Chemical physics of electrochemical energy materials Free

Developing theoretical tools is of great importance in probing the electrochemistry of energy materials. Domínguez-Flores and Melander proposed approximating constant potential density functional theory (DFT) with canonical DFT and electrostatic corrections, which demonstrated great success in modeling CO₂ adsorption on a single-atom catalyst.¹ Pasquale et al. probed the graphene–electrolyte double layer by combining constant potential molecular dynamics (MD) simulations and quantum mechanics/MD simulations, from which a thorough analysis of electrolyte properties at charged interfaces can be obtained.² Le and co-workers contributed an in-depth molecular understanding of the Helmholtz capacitance of both Cu(100) and graphene electrodes.³ According to a comprehensive analysis of the usual solvent capacitance, water chemisorption-induced capacitance, and Pauling repulsion caused gap capacitance, the lower Helmholtz capacitance of graphene compared with Cu(100) is ascribed to two intrinsic factors, based on which valuable strategies were proposed to increase the capacitance of graphene electrodes. Koper and co-workers proposed a computational description of surface hydride phases on Pt(111) electrodes using thermodynamic methods and DFT, which affords fresh insights into the operando surface state during low-potential reduction reactions on Pt(111).⁴ Beyond interfacial properties, bulk properties are also of wide interest and high impact on the electrochemical process. Shao and Zhang obtained accurate Bruce–Vincent transference numbers from MD simulations, taking poly(ethyleneoxide)–lithium (Li) bis(trifluoromethane)sulfonimide system as an example.⁵ 

Besides theoretical tools, experimental characterization tools are of equal importance to unveil the underlying mechanism of energy materials and chemical processes. Electrochemical impedance spectroscopy (EIS) is one of the widely applied experimental characterizations to probe the electrochemical response in energy devices. Zhang and co-workers established the standardized criterion of EIS, which has demonstrated reliable impedance analysis of Li ion batteries (LIBs) and highlights the importance of using three-electrode EIS measurements.⁶ Huang and co-workers proposed the fingerprints of couplings between interfacial electron transfer reactions and ion transportation in the electrolyte solution.⁷ Beyond EIS, Kattwinkel and Magnussen studied the influence of coverage on adsorbate diffusion measurements at electrode surfaces by in situ linear optical diffraction.⁸ The decay of the signal exhibits two-time scales at low and medium sulfur coverage while further ultra-slow decay process or even a complete termination of the decay is observed at high coverage. Cuesta and co-workers probed the kinetics of formic acid dehydration on Pt electrodes by time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode, which possesses high sensitivity and is independent of the transport process.⁹ As a result, HCOO_ad was confirmed as the crucial intermediate in the electro-oxidation of HCOOH on Pt electrodes. Schuhmann and co-workers adopted scanning electrochemical cell microscopy to measure the reorganization energy in a polybromide ionic liquid, which can achieve high-throughput electrochemical measurements using a single electrode with high spatial resolution.¹⁰ 

The emerging functional materials can be further designed with the assistance of characterization and theoretical tools. Rafiq et al. reported a room-temperature synthesis of two-dimensional Ti₃C₂T_x nano-sheets by organic base treatment, which demonstrates a promising route to enhance delamination of MXene materials with a low cost at room-temperature.¹¹ Wu and co-workers increased the interlayer distance of layered double hydroxides by replacing the interlayer nitrate ions with 1,4-benzenedicarboxylic anions, which enhances the rate performance for storing large cations (Na⁺, Mg²⁺, and Zn²⁺) and remains the performance for storing small-radius Li⁺.¹² Liu and co-workers unveiled the acidity and metal complexation of the edge surface of birnessite-type MnO₂, which affords atomic-scale insight into the acid–base chemistry of birnessite and guides the rational design of such energy materials.¹³ The structural evolution of materials at working conditions is also of great importance for designing practical materials. Wei and co-workers explored the surface states of transition metal X-ides (e.g., oxides, nitrides, carbides, and hydroxides) under electrocatalytic conditions, which highlights the indispensable role of applying theoretical calculations (e.g., surface Pourbaix diagram analysis), in situ/operando, and post-reaction experiments to analyze the surface state when probing the underlying working mechanism of functional materials.¹⁴ 

Energy materials have been widely applied to energy storage and conversion devices such as batteries, fuel cells, electrolyzers, and supercapacitors. Among them, Li ion battery is widely applied to electronic devices, electric vehicles, and smart grids. Peng and co-workers systematically explored the origin of oxygen-redox and transition metals dissolution in Ni-rich Li_xNi_0.8Co_0.1Mn_0.1O₂ cathode.¹⁵ The new insights from the vacancy formations, number of unpaired spins, and net charges afford an in-depth understanding of stabilizing the Ni-rich Li_xNi_0.8Co_0.1Mn_0.1O₂ cathode. Zhu and co-workers revealed the correlated factors for Li ion migration in ionic conductors with the face-centered cubic (fcc) anion sublattice, which helps to design solid-state electrolytes with low activation energy.¹⁶ Inspired by the superfast water transport phenomenon in carbon nanotubes (CNTs), Chen and co-workers explored the Li ion transport in both armchair- and zigzag-type CNTs. CNTs with a diameter of 5.5 Å are predicted to deliver an ultralow Li-ion diffusion barrier of about 10 meV due to the ultrahigh chemical environment similarity of a Li ion during its diffusion.¹⁷ Carvalho et al. investigated the lithiation limitation of high-capacity organic battery anodes, i.e., dicarboxylate-based materials, using atomic charge derivative analysis.¹⁸ The charge derivatives can be adopted as a fingerprint to understand the energy storage mechanism and determine the specific capacity of these materials. Besides electrode and electrolyte materials, the production layer between them also has an obvious influence on battery performance. Especially, Cheng and co-workers unveiled the preferential decomposition mechanism of major salt anions in a dual-salt electrolyte to facilitate the formation of organic–inorganic composite solid electrolyte interphase (SEI), which is supposed to stabilize working Li metal anodes.¹⁹ Accordingly, advanced dual-salt electrolytes can be rationally designed and fabricated. Beyond battery chemistry and materials, predicting the working conditions of Li ion batteries is of special interest for their practical applications, especially for smart grids. Liu and co-workers reported a whale optimization algorithm and multi-kernel relevance vector machine to estimate the state of charge (SOC) of Li ion batteries.²⁰ With an overall consideration of charge and discharge voltage, current, and temperature, the SOC can be accurately predicted.

As an important alternative to Li batteries, zinc- and sodium-based batteries with low costs hold great potential for the use of renewable energy resources. Wang and co-workers designed a carbon fiber material to enhance the kinetics of oxygen evolution reaction (OER).²¹ Consequently, the zinc–air batteries with a high peak power density of 147.1 mW cm⁻² were achieved. Ling and co-workers demonstrated Zn single-atom catalysts as effective oxygen reduction reaction catalysts with high anti-poisoning properties for stable seawater batteries.²² The catalysts exhibited a superior advantage in terms of half-wave potential (0.87 V), limiting current (6 mA cm⁻²), and Tafel slope (69.5 mV dec⁻¹). Balbuena and co-workers focused on the formation of SEI in sodium–sulfur batteries, which is usually adopted for electric grids.²³ NaF was confirmed as a critical SEI component to reduce the extent of interfacial charge transfer and decelerate the Na₂S₈ decomposition reaction.

Besides battery materials, electrocatalytic materials have also been widely used in energy applications. Liu and co-workers summarized the recent progress on the electrochemical hydrogen evolution on Pt-based catalysts from a theoretical perspective.²⁴ Wang and co-workers investigated the OER and electrochemical ozone production on heteroatom-doped carbon materials by grand canonical DFT, which unravels the potential effect on the O₂/O selective formation.²⁵ Uosaki and co-workers discovered the size-dependent electrocatalytic activities of h-BN for oxygen reduction reaction (ORR) to water.²⁶ As a result, the smaller the size, the smaller the overpotential for ORR. Besides the active site in the catalyst, the supporting materials and environment also regulate the ORR reactivity a lot. Particularly, Chen and co-workers boosted the ORR on the Pt catalyst with IrO₂ as support in acid under high-temperature conditions, which achieved even lower apparent activation energy than commercial Pt/C.²⁷ 

Similar to OER and ORR, CO₂ reduction has been paid a lot of attention recently, especially under the global consensus of achieving carbon neutrality. Feliu and co-workers investigated CO₂ reduction on bismuth- and copper-decorated platinum stepped surfaces.²⁸ The formation of CO can be regulated due to the varied local charges on the terrace and step sites. Yang and co-workers found that the iodine adsorption on Cu surfaces facilitates electrocatalytic CO₂ reduction.²⁹ The introduction of halogen anions can strengthen the Cu–CO bond and facilitate the hydrogenation process, enhancing the production of CH₄. Beyond the catalyst itself, the electrolyte solvation environment also has an obvious influence on CO₂ reduction. Zhang et al. probed such electrolyte effects through theoretical simulations.³⁰ According to the characteristic vibration frequency of intermediates in different electrolytes, H₂O was confirmed as the component of HCO₃⁻, which promotes CO₂ adsorption and reduction.

The cooperation of photocatalysis and electrocatalysis, e.g., photoelectrochemical water oxidation, is another promising topic in this area. Chen and co-workers modified BiVO₄ with polypyrrole to enhance the photoelectrochemical water splitting, which achieved an incident photon-to-current conversion efficiency of 63% at 430 nm.³¹ Huang and co-workers described the underlying mechanism of Au(III) (hydro-)oxides in promoting plasmon-mediated photoelectrochemical water oxidation using in situ microphotoelectrochemical surface-enhanced Raman spectroscopy and ambient-pressure x-ray photoelectron spectroscopy.³² The spectroscopic results verified that the kinetically promoted water-oxidation on the surface Au(III) (hydro-)oxides was induced by the restricted growth of surface (hydro-)oxides.

The above discussion can hardly cover all the applications of emerging energy materials. It is reasonable to observe other potential promising applications. For instance, the supercapacitor is another kind of important energy device beyond batteries. Sundararaman and Schwarz explored the solvent effects in determining the sign of the charges of maximum entropy and capacitance at silver electrodes.³³ Han and co-workers focused on the hydration of a Nafion membrane using ambient-pressure x-ray photoelectron spectroscopy and ab initio MD simulations.³⁴ A direct connection between the electrical properties and the microscopic mechanism was consequently established.

These articles afford a selective sampling of the growing field of chemical physics of electrochemical energy materials. The fabrication of advanced energy materials and their applications to energy storage and conversion devices, and the development of experimental and theoretical tools to probe underlying chemistry and material science are particularly focused on. The collection is anticipated to provide a complete guidance to the future direction of this field. Here we sincerely appreciate the editorial team of the Journal of Chemical Physics for providing the opportunity to publish this special topic, and we would like to express our deepest gratitude to all authors, reviewers, editors, and readers for their excellent contributions to this special topic.

The authors have no conflicts to disclose.