Most people find it hard to imagine something they drink daily as an energy source.
Yet, a simple glass of water can be more powerful than a solar panel.
We know very well that we couldn’t survive more than a couple of days without water or oxygen-producing vegetation, but we have a hard time delving into nature’s secrets and simulating its dynamic energy generators.
Without a doubt, we have to bow in front of nature’s perfect processes that keep us healthy and alive. However, we are also curious and inquisitive and love borrowing solutions that work well in nature.
Plants are irreplaceable buffers for our reckless fuel use, directly engaged in a continuous spring cleaning of our climate change aftermath. In an indirect way, plants have been included in artificial photosynthesis research, where scientists try to replicate natural photosynthesis in order to create renewable energy sources with the least by-products.
A Brief Lesson in the Power of Plants
The majority of research on artificial photosynthesis (AP) is based on biomimetics and photoelectrochemistry. Biomimetics refers to applying processes as they occur in natural systems to solve human problems. So, when research labs engage in replicating plant photosynthesis, they actually work in finding human ways of converting solar energy, water, and carbon dioxide into a powerful energy source.
But, of course, there are differences between natural and human-made systems. Plants have leaves to absorb sunlight and roots to absorb water. They have complex molecular structures, which makes them impossible to copy from top to bottom. They have chloroplasts and enzymes that catalyze chemical reactions, which split the water into molecules of hydrogen, oxygen, and electrons.
As the hydrogen and the electrons combine forces with carbon dioxide to feed the plant with glucose, the oxygen is expelled and given to us to breathe. It’s a remarkable symbiosis, one that doesn’t cease to amaze me long after middle school. Plants are remarkable climate change fighters.
However, we are not equipped with the same systems as plants. Therefore, we need to be smart and work with what we have.
While plants produce carbohydrates for food and oxygen as a by-product that’s released into the atmosphere, the main product of artificial photosynthesis is hydrogen fuel that can be stored and transported in conventional ways without disproportional energy degradation.
A number of recent research efforts have been focused on producing alternative fuel sources, such as methanol, ethanol, and ethylene as gas, as well as ethylene glycol, which can be used as a greener method for manufacturing polyethylene fibres—and, thus, potentially solve another eco issue we face: the detrimental effects of plastic production and consumption.
What’s in Typical Artificial Photosynthesis Hardware?
Although the AP methodology is complex and versatile, including techniques such as light-powered CO2 reduction and photoautotrophic microorganisms for making biofuels, the majority of the research is concentrated on light-powered water catalysis or photocatalytic water splitting.
Photocatalytic water splitting uses photoelectrochemical cells to convert water into hydrogen and oxygen. A photoelectrochemical cell consists of two electrodes immersed in an electrolyte and connected in an external circuit. The anode is typically a semiconductor, while the cathode is made of metal.
In theory, to perform AP, only three components are needed: water, catalysts, and artificial light or sunlight (photons). In practice, researchers struggle with finding commercially viable materials for the cells. Additionally, plant-based photosynthesis is based on four metal centres. A single-core lab device cannot exactly copy that rate of natural energy conversion with such capacity. Instead, researchers focus on testing catalyst efficiency—for instance, by comparing phosphonates and carboxylates.
By far, the biggest issue in mimicking artificial photosynthesis is finding adequate and inexpensive catalysts to reproduce the exact chemical processes as they occur in plants. Things have moved forward lately, as research teams have succeeded in partially replacing the super-expensive ruthenium and rhenium with the more affordable cobalt, as well as phosphate and iron compounds.
Nanowires and Bionic Leaves
Harvard researchers managed to produce a bionic leaf that has a number of industrial applications. It can be used as a natural solution for soil self-enrichment in the food industry, as well as a device for artificial photosynthesis. An example of a bionic leaf used for photosynthesis is a device made of nanowires that, together with a thin membrane and conductive silicone-like threads, replicate the natural leaf structure. A recent research paper on industry-friendly nanowire device proposes nanowire arrays made of Ga(In)N (indium gallium nitride) as a cost-effective artificial photosynthesis solution.
Finding Cost-effective Catalysts
The point of commercializing AP is to make it more efficient than natural photosynthesis so that solar energy can be supplied from areas with bountiful resources of water and sunlight in order to produce fuels that lower carbon emissions. As mentioned, catalysts play a major role here. Catalysis based on nanocomposite materials made of copper oxide and tiny carbon dots has been used to convert carbon dioxide into methanol, which could be used directly as a fuel or as a component in complex carbon industrial compounds.
Alternative research on AP catalysts includes manganese, which is already naturally found in plants and has, therefore, a strong biomimetic component. Titanium dioxide is a stable and widely used catalyst in dye-sensitized solar cells, called Graetzel cells. Another option is cobalt oxide, which has similar characteristics and is a popular industrial catalyst that has recently been more used in artificial photosynthesis.
Inkjet Printed Catalysts?
The process of inventing and testing catalysts is by itself cumbersome and costly. It’s no wonder that more than 20 research centres, including teams from the Caltech hub and the Lawrence Berkeley National Lab, have been relentless in developing an inkjet printing process for faster catalyst testing. The process includes testing of millions of micro catalyst variations in order to mimic the complex vegetational infrastructure as close as possible. Testing samples are as small as screen pixels. Such innovation can dramatically increase the frequency of conducting different catalyst tests from just a few a year to millions per day.
What strikes as a common denominator for all AP research is that it involves cross-industry benefits and interdisciplinary research, which could potentially solve many of the problems which are currently destroying the planet. It involves a strong ethical component, which holds us responsible for how we interact with the greatest riches of all: the biosphere. There are claims that put artificial photosynthesis among discoveries as valuable, universal, and intriguing as the Higgs boson particle or dark matter. Though it seems we are far from becoming as energy-efficient as plants, precious discoveries are in the making.
Because of this, engineers and product designers need to become more conscious and work hard to implement that new awareness in practice. That’s the only way common practices can change and evolve.
Featured image courtesy of Gamry Instruments.