Switching of Kinetically Relevant Reactants for the Aqueous Cathodic Process Determined by Mass‐transport Coupled with Protolysis

Understanding the Activation Mechanism of RhCrOx Cocatalysts for Hydrogen Evolution with Nanoparticulate Electrodes

Mixed oxides of Rh-Cr (RhCrOx), containing Rh3+ and Cr3+ cations, are commonly used as cocatalysts for the hydrogen evolution reaction (HER) on particulate photocatalysts. The precise physicochemical mechanisms of the HER at the catalytic sites of these oxides are not well understood. In this study, model cocatalyst electrodes, composed of nanoparticulate RhCrOx, were fabricated to investigate the physicochemical mechanisms of the HER. Electroanalytical and X-ray photoelectron spectroscopic measurements revealed that nanoparticulate RhCrOx produces reduced Rh (Rh0) species by maintaining an electrode potential more negative than 0.03 V versus the reversible hydrogen electrode (VRHE). This results in significant enhancement of the HER activity. The catalytic activity for the HER stems from the reduced Rh species, and the inclusion of Cr3+ (CrOx) aided in the electron transfer process at the solid/liquid interface, resulting in a higher current density during the HER. To achieve a solar-to-hydrogen efficiency of over 3%, the conduction band minimum of the particulate photocatalyst should be positioned more negatively than -0.10 VRHE. Moreover, the formation of electron trap states at potentials more positive than 0.03 VRHE should be avoided. This study highlights the importance of understanding the catalytic sites on metal oxide cocatalysts. Moreover, it offers a design strategy for enhancing the efficiency of photocatalytic water splitting.

Mechanistic Insights into Enhanced Hydrogen Evolution of CrOx /Rh Nanoparticles for Photocatalytic Water Splitting

The hydrogen evolution reaction (HER) of Rh nanoparticles (RhNP) coated with an ultrathin layer of Cr-oxides (CrO_x ) was investigated as a model electrode for the Cr₂ O₃ /Rh-metal core-shell-type cocatalyst system for photocatalytic water splitting. The CrO_x layer was electrodeposited over RhNP on a transparent conductive fluorine-doped tin oxide (FTO) substrate. The CrO_x layer on RhNP facilitates the electron transfer process at the CrO_x /RhNP interface, leading to the increased current density for the HER. Impedance spectroscopic analysis revealed that the CrO_x layer transferred protons via the hopping mechanism to the RhNP surface for HER. In addition, CrO_x restricted electron transfer from the FTO to the electrolyte and/or RhNP and suppressed the backward reaction by limiting oxygen migration. This study clarifies the crucial role of the ultrathin CrO_x layer on nanoparticulate cocatalysts and provides a cocatalyst design strategy for realizing efficient photocatalytic water splitting.

Improving the intrinsic activity of electrocatalysts for sustainable energy conversion: where are we and where can we go?

As we are in the midst of a climate crisis, there is an urgent need to transition to the sustainable production of fuels and chemicals. A promising strategy towards this transition is to use renewable energy for the electrochemical conversion of abundant molecules present in the earth's atmosphere such as H₂O, O₂, N₂ and CO₂, to synthetic fuels and chemicals. A cornerstone to this strategy is the development of earth abundant electrocatalysts with high intrinsic activity towards the desired products. In this perspective, we discuss the importance and challenges involved in the estimation of intrinsic activity both from the experimental and theoretical front. Through a thorough analysis of published data, we find that only modest improvements in intrinsic activity of electrocatalysts have been achieved in the past two decades which necessitates the need for a paradigm shift in electrocatalyst design. To this end, we highlight opportunities offered by tuning three components of the electrochemical environment: cations, buffering anions and the electrolyte pH. These components can significantly alter catalytic activity as demonstrated using several examples, and bring us a step closer towards complete system level optimization of electrochemical routes to sustainable energy conversion.

Efficient Hydrogen Delivery for Microbial Electrosynthesis via 3D-Printed Cathodes

The efficient delivery of electrochemically in situ produced H₂ can be a key advantage of microbial electrosynthesis over traditional gas fermentation. However, the technical details of how to supply large amounts of electric current per volume in a biocompatible manner remain unresolved. Here, we explored for the first time the flexibility of complex 3D-printed custom electrodes to fine tune H₂ delivery during microbial electrosynthesis. Using a model system for H₂-mediated electromethanogenesis comprised of 3D fabricated carbon aerogel cathodes plated with nickel-molybdenum and Methanococcus maripaludis, we showed that novel 3D-printed cathodes facilitated sustained and efficient electromethanogenesis from electricity and CO₂ at an unprecedented volumetric production rate of 2.2 L_CH4 /L_catholyte/day and at a coulombic efficiency of 99%. Importantly, our experiments revealed that the efficiency of this process strongly depends on the current density. At identical total current supplied, larger surface area cathodes enabled higher methane production and minimized escape of H₂. Specifically, low current density (<1 mA/cm²) enabled by high surface area cathodes was found to be critical for fast start-up times of the microbial culture, stable steady state performance, and high coulombic efficiencies. Our data demonstrate that 3D-printing of electrodes presents a promising design tool to mitigate effects of bubble formation and local pH gradients within the boundary layer and, thus, resolve key critical limitations for in situ electron delivery in microbial electrosynthesis.

Increasing the rate of the hydrogen evolution reaction in neutral water with protic buffer electrolytes

Electrocatalytic generation of H2 is challenging in neutral pH water, where high catalytic currents for the hydrogen evolution reaction (HER) are particularly sensitive to the proton source and solution characteristics. A tris(hydroxymethyl)aminomethane (TRIS) solution at pH 7 with a [2Fe-2S]-metallopolymer electrocatalyst gave catalytic current densities around two orders of magnitude greater than either a more conventional sodium phosphate solution or a potassium chloride (KCl) electrolyte solution. For a planar polycrystalline Pt disk electrode, a TRIS solution at pH 7 increased the catalytic current densities for H2 generation by 50 mA/cm2 at current densities over 100 mA/cm2 compared to a sodium phosphate solution. As a special feature of this study, TRIS is acting not only as the primary source of protons and the buffer of the pH, but the protonated TRIS ([TRIS-H]+) is also the sole cation of the electrolyte. A species that is simultaneously the proton source, buffer, and sole electrolyte is termed a protic buffer electrolyte (PBE). The structure-activity relationships of the TRIS PBE that increase the HER rate of the metallopolymer and platinum catalysts are discussed. These results suggest that appropriately designed PBEs can improve HER rates of any homogeneous or heterogeneous electrocatalyst system. General guidelines for selecting a PBE to improve the catalytic current density of HER systems are offered.

Water Electrolysis in Saturated Phosphate Buffer at Neutral pH

Hydrogen production from renewable energy and ubiquitous water has a potential to achieve sustainability, although current water electrolyzers cannot compete economically with the fossil fuel-based technology. Here, we evaluate water electrolysis at pH 7 that is milder than acidic and alkaline pH counterparts and may overcome this issue. The physicochemical properties of concentrated buffer electrolytes were assessed at various temperatures and molalities for quantitative determination of losses associated with mass-transport during the water electrolysis. Subsequently, in saturated K-phosphate solutions at 80 °C and 100 °C that were found to be optimal to minimize the losses originating from mass-transport at the neutral pH, the water electrolysis performance over model electrodes of IrOx and Pt as an anode and a cathode, respectively, was reasonably comparable with those of the extreme pH. Remarkably, this concentrated buffer solution also achieved enhanced stability, adding another merit of this electrolyte for water electrolysis.