High-efficiency photoelectrochemical water-splitting devices require the integration of electrocatalysts (ECs) with light-absorbing semiconductors (SCs), but the energetics and charge-transfer ...processes at SC/EC interfaces are poorly understood. We fabricate model EC-coated single-crystal TiO2 electrodes and directly probe SC/EC interfaces in situ using two working electrodes to independently monitor and control the potential and current at both the SC and the EC. We discover that redox-active ion-permeable ECs such as Ni(OH)2 or NiOOH yield 'adaptive' SC/EC junctions where the effective Schottky barrier height changes in situ with the oxidation level of the EC. In contrast, dense, ion-impermeable IrOx ECs yield constant-barrier-height 'buried' junctions. Conversion of dense, thermally deposited NiOx on TiO2 into ion-permeable Ni(OH)2 or NiOOH correlated with increased apparent photovoltage and fill factor. These results provide new insight into the dynamic behaviour of SC/EC interfaces to guide the design of efficient SC/EC devices. They also illustrate a new class of adaptive semiconductor junctions.
Cobalt oxides and (oxy)hydroxides have been widely studied as electrocatalysts for the oxygen evolution reaction (OER). For related Ni-based materials, the addition of Fe dramatically enhances OER ...activity. The role of Fe in Co-based materials is not well-documented. We show that the intrinsic OER activity of Co1–x Fe x (OOH) is ∼100-fold higher for x ≈ 0.6–0.7 than for x = 0 on a per-metal turnover frequency basis. Fe-free CoOOH absorbs Fe from electrolyte impurities if the electrolyte is not rigorously purified. Fe incorporation and increased activity correlate with an anodic shift in the nominally Co2+/3+ redox wave, indicating strong electronic interactions between the two elements and likely substitutional doping of Fe for Co. In situ electrical measurements show that Co1–x Fe x (OOH) is conductive under OER conditions (∼0.7–4 mS cm–1 at ∼300 mV overpotential), but that FeOOH is an insulator with measurable conductivity (2.2 × 10–2 mS cm–1) only at high overpotentials >400 mV. The apparent OER activity of FeOOH is thus limited by low conductivity. Microbalance measurements show that films with x ≥ 0.54 (i.e., Fe-rich) dissolve in 1 M KOH electrolyte under OER conditions. For x < 0.54, the films appear chemically stable, but the OER activity decreases by 16–62% over 2 h, likely due to conversion into denser, oxide-like phases. We thus hypothesize that Fe is the most-active site in the catalyst, while CoOOH primarily provides a conductive, high-surface area, chemically stabilizing host. These results are important as Fe-containing Co- and Ni-(oxy)hydroxides are the fastest OER catalysts known.
This Viewpoint highlights recent advances in oxygen evolution reaction (OER) catalysis using well-defined/characterized systems and outlines a path for possible further improvements. First, we review ...our results on ultra-thin film catalysts and compare them to other systems, emphasizing methods that provide accurate intrinsic catalyst activities. We then discuss reports that catalysts with the highest OER activities (in base) often undergo structural and chemical changes during the OER. These findings have implications on how OER catalysts are studied and designed. We suggest opportunities to control molecular-scale interactions in hydrous layered hydroxide/oxyhydroxide catalysts as well as the control of three-dimensional nano- and microstructures using templating approaches.
Water oxidation is a critical step in water splitting to make hydrogen fuel. We report the solution synthesis, structural/compositional characterization, and oxygen evolution reaction (OER) ...electrocatalytic properties of ~2-3 nm thick films of NiO(x), CoO(x), Ni(y)Co(1-y)O(x), Ni(0.9)Fe(0.1)O(x), IrO(x), MnO(x), and FeO(x). The thin-film geometry enables the use of quartz crystal microgravimetry, voltammetry, and steady-state Tafel measurements to study the electrocatalytic activity and electrochemical properties of the oxides. Ni(0.9)Fe(0.1)O(x) was found to be the most active water oxidation catalyst in basic media, passing 10 mA cm(-2) at an overpotential of 336 mV with a Tafel slope of 30 mV dec(-1) with oxygen evolution reaction (OER) activity roughly an order of magnitude higher than IrO(x) control films and similar to the best known OER catalysts in basic media. The high activity is attributed to the in situ formation of layered Ni(0.9)Fe(0.1)OOH oxyhydroxide species with nearly every Ni atom electrochemically active. In contrast to previous reports that showed synergy between Co and Ni oxides for OER catalysis, Ni(y)Co(1-y)O(x) thin films showed decreasing activity relative to the pure NiO(x) films with increasing Co content. This finding is explained by the suppressed in situ formation of the active layered oxyhydroxide with increasing Co. The high OER activity and simple synthesis make these Ni-based catalyst thin films useful for incorporating with semiconductor photoelectrodes for direct solar-driven water splitting or in high-surface-area electrodes for water electrolysis.
Abstract
Water dissociation (WD, H
2
O → H
+
+ OH
−
) is the core process in bipolar membranes (BPMs) that limits energy efficiency. Both electric-field and catalytic effects have been invoked to ...describe WD, but the interplay of the two and the underlying design principles for WD catalysts remain unclear. Using precise layers of metal-oxide nanoparticles, membrane-electrolyzer platforms, materials characterization, and impedance analysis, we illustrate the role of electronic conductivity in modulating the performance of WD catalysts in the BPM junction through screening and focusing the interfacial electric field and thus electrochemical potential gradients. In contrast, the ionic conductivity of the same layer is not a significant factor in limiting performance. BPM water electrolyzers, optimized via these findings, use ~30-nm-diameter anatase TiO
2
as an earth-abundant WD catalyst, and generate O
2
and H
2
at 500 mA cm
−2
with a record-low total cell voltage below 2 V. These advanced BPMs might accelerate deployment of new electrodialysis, carbon-capture, and carbon-utilization technology.
Abstract
In alkaline and neutral MEA CO
2
electrolyzers, CO
2
rapidly converts to (bi)carbonate, imposing a significant energy penalty arising from separating CO
2
from the anode gas outlets. Here we ...report a CO
2
electrolyzer uses a bipolar membrane (BPM) to convert (bi)carbonate back to CO
2
, preventing crossover; and that surpasses the single-pass utilization (SPU) limit (25% for multi-carbon products, C
2+
) suffered by previous neutral-media electrolyzers. We employ a stationary unbuffered catholyte layer between BPM and cathode to promote C
2+
products while ensuring that (bi)carbonate is converted back, in situ, to CO
2
near the cathode. We develop a model that enables the design of the catholyte layer, finding that limiting the diffusion path length of reverted CO
2
to ~10 μm balances the CO
2
diffusion flux with the regeneration rate. We report a single-pass CO
2
utilization of 78%, which lowers the energy associated with downstream separation of CO
2
by 10× compared with past systems.
Electrochemical double-layer capacitors exhibit high power and long cycle life but have low specific energy compared with batteries, limiting applications. Redox-enhanced capacitors increase specific ...energy by using redox-active electrolytes that are oxidized at the positive electrode and reduced at the negative electrode during charging. Here we report characteristics of several redox electrolytes to illustrate operational/self-discharge mechanisms and the design rules for high performance. We discover a methyl viologen (MV)/bromide electrolyte that delivers a high specific energy of ∼14 Wh kg(-1) based on the mass of electrodes and electrolyte, without the use of an ion-selective membrane separator. Substituting heptyl viologen for MV increases stability, with no degradation over 20,000 cycles. Self-discharge is low, due to adsorption of the redox couples in the charged state to the activated carbon, and comparable to cells with inert electrolyte. An electrochemical model reproduces experiments and predicts that 30-50 Wh kg(-1) is possible with optimization.
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