Chemistry

Graphite-protected CsPbBr three perovskite photoanodes functionalised with water oxidation catalyst for oxygen evolution in water

CsPbBr3 perovskite system preparation

CsPbBr3 is a completely inorganic halide perovskite materials, inherently unstable in water like most different natural–inorganic halide perovskites. Supplementary Fig. 1a, b reveals the X-ray diffraction (XRD) patterns and UV-vis absorption of a CsPbBr3 movie earlier than and after it was dipped in water for 1 s. The preliminary XRD spectrum was in good settlement with the orthorombic perovskite construction43, nonetheless, after immersion in water, its distinct (100), (110) and (200) diffraction peaks quickly disappeared and peaks attribute of the precursor supplies PbBr2 and CsBr appeared. Whereas pristine CsPbBr3 has a bandgap of two.35 eV with an absorption onset similar to 530 nm giving rise to its yellow color, after immersion in water solely a semitransparent white materials stays (PbBr2). Subsequently, in an effort to use CsPbBr3 in aqueous PEC cells for photo voltaic water splitting, efficient safety methods should be developed as for every other halide perovskite.

We ready our perovskite photoanodes based mostly on the structure of a normal carbon perovskite photo voltaic cell. Typically, perovskite photo voltaic cells are made utilizing an absorber materials sandwiched between an electron transport layer (ETL) and a gap transport layer (HTL). In these cells, a compact layer of TiO2 deposited by spray pyrolysis and a doctor-bladed mesoporous carbon layer (m-carbon or m-c) have been used because the ETL and HTL, respectively, as proven in Fig. 1a (Supplementary Fig. 2 reveals a schematic power band diagram). CsPbBr3 photo voltaic cells with comparable carbon-based architectures have already proven good stability over three months in humid air, because of the hydrophobic nature of the m-carbon HTL44. Certainly, the water contact angle (WCA) of the mesoporous carbon layer used right here exceeded 90° (Supplementary Fig. three). Determine 1b reveals a cross-sectional scanning electron microscopy (SEM) picture of the as-prepared CsPbBr3 construction with a 20-μm-thick m-carbon layer performing as conducting contact. Power dispersive X-ray (EDX) mapping pictures of the CsPbBr3 photo voltaic cell are proven in Supplementary Fig. four, confirming the formation of a TiO2|CsPbBr3|m-c stack. Determine 1c reveals the FTO|TiO2|CsPbBr3 interfaces with skinny layers of TiO2 and CsPbBr3 of about 50 and 350 nm, respectively.

Fig. 1Fig. 1

Configuration of CsPbBr3 units. a Construction of normal carbon photo voltaic cell (illuminated by means of the FTO glass as indicated by the yellow arrows). Electron-hole (e−/h+) pairs are generated within the CsPbBr3 skinny layer, then e− and h+ are collected within the TiO2 and m-carbon layers, respectively. b Cross-sectional scanning electron microscopy (SEM) picture of the as-prepared CsPbBr3 perovskite photo voltaic cell (scale bar 10 μm). c Magnification of the cross-sectional SEM picture to indicate the FTO|TiO2|CsPbBr3 interfaces (scale bar 500 nm)

We first ready photoanodes with a TiO2|CsPbBr3|m-c configuration (schematic illustration in Supplementary Fig. 5a), the place the 20 μm-carbon layer aimed to offer waterproof safety to the moisture-sensitive CsPbBr3. Supplementary Fig. 5b reveals the present density-voltage (JV) curve of a typical TiO2|CsPbBr3|m-c photo voltaic cell measured beneath AM 1.5 G. and photo voltaic simulated gentle at 100 mW cm−2. As proven in Supplementary Fig. 5c and Supplementary Observe 1, the TiO2|CsPbBr3|m-c photoanode in pH 9 aqueous electrolyte exhibited a photocurrent density of ~zero.four at 1.23 VRHE. Chronoamperometry was carried out at pH 9 and 13 at 1.23 VRHE and is proven in Supplementary Fig. 6a and Supplementary Fig. 7. Promisingly, the photoanode retained 70% of its preliminary worth over 20 min testing and H2 bubbles have been generated on the Pt counter electrode (Supplementary Fig. 6b). The photocurrent density was additionally measured with monochromatic gentle, exhibiting that CsPbBr3 was nonetheless absorbing seen gentle at wavelengths λ < 530 nm after 15 min of immersion in water at pH 9 (see Supplementary Fig. 6c). This photocurrent density couldn't be assigned to the TiO2 layer or to the formation of PbBr2 since a management photoanode consisting of TiO2|PbBr2|m-c generated a most of zero.067 mA cm−2 at 1.23 VRHE (Supplementary Fig. eight).

Safety of perovskite system in water

In an effort to extra effectively shield CsPbBr3 from water, a 25 μm self-adhesive graphite sheet (GS) was positioned on high of the mesoporous carbon layer, to create TiO2|CsPbBr3|m-c|GS photoanodes (see Fig. 2a). Whereas the mesoporous carbon acts as an efficient HTL attributable to its intimate contact with the perovskite absorber layer, the GS gives a extra compact seal that may shield the halide perovskite extra effectively from liquid water percolation (see Hg porosimetry of m-c and GS in Supplementary Fig. 9 and Supplementary Observe 2 exhibiting variations in porosity). Much like mesoporous carbon, the GS is distinctly hydrophobic (measured WCA is 98°, proven in Supplementary Fig. 10) and it gives wonderful thermal and electrical conductivity (1600 Wm−1 Okay−1 and 20,000 S cm−1, respectively). Determine 2b reveals that the JV curves of the TiO2|CsPbBr3|m-c|GS measured as photo voltaic cells earlier than and after 2 h of PEC operation in water have been unchanged, indicating extraordinary water stability. As proven from the linear sweep voltammetry (LSV) curve in Fig. 2c, the photocurrent steeply rose from zero.6 VRHE on, reaching a photocurrent density of two.5 mA cm−2 at 1.23 VRHE.

Fig. 2Fig. 2

TiO2|CsPbBr3|m-c|GS as photo voltaic cell and photoanode. a Schematic illustration of the CsPbBr3 photoanode for PEC O2 evolution: TiO2|CsPbBr3|m-c|GS makes use of industrial conductive GS on the m-carbon layer; The photoanode is illuminated by means of the FTO glass. b Photovoltaic present density-voltage curves measured beneath simulated AM 1.5 G photo voltaic gentle (100 mW cm−2) earlier than and after 2 h of immersion in water. Curves proven are measured in reverse scan. c Typical linear sweep voltammetry (LSV) measured beneath PEC situations at a scan fee of 20 mV s−1, measured in zero.1 M KNO3 at pH four.three, beneath steady simulated AM 1.5 G photo voltaic illumination, 1 solar

The wavelength dependence of the incident photon-to-current effectivity (IPCE) measured in aqueous resolution in Fig. 3a confirmed an onset wavelength of 530 nm for the TiO2|CsPbBr3|m-c|GS, which corresponds precisely to the onset gentle absorption of CsPbBr3 in Supplementary Fig. 1b. An IPCE effectivity of about 70% was measured at 500 nm. In distinction, an IPCE effectivity decrease than 10% was measured at 500 nm for the TiO2|CsPbBr3|m-c in water (Supplementary Fig. 11), suggesting that the CsPbBr3 partially degraded when protected by the m-c layer by itself. The GS, nonetheless, proved to be a very efficient encapsulation layer which didn’t hinder cost transport. This was confirmed by the JV curves of TiO2|CsPbBr3|m-c measured earlier than and after making use of the GS on the floor, that are proven in Supplementary Fig. 12 and Supplementary Desk 1. The Voc, Jsc and FF of the system decreased by solely about 5% after the GS was utilized on the floor, confirming that holes could possibly be correctly extracted by means of the GS layer. Images in Fig. 3b present O2 and H2 gasoline bubbles evolving from the GS energetic space of the CsPbBr3 based mostly photoanode and the Pt counter electrode, respectively.

Fig. threeFig. 3

TiO2|CsPbBr3|m-c|GS PEC efficiency in water. a Incident photon-to-current effectivity (IPCE) measured in aqueous zero.1 M KNO3 resolution with the pH adjusted to 12 with KOH; electrode was subjected to monochromatic gentle irradiation at 1.23 VRHE. b Images of the PEC cell beneath operation, exhibiting H2 and O2 gasoline bubbles evolving from the counter electrode (Pt) and the photoanode, respectively. c Chronoamperometric hint recorded at an utilized potential of 1.23 VRHE in KOH electrolyte resolution at pH 12.5, beneath chopped simulated photo voltaic gentle irradiation (AM 1.5 G, 100 mW cm−2). d Images of the photoanode immersed within the electrolyte resolution after 18 h of steady operation, exhibiting oxygen bubbles evolving from the energetic space

Determine 3c reveals chronoamperometry recorded at an utilized potential of 1.23 VRHE in electrolyte at pH 12.5. The TiO2|CsPbBr3|m-c|GS photoanodes have been secure beneath chopped illumination in fundamental aqueous electrolyte for greater than 5 h and the photoanode’s present elevated from 1.5 to 2 mA cm−2 throughout this time. This represents the primary instance of halide perovskite-based photoeletrodes, protected with cheap safety layers, which might show secure photocurrents for hours.

To analyze the long-term stability of the CsPbBr3-based photoanode, chronoamperometry was recorded at a relentless utilized potential of 1.23 VRHE in aqueous resolution (zero.1 M KNO3) beneath steady illumination. The evolution of the photocurrent density with time at completely different pH till the tip of life is proven in Supplementary Fig. 13. The system achieved a lifetime of 34 h in an alkaline electrolyte and 23 h in near-neutral resolution, whereas the longest lifetime examined in acidic options was 7.eight h. Determine 3d reveals of the PEC cell after 18 h of operation, with oxygen bubbles nonetheless rising from the energetic space of the photoanode contained in the PEC cell. The top of lifetime of all units examined at completely different pH was at all times brought on by delamination and fracture of the GS floor (Supplementary Fig. 14). In distinction to earlier stories within the literature, which all reported fixed lowering photocurrent beneath steady illumination29,31, we noticed an rising photocurrent throughout the measurement, particularly in near-neutral pH, the place the photoanode’s present density step by step elevated beneath illumination reaching a peak of two.5 mA cm−2 after 16.7 h (Supplementary Fig. 13). We imagine that soaking results are liable for inflicting the rise in photocurrents. Tensile energy measurements confirmed that the GS materials used to encapsulate the moisture-sensitive perovskite layer does expertise modifications in its mechanical properties when uncovered to aqueous electrolyte at pH 7 (as proven in Supplementary Fig. 15). The lower in stiffness noticed probably displays swelling throughout soaking that may expose the next floor space to the electrolyte, thereby rising water oxidation efficiency. To additional confirm this impact of soaking, we examined a TiO2|CsPbBr3|m-c|GS photoanode with intermittent electrolyte soaking in the dead of night at open circuit. The ends in Supplementary Fig. 16 present that the photocurrent density elevated each time the photoanode was illuminated once more following a interval of soaking. One other potential clarification for the rise in photocurrent is feasible structural modification of the CsPbBr3 attributable to halide migration beneath gentle soaking45. Mosconi et al. proposed that Frenkel defects ((_^+ /_^ )) in halide perovskites heal throughout irradiation attributable to decrease power obstacles for halide (X) migration within the photoexcited state46. Certainly, we beforehand confirmed that the effectivity of CsPbBr3 carbon photo voltaic cells elevated beneath gentle soaking41. Gradual enhance in present over time has been additionally not too long ago reported for 2 dimensional/three dimensional perovskite carbon photo voltaic cells and triple-cation perovskite units47.

In an effort to additional optimise the photoanode and improve the steadiness of the GS when uncovered to aqueous electrolyte, we measured the efficiency of TiO2|CsPbBr3|m-c|GS utilizing a 70-μm-thick GS (GS70). Despite the fact that a thicker GS was used, fees have been nonetheless effectively extracted when testing the system as a photo voltaic cell earlier than PEC testing (Supplementary Fig. 17 and Supplementary Desk 2). Determine four reveals chronoamperometry of TiO2|CsPbBr3|m-c|GS70 photoanodes recorded at a relentless utilized potential of 1.23 VRHE in aqueous resolution (zero.1 M KNO3, pH 7) beneath steady illumination. The photoanode exhibited photocurrents above 2 mA cm−2 for about 30 h. Ultimately, the silicone-epoxy resin sealing on the slide edges partly degraded within the electrolyte resolution, letting some water cross by means of it, which slowly degraded the CsPbBr3 materials to PbBr2 (Supplementary Fig. 18). The GS70 was nonetheless intact on the finish of the experiment and as talked about above the failure was attributable to degradation of the sealant. In distinction, failure in units with GS25 defending layers occurred attributable to fracture of the GS after prolonged operation in water. This clearly signifies that thicker GS safety layers, together with higher sealing supplies, can lengthen the lifetime of those composite photoanodes even additional.

Fig. fourFig. 4

Lengthy-term stability of TiO2|CsPbBr3|m-c|GS70 in water. Chronoamperometric hint of TiO2|CsPbBr3|m-c|GS70 recorded at an utilized potential of 1.23 VRHE. zero.1 M KNO3 electrolyte resolution pH 7, beneath steady simulated photo voltaic gentle irradiation (AM 1.5 G, 100 mW cm−2)

Functionalisation of GS with a water oxidation catalyst

To check the flexibility of our encapsulated perovskite-based photoanodes and to additional enhance their effectivity at decrease potentials, the addition of an iridium-based water oxidation catalyst (WOC)48,49,50 was explored. The [Ir(μO)(pyalk)(H2O)2]22+ catalyst chosen has beforehand been proven to quickly and irreversibly adsorb to the floor of indium tin oxide (ITO) and α-Fe2O3 as a minimally skinny molecular monolayer50,51,52,53. Grafitingly, we additionally discovered it to robustly bind to graphitic surfaces after merely floating the GS on an activated WOC resolution at room temperature for 16 h (Fig. 5a). Supplementary Fig. 19 reveals EDX mapping pictures and the basic composition of the GS|WOC, offering direct proof for the presence of the Ir-WOC on the floor and exhibiting its localisation round oxidic edges on the GS floor50. Proof for the presence of the catalyst was additionally offered by electrochemical measurements of the GS|WOC sheet. Determine 5b reveals cyclic voltammetry (CV) with a transparent catalytic wave for water oxidation beginning at about 1.four VRHE54. As beforehand reported for the Ir-WOC in resolution49 and on ITO surfaces50, the quasi-reversible redox course of centred round 1 VRHE seems in acidic media and corresponds to the IrIII/IrIV redox couple50. Evaluating CVs of each naked and functionalised GS throughout a pH vary of two–13 (Supplementary Fig. 20) confirmed the overpotentials for oxygen evolution to extend with rising pH for each supplies, however with distinctly decrease overpotentials within the presence of the Ir-WOC in any respect pH values (Fig. 5c).

Fig. 5Fig. 5

Formation of the WOC and functionalisation of the GS floor. a Oxidative activation of the iridium precursor (high) and functionalisation of the GS floor (backside). b Cyclic voltammetry (CV) scan of GS and GS functionalised with the Ir-WOC measured in zero.1 M KNO3 at pH 2.6, with scan fee of 50 mV s−1. The inset reveals a close-up of the potential axis, the place the IrIII/IrIV couple may be seen. c Electrode overpotentials at 10 μA cm−2 as a operate of the pH of the answer. The error bars symbolize customary deviation

These pre-functionalised GS|WOC sheets might simply be used to manufacture TiO2|CsPbBr3|m-c|GS|WOC photoanodes as illustrated in Fig. 6a utilizing a simple process beneath ambient situations. When measured as a photoanode in aqueous resolution at pH 2.5, a cathodic shift within the onset potential Von of 100 mV was noticed with respect to unfunctionalised TiO2|CsPbBr3|m-c|GS (Fig. 6b). Furthermore, the PEC photocurrent density was discovered to rise quicker with utilized potential in contrast with TiO2|CsPbBr3|m-c|GS, a transparent signal of improved gap transport and O2 evolution kinetics as seen beforehand for a similar Ir-WOC on haematite photoanodes52. The shift within the Von to extra adverse potentials was noticed additionally at increased pHs four and seven (Supplementary Fig. 21 and Supplementary Observe three) and it was in settlement with Tafel plots (Supplementary Fig. 22). Supplementary Fig. 23 reveals CVs for full TiO2|CsPbBr3|m-c|GS|WOC photoanodes at pH 2.5, four and seven. The photocurrent density was increased and the onset potential shifted to extra adverse values at low pH, as already evidenced by the overpotential pattern noticed in Fig. 5c as a operate of pH.

Fig. 6Fig. 6

TiO2|CsPbBr3|m-c|GS|WOC as photo voltaic cell and photoanode. a Schematic illustration of the CsPbBr3 photoanode for PEC O2 evolution: TiO2|CsPbBr3|m-c|GS|WOC makes use of industrial conductive GS functionalised with an Ir-based catalyst. The photoanode is illuminated from the again aspect. b Linear sweep voltammetry (LSV) of a TiO2|CsPbBr3|m-c|GS and a TiO2|CsPbBr3|m-c|GS|WOC measured at a scan fee of 20 mV s−1, in a zero.1 M KNO3 electrolyte resolution with pH 2.5 (pH adjusted with H2SO4)

Open-circuit measurements have been carried out in the dead of night and beneath illumination in numerous electrolytes (Fig. 7a). The distinction between the darkish open-circuit voltage and that beneath illumination determines the photovoltage ΔVph of the photoanode52. TiO2|CsPbBr3|m-c|GS and TiO2|CsPbBr3|m-c|GS|WOC confirmed comparable open-circuit potential (OCP) and ΔVph at equilibrium, exhibiting once more that the improved exercise of TiO2|CsPbBr3|m-c|GS|WOC isn’t a thermodynamic impact however the results of enhanced cost switch kinetics. When 1.23 VRHE have been utilized in a three-electrode PEC system, comparable water oxidation photocurrents for photoanodes with and with out WOC have been obtained (Supplementary Figs 24 and 25 and Supplementary Observe four). The long-term efficiency and stability of TiO2|CsPbBr3|m-c|GS|WOC confirmed the addition of the WOC to afford nearly 20% increased preliminary photocurrents of three.5 mA cm−2 and practically 3 times longer lifetime of 17 h in acidic electrolyte than the naked TiO2|CsPbBr3|m-c|GS beneath the identical situations (Fig. 7b).

Fig. 7Fig. 7

Comparability of TiO2|CsPbBr3|m-c|GS with and with out Ir-WOC. a Open-circuit potential (OCP) of photoanodes measured in zero.1 M KNO3 options with completely different pH; Measurements carried out in the dead of night are proven with strong symbols, whereas those carried out beneath illumination are proven with open symbols; The photovoltage ΔVph measured for every pattern is given. b Chronoamperometric hint of TiO2|CsPbBr3|m-c|GS|WOC recorded at an utilized potential of 1.23 VRHE. The electrolyte was zero.1 M KNO3 adjusted to pH 2.5 with H2SO4, beneath steady simulated photo voltaic gentle irradiation (AM 1.5 G, 100 mW cm−2)

O2 manufacturing with functionalised perovskite photoanode

The quantity of O2 produced over time by a TiO2|CsPbBr3|m-c|GS|WOC was sufficient to be monitored utilizing an oxygen probe within the headspace of a gas-tight PEC cell. Determine 8a reveals the detected and predicted O2 evolution as a operate of time beneath steady simulated photo voltaic gentle illumination (1 solar, AM 1.5 G) at an utilized voltage of 1.23 VRHE, with the anticipated quantity of O2 calculated from the photocurrent generated as proven in Supplementary Fig. 26 and Supplementary Observe 5. Determine 8b reveals the Faradaic effectivity of the light-driven water oxidation, which reached values as excessive as 82.three% utilizing this unoptimised small-scale laboratory setup (Supplementary Dialogue). This worth compares favourably with different photoanode supplies reminiscent of WO3 and TiO255,56, and represents the primary profitable measurement of a Faradaic effectivity of a secure perovskite-based photoanode in aqueous resolution.

Fig. eightFig. 8

O2 evolution efficiency on TiO2|CsPbBr3|m-c|GS|WOC. a Detected (open sq.) and predicted (shut sq.) O2 manufacturing beneath steady simulated photo voltaic gentle irradiation (AM 1.5 G, 100 mW cm−2), in zero.1 M KNO3 adjusted to pH three.5 with H2SO4. b Faradaic effectivity for O2 evolution

The photovoltages of about 1.35 V measured in impartial to alkaline media (pH = eight.eight and 12.5 V) exceeded the thermodynamic 1.23 V required for electrolysis of water. In apply, most PEC units require photovoltages within the vary of 1.7–2 V to successfully overcome the kinetic limitations of proton discount and water oxidation on the electrolyte interface and drive unbiased photo voltaic water splitting57,58. Certainly, when an unbiased two-electrode measurement was carried out with TiO2|CsPbBr3|m-c|GS and Pt electrodes at pH 2.5, very low photocurrent of zero.05 mA cm−2 was measured (Fig. 9a). Excitingly, when a floor functionalised TiO2|CsPbBr3|m-c|GS|WOC was examined beneath the identical situations, a photocurrent of zero.1 mA cm−2 was sustained for greater than 5 min (Fig. 9b).

Fig. 9Fig. 9

Unbiased two-electrode efficiency of TiO2|CsPbBr3|m-c|GS and TiO2|CsPbBr3|m-c|GS|WOC. a Chronoamperometric hint of TiO2|CsPbBr3|m-c|GS recorded in a two-electrode cell. b Chronoamperometric hint of TiO2|CsPbBr3|m-c|GS|WOC recorded in a two-electrode cell (zero.1 M KNO3 with pH adjusted to 2.5 with H2SO4, with no exterior bias voltage utilized)

Sadly the corresponding quantity of O2 produced was beneath the detection restrict of our oxygen probe (theoretically we generated 28 nmol after 2 h in de-aerated resolution, Supplementary Fig. 27), however the photocurrents noticed for TiO2|CsPbBr3|m-c|GS|WOC represent an thrilling glimpse at the potential for unbiased photo voltaic water splitting which may turn into potential with these photoanode architectures after some additional optimisation.

In conclusion, we now have demonstrated a simple versatile and extremely efficient strategy for the fabrication of secure inorganic halide perovskite-based photoanodes utilizing an simply fabricated mesoporous carbon layer and commercially obtainable graphite sheet as gap transport and safety layers. This encapsulation approach was efficient in rising the photocurrent density of the CsPbBr3-based photoanodes from zero.four to greater than 2 mA cm−2 at 1.23 VRHE, and on the similar time protected the perovskite from degradation by the aqueous electrolyte. The so-sealed photoanodes labored effectively in aqueous electrolytes with IPCE values above 70% for direct light-driven water oxidation in aqueous resolution, exhibiting good exercise over a large pH vary of two–13. Photocurrents above 2 mA cm−2 have been obtained for 30 h of steady illumination in alkaline aqueous resolution. Now we have additionally demonstrated the flexibility of our photoanode system by successfully functionalising the electrolyte-facing floor of the GS with an Ir-based WOC to enhance the onset potential of the photoanode by 100 mV in acidic options through improved cost transport kinetics. Excessive Faradaic efficiencies for O2 evolution of as much as 82.three% have been achieved over 2 h of steady simulated daylight irradiation with such a TiO2|CsPbBr3|m-c|GS|WOC photoanode.

These composite cells are remarkably straightforward to synthesise with out the necessity for prime temperature or vacuum strategies. A working system could possibly be fabricated by a single particular person utilizing customary laboratory strategies and cheap supplies and strategies. The GS safety technique used right here for oxygen evolution in water can be used to reinforce the lifetime of perovskite photo voltaic cells, the place the versatile nature of graphite might enable for roll-to-roll processing for environment friendly massive scale manufacturing. We imagine that this design represents a promising lead for utilizing cheap, excessive efficiency however inherently moisture and water-sensitive perovskite supplies in built-in photoelectrochemical cells for photo voltaic power conversion. The flexibleness of the underlying perovskite materials and possibility of including matched oxygen-evolution catalysts permits for a excessive diploma of variability that guarantees software in a variety of various architectures.


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