A catalyst’s effectiveness is indicated by its ‘overpotential’ and ‘turnover frequency’. Lower overpotential and higher frequency suggest a productive and energy-efficient catalyst
| Photo Credit:
iStockphoto

Toyota Chairman Akio Toyoda is reported to have recently said that one electric vehicle (EV) causes as much pollution as 3 hybrid vehicles put together. Given the catastrophic impact of climate change that is already upon us, green fuels such as hydrogen are likely the way forward. 

But if the process of splitting hydrogen from, say, water, releases carbon into the atmosphere, then it won’t contribute to ‘clean’ energy. And, current methods of producing truly green hydrogen are expensive. 

A research team from Ashoka University focused on electrocatalytic hydrogen production – i.e., the process of producing hydrogen by using catalysts to help split water (H₂O) molecules into hydrogen and oxygen. 

The performance of electrocatalysts is crucial in making the process energy-efficient. Electrocatalysts are materials that speed up chemical reactions and the more efficient ones need less energy for these reactions. 

Currently, the most effective electrocatalysts are based on platinum, which is expensive and not easily available. This has driven researchers to find more affordable alternatives, particularly those derived from the likes of nickel, iron, cobalt, copper, and zinc. 

Says Assistant Professor Dr Munmun Ghosh of the research team, “We designed a ligand, combined it with different metals to see how each such complex behaved.” 

What is a ligand? 

In chemistry, a ligand is a molecule that bonds with a metal ion to form a complex. In biochemistry, a good example of a ligand is in haemoglobin. Oxygen that we breathe in combines with iron and this complex helps transport oxygen – which acts as the ligand – from the lungs to other parts of the body. 

And this, says Dr Ghosh, is where her background in biomimetic research helped. “I see what nature has and from there I try to mimic those for scientific purposes.” 

In the experiment with metal ligands, the research team found that the nickel-ligand combination showed the most promise in efficiently producing hydrogen gas. Other complexes tested include copper, cobalt, zinc and iron. 

Overpotential and turnover frequency 

A catalyst’s effectiveness is indicated by what scientists call its ‘overpotential’ and ‘turnover frequency’. Overpotential is the additional electrical energy needed beyond the minimum to drive the reaction. A lower overpotential means the catalyst is more energy-efficient. 

Turnover frequency measures how quickly the catalyst can produce hydrogen. A higher frequency signals a more productive catalyst. 

Why ligand at all? 

Why not use just the metal? Why use ligands to bond with the metal? Says Dr Ghosh, “My overpotential will be much higher with just the metal. A ligand helps decrease the overpotential and make the process more efficient.” 

The team showed that the nickel-ligand complex’s catalytic rates were “comparable to some of the best; and in terms of overpotential, it performs better than certain previously reported nickel and iron systems”. 

Put simply, nickel was the best among the metals tested, in efficiency terms. However, cobalt is best in overpotential which was at 200 millivolt, which, Dr Ghosh says is comparable to good catalysts. Platinum has an overpotential measure of 30 millivolt. Without a ligand, a metal-only catalyst may touch overpotential levels of up to 1 volt, which is too high in such processes, points out Dr Ghosh. 

Assistant Professor Dr Deepak Asthana, a member of the research team, says that experimental evidence showed that the ligand, rather than the metal, was the key ‘participant’ in the hydrogen production process. The metal itself could have done the job but it’s like doing a lot of hard work single-handedly, which results in high overpotential (or requiring a lot of energy) to accomplish the task. The metal-ligand combine helps bring down the energy requirement. That’s why, says Dr Ghosh, “Ligand design is very important.” 

Now, why electrochemistry, is a natural question to ask, says Dr Ghosh. “Electrochemistry helps achieve high atom economy or atom efficiency – meaning all or most of the input material is used in the end product, resulting in minimal waste,” she points out. 

“But if I don’t have ligand, it’s only metal. Just look at that. How much pressure on that metal that I have to do everything. However, if I have ligand then it can help always,” she adds.  

Designing the ligand is key, she says. That means several things. But the simplest way to describe it is that the design helps researchers achieve the end-result. “You can design the ligand in such a way that it releases electrons to the metal, or such that it accepts electrons from the metal. It depends on what you wish to achieve. The ligand plays a role in keeping the metal component stable. Without stability, the catalyst doesn’t survive,” she says. 

The study is only the first step, points out Dr Asthana. “What we have learnt from the study is that narrowing down on nickel-ligand combination is a good first step. It’s not the best solution yet. But it’s competitive. The study shows promise that we can work on the design, modify it and potentially come up with a system that is as efficient and stable as costly, metal-based systems.” 

Published on August 25, 2025