Imagine driving a car for hours, travelling along the beautiful coasts of Sweden, and what is coming out of the exhaust pipe are not toxic and polluting gases but only water. How fun and relieving would it be? This scenario may sound like imagination but, in fact, it is not. It happens now as you read this, with the rise of a hydrogen energy system. Hydrogen, the smallest and most abundant element in the universe, is wonderful. It carries three times more energy than gasoline of the same mass and possesses high energy conversion efficiency. Even better, hydrogen can be produced from water and therefore enables a truly sustainable energy cycle, free from polluting and greenhouse gas emissions.
With these fantastic facts, one may then wonder: “Why don’t we see it being used more often now?” Apparently, there are a lot of reasons. To fully integrate the concept into our lives, there are still a lot of technological advances that have to be accomplished in multiple sectors, from the creation of hydrogen gas, its storage, to its distribution and usage. Most importantly, in each of these sectors, the process has to be done safely as hydrogen is actually a quite dangerous gas: it is flammable! Just a mere 4% of hydrogen gas in a room is enough to start a fire if a tiny spark ignites the hydrogen-air mixture. In fact, the trauma of the Hindenburg disaster in 1937, where a zeppelin balloon filled with hydrogen caught fire and crashed, killing 35 people, hinders to some extent the wide use of hydrogen fuel still today. To move forward, it is thus imperative to have a hydrogen detection system to signal a leak before a flammable hydrogen concentration is reached. As a guideline for this development, many institutions and stakeholders of a hydrogen energy system have released demanding performance standards for hydrogen sensors. In general, they have to be fast, sensitive and stable for many years of application.
Current hydrogen sensors mostly use palladium (often mixed with gold to produce “white gold”) since it can absorb a lot of hydrogen spontaneously, accompanied by the change in its characteristics. However, in this respect palladium hydrogen sensors are commonly characterized by slow response with low sensitivity. Even worse, palladium’s ability to absorb hydrogen disappears when it is exposed to some pollutant gases such as carbon monoxide and nitrogen dioxide, which we find readily in the (urban) air. To fulfill the requirements above, finding new type of sensors with novel materials is thus necessary.
In this thesis I present a hydrogen-sensing platform using light and extremely small particles of alloys of palladium and other metals. To do so, I would like to bring the reader into a world thousand times smaller than a strand of hair, a scale where materials may behave significantly different than how they are commonly known to. For example, in this size regime, palladium particles exhibit a color that depends on their shape and size. I will discuss how these small palladium alloy particles are produced and how they absorb hydrogen, which later modifies their color. Finally I show how we can make use of this phenomenon to realize an optical hydrogen sensor by measuring the color change induced by hydrogen. By combining palladium with other metals such as gold and copper, as well as covering it with different types of polymers (one is the polymer we use in our non-stick frying pans!), I created a fast, sensitive and stable hydrogen sensor. As the key highlight of this thesis, I succeeded to detect as low as 1 hydrogen molecule per 1000 gas molecules in less than 1 second, that is, the fastest hydrogen sensor in the world! And best of all, it still works even under exposure to carbon monoxide and nitrogen dioxide. So sit back, get a drink and enjoy the reading!