Water electrolysis and energy harvesting with zero-dimensional ion-sensitive field-effect transistors

Miniaturized Control of Acidity in Multiplexed Microreactors

The control of acidity drives the assembly of biopolymers that are essential for a wide range of applications. Its miniaturization can increase the speed and the possibilities of combinatorial throughput for their manipulation, similar to the way that the miniaturization of transistors allows logical operations in microelectronics with a high throughput. Here, we present a device containing multiplexed microreactors, each one enabling independent electrochemical control of acidity in ∼2.5 nL volumes, with a large acidity range from pH 3 to 7 and an accuracy of at least 0.4 pH units. The attained pH within each microreactor (with footprints of ∼0.3 mm² for each spot) was kept constant for long retention times (∼10 min) and over repeated cycles of >100. The acidity is driven by redox proton exchange reactions, which can be driven at different rates influencing the efficiency of the device in order to achieve more charge exchange (larger acidity range) or better reversibility. The achieved performance in acidity control, miniaturization, and the possibility to multiplex paves the way for the control of combinatorial chemistry through pH- and acidity-controlled reactions.

High-Rate Alkaline Water Electrolysis at Industrially Relevant Conditions Enabled by Superaerophobic Electrode Assembly

Alkaline water electrolysis (AWE) is among the most developed technologies for green hydrogen generation. Despite the tremendous achievements in boosting the catalytic activity of the electrode, the operating current density of modern water electrolyzers is yet much lower than the emerging approaches such as the proton-exchange membrane water electrolysis (PEMWE). One of the dominant hindering factors is the high overpotentials induced by the gas bubbles. Herein, the bubble dynamics via creating the superaerophobic electrode assembly is optimized. The patterned Co-Ni phosphide/spinel oxide heterostructure shows complete wetting of water droplet with fast spreading time (≈300 ms) whereas complete underwater bubble repelling with 180° contact angle is achieved. Besides, the current collector/electrode interface is also modified by coating with aerophobic hydroxide on Ti current collector. Thus, in the zero-gap water electrolyzer test, a current density of 3.5 A cm^-2 is obtained at 2.25 V and 85 °C in 6 m KOH, which is comparable with the state-of-the-art PEMWE using Pt-group metal catalyst. No major performance degradation or materials deterioration is observed after 330 h test. This approach reveals the importance of bubble management in modern AWE, offering a promising solution toward high-rate water electrolysis.

Bubble energy generator

Bubbles have been extensively explored as energy carriers ranging from boiling heat transfer and targeted cancer diagnosis. Yet, despite notable progress, the kinetic energy inherent in small bubbles remains difficult to harvest. Here, we develop a transistor-inspired bubble energy generator for directly and efficiently harvesting energy from small bubbles. The key points lie in designing dielectric surface with high-density electric charges and tailored surface wettability as well as transistor-inspired electrode configuration. The synergy between these features facilitates fast bubble spreading and subsequent departure, transforms the initial liquid/solid interface into gas/solid interface under the gating of bubble, and yields an output at least one order of magnitude higher than existing studies. We also show that the output can be further enhanced through rapid bubble collapse at the air/liquid interface and multiple bubbles synchronization. We envision that our design will pave the way for small bubble-based energy harvesting in liquid media.

Noise suppression beyond the thermal limit with nanotransistor biosensors

Transistor biosensors are mass-fabrication-compatible devices of interest for point of care diagnosis as well as molecular interaction studies. While the actual transistor gates in processors reach the sub-10 nm range for optimum integration and power consumption, studies on design rules for the signal-to-noise ratio (S/N) optimization in transistor-based biosensors have been so far restricted to 1 µm2 device gate area, a range where the discrete nature of the defects can be neglected. In this study, which combines experiments and theoretical analysis at both numerical and analytical levels, we extend such investigation to the nanometer range and highlight the effect of doping type as well as the noise suppression opportunities offered at this scale. In particular, we show that, when a single trap is active near the conductive channel, the noise can be suppressed even beyond the thermal limit by monitoring the trap occupancy probability in an approach analog to the stochastic resonance effect used in biological systems.

Interface Engineering of Hematite with Nacre-like Catalytic Multilayers for Solar Water Oxidation

An efficient water oxidation photoanode based on hematite has been designed and fabricated by tailored assembly of graphene oxide (GO) nanosheets and cobalt polyoxometalate (Co-POM) water oxidation catalysts into a nacre-like multilayer architecture on a hematite photoanode. The deposition of catalytic multilayers provides a high photocatalytic efficiency and photoelectrochemical stability to underlying hematite photoanodes. Compared to the bare counterpart, the catalytic multilayer electrode exhibits a significantly higher photocurrent density and large cathodic shift in onset potential (∼369 mV) even at neutral pH conditions due to the improved charge transport and catalytic efficiency from the rational and precise assembly of GO and Co-POM. Unexpectedly, the polymeric base layer deposited prior to the catalytic multilayers improves the performance even more by facilitating the transfer of photogenerated holes for water oxidation through modification of the flat band potential of the underlying photoelectrode. This approach utilizing polymeric base and catalytic multilayers provides an insight into the design of highly efficient photoelectrodes and devices for artificial photosynthesis.

Review-Mathematical Formulations of Electrochemically Gas-Evolving Systems

Electrochemically gas-evolving systems are utilized in alkaline water electrolysis, hydrogen production, and many other applications. To design and optimize these systems, high-fidelity models must account for electron-transfer, chemical reactions, thermodynamics, electrode porosity, and hydrodynamics as well as the interconnectedness of these phenomena. Further complicating these models is the production and presence of bubbles. Bubble nucleation naturally occurs due to the chemical reactions and impacts the reaction rate. Modeling bubble growth requires an accurate accounting of interfacial mass transfer. When the bubble becomes large, detachment occurs and the system is modeled as a two-phase flow where the bubbles can then impact material transport in the bulk. In this paper, we review the governing mathematical models of the physicochemical life cycle of a bubble in an electrolytic medium from a multiscale, multiphysics viewpoint. For each phase of the bubble life cycle, the prevailing mathematical formulations are reviewed and compared with particular attention paid to physicochemical processes and the impact the bubble. Through the review of a broad range of models, we provide a compilation of the current state of bubble modeling in electrochemically gas-evolving systems.

Electrochemical Control of pH in Nanoliter Volumes

The electrochemical management of the proton concentration in miniaturized dimensions opens the way to control and parallelize multistep chemical reactions, but still it faces many challenges linked to the efficient proton generation and control of their diffusion. Here we present a device operated electrochemically that demonstrates the control of the pH in a cell of ∼140 nL. The device comprises a microfluidic reactor integrated with a pneumatic mechanism that allows the exchange of reagents and the isolation of protons to decrease the effect of their diffusion. We monitored the pH with a fluorescence marker and calculated the final value from the redox currents. We demonstrate a large pH amplitude control from neutral pH values beyond the fluorescence marker range at pH 5. On the basis of the calculations from the Faradaic currents, the minimum pH reached should undergo pH ∼ 0.9. The pH contrast between neutral and acid pH cells can be maintained during periods longer than 15 min with an appropriate design of a diffusion barrier.

Quantitative impedance characterization of sub-10 nm scale capacitors and tunnel junctions with an interferometric scanning microwave microscope

We present a method to characterize sub-10 nm capacitors and tunnel junctions by interferometric scanning microwave microscopy (iSMM) at 7.8 GHz. At such device scaling, the small water meniscus surrounding the iSMM tip should be reduced by proper tip tuning. Quantitative impedance characterization of attofarad range capacitors is achieved using an 'on-chip' calibration kit facing thousands of nanodevices. Nanoscale capacitors and tunnel barriers were detected through variations in the amplitude and phase of the reflected microwave signal, respectively. This study promises quantitative impedance characterization of a wide range of emerging functional nanoscale devices.