Originally published April 8, 2015
With recent advances in science, the ability to artificially split water – much like what plants do all the time – might be humanity’s way of finally having access to unlimited clean energy.
Looking back, one of the most important scientific developments of the 20th century was the industrial synthesis of the compound ammonia from nitrogen and hydrogen. Analogous to the oxygen atom in water, H2O, ammonia, or NH3 is the most basic combination of nitrogen with the simplest element, hydrogen. Developed initially for military applications, the revolutionary Haber-Bosch process, compresses nitrogen gas and hydrogen gas under extreme conditions to yield ammonia.
The ability to produce ammonia from its elements facilitated an unprecedented boom in human population. Living things require useable forms of nitrogen for making proteins and DNA, but most of the nitrogen on the planet is in the form of unreactive nitrogen gas, which is the bulk of the air we breathe. Unlike the oxygen, which most organisms use for metabolism, nitrogen gas is almost completely unreactive. Only a handful of microorganisms are capable of “fixing” the N2, by breaking its very strong nitrogen-nitrogen bond so that the individual nitrogen atoms can be used in more complex molecules required by the organism. Before the Haber-Bosch process was developed, the amount of usable nitrogen on the planet was limited by the rate at which these microorganisms could enrich the soil with usable nitrogen to incorporate into the food chain, which by then had essentially plateaued. Once human beings mastered the ability to fix nitrogen on their own, the bottleneck that had been keeping population growth in check was gone, and in only one hundred years, 1.6 billion people grew to over 6 billion.
Such a dramatic and sudden increase in the population has led to fears of overpopulation, and the consequences for the environment of the additional demand for resources and surplus waste. Scientists have long recognized the need for sustainable energy; most current sources, such as burning fossil fuels, are not renewable and release excess waste that is unlikely to be inconsequential on this massive global scale. One of the most promising sources of sustainable energy is sunlight, and provided we can find ways to convert that light energy into solar power we can use efficiently, could sustain all of the planet’s energy needs for generations to come.
Plants, and a handful of microorganisms have been capturing and harvesting solar energy for millions of years through photosynthesis. In this process, energy from sunlight is used to convert carbon dioxide and water into carbohydrates, or sugars. The waste product is molecular oxygen, the essential component of the air we breathe. The energy is stored in the chemical bonds until they are broken during metabolism. In effect, all of the oxygen on Earth originated through this process, and allowed for the evolution of complex life.
Whether it occurs in plants or bacteria, photosynthesis begins with the absorption of light. The light energy is transferred to an electron in the light-absorbing molecule. In this energetically excited state, the electron can be captured by a neighboring acceptor molecule, leaving a positive “hole” behind. The electrons to neutralize these positive holes are harvested from water by the oxygen-evolving complex, or OEC. The OEC contains four manganese ions and one calcium, arranged in a cube-like shape, along with several oxygen atoms and water molecules. The cluster takes two water molecules and combines them into molecular oxygen, O2. The process has been studied thoroughly, but many of the details are not completely understood. We do know that the process occurs in steps, and the positively charged hydrogen ions, H+, are released in sequence, and kept separated from the negatively charged electrons in order to generate a potential across the cell, that it can use for energy.
Both metabolism and combustion of fossil fuels are effectively the reverse of photosynthesis. In the presence of oxygen, molecules that contain carbon and hydrogen are converted into carbon dioxide and water. Despite debates over the existence or causes of climate change, scientists have long anticipated the need for sustainable sources of renewable energy, and have recognized that one of the most promising paths is to learn from nature and develop technology for artificial water splitting and solar energy capture.
Significant progress has been made in recent years, with, for instance, the appearance of so-called “artificial leaves”. Usually, in artificial systems and plants alike, each of the overall reaction steps is accomplished by separate components. One component absorbs light, another generates oxygen and still another for managing protons or hydrogen production, in addition to other components that may be needed to keep the oxygen and hydrogen reactions separated, as both do not usually occur under the same conditions.
One particularly exciting system was recently reported by researchers in China and Israel. These scientists, led by Zhenhui Kang, have discovered that by combining two materials, carbon nitride and carbon nanodots, an indirect route involving hydrogen peroxide allows for a synergistic effect. While each component alone is a capable catalyst, carbon nitride is quickly deactivated by the products it generates. When combined however, each component alleviates shortcomings of the other. This solid material, when placed in water and irradiated with light, generates a 2:1 ratio mixture of H2 and O2 gas. This particular system is difficult to understand, and almost seems too good to be true.
The major drawback is that this mixture of gases is explosive, ideally they would be generated in separate, isolated components. Another weak point is the overall efficiency, when comparing the energy stored in the chemical bonds of O2 and H2 that are formed to the energy in the light needed to drive the process. While the efficiency is far from optimal, the material is made of abundant elements and is extremely stable, and after 200+ days of continuous use, the material continues to steadily split water using sunlight.
Nature often serves as a source of inspiration or guidance, but artificial systems address different needs. While it is amusing to imagine putting water into the gas tank of a solar-powered car, with only water vapor on the tailpipe end, the need for technology for splitting water into O2 and H2 is more extensive than that. The Haber-Bosch process, after all, relies on the reaction of N2 and H2, and the ultimate sources of H2 are typically fossil fuels. Water splitting technology, especially if powered by sunlight, holds enormous promise for changing the way we use resources by closing loops in inefficiency and reducing or even eliminating unnecessary waste. Solar water splitting continues to be an active and exciting field of research, and in the face of impending urgency for green technology, may one day be called the most important scientific development of this century.