Three-dimensional Inkjet Printed Solid Oxide Electrochemical Reactors. I. Yttria-stabilized zirconia Electrolyte

Solid oxide fuel cell (SOFC) and electrolyser (SOE) performances can be enhanced significantly by increasing the densities of (electrode | electrolyte | pore) triple phase boundaries and improving geometric reproducibility and control over composite electrode | electrolyte microstructures, thereby also aiding predictive performance modelling. We developed stable aqueous colloidal dispersions of yttria-stabilized zirconia (YSZ), a common SOFC electrolyte material, and used them to fabricate 2D planar and highly-customisable 3D microstructures by inkjet printing. The effects of solids fraction, particle size, and binder concentration on structures were investigated, and crack-free, non-porous electrolyte planes were obtained by tailoring particle size and minimising binder concentration. Micro-pillar arrays and square lattices were printed with the optimised ink composition, and a minimum feature size of 35 μm was achieved in sintered structures, the smallest published to-date. YSZ particles were printed and sintered to a 23 μm thick planar electrolyte in a Ni-YSZ|YSZ|YSZ-LSM|LSM electrolyser for CO2 splitting; a feed of 9:1 CO2:CO mixture at 1.5 V and 809 °C produced a current density of −0.78 A cm−2 even without more complex 3D electrode | electrolyte geometries

Developments in electrochemical processes for recycling lead-acid batteries

The lead-acid battery recycling industry is very well established, but the conventional pyrometallurgical processes are far from environmentally benign. Hence, recent developments of lead-acid battery recycling technologies have focused on low-temperature (electro-)hydrometallurgical processes, the subject of this review, covering modified electrolytes, improved reaction engineering, better reactor design and control of operating conditions

Fabrication of 3D NiO-YSZ structures for enhanced performance of solid oxide fuel cells and electrolysers

Increasing densities of (electrode–electrolyte-pore) triple phase boundaries (TPBs) / reaction sites enhance performances of solid oxide electrochemical reactors (SOERs) in both fuel cell (SOFC) and electrolyser (SOE) modes. Inkjet 3D printing is capable of construction of ceramic microstructures on support layers, enabling fabrication of SOERs with enhanced active area to geometric area ratios, thereby up-scaling effective areas / TBP lengths per unit volume. A Ni(O)-YSZ functional layer was designed and 3D inkjet printed with a surface of circular pillars, a facile geometry for printing that increased the interfacial to geometric area ratio. Deposition of further functional layers and sintering resulted in fully fabricated reactors with structures: H2O-H2 | Ni(O)-YSZ support | Ni(O)-YSZ pillars | YSZ | YSZ-LSM | O2, Air. The corresponding planar structured cell also was fabricated with the same components, for comparison of its electrochemical performance with that of the pillar-structured cell. The latter exhibited performance enhancement over its planar counterpart by factors of ca. 1.5 in fuel cell mode, ca. 3 in steam electrolysis mode, and ca. 4–5 in CO2 electrolysis mode, thereby demonstrating the potential of geometric structuring of electrode | electrolyte interfaces by 3D printing for developing higher performance SOERs

In situ determination of polysulfides in alkaline hydrogen sulfide solutions

A method was developed to determine low concentrations of polysulfide ions (Sn2- expressed as zero-valent sulfur) in situ and in the presence of high concentrations (0.5 mol dm-3) of hydrogen sulfide ions, HS-, at pH 14. UV-visible spectrophotometry was used to determine absorbances at 295 and 420 nm using an immersion probe, designed for highly corrosive environments. Three absorbance trends were found, corresponding to three concentration ranges of zero-valent sulfur: low (0 – 1.2 10-3 mol dm-3), medium (1.2 – 3.6 10-3 mol dm-3) and high (3.6 – 10 10-3 mol dm-3). The non-linear dependence of absorbance on concentration over the range studied was due to disproportionation of polysulfides. Determination of these species is well known to be problematic at low concentrations due to the effects of adventitious oxygen in solution, meta-stability and speciation of polysulfide species: S22- – S82-. Oxygen concentrations must be minimised in the inert gas used to de-oxygenate sulfide solutions and for the same reason, their contact with atmospheric oxygen should be minimised. During potentiostatic oxidation of alkaline solutions containing HS- ions in the anolyte of electrochemical reactors incorporating cation-permeable membranes, temporal changes in anolyte absorbance and charge were used to estimate polysulfide concentrations. Charge yields for sulfide to polysulfide oxidation were close to unity, confirming the utility of the technique developed. Molar attenuation coefficients of the predominant polysulfide ions S32- at 420 nm and S42- at 295 nm were also estimated as 289 and 3609 dm3 mol-1 cm-1, respectively, and comparable to values of (190, 206) and (3420, 3690) dm3 mol-1 cm-1 reported previously

3-D inkjet printed solid oxide electrochemical reactors III. cylindrical pillared electrode microstructures

Inkjet printing is a scalable technique that can fabricate customised three-dimensional microstructures, reproducibly, accurately, and with high material utilisation, by printing multiple layers sequentially onto previously printed layers, to produce architectures tailored in this case to electrochemical reactors. Printable yttria-stabilised zirconia (YSZ) and lanthanum strontium manganite (LSM) inks were formulated to enable fabrication of solid oxide electrochemical reactors (SOERs): H2O-H2 | Ni(O)-YSZ | YSZ | YSZ pillars | LSM | O2. Of the geometries studied, equi-sized, hexagonally-arranged cylindrical pillars were predicted to produce the largest ratio of interfacial to geometric (cross-sectional) areas. However, this neglects effects of potential and current density distributions that constrain up-scaling to more modest factors. Hence, using kinetic parameter values from the literature, finite element computational simulations of the pillared SOER in (H2 - O2) fuel cell mode predicted peak power densities of 0.11 W cm−2 at 800 °C, whereas its counterpart with only a planar electrolyte layer produced only 0.05 W cm−2; i.e. the pillars were predicted to enhance peak power densities by ca. 2.3. Arrays of several thousand YSZ cylindrical pillars were printed, with post-sintering diameter, height, and spacing of 25, 95 and 63 μm, respectively. LSM was inkjet-printed onto the pillars, and sintered subsequently, to produce contiguous films ca. 4 μm thick. In (H2 - O2) fuel cell mode at 725, 770, and 795 °C, these reactors produced peak power densities of 0.09, 0.21, 0.30 W cm−2, respectively, 3–6 times greater than the performance of ‘benchmark’ Ni(O)-YSZ | YSZ | LSM reactors inkjet-printed with planar cathodes operating under the same conditions, thereby demonstrating the benefit of inkjet printing as a fabrication technique for SOERs

Predicting optimal geometries of 3D-printed solid oxide electrochemical reactors

Solid oxide electrochemical reactors (SOERs) may be operated in fuel cell (SOFC) or electrolyser (SOE) modes, at temperatures > 800 K, depending on electrolyte and electrode materials. In electrolyser mode, current densities of ≥ ca. 104 A m−2 are achievable at potential differences ideally at the thermoneutral values of 1.285 V for steam splitting or 1.46 V for CO2 splitting at 750 °C. As for large scale chemical processes in general, such reactors are required to be energy efficient, economic, of scalable design and fabrication, and durable ideally over ≥ ca. 10 years. Increasing densities of electrode | electrolyte interfacial areas (and electrode | electrolyte | pore triple phase boundaries) of solid oxide fuel cells or electrolysers offers one means of increasing performance, reproducibility, durability and potentially decreasing cost. Three-dimensional structuring of those interfaces can be achieved by 3D printing, but modelling is required to optimise geometries. Using kinetic parameter values from the literature, COMSOL Multiphysics® finite element software was used to predict effects of 3D geometries, increasing interfacial to geometric area ratios, on SOER performances for YSZ ((ZrO2)0.92(Y2O3)0.08) oxide ion conducting electrolyte and Ni-YSZ electrode based cells, relative to corresponding planar structures with < 10 μm thick planar YSZ electrolyte. For the negative electrode, electrolyte and electrode layers were inkjet printed on Ni(O)-YSZ substrate precursors, then sintered. For the positive electrode, porous lanthanum strontium manganite (LSM: La0.8Sr0.2MnO3-δ) was brush-coated over the (gas-tight) YSZ, then sintered to produce complete SOERs: H2O-H2 | Ni(O)-YSZ | YSZ-YSZ pillars | YSZ-LSM | LSM | O2. Results are reported showing that, in the case of solid YSZ pillars, despite interfacial electrode | electrolyte areas being up scaled by factors of 10–150 depending on height (10–150 μm), current densities were predicted to increase by only ca. 1.14 in electrolysis mode and peak power densities were predicted to increase by ca. 1.93 in fuel cell mode. This was due to increased ionic current path length along the pillars, increasing ohmic potential losses relative to faradaic impedances; as expected, such predictions depend strongly on electrode kinetic parameter values. After sintering the porous Ni(O)-YSZ pillars and their subsequent reduction with H2 to nickel, they were assumed to constitute equipotential surfaces, depending on current collector design. Predicted current densities were up to 1011 mA cm−2, far greater than in solid YSZ pillars, ultimately limited by reactant or product mass transport through porous pillars of increasing height

Inkjet 3D-printing of functional layers of solid oxide electrochemical reactors: a review

The review paper overviews principles of inkjet printing and ink formulation, subsequently a literature summary on inkjet-printed solid oxide electrochemical reactors printed with 2D and 3D structures, followed by challenges limiting the technique