Easing water apart
Traditional setups for splitting water into hydrogen and oxygen operate either in acid or in base. A bipolar membrane can potentially enhance efficiency by bridging acidic hydrogen ...evolution to basic oxygen evolution. Oener
et al.
undertook a systematic study of how catalysts paired with such a membrane might accelerate the preliminary step of water dissociation into protons and hydroxide ions. Using insights from this study for optimal catalyst integration, they were able to substantially lower the overpotential of a bipolar membrane electrolyzer.
Science
, this issue p.
1099
A systematic study of water-dissociation catalysts enhanced performance of bipolar membranes for electrolysis.
Catalyzing water dissociation (WD) into protons and hydroxide ions is important both for fabricating bipolar membranes (BPMs) that can couple different pH environments into a single electrochemical device and for accelerating electrocatalytic reactions that consume protons in neutral to alkaline media. We designed a BPM electrolyzer to quantitatively measure WD kinetics and show that, for metal nanoparticles, WD activity correlates with alkaline hydrogen evolution reaction activity. By combining metal-oxide WD catalysts that are efficient near the acidic proton-exchange layer with those efficient near the alkaline hydroxide-exchange layer, we demonstrate a BPM driving WD with overpotentials of <10 mV at 20 mA·cm
−2
and pure water BPM electrolyzers that operate with an alkaline anode and acidic cathode at 500 mA·cm
−2
with a total electrolysis voltage of ~2.2 V.
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.
Developing low-cost and efficient electrocatalysts to accelerate oxygen evolution reaction (OER) kinetics is vital for water and carbon-dioxide electrolyzers. The fastest-known water oxidation ...catalyst, Ni(Fe)O
H
, usually produced through an electrochemical reconstruction of precatalysts under alkaline condition, has received substantial attention. However, the reconstruction in the reported catalysts usually leads to a limited active layer and poorly controlled Fe-activated sites. Here, we demonstrate a new electrochemistry-driven F-enabled surface-reconstruction strategy for converting the ultrathin NiFeO
F
nanosheets into an Fe-enriched Ni(Fe)O
H
phase. The activated electrocatalyst shows a low OER overpotential of 218 ± 5 mV at 10 mA cm
and a low Tafel slope of 31 ± 4 mV dec
, which is among the best for NiFe-based OER electrocatalysts. Such superior performance is caused by the effective formation of the Fe-enriched Ni(Fe)O
H
active-phase that is identified by
Raman spectroscopy and the substantially improved surface wettability and gas-bubble-releasing behavior.
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.
Fe cations dramatically enhance oxygen evolution reaction (OER) activity when incorporated substitutionally into Ni or Co (oxy)hydroxides, serving as possible OER active sites. Pure Fe ...(oxy)hydroxides, however, are typically thought to be poor OER catalysts and are not well-understood. Here, we report a systematic investigation of Fe (oxy)hydroxide OER catalysis in alkaline media. At low overpotentials of ∼350 mV, the catalyst dissolution rate is low, the activity is dramatically enhanced by an AuO x /Au substrate, and the geometric OER current density is largely independent of mass loading. At higher overpotentials of ∼450 mV, the dissolution rate is high, the activity is largely independent of substrate choice, and the geometric current density depends linearly on loading. These observations, along with previously reported in situ conductivity measurements, suggest a new model for OER catalysis on Fe (oxy)hydroxide. At low overpotentials, only the first monolayer of the electrolyte-permeable Fe (oxy)hydroxide, which is in direct contact with the conductive support, is OER-active due to electrical conductivity limitations. On Au substrates, Fe cations interact with AuO x after redox cycling, leading to enhanced intrinsic activity over FeOOH on Pt substrates. At higher overpotentials, the conductivity of Fe (oxy)hydroxide increases, leading to a larger fraction of the electrolyte-permeable catalyst film participating in catalysis. Comparing the apparent activity of the putative Fe active sites in/on different hosts/surfaces supports a possible connection between OER activity and local structure.
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 Co1–y O x , Ni0.9Fe0.1O 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. Ni0.9Fe0.1O 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 Ni0.9Fe0.1OOH 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 Co1–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.
Fe plays a critical, but not yet understood, role in enhancing the activity of the Ni-based oxygen evolution reaction (OER) electrocatalysts. We report electrochemical, in situ electrical, ...photoelectron spectroscopy, and X-ray diffraction measurements on Ni1–x Fe x (OH)2/Ni1–x Fe x OOH thin films to investigate the changes in electronic properties, OER activity, and structure as a result of Fe inclusion. We developed a simple method for purification of KOH electrolyte that uses precipitated bulk Ni(OH)2 to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni(OH)2/NiOOH reveals new Ni redox features and no significant OER current until >400 mV overpotential, different from previous reports which were likely affected by Fe impurities. We show through controlled crystallization that β-NiOOH is less active for OER than the disordered γ-NiOOH starting material and that previous reports of increased activity for β-NiOOH are due to incorporation of Fe-impurities during the crystallization process. Through-film in situ conductivity measurements show a >30-fold increase in film conductivity with Fe addition, but this change in conductivity is not sufficient to explain the observed changes in activity. Measurements of activity as a function of film thickness on Au and glassy carbon substrates are consistent with the hypothesis that Fe exerts a partial-charge-transfer activation effect on Ni, similar to that observed for noble-metal electrode surfaces. These results have significant implications for the design and study of Ni1–x Fe x OOH OER electrocatalysts, which are the fastest measured OER catalysts under basic conditions.
Heterogeneous electrocatalysts for the oxygen evolution reaction (OER) are complicated materials with dynamic structures. They can exhibit potential-induced phase transitions, potential-dependent ...electronic properties, variable oxidation and protonation states, and disordered local/surface phases. These properties make understanding the OER, and ultimately designing higher efficiency catalysts, challenging. We report a series of procedures and measurement techniques that we have adopted or developed to assess each of the above challenges in understanding materials for the OER. These include the targeted synthesis of hydrated oxyhydroxide phases, the assessment and elimination of electrolyte impurities, the use of a quartz crystal microbalance to monitor film loading and dissolution, and the use of an in situ conductivity measurement to understand the flow of electrons from the catalyst active sites to the conductive support electrode. We end with a recipe for the synthesis and characterization of a “standard” Ni(Fe)O x H y catalyst that can be performed in any laboratory with a basic electrochemical setup and used as a quantitative comparison to aid the development of new OER catalyst systems.
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.
One practical metric for electrocatalyst performance is current per geometric area at a given applied overpotential. An obvious route to increase performance is to increase the catalyst mass ...loadingas long as the intrinsic performance (i.e., specific activity or turnover frequency) of the catalyst is independent of loading, and other electrical, ionic, or mass-transfer resistances are not severe. Here we report the geometric and intrinsic oxygen evolution reaction (OER) activities of Ni(Fe)OOH films, the fastest known water oxidation catalyst in basic media, as a function of mass loading from 0 to ∼100 μg cm–2. We discuss practices for measuring and reporting intrinsic activities, highlighting experimental conditions where the film activity on a per-metal-cation basis can be accurately measured and where capacitance measurements of electrochemically active surface area fail. We find that the electrochemical reversibility of the (nominally) Ni2+/3+ redox wave correlates with the apparent intrinsic activity as a function of loading. We report a pulsed-electrodeposition method that dramatically improves the catalyst reversibility and performance at high loading compared to continuous electrodeposition, which we attribute to improved connectivity in the micro/nanostructure and better composition control. Pulse electrodeposited films are shown to have geometric performance similar to a number of advanced composite electrocatalyst structures and to maintain effective per-metal turnover frequencies of >0.4 s–1 at 300 mV overpotential, even for loadings of ∼100 μg cm–2.
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