First-principles calculations of molecular adsorption energies at various contact configurations are currently playing a critical role in obtaining atomic-level understandings in surface science, electrochemistry, and other related fields, and they have huge technological implications for the development of advanced energy storage/conversion devices, semiconductor manufacturing processes, drug delivery, environmental remediation, etc.

However, while catalysts, batteries, supercapacitors, chemical vapor deposition (CVD), molecular beam epitaxy (MBE), etc. are operating under non-equilibrium conditions, so far only the equilibrium adsorption energy has remained a well-defined concept and could be accurately estimated from density functional theory (DFT) and related ab initio calculation schemes.

The reason why the non-equilibrium adsorption energy remains an ill-defined concept is mainly because DFT which provides the basis to calculate the adsorption energy is fundamentally an equilibrium theory. In a paper published today, based on the multi-space constrained-search DFT (MS-DFT) framework we have developed at KAIST after our decade-plus effort, we formulated an ab initio theory to calculate the electric enthalpy of a molecular system in contact with a biased electrode.

One of the key properties of MS-DFT is that the non-equilibrium total energy of a nanoscale electrode-channel-electrode junction is defined. This then allows us to define for the first time the non-equilibrium electrode-molecule adsorption energy as the difference between the (non-equilibrium) electric enthalpy of the electrode-molecule-electrode junction and the equilibrium total energy of the molecule.

Applying the newly formulated theory, we show that the bias-dependent rearrangement behavior of water on graphene is very different from that on normal metals. This might prove to be an important hint in resolving controversies associated with the water configurations on electrified graphene and related atomically thin two-dimensional materials.

By creating a new paradigm of non-equilibrium adsorption energy and establishing a corresponding ab initio calculation scheme, our work advances significantly the state-of-the-art of the first-principles computation field and will contribute to the development of next-generation energy and electronic devices.