splitting, low-cost and facile fabrication of electrocatalysts, active for oxygen evolution reaction (OER) without involving precious metals (e.g., Ru and Ir), is therefore the central interests. [8,9] General approaches and knowledge capable of enhancing atom-efficiency in OER are also essential to conserve all the elements used in electrocatalysts regardless the individual nature abundance.As compared to colloidal electrocatalysts, scalable thin film electrocatalysts have been recognized by rapid mass transfer and robust adhesion with electrodes against peeling of electrocatalytic materials during oxygen bubbling, essential to achieve durable water splitting, high power output, and commercial demand of coating on various substrates. [10] The previous OER studies have concentrated on exploring continuous, void-free thin films of Ni, Co, Fe, and Mn oxides in alkaline conditions, [5,[10][11][12][13][14][15][16][17][18][19] revealing the important electrocatalytic influence regarding crystal structures, synergistic behaviors between elements, catalytic roles of active species, alternation of orbitals and electron transfer, and deposition methodologies. As electrocatalytic OER requires electricity (anodic current) and mass transport of reactants (water/OH − ) and product (O 2 ) at catalytic sites to proceed, this so-called "triple-phase boundary region," [20] usually at the heterojunction edges, is anticipated to deliver the best OER activities. By dividing a continuous thin film into fractured, discontinuous pieces (as a concept of "thin film imperfection"), the additionally exposed edge sites can thus greatly improve OER performance and atom efficiency. Recent designs of single atom catalysts may further support this idea. [21,22] In fact, strong edge effects on improving hydrogen evolution reaction (the other half reaction in water splitting) have been well recognized. [23,24] However, this concept of edge exposure in thin film has received much less attentions possibly due to the investigation complexity and coverage uncertainty of discontinuous deposition. With the example of photolithography-patterned electrocatalysts toward improved OER, [25] correlation of OER performance to systematic edge exposure normalizing to unit area is then becoming the most desired information. Recent study on the edge-site atom population of discrete, atom-scale deposition of iron oxides shows a proportional relationship to OER activities. [26] Despite the emerging of edge-dependent OER, reliable deposition over large area with versatile coverage/continuity to achieve Thin film electrocatalysts allow strong binding and intimate electrical contact with electrodes, rapid mass transfer during reaction, and are generally more durable than powder electrocatalysts, which is particularly beneficial for gas evolution reactions. In this work, using cobalt manganese oxyhydroxide, an oxygen evolution reaction (OER) electrocatalyst that can be grown directly on various electrodes as a model system, it is demonstrated that breaking a continuous...
