precursors graphitize around 1000°C⁶⁹, striking down necessity for such work.

Fuel cells are a viable biomass-derived carbon energy storage application. Commercial cells often use carbon supported platinum as an electrocatalyst, which in turn makes the large fuel cells expensive⁷⁰. Grafting of Pt nanoparticles into a high surface carbon matrix has reduced the amount of platinum required in the cell, however the ideal scenario for commercial mass-produced cells would be to dispose of platinum altogether⁷¹. With this scenario in mind, relevant research has been made into the area of platinum metal group (PMG) free electrodes. The general approach towards the construction of a PMG-free electrode is to replace platinum with adatoms of iron⁷², cobalt⁷³, nickel⁷⁴, nitrogen⁷⁵ and/or sulphur⁷⁶ embedded into a turboestratic carbon matrix (or a form of nanostructured carbon). There have been some promising results with regards to speeding up the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER) both critical reactions in the production of hydrogen from water⁷⁷, however the durability of these electrodes is still highly suspect⁷⁸. Section 2.2.1 will describe in further detail how the improvement of electrocatalytic activity in a carbon electrode through the addition of adatoms tends to have a negative effect on its durability. In the case of Fuel Cells, this tradeoff is markedly more painful due to their smaller cycle life (10,000 cycles for Fuel Cells and over a million for EDLC capacitors). The trade-off between a wanted property such as energy density or catalytic activity and durability is at the crux of modern carbon-related energy storage device research. Whenever the reader engages with the literature, benchmarking against commercially available devices and observing, critically, the applicability of the results presented in the research should be priority. Maintaining a first principles approach towards electrochemical phenomena enables the differentiation of good, applicable, research from the mediocre. Regrettably, it has become common practice in energy storage device literature to report eye catching numbers that do not translate well into commercial systems.

(16)

2.2.1 Electrochemical Capacitors

Batteries and capacitors are closely related devices, what differs between the two the energy storage method. Capacitors favor capacitive storage processes over the faradic counterpart observed in batteries. Faradic processes involve redox reactions at the anode and the cathode while capacitive energy storage comes from the reversible adsorption of ions at the electrode surface during the charge and discharge cycles of the device⁷⁹. The fundamental result of these difference is best described through a Ragone plot of capacitors against batteries, a Ragone plot is the logarithmic graph of specific power vs specific energy. Figure 3 displays Ragone Plot from Simone and Gogotsi’s seminal review in the matter published in Nature Materials in 2008⁸⁰. In essence, the storage mechanism of capacitors enables much quicker charge and discharge cycles, which implies an ability to supply high rates of electric power, however, due the nature of the interaction with charges with the electrode, energy storage is limited.

Fig. 3, Ragone Plot of electrochemical capacitors vs various battery cells. In terms of electric vehicles, specific power translates how fast can a given machine

go while specific energy displays how far can it go on a single charge. The vertical lines show the time constants of the devices which are obtained by

dividing energy density by the power⁸⁰.

(17)

Capacitors can be roughly divided in two categories, electrochemical and electrolytic. The former can be further divided into electrochemical double layer capacitors and electrochemical pseudocapacitors⁸¹; both these are referred to commercially as supercapacitors or ultracapacitors. Figure 4 displays a class schematic of an electrochemical capacitor.

Fig. 4 Schematic of a charged electrochemical capacitor, two symmetrical carbon electrodes in the presence of electrolytes and a membrane⁸².

2.2.2 Electrochemical Characterization of Electrochemical Capacitors

The following section summarizes the experimental techniques described by Simon and Brousse in a special edition review commissioned by the Electrochemical Society on electrochemical capacitor electrode characterization and synthesis⁸². One of the most convenient methods for analyzing ECs is cyclic voltammetry in the standard three electrode setup. The specific capacitance (C) can be calculated roughly by integrating the curve of the voltammogram (which results in voltametric charge Q) and dividing it by the mass of the active material on the electrode (m) and the length of the potential window (∆E).

(18)

C = Q / (∆E*m) (1) Charge and discharge curves can also be utilized provided that the current remains constant.

C = (I*td) / (∆E*m) (2)

Where I is the static current and td is the discharge time. With modern software it is also possible to calculate accurately from the slope of the discharge curve (dV / dt) through the following equation.

C = I / (dV / dt) (3)

Specific capacitance values for a single electrode provide a neat comparison between EC materials, however it’s more relevant for applications to determine the capacitance of the two-electrode system (Ctot).

(4) 1 / Ctot = (1/C+) + (1/C-)

Equation (4) implies that for symmetric ECs (same material in the same quantity) the gravimetric capacitance will be Ctot/2.

While measurements of electrode and cell capacitance are useful for comparison, the most important metrics that can be derived from a two-electrode system are the real power density (W/kg) and the real energy density (W h/kg). Using the same technique of constant current charge and discharge cycles as in (3), the power density can be calculated as

Preal = (∆E*I) / m, (5)

where ∆E = (Emax + Emin) / 2 intuitively, Emax is the potential at the end of charge and Emin

is the potential at the end of discharge.

(6) Ereal = Preal * t

gives the real energy density, where t is the measured discharge time of the cell.

Conversely, the maximum specific power Pmax and specific energy Emax can also be calculated through the following equations

Pmax = (U0)²/ 4*R*m, (7)

Emax = C*(Umax)²/ 2, (8)

(19)

where U0 is the potential before ohmic drop, R is the internal resistance of the cell, C is the capacity of the system, and Umax is the potential at the end of discharge. Two other parameters that give a clear description of EC capacitor are the coulombic efficiency and the equivalent series resistance (ESR). Coulombic efficiency is calculated by the discharge time vs the charge time ratio. A Coulombic efficiency of 1 implies that there are no side reactions on either step of the cycle.

There are two methods for measuring ESR, current interrupt technique and electrochemical impedance spectroscopy (EIS). Under an applied current the potential of the cell is ESR*I + Q/C and when the current is stopped the potential drops to Q/R hence

ESR = (V0 – (Q/C)) / I (9)

where V0 is the cell potential when current is being applied. Alternatively, ESR can also be measured at an AC frequency of 1kHz by EIS.

EIS is a critical technique for que characterization of EC systems. Due to the high-power nature of the devices, important results can be derived from the changes of capacitance and resistance with the frequency under AC polarization. Fig. 5.a shows a Nyquist plot, which describes, starting from the bulk of an electrolyte solution, all the way to the inner depths of a porous electrode, the impedance behavior of the cell. This behavior can be modeled as a succession of RC circuits, both in parallel and in series. The exact composition of this hypothetical RC circuit depends on the specific nature of the pore surfaces and channels. However, it’s important to note that, regardless of the system, the dependance of the capacitance on the frequency can be directly extracted from the Nyquist plot through the complex capacitance model

C’(ω) = Z’’ (ω) / ω* |Z(ω)|^2, (10)

where |Z(ω)|²is the modulus of the impedance Z(ω) and C’(ω) stands for the real part of the capacitance.

Figure 5.b shows the real part of the capacitance vs the frequency. The physical interpretation is that, at lower frequencies, more surface area of the carbon is accessible within the depths of the electrode. Every system has a knee frequency which is the point where the capacitance becomes visibly dependent on frequency, at frequencies lower than the knee frequency, the capacitance should be constant. The knee frequency is also a useful performance metric for EDLCs, it depends on the type of porous carbon, the electrolyte and the cell assembly. Knowing how EC capacitors respond to frequencies is important for pulse-power applications and EIS is a simple and efficient technique to obtain that information

(20)

.

Fig. 5 (a) Nyquist plot for a symmetrical cell of high surface area carbon electrodes in 1.5 M (C2H5) BF4 in acetonitrile. Frequency range from 10kHz to 13mHz. (b) Evolution of the real part of the capacitance C’(ω) vs frequency, same

electrochemical system⁸². 2.2.3 EC fundamental structure

Building on the previous discussions on the unique position of electrochemical capacitors within the broader spectrum of energy storage devices and the electrochemical methods utilized characterize them, further the development of the subject requires developing a functional understanding of physical principles that guide the design and engineering of and electrochemical capacitor cell.

Double layer capacitance was first described by Helmholtz in 1853 while discussing the charge separation that occurs during the polarization of the electrode-electrolyte surface interface when an electrochemical potential is applied⁷⁹. This capacitance was described by the following equation:

C = εr* ε0*A / d (11)

where εr is the electrolyte dielectric constant, ε0 is the dielectric constant of vacuum, d is the effective thickness of the double layer and A is the electrode surface area. Equation (11) correlates surface area with capacitance and the electrolytes dielectric constant, however one must not forget that supercapacitors, are, after all, energy devices and are subject to the broader equation

E = ½ C*V² (12)

(21)

where E is the energy and V is the operating voltage. Equation (12) is of special significance when optimizing supercapacitive cells, because it implies that higher specific capacitance values need not necessarily translate better energy storage devices. Organic electrolytes are used more widely in the industry in spite of having a smaller specific capacitance than aqueous alkaline or acidic electrolytes because organic electrolytes can sustain larger operation voltage window while water undergoes ORR and OER at 1.24 volts and hydrogen evolution reaction (HER) at 0 volts⁸³. Keeping application in mind is key, often having a wider operation voltage window is more important than the sheer specific capacitance⁸⁴. This is an important consideration given that many articles neglect to report actual power or energy values in favor of exaggerated specific capacitance values.

From equation (11), (12), and the intuition provided by the Ragone plot in Figure 4 it’s possible to visualize two general approaches to EDLC design. The first approach is to try and synthesize different organic electrolytes that are stable at wide operational windows and the second approach is to improve the electrochemical properties and surface area of the EC electrodes. Given that the focus on the thesis is on using biomass-derived carbon for commercially viable applications, special focus will be given to the latter strategy over the former however, for the sake of completeness, a brief survey on supercapacitor electrolytes will be provided, however, readers are encouraged to delve deeper onto the topic from other sources^85–87.

2.2.4 Electrolytes

Electrolytes for electrochemical capacitors fall into one of four categories, aqueous electrolytes, organic solvent electrolytes, ionic liquids or polymer and gel electrolytes. As mentioned in the previous subsection, the regular voltage range for aqueous electrolytes is 1.24 volts, however, through the use asymmetric capacitors (which will be discussed in section 2.2.5) voltages of up to 2V have being reported⁸².

Organic solvents such as acetonitrile, Ethymethy-carbonate and sulfolane have wide ranges of operable voltages ranging from 2V to 3.3V. While heating and durability are of concern with these types of systems, the energy advantages are often worth engineering around the limitations of the system⁸⁵. Common electrolytes for these systems include quaternary ammonium salts, tetrafluoroborates and hexafluorophosphates⁸².

Ionic liquids have attracted research interest in the field of supercapacitors because of their highly concentrated electrolytes and nonvolatility. The general problem of this systems is their high viscosity. Efforts to improve fluidity include high temperature cells and tailoring electrodes to better fit the ions⁸⁸.

The replacement of liquid electrolytes with solid ones greatly improves commercial device reliability however solid polymer electrolytes and inorganic electrolytes are inadequate due to their inability to interface properly with porous electrodes, air bubbles would result in increased resistance and poor performance. In contrast, gel polymer electrolytes have

(22)

been successfully reported⁸⁹. Using hydrogels, however brings to the forefront the same limitations of aqueous materials (limited temperature range and operation voltage)⁹⁰. Nonaqueous gels have also been reported with improved operation voltage ranges⁹¹. 2.2.5 Supercapacitor electrodes

Electrochemical capacitor cells can be described as electrochemical double layer capacitors (where no faradic reactions happen), pseudocapacitors in which redox reactions do occur, but do so reversively, and hybrid systems⁸⁰. There are two cell designs that are categorized as hybrid systems, the first combines high power electrodes (EDLC or pseudocapacitive) with high energy electrodes such as lithium intercalated electrodes in a single cell; the second approach consists on synthesizing EDLC carbons directly with pseudocapacitive materials on top of them (either as deposits or as coats) within a single electrode⁸³.

2.2.5.1 Electrochemical double layer capacitors

Fig. 6 Specific capacitance normalized by specific surface area as a function of pore size distribution across different carbon samples with the same electrolyte (NEt4+, BF4- in acetonitrile, concentrations in key). The symbols display how capacitance can be increased through the removal of the

solvation shell at extremely small pore sizes (zone I and II which are modeled as electric wire-in-

cylinder capacitor). Traditional EDLC behavior in zone III⁹².

EDLCs are, as described previously, simple carbon electrodes where surface area should be maximized in order to improve capacitance. From activated carbon to graphene, through carbon aerogels, nanotubes, nano-onions, nano-horns and pretty much any other structure in between. Commercially, activated carbons dominate the market of supercapacitors due to their moderate costs and availability^80,84,87. An important caveat must be taken into consideration when discussing EDLC carbons, while at first glance,

(23)

the goal for engineering this type of electrodes might seem to be a trivial matter of maximizing surface area, the reality is more complex and with finer details than that. What actually must be optimized is the size of the micropores within the carbon structure. When the micropores match up with the general size of the ion in question the removal of the solvation layer allows for a different model of capacitance to take place as shown in Fig.

6 and described Huang et al. in the following equation⁹²

C/A = εr* ε0 / b*ln(b/a0) (13)

Fig.7 Normalized capacitance change as a function of carbon-derived-carbide samples. Samples were prepared at different temperatures in ethyl-

methylimidazolium / trifluoro-methane-sulphonylimide (EMI, TFSI) ionic liquid at 60 °C. Inset shows the structure and size of the and TFSI ions. The maximum capacitance is obtained when the pore size is in the same range as the maximum

ion dimension⁹².

where b is the pore radius and a0 is the effective size of the desolvated ion. Fig. 7 shows further proof upon the use of pore tailored carbon that matching micropore size with ion size yields the so called “electric wire-in-cylinder” capacitor behavior. These results match up nicely with the prior discussion on tailoring pores in section 1.1 where techniques for such processes where described for the application of molecular sieving.

(24)

2.2.5.2 Pseudocapacitors

As mentioned previously, pseudocapacitors use fast, reversible redox reactions at the surface of the electrode to hold on to charge. Conductive polymers and metal oxides such as RuO2, MnO2, Fe3O4 and V2O5 have been extensively studied for their ability to store energy⁸⁴. Pseudocapacitors benefit from a higher energy density than EDLCs however, due to the chemical nature of their energy storage mechanism (in contrast to the electrostatic charge in double layer capacitance), pseudocapacitors often degrade much faster than their carbon counterparts.

Fig. 8 Schematic representation of cyclic voltammetry for a MnO2 cell in 0.1 K2SO4. Multiple successive surface redox reactions store charge⁸⁰.

2.2.5.3 Hybrid Electrochemical Capacitors

Commercial hybrid electrochemical capacitors combine Li-ion-battery graphite anode with a porous-carbon capacitive cathode. These devices are known as Lithium-Ion Capacitors (LiCs) and have a high energy density (beyond 20 Wh kg^-1)⁹³. As with all devices, the high energy density comes at a price, LiCs produce a few kilowatts per

(25)

kilogram. These devices bridge the gap between more traditional supercapacitors and lithium batteries.

Fig. 9 Concept of a LiC, combining graphite anode with porous carbon cathode.

The cell voltage is increased relative to an EDLC capacitor using asymmetrical electrodes⁸⁷.

Other similarly inspired devices that use pseudocapacitor cathodes and Li-ion intercalated carbon anodes have also been reported, however the other broad category of hybrid capacitors consist on porous carbon and pseudocapacitor composites, as seen in Figure 10.

(26)

Fig. 10 Decoration of activated carbon grains with pseudocapacitive materials⁸⁰ These approaches often also find a middle ground between the sheer durability and high power of regular EDLCs and the improved energy density of pseudocapacitive materials.

Consistently enough, trade-offs are still unavoidable when designing supercapacitor cells⁸⁷.

2.2.5.4 2D Material Electrodes

Ever since Richard Feynman told the world that there was plenty of room at the bottom⁹⁴, material scientists, physicists and chemists have been racing towards it to realize the promise of materials engineered at the atomic scale. Few materials exemplify how far humanity has progressed in this journey from the microscopic to the nanoscopic better than graphene. It’s been well over a decade since Geim and Novoselov first isolated graphene from graphite using adhesive tape⁹⁵, opening the floodgates for the development of other atomic 2D materials such as silicene⁹⁶, germanene⁹⁷ and stanene⁹⁸. On top of those graphene analogues, a completely new type of 2D material has also been reported, MXenes, these ceramics were developed from 3D structures called MAX phases⁹⁹ (where M stands for transition metal, A Is a group III or IV element and X is carbon or nitrogen), MXenes are created by etching the A layer by means of vacuum heating, molten salt, metal heating or using strong chemical etchants such as fluorhydric acid or elemental chlorine¹⁰⁰. Most MXenes are less than 1nm thick and form structures in the order of microns¹. Both Graphene and MXenes are premier materials for supercapacitor electrode materials that are somewhat held back by manufacturing complications and sluggish industry adoption^101,102. In this subsection the state of the art of both materials and their composites will be surveyed within the scope scalability concerns in energy storage device applications.

2.2.5.4.1 Graphene

Because of its status as the first 2D material synthesized, graphene has a natural lead over the rest of the pack and yet, in spite of it’s thermal, mechanical, electrical and chemical properties¹⁰³, the story of graphene manufacturing has been far from an overnight success. Within the mass production of graphene, there are four mayor industrial products in the global market of graphene: nanoflakes (non-oxidized), graphene

(27)

oxide (GO) or reduced GO (rGO) and graphene films^104,105. These products have significant differences in thickness, uniformity, yield, horizontal size and production costs, which results in different applications and market niches for every single one of them¹⁰⁶. Generally speaking, there is a tradeoff, between film techniques based in epitaxial lateral growth of graphene through CVD¹⁰⁷, and chemical exfoliation of graphene or it’s oxide into nanoflakes; CVD makes for better uniform films at elevated costs while nanoflakes are relatively cheap but more defect prone¹⁰¹. Even techniques trying to develop faster CVD procedures, such as roll to roll CVD, cannot escape the limiting reality that creating a continuous graphene film with few defects requires a slow(minutes per millimeter) and deliberate process with limited scalability¹⁰⁸. It is for that very reason that commercial EDLC electrodes that involve graphene often use it as nanoflakes suspended in a paste.

Fig 11. Broad schematic of Hummer’s method

Within the literature, a modified version Hummer’s method¹⁰⁹, shown in Figure 11, is the predominant method of synthesis due to its reliability and simplicity¹¹⁰. Greener, i.e. less reliant on strong oxidizing agents, electrochemical alternatives have also been reported in the literature^111,112. For composite graphene electrodes, after graphene nanoflakes or GO nanoflakes are synthesized and in the case of GO further reduced, an adatom¹¹³, a pseudocapacitive material^114,115 or a polymer¹¹⁶ is grafted into the graphene for the development of a double action hybrid capacitor, combining the effects of double layer capacitance with the reversible reactions of a pseudocapactior for an additive effect, just as it’s observed in traditional AC composites³⁶. An important fact that must not be overlooked when designing composite electrodes is that that excellent results have been reported by EDLC capacitors made from curved graphene paste, without any composite whatsoever. Proper selection of organic electrolytes can often be just as successful if not more than composite electrodes as demonstrated Bor Z. Jang’s group with their high

(28)

energy density (85.6 Wh kg^-1 at 1 A g^-1) and high gravimetric capacitance for ionic liquid (154.1 F g^-1 at 1 A g^-1) dating as far back as 2010¹¹⁷. The key take way is that merely competing for high capacitance values or energy density while being devoid of a proper application often makes for research that lacks applicability or even justification within a wider context. Work by Negre and coworkers demonstrates this principle excellently¹¹⁸, the carbon-carbon composite supercapacitors designed with a ionogel acting both as electrolyte and separator which operates in a wide temperature range of -40⁰C (34 F g^-1) to 60⁰C (96 F g^-1). Application-minded research often leads to inspired cell design, which in turn pushes forward relevant scientific results with broader applicability. The underlying area of opportunity with studies of this nature is the lack of comparison between the simple graphene and the composite. Typically, only the composite is characterized electrochemically, which is problematic when trying to discern the actual contribution of the composite part of the electrode versus that of the singular carbon structure.

2.2.5.4.2 Hierarchical Graphene Structures

Another approach to graphene electrodes is that of hierarchical 3D structures. These architectures have the intrinsic advantage of maximizing the exposed surface area of the graphene and hence, increasing the overall capacitance. One successful commercial approach has been that of carbon – carbon matrices. Specifically, activated carbon (AC) combined with graphene. Patents from industry leader in energy saving supercapacitor systems, Skeleton Technologies show the use of both activated carbon and curved graphene in the electrode paste of the cathode which then is inserted into an asymmetrical setup and an organic electrolyte¹¹⁹. In the scientific literature inventive approaches have been developed to create a two in one composite graphene/AC electrode by dispersing graphene into pyrolysable materials. Zhou and collaborators utilized this strategy to great effect by dispersing graphene oxide into a glucose solution that was hydrothermally carbonized and then activated chemically with KOH in a two-step process for a final capacitance of 103 F g^-1 in organic electrolyte¹²⁰.

Another interesting hierarchical structure is the graphene foam, also known as 3D Graphene-Based Macrostructures. There is a wide variety of ways to assemble rGO into graphene foams that can be used as electrode materials. The following list is not exhaustive but covers the most promising approaches: template-assisted¹²¹, laser induced¹²², electrophoretic deposition¹²³ and electrochemical deposition¹²⁴. Pioneering work by Z. Chen, et al, demonstrated the possibility of creating highly conductive and resilient graphene foams by using nickel foam as a template⁹⁰. From there on CVD has often being used for the creation of EDLC capacitors utilizing analogous, procedures¹²⁵, while the excellent properties of said materials cannot be denied, especially in comparison to other methodologies, there is a need to emphasize the difficulties of scaling said procedures. Other techniques have sought to emulate procedure through different means, self-assembly into the template was reported by Ji Chen and collaborators, which showed a promising one pot electrode approach, where GO was reduced into the submerged nickel foam matrix from solution, making a tandem collector-electrode system⁹⁰. A similar inspired approach was reported by Kaiwu Chen and his

(29)

collaborators¹²⁴, where they electrochemically reduced and deposited GO into a copper electrode, resulting in graphene hierarchical structures.

However, perhaps the most groundbreaking technique of manufacturing graphene foams is through laser induction. The sheer scalability and flexibility of this technique makes it the most promising approach to the industrial production of graphene foams. Pioneering work by Kaner and his research group first demonstrated the possibility of reducing a thin film of GO directly through the use of the infrared laser of a LightScribe CD/DVD optical drive¹²⁶. In another critical breakthrough Yakobson, Tour and their collaborators demonstrated that infrared lasers could be used to irradiate commercial polyimide to produce similar laser induced graphene (LIG)¹²², which means this technique could result in roll to roll production of custom made LIG micro supercapacitors. Further work by Tour, et al. demonstrated that most lignocellulose materials could be used as substrates for LIG. Wood, Potatoes, coconuts, bread, cork and paper were some of the substrates demonstrated to be consistently transformed into LIG under ambient conditions¹²⁷. While roll to roll manufacturing would limit this technique to 2D structures, 3D Printed LIG Foams have also been reported and studied not only as capacitor electrodes but as embeddable stress and gas sensors¹²⁸. Even if LIG continues to be a promising avenue for the growing field of micro supercapacitors, concerns about low capacitance values, have been tackled through the tried and true method of compositing carbon EDLC with pseudocapacitors.

Kaner, et al. used electrochemical deposition of MnO2 to improve the capacitance value to 1,380 F g^{-1 129}. Concerns previously stated about post-10,000 cycle lifetime stability of composite EDLC and pseudocapacitor hybrids still apply to LIG, and an important challenge is to study how the laser intensity can be finely tuned to manipulate the foam morphology into structures that favor the disruption of the solvation shell as seen in modern activated carbon EDLCs. Another important consideration is that, if intended as wearable technology electrodes¹³⁰, adding heavy metal oxides to the electrodes is a questionable approach given that the product lifecycle might lead to environmental damage¹³¹.

2.2.5.4.3 MXenes

There is another family of 2D atomic structures that has garnered a large amount of interest in the energy community, MXenes. First reported in 2011¹³², MXenes are a family of metal carbides, nitrides and carbonitrides, with a hollow plate structure as shown in Figure 12. Their precursor materials, MAX phases, were first reported in 1996¹³³ and consist of the molecular formula Mn+1AXn where M is a transition metal, A is aluminum (or rarely an analogous atom) and X is either carbon or nitrogen. By selectively etching the A from the sandwich structure of a MAX face the structure becomes a stack with empty layers described by the formula Mn+1XnTx where T is a termination (O, -OH, -F or Cl).

What is remarkable about this hollow 2D plane where the aluminum atoms stood is its ability to electrostatically hold charges, making it a prime candidate for supercapacitor electrodes, among many other commercial applications.

(30)

Fig. 12 Production of MXenes through etching¹⁰⁰

Gogotsi, et al. reported a remarkable capacitor electrode made from Titanium Carbide MXene paper, with a high volumetric capacitance (350 F cm^-3), durability and mechanical flexibility¹³⁴. The sheer amount of possible atomic combinations has made MXene computational simulation a booming field¹³⁵, and strong strides have been made towards the demonstration of the general scalability of MXenes⁶⁶. Murata Manufacturing Co., Ltd.

is working on a kg scale

pilot prototype production line, giving general credence to the commercial interest and scalability of the chemical process. Given the exhaustive amount of work already done in graphene, and how redundant has become to simply add a given set of adatoms to improve its electrical properties as proven by the most elegant work by Pumera and his collaborators, where bird droppings were used to enhance the electrocatalytic effect of graphene¹³⁶; MXenes are the electrode materials that would benefit from more exploratory research, focusing on finding unique formulations based on computationally predicted properties and ingenious approaches to material synthesis.

While MXenes remain one of the most promising areas of supercapacitor electrode research, there are a few points that must be taken into account as MXenes transition from the lab into the factory. The toxicity of certain metal carbides and nitrides must be taken into account when choosing which MXene candidates to pursue as products. Given that the strength of MXenes lies on their volumetric capacitance, it’s important to select

Carbon derived from biomass: a precursor-based approach towards enhanced carbon surface chemistry

Carbon derived from biomass: a precursor-based approach towards enhanced carbon surface chemistry

by

Julio Fredin Ortiz

List of Figures

List of Tables

Contents

Chapter 1

Introduction

Chapter 2

Literature Review