Water splitting is a promising approach to the efficient and cost-effective production of renewable fuels, but water oxidation remains a bottleneck in its technological development because it largely relies on noble-metal catalysts. Although inexpensive transition-metal oxides are competitive water oxidation catalysts in alkaline media, they cannot compete with noble metals in acidic media, in which hydrogen production is easier and faster. Here, we report a water oxidation catalyst based on earth-abundant metals that performs well in acidic conditions. Specifically, we report the enhanced catalytic activity of insoluble salts of polyoxometalates with caesium or barium counter-cations for oxygen evolution. In particular, the barium salt of a cobalt-phosphotungstate polyanion outperforms the state-of-the-art IrO catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm. In addition, we find that a carbon-paste conducting support with a hydrocarbon binder can improve the stability of metal-oxide catalysts in acidic media by providing a hydrophobic environment.

The efficient integration of photoactive and catalytic materials is key to promoting photoelectrochemical water splitting as a sustainable energy technology built on solar power. Here, we report highly stable water splitting photoanodes from BiVO4 photoactive cores decorated with CoFe Prussian blue-type electrocatalysts (CoFe-PB). This combination decreases the onset potential of BiVO4 by ∼0.8 V (down to 0.3 V vs reversible hydrogen electrode (RHE)) and increases the photovoltage by 0.45 V. The presence of the catalyst also leads to a remarkable 6-fold enhancement of the photocurrent at 1.23 V versus RHE, while keeping the light-harvesting ability of BiVO4. Structural and mechanistic studies indicate that CoFe-PB effectively acts as a true catalyst on BiVO4. This mechanism, stemming from the adequate alignment of the energy levels, as showed by density functional theory calculations, allows CoFe-PB to outperform all previous catalyst/BiVO4 junctions and, in addition, leads to noteworthy long-term stability. A bare 10–15% decrease in photocurrent was observed after more than 50 h of operation under light irradiation.

Catalyst modification of metal oxide photoanodes can result in markedly improved water oxidation efficiency. However, the reasons for improvement are often subtle and controversial. Upon depositing a CoFe Prussian blue (CoFe-PB) water oxidation catalyst on BiVO 4 , a large photocurrent increase and onset potential shift (up to 0.8 V) are observed, resulting in a substantially more efficient system with high stability. To elucidate the origin of this enhancement, we used time-resolved spectroscopies to compare the dynamics of photogenerated holes in modified and unmodified BiVO 4 films. Even in the absence of strong positive bias, a fast (pre-ms), largely irreversible hole transfer from BiVO 4 to CoFe-PB is observed. This process retards recombination, enabling holes to accumulate in the catalyst. Holes in CoFe-PB remain reactive, oxidizing water at a similar rate to holes in pristine BiVO 4 . CoFe-PB therefore enhances performance by presenting a favorable interface for efficient hole transfer, combined with the catalytic function necessary to drive water oxidation.

The structure and magnetic properties of novel hybrid molecule-based magnets formed by combination of two magnetically active sublattices, the bimetallic oxalatobridged honeycomb net [Mn II M III (ox) 3 ] (M II = Mn, Fe, Co, Cr, Ni, Cu; M III = Cr, Fe) and the organometallic cation decamethylferrocenium, are reported.Much interest has been shown in oxalate complexes in the search for new molecule-based magnets since the discovery at the beginning of the nineties of new polymeric bimetallic phases of variable dimensionality. 1 Especially interesting are the two-dimensional bimetallic phases A[M II M III (ox) 3 ] (M II = Mn, Fe, Co, Cr, Ni, Cu; M III = Cr, Fe) because they behave as ferro-, 2 ferri-3 or canted antiferro-magnets 4 with critical temperatures ranging from 5 up to 44 K. Their structure 5 consists of extended oxalate-bridged layers of the two metal atoms separated by A + , an electronically 'innocent' counter ion of the type [XR 4 ] + (X = N, P; R = Ph, Pr n , Bu n ), which may act as a template controlling the formation of the net structure and thus determining the interlayer separation. 4b The change of this cation to an electroactive one could create new hybrid molecular materials with interesting properties or combinations of properties, depending on the nature of the cation and the possible interactions with the magnetic network. Very recently we reported an example of such hybrid materials: the 6 in which the cation is the organic p-donor molecule tetrathiafulvalene (TTF), although instead of the extended two-dimensional network, a molecular layer of oxalato-bridged bimetallic trimeric clusters is formed.Here we show that it is possible to combine the layered bimetallic oxalate complexes with the magnetically interesting organometallic cation [Fe(Cp*) 2 ] + (Cp* = decamethylferrocenium), which has spin S = 1/2. It is to be noticed that other salts formed by decamethylmetallocenium cations and twodimensional molecule-based magnets have been very recently reported, but the structure of these materials is unknown. 7 The new family of organometallic-inorganic magnetic compounds has the formula [Fe(Cp* Crystals of good quality were obtained by diffusion, and the crystal structure was solved for the Fe III Mn II derivative. The structure † consists of anionic layers of the well known honeycomb bimetallic oxalato-bridged network with decamethylferrocenium cations intercalated in between [ Fig. 1(a)]. The bimetallic layers are eclipsed by each other creating hexagonal channels running along the c axis. The organometallic cations exhibit a staggered configuration (symmetry D 5d ). They are ordered within the interlayer space. In fact, they are located at the center of the hexagonal channels forming a pseudo-hexagonal arrangement. These cations are tilted with respect to the layer by an angle of 32.6° (angle defined by the fivefold symmetry axis of the cation with the normal to the bimetallic plane) in such a way that the two pentamethylcyclopentadienyl molecules of each [Fe(Cp*) 2 ] + cation are pointin...