The effective surface pH during reactions at the solid–liquid interface

Direct Seawater Electrolysis: From Catalyst Design to Device Applications

Direct seawater electrolysis (DSE) for hydrogen production, using earth-abundant seawater as the feedstock and renewable electricity as the driving source, paves a new opportunity for flexible energy conversion/storage and smooths the volatility of renewable energy. Unfortunately, the complex environments of seawater impose significant challenges on the design of DSE catalysts, and the practical performance of many current DSE catalysts remains unsatisfactory on the device level. However, many studies predominantly concentrate on the development of electrocatalysts for DSE without giving due consideration to the specific devices. To mitigate this gap, the most recent progress (mainly published within the year 2020-2023) of DSE electrocatalysts and devices are systematically evaluated. By discussing key bottlenecks, corresponding mitigation strategies, and various device designs and applications, the tremendous challenges in addressing the trade-off among activity, stability, and selectivity for DSE electrocatalysts by a single shot are emphasized. In addition, the rational design of the DSE electrocatalysts needs to align with the specific device configuration, which is more effective than attempting to comprehensively enhance all catalytic parameters. This work, featuring the first review of this kind to consider rational catalyst design in the framework of DSE devices, will facilitate practical DSE development.

Long-Term Robustness and Failure Mechanisms of Electrochemical Stripping for Wastewater Ammonia Recovery

Nitrogen in wastewater has negative environmental, human health, and economic impacts but can be recovered to reduce the costs and environmental impacts of wastewater treatment and chemical production. To recover ammonia/ammonium (total ammonia nitrogen, TAN) from urine, we operated electrochemical stripping (ECS) for over a month, achieving 83.4 ± 1.5% TAN removal and 73.0 ± 2.9% TAN recovery. With two reactors, we recovered sixteen 500-mL batches (8 L total) of ammonium sulfate (20.9 g/L TAN) approaching commercial fertilizer concentrations (28.4 g/L TAN) and often having >95% purity. While evaluating the operation and maintenance needs, we identified pH, full-cell voltage, product volume, and water flux into the product as informative process monitoring parameters that can be inexpensively and rapidly measured. Characterization of fouled cation exchange and omniphobic membranes informs cleaning and reactor modifications to reduce fouling with organics and calcium/magnesium salts. To evaluate the impact of urine collection and storage on ECS, we conducted experiments with urine at different levels of dilution with flush water, extents of divalent cation precipitation, and degrees of hydrolysis. ECS effectively treated urine under all conditions, but minimizing flush water and ensuring storage until complete hydrolysis would enable energy-efficient TAN recovery. Our experimental results and cost analysis motivate a multifaceted approach to improving ECS's technical and economic viability by extending component lifetimes, decreasing component costs, and reducing energy consumption through material, reactor, and process engineering. In summary, we demonstrated urine treatment as a foothold for electrochemical nutrient recovery from wastewater while supporting the applicability of ECS to seven other wastewaters with widely varying characteristics. Our findings will facilitate the scale-up and deployment of electrochemical nutrient recovery technologies, enabling a circular nitrogen economy that fosters sanitation provision, efficient chemical production, and water resource protection.

Fine Tuning Water States in Hydrogels for High Voltage Aqueous Batteries

Hydrogels are widely used as quasi-solid-state electrolytes in aqueous batteries. However, they are not applicable in high-voltage batteries because the hydrogen evolution reaction cannot be effectively suppressed even when water is incorporated into the polymer network. Herein, by profoundly investigating the states of water molecules in hydrogels, we designed supramolecular hydrogel electrolytes featuring much more nonfreezable bound water and much less free water than that found in conventional hydrogels. Specifically, two strategies are developed to achieve this goal. One strategy is adopting monomers with a variety of hydrophilic groups to enhance the hydrophilicity of polymer chains. The other strategy is incorporating zwitterionic polymers or polymers with counterions as superhydrophilic units. In particular, the nonfreezable bound water content increased from 0.129 in the conventional hydrogel to >0.4 mg mg-1 in the fabricated hydrogels, while the free water content decreased from 1.232 to ∼0.15 mg mg-1. As a result, a wide electrochemical stability window of up to 3.25 V was obtained with the fabricated hydrogels with low concentrations of incorporated salts and enhanced hydrophilic groups or superhydrophilic groups. The ionic conductivities achieved with our developed hydrogel electrolytes were much higher than those in the conventional highly concentrated salt electrolytes, and their cost is also much lower. The designed supramolecular hydrogel electrolytes endowed an aqueous K-ion battery (AKIB) system with a high voltage plateau of 1.9 V and contributed to steady cycling of the AKIB for over 3000 cycles. The developed supramolecular hydrogel electrolytes are also applicable to other batteries, such as aqueous lithium-ion batteries, hybrid sodium-ion batteries, and multivalent-ion aqueous batteries, and can achieve high voltage output.

Microzone-Acidification-Driven Degradation Mechanism of the NiFe-Based Anode in Seawater Electrolysis

The anode stability is critical for efficient and reliable seawater electrolyzers. Herein, a NiFe-based film catalyst was prepared by anodic oxidation to serve as a model electrode, which exhibited a satisfactory oxygen evolution performance in simulated alkaline seawater (1 M KOH + 0.5 M NaCl) with an overpotential of 348 mV at 100 mA cm-2 and a long-term stability of over 100 h. After that, the effects of the current density and bulk pH of the electrolyte on its stability were evaluated. It was found that the electrode stability was sensitive to electrolysis conditions, failing at 20 mA cm-2 in 0.1 M KOH + 0.5 M NaCl but over 500 mA cm-2 in 0.5 M KOH + 0.5 M NaCl. The electrode dissolved, and some precipitates immediately formed at the region very close to the electrode surface during the electrolysis. This can be ascribed to the pH difference between the electrode/electrolyte interface and the bulk electrolyte under anodic polarization. In other words, the microzone acidification accelerates the corrosion of the electrode by Cl-, thus affecting the electrode stability. The operational performances of the electrode under different electrolysis conditions were classified to further analyze the degradation behavior, which resulted in three regions corresponding to the stable oxygen evolution, violent dissolution-precipitation, and complete passivation processes, respectively. Thereby increasing the bulk pH could alleviate the microzone acidification and improve the stability of the anode at high current densities. Overall, this study provides new insights into understanding the degradation mechanism of NiFe-based catalysts and offers electrolyte engineering strategies for the application of anodes.

Recent Advances in Hybrid Seawater Electrolysis for Hydrogen Production

Seawater electrolysis (SWE) is a promising and potentially cost-effective approach to hydrogen production, considering that seawater is vastly abundant and SWE is able to combine with offshore renewables producing green hydrogen. However, SWE has long been suffering from technical challenges including the high energy demand and interference of chlorine chemistry, leading electrolyzers to a low efficiency and short lifespan. In this context, hybrid SWE, operated by replacing the energy-demanding oxygen evolution reaction and interfering chlorine evolution reaction (CER) with a thermodynamically more favorable anodic oxidation reaction (AOR) or by designing innovative electrolyzer cells, has recently emerged as a better alternative, which not only allows SWE to occur in a safe and energy-saving manner without the notorious CER, but also enables co-production of value-added chemicals or elimination of environmental pollutants. This review provides a first account of recent advances in hybrid SWE for hydrogen production. The substitutional AOR of various small molecules or redox mediators, in couple with hydrogen evolution from seawater, is comprehensively summarized. Moreover, how the electrolyzer cell design helps in hybrid SWE is briefly discussed. Last, the current challenges and future outlook about the development of the hybrid SWE technology are outlined.

Measuring Local pKa and pH Using Surface Enhanced Raman Spectroscopy of 4-Mercaptobenzoic Acid

We report spectroscopic measurements of the local pH and pKa at an electrode/electrolyte interface using surface enhanced Raman scattering (SERS) spectroscopy of 4-mercaptobenzoic acid (4-MBA). In acidic and basic solutions, the protonated and deprotonated carboxyl functional groups at the electrode surface exist in the solution as -COOH and -COO-, which have different Raman active vibrational features at around 1697 and 1414 cm-1, respectively. In pH neutral water, as the applied electrochemical potential is varied from negative to positive, the acidic form of the 4-MBA (i.e., -COOH) decreases in Raman intensity and the basic form (i.e., -COO-) increases in Raman intensity. The change in local ion concentration is due to the application of electrochemical potentials and the accumulation of ions near the electrode surface. Under various applied potentials, the ratio of 1697 and 1587 cm-1 (pH-independent) peak areas spans the range between 0.7 and 0, and the ratio of the 1414 and 1587 cm-1 peak areas ranges from 0 to 0.3. By fitting these data to a normalized sigmoid function, we obtain the percentage of surface protonation/deprotonation, which can be related to the pKa and pH of the system. Thus, we can measure the local pKa at the electrode surface using the surface enhanced Raman signal of the 4-MBA.

Ion sieving membrane for direct seawater anti-precipitation hydrogen evolution reaction electrode

In seawater, severe hydroxide-based precipitation on the hydrogen evolution reaction (HER) electrode surface is still a major stumbling block for direct seawater electrolysis. Here, we design a direct seawater HER electrode with excellent anti-precipitation performance based on an Ni(OH)2 nanofiltration membrane in situ grown on nickel foam (NF) at room temperature. The positively charged Ni(OH)2 membrane with nanometer-scale cracks realises an ion sieving function, which apparently hinders the transfer of Mg2+/Ca2+ ions to suppress precipitation, while rapidly transporting OH- and H2O to ensure HER mass transfer. Therefore, the Ni(OH)2-membrane-decorated seawater HER electrode reduces precipitation by about 98.3% and exhibits high activity and stability. Moreover, in the application of a direct seawater electrolyser and magnesium seawater battery, the Ni(OH)2 membrane-decorated electrode also shows low precipitation and high stability. This work highlights a potential strategy to solve HER electrode precipitation in seawater via an ingenious electrode structure design.

Buffering the local pH via single-atomic Mn-N auxiliary sites to boost CO2 electroreduction

Electrocatalytic CO2 reduction driven by renewable energy has become a promising approach to rebalance the carbon cycle. Atomically dispersed transition metals anchored on N-doped carbon supports (M-N-C) have been considered as the most attractive catalysts to catalyze CO2 to CO. However, the sluggish kinetics of M-N-C limits the large-scale application of this type of catalyst. Here, it is found that the introduction of single atomic Mn-N auxiliary sites could effectively buffer the locally generated OH- on the catalytic interface of the single-atomic Ni-N-C sites, thus accelerating proton-coupled electron transfer (PCET) steps to enhance the CO2 electroreduction to CO. The constructed diatomic Ni/Mn-N-C catalysts show a CO faradaic efficiency of 96.6% and partial CO current density of 13.3 mA cm-2 at -0.76 V vs. RHE, outperforming that of monometallic single-atomic Ni-N-C or Mn-N-C counterparts. The results suggest that constructing synergistic catalytic sites to regulate the surface local microenvironment might be an attractive strategy for boosting CO2 electroreduction to value-added products.

Quantifying local pH changes in carbonate electrolyte during copper-catalysed [Formula: see text] electroreduction using in operando [Formula: see text] NMR

The electrochemical carbon dioxide reduction on copper attracted considerable attention within the last decade, since Cu is the only elemental transition metal that catalyses the formation of short-chain hydrocarbons and alcohols. Research in this field is mainly focused on understanding the reaction mechanism in terms of adsorbates and intermediates. Furthermore, dynamic changes in the micro-environment of the catalyst, i.e. local pH and [Formula: see text] concentration values, play an equivalently important role in the selectivity of product formation. In this study, we present an in operando [Formula: see text] nuclear magnetic resonance technique that enables the simultaneous measurement of pH and [Formula: see text] concentration in electrode vicinity during electroreduction. The influence of applied potential and buffer capacity of the electrolyte on the formation of formate is demonstrated. Theoretical considerations are confirmed experimentally and the importance of the interplay between catalyst and electrolyte is emphasised.

Gas Crossover Regulation by Porosity-Controlled Glass Sheet Achieves Pure Hydrogen Production by Buffered Water Electrolysis at Neutral pH

Near-neutral pH water electrolysis driven by renewable electricity can reduce the costs of clean hydrogen generation, but its low efficiency and gas crossover in industrially relevant conditions remain a challenge. Here, it was shown that electrolyte engineering could suppress the crossover of dissolved gases such as O₂ by regulating their diffusion flux. In addition, a hydrophilized mechanically stable glass sheet was found to block the permeation of gas bubbles, further enhancing the purity of evolved gas from water electrolysis. This sheet had a lower resistance than conventional diaphragms such as Zirfon due to its high porosity and small thickness. A saturated K-phosphate solution at pH 7.2 was used as an electrolyte together with the hydrophilized glass sheet as a gas-separator. This led to a near-neutral pH water electrolysis with 100 mA cm^-2 at a total cell voltage of 1.56 V with 99.9 % purity of produced H₂ .

Classical and Nonclassical Nucleation and Growth Mechanisms for Nanoparticle Formation

All solid materials are created via nucleation. In this evolutionary process, nuclei form in solution or at interfaces and expand by monomeric growth, oriented attachment, and phase transformation. Nucleation determines the location and size of nuclei, whereas growth controls the size, shape, and aggregation of newly formed nanoparticles. These physical properties of nanoparticles can determine their functionalities, reactivities, and porosities, as well as their fate and transport. Recent advances in nanoscale analytical technologies allow in situ real-time observations, enabling us to uncover the molecular nature of nuclei and the critical controlling factors for nucleation and growth. Although a single theory cannot yet fully explain such evolving processes, we have started to better understand how both classical and nonclassical theories can work together, and we have begun to recognize the importance of connecting these theories. This review discusses the recent convergence of knowledge about the nucleation and the growth of nanoparticles. Expected final online publication date for the Annual Review of Physical Chemistry, Volume 73 is April 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Electroreductive 5-Hydroxymethylfurfural Dimerization on Carbon Electrodes

The electrochemical conversion of biomass-based compounds to fuels and fuel precursors can aid the defossilization of the transportation sector. Herein, the electrohydrodimerization of 5-hydroxymethylfurfural (HMF) to the fuel precursor 5,5'-bis(hydroxymethyl)hydrofuroin (BHH) was investigated on different carbon electrodes. Compared to boron-doped diamond (BDD) electrodes, on glassy carbon (GC) electrodes a less negative HMF reduction onset potential and a switch in product selectivity from BHH to the electrocatalytic hydrogenation product 2,5-di(hydroxymethyl)furan (DHMF) with increasing overpotential was found. On BDD, the electrohydrodimerization was the dominant process independent of the applied potential. An increase in the initial HMF concentration led to suppression of the competing hydrogen evolution reaction and DHMF formation, resulting in higher BHH faradaic efficiencies. In contrast, BHH selectivity decreased with higher initial HMF concentration, which was attributed to increased electrochemically induced HMF degradation. Finally, it was demonstrated that even a simple graphite foil can function as an active HMF electroreduction catalyst.

Understanding the role of mass transport in tuning the hydrogen evolution kinetics on gold in alkaline media

In this work, we present an in-depth study of the role of mass transport conditions in tuning the hydrogen evolution kinetics on gold by means of rotation rate control. Interestingly, we find that the hydrogen evolution reaction (HER) activity decreases with the increasing rotation rate of the electrode. As we increase the rotation (mass transport) rate, the locally generated hydroxyl ions (2H₂O +2e^- → H₂ + 2OH^-) are transported away from the electrode surface at an accelerated rate. This results in decreasing local pH and, because of the need to satisfy local electroneutrality, decreasing near-surface cation concentration. This decrease in the near-surface cation concentration results in the suppression of HER. This is because the cations near the surface play a central role in stabilizing the transition state for the rate determining Volmer step (*H-OH^δ--cat⁺). Furthermore, we present a detailed analytical model that qualitatively captures the observed mass transport dependence of HER solely based on the principle of electroneutrality. Finally, we also correlate the cation identity dependence of HER on gold (Li⁺ < Na⁺ < K⁺) to the changes in the effective concentration of the cations in the double layer with the changes in their solvation energy.

Improving the performance stability of direct seawater electrolysis: from catalyst design to electrode engineering

Direct seawater electrolysis opens a new opportunity to lower the cost of hydrogen production from current water electrolysis technologies. To facilitate its commercialization, the challenges of long-term performance stability of electrochemical devices need to be first addressed and realized. This minireview summarised the common causes of performance decline during seawater electrolysis, from chemical reactions at the electrode surface to physical damage to the cell. The problems triggered by the impurities in seawater are specifically discussed. Following these issues, we further outlined the ongoing effort of counter-measurements: from electrocatalyst optimization to electrode engineering and cell design. The recent progress in selectivity tuning, surface protection, gas diffusion, and cell configuration is highlighted. In the final remark, we emphasized the need for a consensus on evaluating the stability of seawater electrolysis in the current literature.

Electrification of water interface

The surface charge of a water interface determines many fundamental processes in physical chemistry and interface science, and it has been intensively studied for over a hundred years. We summarize experimental methods to characterize the surface charge densities developed so far: electrokinetics, double-layer force measurements, potentiometric titration, surface-sensitive nonlinear spectroscopy, and surface-sensitive mass spectrometry. Then, we elucidate physical ion adsorption and chemical electrification as examples of electrification mechanisms. In the end, novel effects on surface electrification are discussed in detail. We believe that this clear overview of state of the art in a charged water interface will surely help the fundamental progress of physics and chemistry at interfaces in the future.

Facing Seawater Splitting Challenges by Regeneration with Ni-Mo-Fe Bifunctional Electrocatalyst for Hydrogen and Oxygen Evolution

Hydrogen, produced by water splitting, has been proposed as one of the main green energy vectors of the future if produced from renewable energy sources. However, to substitute fossil fuels, large amounts of pure water are necessary, scarce in many world regions. In this work, we fabricate efficient and earth-abundant electrodes, study the challenges of using real seawater, and propose an electrode regeneration method to face undesired salt deposition. Ni-Mo-Fe trimetallic electrocatalyst is deposited on non-expensive graphitic carbon felts both for hydrogen (HER) and oxygen evolution reactions (OER) in seawater and alkaline seawater. Cl^- pitting and the chlorine oxidation reaction are suppressed on these substrates and alkalinized electrolyte. Precipitations on the electrodes, mainly CaCO₃ , originating from seawater-dissolved components have been studied, and a simple regeneration technique is proposed to rapidly dissolve undesired deposited CaCO₃ in acidified seawater. Under alkaline conditions, Ni-Mo-Fe-based catalyst is found to reconfigure, under cathodic bias, into Ni-Mo-Fe alloy with a cubic crystalline structure and Ni : Fe(OH)₂ redeposits whereas, under anodic bias, it is transformed into a follicular Ni:FeOOH structure. High productivities over 300 mA cm^-2 and voltages down to 1.59 V@10 mA cm^-2 for the overall water splitting reaction have been shown, and electrodes are found stable for over 24 h without decay in alkaline seawater conditions and with energy efficiency higher than 61.5 % which makes seawater splitting promising and economically feasible.

CO2 electrolysis to multicarbon products in strong acid

Carbon dioxide electroreduction (CO₂R) is being actively studied as a promising route to convert carbon emissions to valuable chemicals and fuels. However, the fraction of input CO₂ that is productively reduced has typically been very low, <2% for multicarbon products; the balance reacts with hydroxide to form carbonate in both alkaline and neutral reactors. Acidic electrolytes would overcome this limitation, but hydrogen evolution has hitherto dominated under those conditions. We report that concentrating potassium cations in the vicinity of electrochemically active sites accelerates CO₂ activation to enable efficient CO₂R in acid. We achieve CO₂R on copper at pH <1 with a single-pass CO₂ utilization of 77%, including a conversion efficiency of 50% toward multicarbon products (ethylene, ethanol, and 1-propanol) at a current density of 1.2 amperes per square centimeter and a full-cell voltage of 4.2 volts.

Anticatalytic Strategies to Suppress Water Electrolysis in Aqueous Batteries

Aqueous electrolytes are the leading candidate to meet the surging demand for safe and low-cost storage batteries. Aqueous electrolytes facilitate more sustainable battery technologies due to the attributes of being nonflammable, environmentally benign, and cost effective. Yet, water's narrow electrochemical stability window remains the primary bottleneck for the development of high-energy aqueous batteries with long cycle life and infallible safety. Water's electrolysis leads to either hydrogen evolution reaction (HER) or oxygen evolution reaction (OER), which causes a series of dire consequences, including poor Coulombic efficiency, short device longevity, and safety issues. These are often showstoppers of a new aqueous battery technology besides the low energy density. Prolific progress has been made in the understanding of HER and OER from both catalysis and battery fields. Unfortunately, a systematic review on these advances from a battery chemistry standpoint is lacking. This review provides in-depth discussions on the mechanisms of water electrolysis on electrodes, where we summarize the critical influencing factors applicable for a broad spectrum of aqueous battery systems. Recent progress and existing challenges on suppressing water electrolysis are discussed, and our perspectives on the future development of this field are provided.

Delivering the Full Potential of Oxygen Evolving Electrocatalyst by Conditioning Electrolytes at Near-Neutral pH

This study reports on the impact of identity and compositions of buffer ions on oxygen evolution reaction (OER) performance at a wide range of pH levels using a model IrOx electrocatalyst. Rigorous microkinetic analysis employing kinetic isotope effects, Tafel analysis, and temperature dependence measurement was conducted to establish rate expression isolated from the diffusion contribution of buffer ions and solution resistance. It was found that the OER kinetics was facile with OH- oxidation compared to H2 O, the results of which were highlighted by mitigating over 200 mV overpotential in the presence of buffer to reach 10 mA cm-2 . This improvement was ascribed to the involvement of the kinetics of the local OH- supply by the buffering action. Further digesting the kinetic data at various buffer pKa and the solution bulk pH disclosed a trade-off between the exchange current density and the Tafel slope, indicating that the optimal electrolyte condition can be chosen at a different range of current density. This study provides a quantitative guideline for electrolyte engineering to maximize the intrinsic OER performance that electrocatalyst possesses especially at near-neutral pH.

Reactive electrically conducting membranes for phosphorus recovery from livestock wastewater effluents

We present a novel 'proof-of-concept' electrochemically based membrane filtration process for the recovery of nitrogen and phosphorus from livestock wastewater following an anaerobic digestion step. Reactive electrically conducting membranes are shown to precipitate and separate struvite, an eco-friendly fertilizer from synthetic livestock wastewater, resulting in the production of a solid fertilizer and a high-quality water stream, fit for irrigation. The recovery process is based on electrochemical hydrolysis and control of local pH in proximity to the surface of the membrane, and therefore, does not require chemical additives for pH adjustment. The system was assessed at varying concentrations of nitrogen and phosphorus corresponding to diluted and concentrated livestock wastewater (up to 1000 mg/L of N and P). Experimental results show up to 65% removal of phosphorus and nitrogen in the first 30 min of electrochemical filtration, and the precipitates were analytically confirmed to be struvite. In addition, the recovery process was shown efficient as it resulted in limited membrane fouling and flux reduction. Fouling and precipitation results were explained by a mathematical model describing the concentration of N, P, Mg ions in the presence of an external electric field. Accordingly, precipitation takes place in proximity to the membrane's surface but not directly on it, thus, limiting surface fouling. The electrochemical filtration system does not require chemical additives for pH adjustment, and the cost associated with electrochemical membrane-based struvite recovery was calculated to be $158 per ton of dry struvite, which is about 1.4 times lower in comparison to conventional recovery approaches. Overall, the electrochemical filtration system may be a promising alternative for nutrient recovery from livestock wastewater in terms of operational costs, recovery efficiency, and fouling mitigation.

How Buffers Resist Electrochemical Reaction-Induced pH Shift under a Rotating Disk Electrode Configuration

In mild acidic or alkaline solutions with limited buffer capacity, the pH at the electrode/electrolyte interface (pH^s) may change significantly when the supply of H⁺ (or OH^-) is slower than its consumption or production by the electrode reaction. Buffer pairs are usually applied to resist the change of pH^s during the electrochemical reaction. In this work, by taking H₂X ⇄ 2H⁺ + X + 2e^- under a rotating disk electrode configuration as a model reaction, numerical simulations are carried out to figure out how pH^s changes with the reaction rate in solutions of different bulk pHs (pH^b in the range from 0 to 14) and in the presence of buffer pairs with different pK_a values and concentrations. The quantitative relation of pH^s, pH^b, pK_a, and concentration of buffer pairs as well as of the reaction current density is established. Diagrams of pH^s and ΔpH (ΔpH = pH^s - pH^b) as a function of pH^b and the reaction current density as well as of the j_max-pH^b plots are provided, where j_max is defined as the maximum allowable current density within the acceptable tolerance of deviation of pH^s from that of pH^b (e.g., ΔpH < 0.2). The j-pH^s diagrams allow one to estimate the pH^s and ΔpH without direct measurement. The j_max-pH^b plots may serve as a guideline for choosing buffer pairs with appropriate pK_a and concentration to mitigate the pH^s shift induced by electrode reactions.

Effect of the Morphology of the High-Surface-Area Support on the Performance of the Oxygen-Evolution Reaction for Iridium Nanoparticles

The development of affordable, low-iridium-loading, scalable, active, and stable catalysts for the oxygen-evolution reaction (OER) is a requirement for the commercialization of proton-exchange membrane water electrolyzers (PEMWEs). However, the synthesis of high-performance OER catalysts with minimal use of the rare and expensive element Ir is very challenging and requires the identification of electrically conductive and stable high-surface-area support materials. We developed a synthesis procedure for the production of large quantities of a nanocomposite powder containing titanium oxynitride (TiON x ) and Ir. The catalysts were synthesized with an anodic oxidation process followed by detachment, milling, thermal treatment, and the deposition of Ir nanoparticles. The anodization time was varied to grow three different types of nanotubular structures exhibiting different lengths and wall thicknesses and thus a variety of properties. A comparison of milled samples with different degrees of nanotubular clustering and morphology retention, but with identical chemical compositions and Ir nanoparticle size distributions and dispersions, revealed that the nanotubular support morphology is the determining factor governing the catalyst's OER activity and stability. Our study is supported by various state-of-the-art materials' characterization techniques, like X-ray photoelectron spectroscopy, scanning and transmission electron microscopies, X-ray powder diffraction and absorption spectroscopy, and electrochemical cyclic voltammetry. Anodic oxidation proved to be a very suitable way to produce high-surface-area powder-type catalysts as the produced material greatly outperformed the IrO2 benchmarks as well as the Ir-supported samples on morphologically different TiON x from previous studies. The highest activity was achieved for the sample prepared with 3 h of anodization, which had the most appropriate morphology for the effective removal of oxygen bubbles.

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.

A Comparison of “Bottom-Up” and “Top-Down” Approaches to the Synthesis of Pt/C Electrocatalysts

Three 40 wt % Pt/C electrocatalysts prepared using two different approaches—the polyol process and electrochemical dispersion of platinum under pulse alternating current—and a commercial Pt/C catalyst (Johnson Matthey prod.) were examined via X-ray diffraction (XRD) and transmission electron microscopy (TEM). The stability characteristics of the Pt/C catalysts were studied via long-term cycling, revealing that, for all cycling modes, the best stability was achieved for the Pt/C catalyst with the largest platinum nanoparticle sizes, which was synthesized via electrochemical dispersion of platinum under pulse alternating current. Our results show that the mass and specific electrocatalytic activities of Pt/C catalysts toward ethanol electrooxidation are determined by the value of the electrochemically active Pt surface area in the catalysts.

Direct PtSn Alloy Formation by Pt Electrodeposition on Sn Surface

Electrochemical deposition is a viable approach to develop novel catalyst structures, such as Pt thin films on conductive support materials. Most studies, reaching out to control electrochemical deposition of Pt to monolayer quantities focus on noble metal substrates (e.g., Au). In contrast, conductive oxides, such as antimony doped tin oxide (ATO), are considered as support material for different applications, e.g., as fuel cell catalysts. Herein, we investigate the deposition process of Pt on Sn, used as a model system for the electrochemical deposition of Pt on non-noble metal oxide supports. Doing so, we shade some light on the differences of a metallic Sn surface and surface oxide species in electrochemical deposition processes. With respect to a borate buffer solution, containing K₂PtCl₄ as Pt precursor, we report for the first time that surface oxides have the capability to fully inhibit the electrochemical deposition of Pt. Furthermore, direct alloying of the deposited Pt with the Sn support during the electrodeposition process yielded a catalyst with a high activity for the oxidation of CO.

Ab Initio Simulation of pH-Sensitive Biomarkers in Magnetic Resonance Imaging

An ab initio simulation scheme is introduced as a theoretical prescreening approach to facilitate and enhance the research for pH-sensitive biomarkers. The proton ¹H and carbon ¹³C nuclear magnetic resonance (NMR) chemical shifts of the recently published marker for extracellular pH, [1,5-¹³C₂]zymonic acid (ZA), and the as yet unpublished ( Z)-4-methyl-2-oxopent-3-enedioic acid (OMPD) were calculated with ab initio methods as a function of the pH. The influence of the aqueous solvent was taken into account either by an implicit solvent model or by explicit water molecules, where the latter improved the accuracy of the calculated chemical shifts considerably. The theoretically predicted chemical shifts allowed for a reliable NMR peak assignment. The p K_a value of the first deprotonation of ZA and OMPD was simulated successfully whereas the parametrization of the implicit solvent model does not allow for an accurate description of the second p K_a. The theoretical models reproduce the pH-induced chemical shift changes and the first p K_a with sufficient accuracy to establish the ab initio prescreening approach as a valuable support to guide the experimental search for pH-sensitive biomarkers.