Hundred years ago, ammonia came to humanity’s rescue as a fertiliser and averted a major food crisis. Now it is poised to return in a new avatar as fuel and bail us out of the looming disaster due to climate change. Green Ammonia produced using renewable energy holds much promise to decarbonise the world economy in the coming decades.

The Haber-Bosch process for the synthesis of ammonia is widely regarded as the most important invention of the 20thCentury. As the Germans put it dramatically, ammonia made it possible to make “bread from air”. But, for ammonia-based fertilisers, the explosive growth of population in the last century would not have been possible. Now as humanity is facing yet another gargantuan crisis of climate change, ammonia is emerging as the saviour yet again in a new avatar.

Green Ammonia
Currently, almost all the ammonia in the world is derived from fossil fuels. Ammonia manufacture emits more carbon dioxide than any other process in the chemical industry. When green hydrogen, produced by electrolysis of water using renewable energy, is used to synthesise ammonia, the latter also acquires the “green” label. Both hydrogen and ammonia can be used as fuels without emitting carbon dioxide and thus hold the key to a decarbonised world.

Energy Carrier
As a carrier of energy, ammonia has several advantages over hydrogen. Whereas liquid hydrogen has to be stored at a very energy intensive minus 253 degrees C, liquid ammonia needs only minus 33 degrees C. Ammonia is much less flammable than hydrogen. Liquid ammonia has higher volumetric energy density at 3.75 kWh/L compared to 2 kWh/L for liquid hydrogen. Unlike hydrogen, ammonia has been widely transported for decades by ships, rail cars, road tankers and pipelines. It is the second most widely produced commodity chemical in the world. The infrastructure for distributing ammonia is already robust and is very well understood. Several ports across the world have terminals that are fully capable of despatching and receiving liquid ammonia. The energy from ammonia can be tapped in two ways – direct combustion or through a fuel cell.

Direct Combustion
The earliest idea of using ammonia as a fuel in an internal combustion engine appear in patents as far back as 1905, around the same time when the Haber-Bosch process was being developed. The diesel shortage during World War II resulted in the first practical use of ammonia as a motor fuel by buses in Belgium in 1943. Interest in using ammonia as a direct fuel has revived in recent years and considerable research has been carried out in optimising the performance of ammonia-fuelled engines. Ammonia’s poor combustion characteristics are a major stumbling block in its widespread deployment as an engine fuel. These include high auto-ignition temperature, low flame speed, narrow flammability limits and high heat of vapourisation. Compression ratios of 35:1 and more are required for successful use of ammonia in a compression-ignition engine. To lower the compression ratios, ammonia has been co-combusted with other fuels like diesel and dimethyl ether. Hydrogen has also been successfully used as a complementary fuel. Luckily, hydrogen can be produced by in-situ cracking of ammonia.

NOx emission is the other major problem of using ammonia as an engine fuel. If combustion temperature  is lowered to reduce NOx level, concentration of unburned ammonia goes up. NOx levels in ammonia engine exhaust are as high as 1000 ppm. Researchers are trying different fuel injection techniques to simultaneously reduce NOx and unburned ammonia. Another post-combustion technique is to use selective catalytic reduction and reduce both NOx and ammonia in the exhaust gases.

Fuel Cells
An alternative way of recovering the chemical energy from ammonia is through fuel cells. Fuel cells can be indirect or direct. The indirect method uses in-situ thermal cracking of ammonia to yield hydrogen, which is subsequently processed in a fuel cell to produce power. The second method, as the name suggests, directly processes ammonia inside the fuel cell. Direct ammonia fuel cells are of many types depending on the electrolyte used. The earliest direct ammonia fuel cells were Alkaline Fuel Cells (AFC), which initially used KOH solution and subsequently molten mixtures of NaOH and KOH as the electrolyte. Oxygen from the air supplied to the cathode generate hydroxyl ions, which are transported through the electrolyte to the anode, where they encounter ammonia. The ammonia reacts with the hydroxyl ions to form nitrogen, water and free electrons, which circulate to the cathode through the external electrical load producing power. Noble metals like platinum and ruthenium are used for the electrodes. The main drawback of these cells is the absorption of carbon dioxide in the air by the alkaline electrolytes and consequent drop in efficiency.

SOFC
Solid Oxide Fuel Cells (SOFC) use several mixed conducting ceramics as the electrolyte and operate at high temperatures, typically 650 to 1000 degrees C. A commonly used  lectrolyte is yttria-stabilised zirconia. They do not require noble metals as electrodes; instead cheaper nickel-based materials are used. Because of their high efficiency, they have been extensively researched in recent years and a commercial breakthrough is expected very soon. SOFCs come in twovariants: SOFC-O and SOFC-H. Oxygen anions are transported through the electrolyte in SOFC-O, whereas the SOFC-H uses protons as charge carriers. Ammonia is introduced at the anode where it splits into hydrogen and nitrogen. Air is introduced at the cathode. In SOFC-O, the oxygen at the cathode is reduced to oxygen anions,which travel through the electrolyte to the anode where they react with hydrogen to form water. Since water is formed at the anode, where ammonia is present, side reactions occur leading to nitric oxide generation. In SOFC-H, hydrogen at the anode is stripped off electrons to yield protons, which travel through the electrolyte to the cathode to react with the oxygen to produce water. Nitric Oxide formation is liminated in SOFC-H. Also, 20-30 percent higher peak power output has been achieved in SOFC-H compared to SOFC-O.

Going Green
The shipping industry, which currently accounts for nearly three percent of the global carbon dioxide emissions, has been quick to embrace ammonia as an alternate fuel in its efforts to decarbonise its operations. Five leading shipping companies have come together to sign a MOU to establish a supply chain for green ammonia. If all long-distance shipping were to convert to ammonia, the global production would have to nearly treble from the present 180 million tonnes per annum. Wartsila and MAN, the major engine manufacturers, have announced plans to undertake long term trials of ammonia in a marine combustion engine. Wartsila hopes to have its engine installed on a ship by 2023. Yara, the world’s leading producer of ammonia is betting big on going green. It has announced plans to set up the first large-scale green ammonia project in Europe. Other countries like USA and Saudi Arabia, have also chalked out plans to manufacture green ammonia. The Indian government has plans to invite bids for setting up green ammonia projects with a view to reduce dependence on fossil fuels.

Epilogue
As nations compete with each other in announcing roadmaps for decarbonisation of their economy by middle of this century, hydrogen and ammonia will play a stellar role in fructification of these ambitions plans.

Readers’ responses may be sent to:
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The 50th edition of this column examines some salient facets of the material revolution that is silently shaping our lives. Materials have always been at the forefront of our progress from prehistoric times and will continue to lead us forward in an uncertain future. We need novel materials with unique properties to solve many tough challenges facing us, for which we need to place the cart before the horse.

Fifty years ago, Dupont launched a fibre that is five times tougher than steel. Kevlar was born in a cloudy cocktail of chemistry and curiosity garnished liberally with patience and perseverance. Today, Kevlar is a de facto material in bulletproof vests and other protective apparel worn by military personnel. Kevlar is an astonishingly strong fibre that belongs to the family of aromatic polyamides (aramids) and has a myriad other applications ranging
from racing car tyres to the bow of the violin. When molten Kevlar is spun into fibres, the molecules of the polymer orient themselves in a crystalline arrangement with a high degree of order and symmetry. The fibre owes its extraordinary strength to the near perfect orientation of the molecules of poly-para-phenylene terephthalamide.

Material Revolution
The evolution of human race is inextricably tangled with materials, so much so that the three main stages of pre-historic human civilisation are categorised after materials of that
age – stone, bronze and iron. Over many centuries, our lives have been continuously reshaped by materials; materials like paper and leather, glass and concrete, alloys and plastics. Today, we are living in the silicon and lithium age. Life, without these two metals appears inconceivable. We are currently in the midst of a material revolution. Advances in
materials are fundamental to breakthroughs in science and technology that can offer remedies to some of the most challenging problems confronting us. We are seeking materials with unique and customisable mechanical, thermal, chemical, optical,electrical,  electronic and magnetic properties. Materials that can solve difficult problems related to
energy storage and climate change. Materials that will improve the performance of batteries, fuel cells and photovoltaic cells. Materials that are lighter, yet stronger. Materials that are cheaper to produce and easier to process. Materials that are green and sustainable. Materials that will work in a circular economy. Materials that can be 3D printed. It is a long wish list.

Nanomaterials
Nanoparticles are interesting because they display properties that are very different from those of their parent bulk material. Ensembles of nanoparticles, in turn, exhibit properties
that are different from the discrete nanoparticles. The novel mechanical, thermal, optical, electronic and magnetic properties of nanoparticle assemblies are attributed to the coupling and interaction among individual nanoparticles and these can be tweaked and fine-tuned. Previously unknown properties can also emerge from the strong interaction between assembled nanoparticles. Nanostructures with identical composition, but different morphologies exhibit entirely different properties.

Self-assembly
Self-assembly is the process where nanoscale building blocks spontaneously organise themselves into ordered structures or patterns without any human intervention. The self-assembly process can be adjusted and modified to build nanostructures with bespoke properties. Sophisticated architectures with extremely useful properties have been achieved by influencing the self-assembly process through various types of external forces like electrical, magnetic, optical and ultrasonic. Hard templates have been used to obtain nanostructures in well-defined shapes like wires, rods, tubes, spheres, cubes, etc. Various surfactants have been used to construct nanostructures with desired porosity and texture.

Nanostructure Applications
The customised nanoparticle assemblies have several potential applications in energy storage, photocatalysis, photovoltaic cells, sensing etc. Research is underway to build nanostructures for photocatalytic water splitting and for photocatalytic capturing of carbon dioxide. Engineered nanostructures are being keenly investigated to increase the efficiency of dye-sensitised solar cells and quantum-dot sensitised solar cells. Self-assembled nanoparticle structures have been used as bio and chemical sensors. Compared to conventional sensing technologies, the techniques based on properties of self-assembled nanoparticles exhibit superior selectivity and sensitivity with an unlimited lifetime. Last year, a group of Chinese researchers obtained a new insight into nanoparticle assemblies when they discovered that they can mimic living organisms and adapt to changes in their
environment though motion, perception, self-replication, self-regulation and self-repair. These animate characteristics of nanoparticle assemblies open up an exciting future.

Self-healing Materials
Self-healing materials (SHM) have an intrinsic ability to substantially recover their mechanical properties after damage. The interest in these materials has been growing exponentially as they form an important pillar for sustainable infrastructure through a significant increase in the service life of a structural member. Most living materials exhibit a self-healing property and thus have been a great source of inspiration in the development of SHM. Many techniques have been used to impart self-healing property in concrete and polymers. A commonly adopted approach to instill self-healing in a composite material is to embed microcapsules containing healing agents. When the composite is damaged, the microcapsules rupture and release the healing agents into the network to substantively restore the original properties. Another interesting approach to self-healing is through the use of materials that have shape memory. Shape memory alloys and polymers can remember their original form and regain it when an external stimulus like heat or light is applied.

Composites
Composites characterised by high strength to weight ratio, are increasingly used by automotive and aerospace industries for achieving energy efficiency and reducing  missions. They are also a growing favourite in infrastructure projects because of higher corrosion resistance, and hence reduced maintenance and onger life span. The EV  evolution will provide further fillip to lightweighting technologies that are already in vogue in most manufacturing industries. The battery constitutes as much as 20 per cent of the gross weight of an electric vehicle. To overcome this weighty issue, battery developers are looking at structural batteries, where the batteries also double up as a load bearing member of the vehicle. Carbon fibre composites with excellent mechanical and electrical properties have shown great promise for structural batteries. The prototype structural battery that is currently in active trial uses carbon fibre composite anode and lithium iron phosphate cathode with a fiberglass matrix separating the electrodes and serving as the electrolyte. Composites exhibit anisotropic behaviour, with their strength, stiffness and other desirable properties strongly influenced by the way the fibres are oriented in the polymer matrix. Understanding this anisotropic performance is essential to the design and manufacture of composite parts and components.

Material Discovery
Bakelite, the first plastic, was discovered accidentally. Historically speaking, materials were discovered first; their applications came later. But, when it comes to inventing materials for the new decade, we have to put the cart before the horse. We need to find materials with an exact property for a specific application. Designing materials that possess a desired property is in the realm of fantasy. The materials genome Initiative launched by the US Government in 2011 threw a challenge to the scientific and engineering community to accelerate the pace of discovery and deployment of advanced materials. The MGI paradigm for new materials discovery combines theory, experiments and computational predictions in a synergistic fashion.

Epilogue
Advanced materials with novel properties are tumbling out of laboratories across the world at an ever faster pace. It increases the belief that there exists a panacea for every problem confronting us. We have to go out and find it before the problem consumes us.

Readers’ responses may be sent to:
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As solar power is poised to explode in the coming decade, a slew of exciting technologies is emerging out of labs to create the 3rd generation of photovoltaic cells, which are cheaper and easier to fabricate. Underlying these cells are some very complex chemistries shaped by high performance molecules. A striking feature of these thin-film solar cells is their flexibility which allows them to be integrated with various other products

In its report late last year, the International Energy Agency (IEA) declared that photovoltaic power has become cheapest ever in history. Yet, its share in the contribution of global electricity generation is a measly 2%. But solar power is tipped to grow at a frenetic pace this decade and together with wind is likely to knock coal off its pedestal as the most dominant energy resource for electricity production.

Bulk of the photovoltaic cells today are based on silicon. There are two main reasons why we need to look beyond silicon for our photovoltaic power in this decade. Firstly, the efficiency of silicon cells has peaked and is currently very close to the theoretical maximum stipulated by the Shockley- Queisser limit of 30%. Secondly, silicon production and purification are extremely energy intensive. Energy Payback Time (EPBT) is a good metric of sustainability and is defined as the time taken for an energy device to generate the amount of energy equivalent to that required for its manufacture. EPBT of silicon solar cells range between 2 to 3 years.

There are quite a few contenders to replace silicon as the key constituent in the third generation photovoltaic cells. Among them are perovskite solar cell, organic solar cell, dye-sensitised solar cell and quantum dots.

OPV Cells
Organic Solar Cells, commonly known as OPV (Organic Photovoltaic) cells are flexible, lightweight and cheaper and easier to fabricate. Their efficiency is currently low, but still their EPBT is estimated to be of the order of few months, compared to 2-3 years for silicon cells. Earlier this year, researchers claimed to have realised a record efficiency of 18% from an OPV cell. OPV cells are typically fabricated by spraying or coating the active substances on to a rigid or flexible substrate. The active layer of an OPV cell is a mixture of electron donor (a material with low ioinisation energy) and an electron acceptor (a material with high electron affinity).

The earliest electron acceptors in OPV cells were derivatives of Fullerene, the molecule with 60 carbon atoms, that was discovered in 1985 earning a Nobel Prize for its 3 discoverers in 1996. But Fullerene derivatives had serious limitations which prevented the OPV cells from achieving commercially viable efficiencies. This kick-started a major research programme resulting in the development of a class of non-Fullerene acceptors termed as Fused Ring Electron Acceptors (FREA). The benchmark molecule of FREA was synthesised in 2015 and is popularly known as ITIC with the chemical formula of C94H82N4O2S4. It opened the floodgates for a second generation of more efficient non-Fullerene OPV cells. These new class of OPV cells not only have higher efficiency, but better thermal and photochemical stability and improved longevity. There is a huge number of potential electron acceptors in the FREA family that can deliver even better performance. Scientists are now resorting to machine-learning models to sift through the impossibly large number of candidates and come up with high performance molecules to build more efficient OPV cells.

DSSC
Dye-Sensitised Solar Cells (DSSC), also known as Grätzel Cells after the Swiss chemist Michael Grätzel, who did pioneering work on their development, can operate under varied lighting conditions including low and diffused light. They are thus ideal for indoor applications. DSSC use a redox electrolyte sandwiched between two transparent semiconductor electrodes. Dye molecules, absorbed as a monolayer to the back of one of the electrodes, are excited by light and inject electrons into the conduction band of the electrode. The dye is restored to its original state by the redox electrolyte. Electrons moving through the external circuit reduce the oxidised redox agent of the electrolyte at the opposite electrode to complete the cycle. Polypyridine complexes of Ruthenium were the first dyes to be extensively studied in DSSC.

Metallo-porphyrins and metallophthalocyanines are other dyes investigated. Typical efficiency of these cells range between 10% to 14%. The current thrust of research is to develop greener and more sustainable alternatives of the many components that go into the making of the DSSC, in addition to improving the efficiency numbers.

Perovskites
Among the various emerging alternatives to silicon, none is more exciting than Perovskites. A perovskite coated silicon cell could hit the market as early as first quarter of 2022. Perovskite was originally a mineral discovered in the Ural Mountains of Russia in 1839 and named after the Russian mineralogist, Lev Perovski. Today it is synthesised in the laboratories and refers to a class of materials that have the same crystal structure as Calcium Titanate, the original mineral. A typical Perovskite structure is ABX3, where A is an organic cation, B is a metal (usually lead) and X is a halide anion. The most widely studied perovskite today is methylammonium lead iodide. Recently, a start-up spawned by Oxford University, announced a record-breaking efficiency of 29.5% for a solar cell, compared to 15-25% for today’s commercial cells. This feat was accomplished by coating the silicon cells with a thin film of perovskite. The higher efficiency is due to the fact that the coated cells are able to absorb more of solar radiation. The thin perovskite layer absorbs shorter wavelengths and silicon absorbs the longer ones. Perovskite can also be used on a stand-alone basis as a thin-film cell and this has exciting possibilities. A solution of perovskite can be spray coated on virtually any surface turning it into a potential power generator.

Quantum Dots
Quantum Dots (QD) are nanostructures imbued with many interesting properties depending upon the material and shape. One of their myriad applications is deployment in solar cells. A decade ago, the efficiency of a QD solar cell was less than 3%. But continuous improvement over the decade has transformed QD Cell from a laboratory curiosity to commercially exciting prospect. Earlier QD cells were made from toxic heavy metals like cadmium and selenium. Researchers are now engaged in developing environmentally more benign alternatives, in addition to optimising the size and density of the Quantum Dots.

Epilogue
Third Generation photovoltaic cells are characterised by ease of fabrication and low environmental footprint. Their EPBT is significantly lower than that of silicon cells. They are lightweight, flexible and cheaper. The flexibility allows them to be incorporated into many different products. They can be rolled up or folded up. They can be coated on windows or painted on façade of buildings, turning them into power generators.It is even possible to have these solar cells on the apparel we wear. The impressive efficiencies chalked up by these cells today are limited to laboratories and much work needs to be done before they are rolled out commercially. Their robustness in real world situations of temperature and humidity has to be established if they can compete with silicon cells which have a proven longevity of 25 years and more. Underlying the many competing technologies of the third generation photovoltaic cells are complex chemistries, greatly shaped by material properties and characteristics. Much fundamental research at molecular level is underway in laboratories across the globe as start-ups flush with funds from green investors are competing with each other to be the first to enter the huge market. The race to capture sunlight and the market is heating up.

Readers’ responses may be sent to:
k.sahasranaman@gmail.com or
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