THE MELODIES OF MATERIALS

This summer, I flew halfway across the country away from my family to conduct research that could make a real difference. I spent 2 months in Houston to intern at the Ajayan Research Group at Rice University, a global powerhouse laboratory (one of the most famous at Rice!) focused on nanomaterials, advanced composites for energy storage, and much, much more.

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I first want to thank Dr. Pulickel Ajayan, the group leader, and Dr. Robert Vajtai, the research professor, for this rare opportunity. My summer may not have been traditional, but it was incredibly fulfilling. The lessons I learned, the people I met, and the insights I gained are things I wouldn’t trade for the world. Today, instead of just talking about my “day in the life” as a high school researcher in the Ajayan Lab, I want to talk about something more meaningful for you all–the next generation of clean energy. This was precisely what my research focused on, and I cannot wait to continue it as an undergraduate next year.

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Hydrogen is the Next Big Thing

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When you hear hydrogen, many of you may have nostalgia for high school chemistry class. Some of you may also immediately think of water, H2O! The truth is that, with rising concerns of global warming and the burning of fossil fuels, the answer to our problems for clean energy might just be the most common element in our entire universe: hydrogen. When hydrogen gas (H2) is combusted or goes through a fuel cell, its only byproduct is water vapor, leaving behind zero carbon emissions. Hydrogen serves as a promising resource for clean energy that our world craves. 

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Currently, batteries work pretty well for small cars and similar vehicles, but hydrogen is a powerful source that can help move large trucks, cargo ships, or similar vehicles that need to travel long distances. The issue is, as with many things in our world, the production of hydrogen gas is expensive! Clean hydrogen can often cost more to produce than standard fossil fuels. Let’s see how we can work towards combatting this.

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The Chemistry Behind it All

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One source of production of hydrogen gas is called water-splitting. It’s exactly what it sounds like. Take a water molecule (H2O), and split it into its constituent elements, giving you hydrogen and oxygen. It sounds ideal in principle, but it's very unfavorable technically-speaking. It’s literally what we call an unfavorable process in chemistry because we have to supply external energy to split apart a strongly-bonded molecule such as water. In this case, we need to use electricity, or voltage, to drive an electrochemical cell that can perform the water splitting reaction. This is called water electrolysis.

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Voltage drives water splitting by supplying the electrical energy needed to break stable water molecules apart. At the positive anode, this potential pulls electrons away from water through oxidation to release oxygen gas and protons. The external power source then pumps those electrons through the circuit to the negative cathode, where they combine with the protons through reduction, to finally generate hydrogen gas. Chemists have mathematical formulas to calculate the voltage needed to run reactions at the anode and cathode, called the cell potential. However, in practice, we end up needing more energy than predicted by mathematics, because the equations predict values under perfect, or ideal, conditions. This excess is called overpotential, representing the energy lost to heat in overcoming kinetic barriers or electrical resistance. The Nernst Equation, a modified version, takes into account temperature and solution concentration to tell us how much energy is really needed when conditions change.

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Wait, materials can solve this issue?

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You’ve heard me talk about it a lot on this blog already, but I’m a strong believer that many of our world’s engineering problems boil down to materials! Materials are what we can use to mitigate this issue of overpotential, and this is what I synthesized and tested during my time at the Ajayan Research Group.

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Not just any materials though, but nanomaterials specifically (my favorite!). Nanomaterial catalysts can lower overpotentials by providing exceptionally high surface area with an abundance of active catalytic sites, which accelerates reaction kinetics. Their nanoscale structure can be engineered to optimize the Gibbs free energy of intermediate adsorption, making the bond-breaking and bond-forming steps of the reaction require less energy, and less energy = less money spent. Additionally, these materials can improve electrical conductivity and facilitate rapid mass transport of reactants and gas bubbles away from the electrode surface (where the material actually is placed).

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Currently, some of the best catalyst materials include platinum and iridium. This introduces a new issue. These materials are among some of the rarest on earth and have soaring prices that make it unfeasible to deploy hydrogen production through electrocatalysis at a larger scale. Thus, what we focused on at Rice was to make similar-performing alternatives through the combination of more commonly abundant materials in our world.

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The Role of High-Entropy Materials

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Traditional alloys, materials made by combining a primary metal with other elements to improve performance, often only use small amounts of other metals to tweak properties. High-entropy materials (HEMs), however, use five or more main elements in roughly equimolar amounts. When 5+ elements are mixed equally rather than utilizing just one, the higher entropy (randomness and thermodynamic favorability) from all possible ways the elements’ atoms can arrange themselves is maximized. This provides the material with a strong stability that binds elements together rather than separating them. Below are some crucial properties of HEMs:

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1. High-Entropy Effect: The larger mixing entropy allows for higher stability of the material under harsh conditions during the reaction.

2. Lattice Distortion: Given the difference in sizes of the five combined elements, the electronic structure of the surface atoms is altered, which we can tune to ideally bind to reaction intermediates (temporary substances that form during steps in a reaction before turning into the final product).

3. Sluggish Diffusion: Since HEMs consist of 5+ elements, every atom sits in a disordered chemical environment. This causes atoms to be surrounded by a random mix of atoms. Some pull stronger and weaker, so overall, atoms in HEMs move through the lattice slowly, making HEMs more stable and retaining their structure.

4. Cocktail Effect: HEMs have extraordinary traits that could not be assumed by analyzing one constituent element alone.

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We’ve synthesized these HEMs, but how do we test them?

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There are several ways that we quantitatively evaluate HEMs performance under real working conditions. While our lab doesn’t focus on the actual harvesting of the hydrogen gas from the electrochemical reactions, we excel at synthesizing and testing materials across the department. 

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1. Linear Sweep Voltammetry (LSV): In a three-electrode cell, we have a working electrode (the desired material catalyst), reference electrode (constant), and a counter electrode (completes the circuit). In LSV, we sweep the voltage in one direction at a constant rate, starting at a low voltage potential and increase it rapidly while recording the current. From the LSV curve, we can identify the overpotential through mathematical conversion with the Reversible Hydrogen Electrode conversion. A steep LSV slope can also inform fast kinetics, while a slow LSV slope can inform us of sluggish kinetics.

2. Cyclic Voltammetry (CV): In CV, we sweep voltage back and forth repeatedly between two voltage limits instead of just sweeping in one direction like LSV. CV evaluates the catalyst by tracking current fluctuations during these sweeps. This technique reveals critical data regarding the catalyst’s active surface area, multi-metal redox activity, and dynamic surface transformations during reactions. Ultimately, these electrochemical insights allow researchers to assess the operational stability of the material.

3. Electrochemical Surface Area (ECSA): ECSA tells us how much of the surface area of the catalyst is actually active for the reaction. More surface area leads to a larger capacitor, resulting in more current and higher ECSA. We can maximize ECSA by reducing particle size (more surface area in general), using 2D structures where atoms are perfectly exposed along the atomically thin layers, among other methods.

4. Stability Testing: Stability testing allows us to determine the life of the catalyst on a broader scale. By running the CV tests discussed previously rapidly and repeatedly, we put immense stress on the material, simulating real-life conditions and lifespan. Essentially, we start with a LSV curve to record the overpotential, run thousands of CV cycles fast to induce stress, take another LSV and record the new overpotential afterwards, and finally compare the LSV curves to assess degradation or enhancement in performance.

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The CV curve can also inform us on which species are performing the catalysis within the HEM through redox features. At certain voltages during our sweep, the metals in the catalyst can gain or lose electrons (reduction-oxidation). When oxidation states change, a peak occurs on the CV curve.

Another core test is the Tafel slope, which shows us the kinetic mechanism. If you plot the overpotential from the LSV curve vs. the logarithm of the current density, the slope of the line in mV/dec (millivolts per decade) provides the Tafel slope. We typically desire a low Tafel slope because it means the catalyst is active, requiring only a small voltage to increase reaction rate. The value of the slope can inform us whether the Volmer, Heyrovsky, or Tafel steps (don’t worry about understanding these yet) are the rate-limiting step in the reaction kinetics.

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What do we do if our HEM is poor?

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There are many different things materials scientists can do when an HEM doesn’t perform ideally. Again, the goal is to get overpotentials, tafel slopes, and other performance metrics of HEMs as close as possible to ideal catalysts like Platinum or Iridium. Some are below:

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1. Compositional Fine-Tuning: We can alter the elemental ratios or substitute specific elements to alter the surface electronic states and better optimize intermediate binding energies.

2. Defect & Nanostructure Engineering: Introducing surface vacancies or lattice distortions can increase the density of exposed, highly active catalytic sites.

3. Conductive Supports: We can integrate the HEM nanoparticles with highly conductive substrates (like graphene) to overcome poor charge-transfer kinetics and maximize surface area usage.

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Takeaways

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As I leave Rice to return to school for my senior year, I am excited to continue contributing to ongoing research at the lab. For one, I am helping to develop a comprehensive dataset, compiling different HEMs and their performance metrics from reams of published literature, which I am working to combine with machine learning predictive analytics. This system might help mitigate the testing time to come up with unique HEM combinations by predicting the performance of existing HEMs or even predicting new HEM compositions based on input parameters.

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Hydrogen fuel is extremely promising as a gateway to a new future in clean energy. Its drawbacks limit its scalability right now, but I cannot imagine how far we can develop this new technology in the near future. Again, the answer may not lie just in advances in chemical, biological, and physical systems, but rather, in the smallest of materials. This summer was amazing, and I hope to continue this kind of discovery through materials the rest of my senior year in high school and early-on in my undergraduate years. But until dhin . . . stay upbeat, and stay tuned.

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Sources

1. https://www.technologyreview.com/2026/08/20/1142512/geologic-hydrogen-hunt/

2. https://energycapitalhtx.com/future-of-green-hydrogen-production

3. https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202512191

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